# '65 GTO losing vacuum, stalling, when hard braking



## JesseLyon (Sep 13, 2020)

Hey all, I posted this in another thread, but starting a new post here for more visability.

I have a '65 GTO, tripower, 4spd, 4 wheel power disc brakes. Can ran great on the 4 barrel Edelbrock setup, but since swapping in '66 tri-power carbs and intake, having issues with the car losing vacuum and stalling out when hard braking. Oddly this does not happen when stopped and idling, vacuum does not change when I press the brake pedal in neutral, only while driving. Heres some video reference, hoping someone can help to lead me in the right direction...

Vacuum at idle with brake booster disconnected is around15/16...





Vacuum at idle with booster connected drops down to 10-12....(apologies for the tilted video)






Vacuum while driving seems ok, but you can see when I hit the brakes hard near the end of the video, vacuum drops to almost 0 and nearly stalls out....(phone fell off dash a few times, time to get a mount ). I have to feather the throttle in order to keep the car from stalling out while braking, and its a bit frustrating, if not a bit dangerous.






So all told, I guess this points to a leak at the brake booster. I am also seeing a bit of brake fluid dripping down the front of the booster too. Is there a kit available just for the seals on these, or would people here recommend swapping out the enitre booster and/or master cylinder? Or is there something else I should be looking into first?


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## 67ventwindow (Mar 3, 2020)

Your videos show up as private.


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## JesseLyon (Sep 13, 2020)

67ventwindow said:


> Your videos show up as private.


Newb error...thank you! can you see them now?


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## BearGFR (Aug 25, 2008)

Well if it was truly 'fine' before the tripower swap and isn't now, then the problem has to be either with the tripower itself or something else you "touched" when you made the swap. 

One question: is the 'disappearing' engine vacuum a cause or is it an effect of something else? 

My first suspicion would be fuel slosh under braking. Float levels "off" or dinner other problem that allows excess fuel to spill over and flood it, or spilling 'out' and causing it to go severely lean. 

Related question: How did you get your tripower? If you bought it from someone else, perhaps the reason it was for sale was because they never could get it to run right either. 

Bear


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## PontiacJim (Dec 29, 2012)

BearGFR said:


> Well if it was truly 'fine' before the tripower swap and isn't now, then the problem has to be either with the tripower itself or something else you "touched" when you made the swap.
> 
> One question: is the 'disappearing' engine vacuum a cause or is it an effect of something else?
> 
> ...



Hey Bear,

Here is what has already been posted - condensed without the video's breaking up the observations. Appears it could be the MC, Brake Booster gone bad, or even the check valve/rubber seal going into the brake booster if it has this - or all of the above.


*JesseLyon*

"OK, so heres what Im seeing with some video reference...

Vacuum at idle with brake booster disconnected is around15/16...

Vacuum at idle with booster connected drops down to 10-12....(apologies for the tilted video)

Vacuum while driving seems ok, but you can see when I hit the brakes hard, vacuum drops to almost 0 and nearly stalls out....(phone fell off dash a few times, time to get a mount )

So all told, I guess this points to a leak at the brake booster. I am also seeing a bit of brake fluid dripping down the front of the booster too, so I guess its time to spend some money. Is there a kit available just for the seals on these, or would people here recommend swapping out the enitre booster and/or master cylinder?"


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## JesseLyon (Sep 13, 2020)

Ive set the videos to "Public" now, so should be able to see them.

The carbs and intake were purchased used on Craigslist and I had them all rebuilt by a reputable specialist. He said they were all in fine shape, and that I "shouldnt have any problems". Car runs great when Im on the throttle! The fluid leakin gfrom the booster is relatively new, so I think all things point to that, just wanted some other input before I started pulling everything apart.

The fuel slosh and float levels could be something. Im also noticing some fuel leaking out of the top of the front carb after hard accleration. 

Ive tested the booster check valve, I can blow air through one way and not the other, so appears to be fine

If you guys have a chance, please take a look at the videos I posted above. Im hoping you might se something that I dont.


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## PontiacJim (Dec 29, 2012)

JesseLyon said:


> Ive set the videos to "Public" now, so should be able to see them.
> 
> The carbs and intake were purchased used on Craigslist and I had them all rebuilt by a reputable specialist. He said they were all in fine shape, and that I "shouldnt have any problems". Car runs great when Im on the throttle! The fluid leakin gfrom the booster is relatively new, so I think all things point to that, just wanted some other input before I started pulling everything apart.
> 
> ...


First video, were you cracking open the center carb only? Did not sound "snappy" in response. Sounded like it wanted to die a bit rather than race up in RPM's. Timing issues or accelerator pump issue? Vacuum of 15/16 inches is OK at idle, and assume the engine has a cam. Stock vacuum on a Pontiac, let's say grandma's car, should be around 20-22 Hg. 

Your idle may be too low. What is your idle in Drive?

Have you timed the engine for maximum vacuum? Have you adjusted the center carb idle mixture screws for highest vacuum? May want to actually make the idle mixture screws richer to test if that helps.

Could be the MC/ power booster. Usually when you see fluid, the seal on the MC is leaking into the power booster. I don't recall if this can cause an issue with the power booster or not, can't recall if the MC is sealed at the power booster so it does not leak in or not. In any case, MC needs to be replaced.

Check valve test seems to indicate it is OK. Your power booster may be ok as typically when the power booster doesn't hold vacuum, brake pedal becomes hard and/or it's height will change - I'm thinking the pedal goes further down to the floor. You may be able to test the booster using one of the hand pump brake/vacuum guns bought at a auto store - they typically have adapters that should allow you to adapt it to the hose going into the brake booster, build vacuum, and see if it holds - that will stop all the guessing.

Third video looks normal. 15/16 driving, let off the gas, jumps up 20-25 when carb throttle plate is shut. Drops like a stone when braking. I don't think that is good as the booster should have enough vacuum in it to operate the brakes and not drop like that. Even though the check valve seemed OK, that check valve should hold vacuum in the booster for a few applications and then draws vacuum from the intake as needed. If the booster is flat out bad, then it would not hold vacuum, the check valve would still be good and doing its job, but the brake booster is drawing all the engine vacuum through the check valve (as it should) and does not stop drawing vacuum because it is bad, ie may have a rip or tear in the diaphragm.

What happens if setting still in Park and you press on the brake pedal without moving? If the brake booster is bad, with the vacuum gauge hooked up, you should see vacuum drop and/or the engine run rough as it loses vacuum.

Have you tested your vacuum advance can on the distributor to make sure it is working? Here again you can use the brake/vacuum gun and with a timing light, shoot the timing mark on the balancer to see the initial timing (should be 9-12 degrees), and with the vacuum port on the intake/carb plugged, attach the vacuum line from the distributor onto the vacuum gun. When you begin to build vacuum on the gun, this will in turn pull vacuum on the vacuum can in the distributor. Watch your timing mark on the balancer and it will increase as you pull vacuum on the vacuum can - you are using the vacuum can to advance the timing at the distributor. If it moves, make note how many degrees, your timing mark on the balancer should move from 9-12 degrees and move up to 19-22 or so degrees. This means it is working. If not, then you may have distributor issues and you are not getting the advance the engine needs - which could be a cause for stalling.

Some Pontiac, 1960-1967, used a carburetor dashpot on the carb. This was used to slow the rate at which the carb closed. Some cars would stall out when you did a panic brake. It prevents the engine from decelerating too quickly. It *is* composed of a cylinder, spring and a shaft that *is* attached to the throttle. This device was used for that so the engine would not stall and you could safely brake. (Sort of the same idea as the later electric carb solenoid Pontiac used). Picture enclosed. You may need to add this.


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## JesseLyon (Sep 13, 2020)

Hi Jim, thanks very much for the detailed response! Lets see If I can answer some of your questions to try and get to the bottom of this....



PontiacJim said:


> First video, were you cracking open the center carb only? Did not sound "snappy" in response. Sounded like it wanted to die a bit rather than race up in RPM's. Timing issues or accelerator pump issue? Vacuum of 15/16 inches is OK at idle, and assume the engine has a cam. Stock vacuum on a Pontiac, let's say grandma's car, should be around 20-22 Hg.


- I believe I was opening the center carb only during those idle tests. The engine has a bit of a cam, so looks like Im ok around 15/16. It does say timing needs to be adjusted in that area on the vacuum gauge, but It was set a few weeks ago along with air/fuel mixture when I took it into my local shop. They had a bunch of classics in there and came highly recommended, so I do trust they know what they're doing.



> Your idle may be too low. What is your idle in Drive?


- Car is 4spd manual M21, idle in neutral is around 800-900rpm



> Have you timed the engine for maximum vacuum? Have you adjusted the center carb idle mixture screws for highest vacuum? May want to actually make the idle mixture screws richer to test if that helps.


- as above, timing and idle mix was set by the shop, and I assume they did it properly. It came out of there running much better then how I had it setup (other then this stalling issue)



> Could be the MC/ power booster. Usually when you see fluid, the seal on the MC is leaking into the power booster. I don't recall if this can cause an issue with the power booster or not, can't recall if the MC is sealed at the power booster so it does not leak in or not. In any case, MC needs to be replaced.


- I have a new master and booster on order, this is my next step. I should likely replace the master due to the leaking fluid, even if its not causing my vacuum issues.



> Check valve test seems to indicate it is OK. Your power booster may be ok as typically when the power booster doesn't hold vacuum, brake pedal becomes hard and/or it's height will change - I'm thinking the pedal goes further down to the floor. You may be able to test the booster using one of the hand pump brake/vacuum guns bought at a auto store - they typically have adapters that should allow you to adapt it to the hose going into the brake booster, build vacuum, and see if it holds - that will stop all the guessing.


- Pedal seems to hold a good amount of braking power at all times. Pump pedal a few times with car off to build pressure, start car, pedal drops an inch or so, and maintains constant pressue while driving and hard braking. Also points to booster and master being ok...confusing.

- Will look for one of these hand pumps to test booster, but hesitant to buy yet another tool I will use once that sits in storage. Maybe I can remove the booster and have a shop test, but they will likely charge me that same price as buying the tool...



> Third video looks normal. 15/16 driving, let off the gas, jumps up 20-25 when carb throttle plate is shut. Drops like a stone when braking. I don't think that is good as the booster should have enough vacuum in it to operate the brakes and not drop like that. Even though the check valve seemed OK, that check valve should hold vacuum in the booster for a few applications and then draws vacuum from the intake as needed. If the booster is flat out bad, then it would not hold vacuum, the check valve would still be good and doing its job, but the brake booster is drawing all the engine vacuum through the check valve (as it should) and does not stop drawing vacuum because it is bad, ie may have a rip or tear in the diaphragm.


- Maybe this is the case. I did notice that when I pulled the check valve off the booster, there was a big release of air pressure through the hole, which makes me think everything actually might be ok with the booster...again, very confusing.



> What happens if setting still in Park and you press on the brake pedal without moving? If the brake booster is bad, with the vacuum gauge hooked up, you should see vacuum drop and/or the engine run rough as it loses vacuum.


- I think I mentioned much earlier in the thread, there is absolutely no drop in RPM when idling and I press the brake pedal. Only while driving. I also find this to point to something other then the booster.



> Have you tested your vacuum advance can on the distributor to make sure it is working? Here again you can use the brake/vacuum gun and with a timing light, shoot the timing mark on the balancer to see the initial timing (should be 9-12 degrees), and with the vacuum port on the intake/carb plugged, attach the vacuum line from the distributor onto the vacuum gun. When you begin to build vacuum on the gun, this will in turn pull vacuum on the vacuum can in the distributor. Watch your timing mark on the balancer and it will increase as you pull vacuum on the vacuum can - you are using the vacuum can to advance the timing at the distributor. If it moves, make note how many degrees, your timing mark on the balancer should move from 9-12 degrees and move up to 19-22 or so degrees. This means it is working. If not, then you may have distributor issues and you are not getting the advance the engine needs - which could be a cause for stalling


- Im running MSD ignition and distributor, so no vacuum advance.



> Some Pontiac, 1960-1967, used a carburetor dashpot on the carb. This was used to slow the rate at which the carb closed. Some cars would stall out when you did a panic brake. It prevents the engine from decelerating too quickly. It *is* composed of a cylinder, spring and a shaft that *is* attached to the throttle. This device was used for that so the engine would not stall and you could safely brake. (Sort of the same idea as the later electric carb solenoid Pontiac used). Picture enclosed. You may need to add this.


- I am finding that the RPMs drop very slowly when I release the throttle at high RPM in neutral. I also noted this in another thread I had going a few weeks back. Is this the pot that attaches to the choke, circled in red below? Or is that something else and only related only to the choke? Maybe this is catching and needs to be replaced? For whats its worth, the car is very hard to start now when hot. Almost like its flooded, and when it does finally start up, theres a big cloud of exhaust smoke. Thinking this is pointing to the choke maybe not operating as it should when the engine is warm. Could all these issues be related?

Thanks for all of your advice and wisdom Jim, its very much appreciated!


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## PontiacJim (Dec 29, 2012)

The item circled in red should be the choke pull-off. Not related to the dashpot, but looks the same and I believe may operate the same. Your choke is operated by the rod coming out of the bi-metal spring found in that "box" on the manifold. The choke pull-off is operated off vacuum and is designed to open the *choke* plate slightly after the engine starts - as the choke should be set closed initially for start up. If it were to remain closed after the engine fires, the engine would quit, so the choke pull-off cracks the choke enough to allow air through for the engine to run. As the engine heats up, hot exhaust gases pass through the exhaust crossover under the manifold and the bi-metal spring, and the bi-metal spring expands and pushes on the choke rod to open the choke. The choke should be fully open when warm/hot.









1966 choke pull-off


New white plastic choke diaphram and plated bracket.I also stock the short pull-off rod and the bracket at end of the choke shaft. Let me know if you need either of these.



pontiactripower.com





The dahspot would fit at the end of the throttle rod or act upon the carb linkage in some manner to hold the throttle RPM at a higher RPM, like the electric solenoid, and then when engine vacuum is diminsished, it would slowly drop down to a lower Idle setting, such as the idle with the pin activated fully out might be high idle might be 800 RPM's and when vacuum is lost the pin would slowly retract and drop down to maybe 600 RPM's. What this does is keep a minimum idle speed setting of 800RPM's and when you let off the gas, it does not immediately drop down to 600 RPM's or low idle - which can cause engine stalling. HOWEVER, you have your idle speed set at 800 RPM so this may not do anything for your situation. If you had problems with the engine continuing to run-on when you shut off the key, then this would help and was the purpose of the later electric solenoids Pontiac used.

Hard starting, such as the starter is laboring to turn the engine over can mean the timing is too advanced. Did it do this before sending it to the shop?

If it is turning over good and is hard starting because it takes a bit for the engine to fire, then that could indicate a rich fuel condition such as flooding, leak, or choke setting. It is possible to be timing related as well.

Did the shop give you what they set the timing at? Initial timing at the balancer with the vacuum advance disconnected and then connected? Total mechanical timing (Initial plus the distributor weights) with vacuum advance disconnected? And at what engine RPM was Total timing achieved (2,500 RPM, 2,800 RPM, 3,000 RPM)?

Blue smoke from the exhaust after the car has sat and you first fire it up is an indicator of worn valve guides OR valve seals. When the car sits, oil will seep down along the valves past the guides, and into the combustion chamber where it gets burned off upon initial start up. The tolerances between the valve stem/guide do wear and open up, so this allows oil to find its way down the guides. However, Pontiac did use a generous valve guide clearance spec to purposely lubricate the valve guides. So some is OK. I have seen when the heads are rebuilt the "oil shields" are left off which are used to help direct oil off the valve stems and prevent excessive oil splash onto the valve stems and down the guides. The rubber O-ring type valve seals can get brittle and crack/shrink and allow oil to seep down the valve stem even with the oil shields in place. Again, when the car sits after shutting it off, oil seeps down the valves and into the combustion chambers - not much at all in either cases, but a few drops here and there is all that is needed.

The blue smoke only becomes a problem when it is excessive, such as present when the engine is running after initial start up or oil consumption is noticeable and you are adding a quart of oil often to keep it filled. Again, did it do this before having the timing adjusted or is it new since the timing has been adjusted?

Could it be more black like and maybe excessive fuel burning off, ie flooding and connected to the hard start issue? You could do an easy check by pulling the spark plugs and check for wetness from any excessive gas on them. You should also be able to smell a strong odor of gas on them as well if flooding or a carb leaking.

It could also be boil over of the gas in the carb bowls. Underhood temps increase when you shut the engine off. The heat can literally cause the gas to percolate just like hot coffee. This would force raw gas out from the carb and into the intake manifold, just like a leak would. So this happening could fool you into thinking you have a carb leak when in fact you don't, but the fuel is escaping from some place within the carb just as it would a leak. This is why some will install the phenolic spacers under the tri-power carbs and even openup the hood scoop ala ram air, so the underhood heat can escape.

Do you know what temps your engine runs?

So you may have several issues going on all at once. Power booster/MC would be first so as to see if that cures the vacuum issue and you want a new MC anyway since yours is leaking.

Then go from there. Don't make a bunch of changes at once - do only 1 at a time and see what the results are first, then go to the next item and tackle it.


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## JesseLyon (Sep 13, 2020)

Awesome, so much great advice. Thanks very much for taking the time to respond. The hard starting when hot is a relatively new thing, was not doing this for a while since I setup the tri-power and had it tuned by the shop. It seems to have started doing this since I noticed some fuel leaking from the top gasket of the front carb. Im also wondering if its choke related, and maybe the choke is sticking on once warmed up. It turns over fine, battery and starter are good, but I need to really pump the gas for a while to get it going...and this happns only when hot and I turn it off for a few mins, and get a puff of white smoke out the back.

In any case, I picked up new M/C and booster today, and going to attempt the replacement this weekend. Hoping that cures some of the vaccum issues. Ive never bled brakes before though, so that will be interesting. Luckily YouTube has everything covered 

Thanks again, and will update next week.


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## PontiacJim (Dec 29, 2012)

JesseLyon said:


> Awesome, so much great advice. Thanks very much for taking the time to respond. The hard starting when hot is a relatively new thing, was not doing this for a while since I setup the tri-power and had it tuned by the shop. It seems to have started doing this since I noticed some fuel leaking from the top gasket of the front carb. Im also wondering if its choke related, and maybe the choke is sticking on once warmed up. It turns over fine, battery and starter are good, but I need to really pump the gas for a while to get it going...and this happns only when hot and I turn it off for a few mins, and get a puff of white smoke out the back.
> 
> In any case, I picked up new M/C and booster today, and going to attempt the replacement this weekend. Hoping that cures some of the vaccum issues. Ive never bled brakes before though, so that will be interesting. Luckily YouTube has everything covered
> 
> Thanks again, and will update next week.



Sounds like a plan. Make sure you "bench bleed" the master cylinder before you install it. This will ensure you get all the air out of it.


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## JesseLyon (Sep 13, 2020)

PontiacJim said:


> Sounds like a plan. Make sure you "bench bleed" the master cylinder before you install it. This will ensure you get all the air out of it.


BTW, to answer your question re temps, Im running electric fans, 180 degree thermostat, fan temp switch sets to open at 200-210, so temps seem to hover around 190-200. Seems pretty good, no overheating. I just installed a fan override switch under the dash so I can turn it on manually if things get ugly 

Also. re the shop setting the timing, I am not sure what they ended up setting it at, I should have asked. Maybe will call them up and see if they know.


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## JesseLyon (Sep 13, 2020)

PontiacJim said:


> Sounds like a plan. Make sure you "bench bleed" the master cylinder before you install it. This will ensure you get all the air out of it.


Maybe dumb question, but I dont really have the abillity to bleed the rest of the brake system where my car is currently sitting, only to do the M/C bench bleed. In your opinion, would it be safe to drive the car after just the bench bleed, and then do the rest of the system elsewhere? Or would you say only option is to bleed the entire system before driving the car? (In which case, Im going to take the car somewhere else to do this work)


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## Lemans guy (Oct 14, 2014)

Not safe to drive unles you bleed all four wheels. MC first.

Also you may be experiencing”Heat soak” regarding your hot starting problem and pumping the gas pedal only makes that worse.

Here is what may be causing your hot start problem. When you shut the car off, the temperature of the engine, the engine compartment and everything around it heat up. Modern gas has a comparatively low boiling point. And since your Tri-power does not have phenolic spacers between the carb and the intake. It heats up real fast too.

Thengas boils in the carb when shutoff, it boils over into the intake and sometimes you can see gas leaking out the throttle blade shafts.

15 minutes later you return and now you are flooded. It won’t start. So you furiously pump the pedal to make it start, but each pump activates the accelerator pump which shoots more fuel into your flooded intake. Eventually you get it to start and a big puff of white & Blue smoke comes out the tailpipe, from too much fuel.

so what do you do. First follow everything PJ described about the choke, choke pull off etc. and. Any adjustments he recommended.

then take a test drive stop somewhere and put on your electric fans with your remote switch whether the coolant is too hot or not. You can also open the hood as this aids cooling. then tell everybody who stops to help you are Ok. After 15 minutes close the hood and shutoff the fans. Now don’t pump the gas at all just try to start the car no pedal.


If it does not start on the first try SLOWLY without activating the accelerator pump, push the pedal halfway down and try to crank again. Should start.

If it works, you may be experiencing heat soak. And the reason it worked is that your fans and open hood kept the temp down enough to keep the fuel from boiling into the intake, when you half opened the pedal it allowed enough air to come into the intake to overcome the fuel that boiled out.

so see if that works. A choke stuck closed will also cause car to hard start when hot, so make sure it is fully open when hot like PJ said!

One last thing, make sure that you don’t have gasoline left in the tank from March or April. As that is winter blend gas and it has a very low volatility so it will start in cold weather. But when hot it boils easier than summer blend gas.....you need fresh currently blended fuel.

Good luck you will get it


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## JesseLyon (Sep 13, 2020)

Lemans guy said:


> Not safe to drive unles you bleed all four wheels. MC first.
> 
> Also you may be experiencing”Heat soak” regarding your hot starting problem and pumping the gas pedal only makes that worse.
> 
> ...


I guess this could also explain the fuel leaking out the top of the front carb as well? I'll try this hot starting method you mentioned next time Im out and see what happens. If I start the car up right away after shutting it off or after it stalls out, seems to fire up right away. Only occurs when I let it still for 10-15mins and its cooled off a bit, but not entirely, so I think everything you are saying makes sense. Thanks!

Also, thanks for the advice re. brake bleeding. I will take the car somewhere I can bleed the entire system before starting the work.


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## Lemans guy (Oct 14, 2014)

Pontiac Jim said it here first that it might be heat soak. he also lined out many other possibilities and he is spot on.

so I am concurring with PJ, the reason I think you have heat soak is 1) starts good cold 2) runs good 3) starts right up hot after shutdown 4) won’t start or hard start hot after 15 or twenty minutes 5) you don’t have a phenolic spacer for heat shielding

when you shut off a hot engine timing will not be different if you start t immediately or in twenty minutes (Unless you break something) but this is a repeatable problem.

i


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## Lemans guy (Oct 14, 2014)

So had to break off sorry, yes I am betting that it is heat soak. So try to he test and we can later talk about how to beat it.


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## JesseLyon (Sep 13, 2020)

Update on this thead...hard start issue is definitely heat soak, as it starts much easier with the recommendations outlined by Lemans Guy and Pontiac Jim, so thanks very much for the advice.

Re. the stalling out when hard braking...Ive deternined that the vacuum is fine on all carbs, and brake booster, so its not related to that. I did however notice today when coasting down our parkade in neutral, at idle, that the car also wanted to die. Car does not want to stall out when its idling with foot on the brake on level ground. The parkade has a fairly steep grade, so I wonder, could this point more towards float levels? Should I try setting the center carb float level a bit higher or lower to see if it helps?


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## Lemans guy (Oct 14, 2014)

Yes , it could be flooding and then stalling. When you described the vacumn drop on hard braking and now established that it is not the brake system & booster that are leaking vacumn. Nevertheless your vacumn on the video make a deep drop on hard braking, but the stall could have proceeded that, and therefore the loss of vacumn occurred immediately. Making it seem that the cause was the vacumn loss, while it was actually the stall.

Maybe,.....your description of brake fluid leaking at thebooster and vacumn loss made the booster suspect. Sometimes a booster will only leak vacumn when the brake pedal is depressed and not sitting with the pedal up. But now you have tested at idle with foot on the brake. (make sure you test it at idle with hard foot on the brake) to be absolutely sure.

anthorenway is to drive the car with the vacumn booster not connected and plugged. You won’t have power brakes, but your brakes will work fine, just tougher and be sure.

but float levels are suspect here, even fuel pump pressure.


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## BearGFR (Aug 25, 2008)

JesseLyon said:


> Update on this thead...hard start issue is definitely heat soak, as it starts much easier with the recommendations outlined by Lemans Guy and Pontiac Jim, so thanks very much for the advice.
> 
> Re. the stalling out when hard braking...Ive deternined that the vacuum is fine on all carbs, and brake booster, so its not related to that. I did however notice today when coasting down our parkade in neutral, at idle, that the car also wanted to die. Car does not want to stall out when its idling with foot on the brake on level ground. The parkade has a fairly steep grade, so I wonder, could this point more towards float levels? Should I try setting the center carb float level a bit higher or lower to see if it helps?


Having the problem on an incline is more evidence that you've got a fuel slosh problem.

Before you start adjusting things, make sure the float/needle/seat assemblies in all 3 carbs are in good working order and adjusted correctly. Contrary to popular belief, Pontiac engineers weren't dummies. When everything is in good condition and adjusted to factory specs, these problems won't happen. 

Make sure all float levels are set correctly. 

Next make sure none of the floats are leaking (if they're brass, take them out and shake - if you hear fuel sloshing, it's leaking and needs to be replaced. If they're composite/foam, look for nicks or scratches on the surface. If present that can let them soak up fuel like a sponge, get heavier, and not work correctly. If you have an accurate scale you can also weigh each one IMMEDIATELY after you remove it - before any absorbed fuel has a chance to evaporate. They should all weigh the same.)

Visually inspect all the seats and needles, looking for "wear rings" or other nicks/problems that might allow them to leak. Here's a great video that shows how you can test for s positive seal.
Testing a carburetor needle and seat

I'm not that familiar with tripower carbs so I don't know if they're supposed to have any sort of internal baffle to prevent fuel slosh. That would be worth researching and checking out too.

Make sure all the gaskets are in good condition and sealed.

If you really do have a fuel slosh problem, and it sounds like you do, then your challenge now is to find where it is.

Only after you've made certain that everything having to do with controlling fuel levels in the carbs is all correct, can you consider making float level changes--- and you'd want to LOWER the level, not raise it. But, be aware that changing the float level is also going to change how the carb meters fuel and your fuel mixture. A lower float level is going to make it run leaner, so if you REALLY insist on trying that, don't be surprised if you create a lean misfire condition and end up having to put larger jets in the carb to compensate.

Bear


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## JesseLyon (Sep 13, 2020)

BearGFR said:


> Having the problem on an incline is more evidence that you've got a fuel slosh problem.
> 
> Before you start adjusting things, make sure the float/needle/seat assemblies in all 3 carbs are in good working order and adjusted correctly. Contrary to popular belief, Pontiac engineers weren't dummies. When everything is in good condition and adjusted to factory specs, these problems won't happen.
> 
> ...


Thanks for the info Bear and LeMansGuy....looks like I have some more detective work to do....time to enjoy the hobby that keeps me broke and frustrated but at least out of trouble!


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## JesseLyon (Sep 13, 2020)

BearGFR said:


> Having the problem on an incline is more evidence that you've got a fuel slosh problem.
> 
> Before you start adjusting things, make sure the float/needle/seat assemblies in all 3 carbs are in good working order and adjusted correctly. Contrary to popular belief, Pontiac engineers weren't dummies. When everything is in good condition and adjusted to factory specs, these problems won't happen.
> 
> ...


Is there ever some kind of diffuser or sponge in the carb to help with fuel slosh?


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