# what would cause gas to boil in a carburator



## trainman115 (Jul 31, 2014)

Does anyone know what would cause gas to boil in the carburator?? should I block off the crossover?? I do not have a cross over pipe in the exchaust system would this do it?? could the mufflers be blocked ??? maybe not on right?? I have no arrow or A&B on my mufflers, This is the way I got the car the exchaustr is new as well as the gas tank, lines, fuel pump. any help will be greatly appreciated Ed working on a 1969 GTO 350 engine just until I can get money to have the 400 rebuilt


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## Roger that (Feb 6, 2010)

Ethanol fuel. It seems that many are using Phenolic insulators under the thottle body to reduce this problem. Ethanol fuel has a boiling point around 20 to 30 degrees below straight gasoline's boiling temp.


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## Lemans guy (Oct 14, 2014)

trainman; I blocked the crossover in my 66 lemans and I do think that helps in summer months. First determine is it overheating or running at correct thermostat temp? Temp gauge or infared handheld themometer. Cooling systems things first, water pump, thermostat, Radiator, fan, fan clutch...is Radiator clear for air and water flow...Check the fins.

If that is OK than things like ethanol gas and a real hot carb..can contribute to that. Timing as well as retarded timing makes the engine run hotter than advanced at idle


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## Rukee (Feb 8, 2007)

Along with everything else mentioned run ethanol free fuel, it will help


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## geeteeohguy (Feb 2, 2008)

Some of us can't....ethanol contaminated fuel is all that is available to me, for instance. What these guys said: insulate the carb. Blocking off the heat crossover on a street driven car is ill-advised.


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## trainman115 (Jul 31, 2014)

*carb*

Thanks Guys i will try it and see what happens I did look up the carb mounting gasket and the one it show for the 69 has 4 holes where mine is all open except around the edge so I think I will order the right gasket as well and I aslo pulled off the 2 mufflers and they are different one is longer then the other and the othere one dont have much air flow at all so I am going to put new mufflers on as well not sure what kind to go with but I want something thats sounds great Thanks again for all your help Ed


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## 05GTO (Oct 6, 2004)

In addition to the suggestions mentioned above, check the routing of the fuel line and make sure it is inside the frame from the frame u-channel at the driver's floorboard up to the cross member under the fuel pump. If the fuel line is too close to the exhaust manifold or pipes the fuel will vaporize.


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## Lemans guy (Oct 14, 2014)

all good points, on my 66 lemans it came with AC. I took it out but left in the 1/4 inch return fuel line which Pontiac engineers put in to keep the fuel cooler on an AC car that constant return of fuel helps cool it. As I mentioned timing, I run full manifold vacumn at carb at idle for 25 degrees BTDC with Vacumn advance.

snooth and cool idle, with that large radiator 4 core and sven bladed fan that the AC cars had


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## trainman115 (Jul 31, 2014)

Thanks for the reply Ya mine has the 4 core and I will try what you said Thanks again Ed


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## Lemans guy (Oct 14, 2014)

trainman; Also of note on the 66 AC cars Pontiac engineers were trying to defeat percolation of the gas at idle due to the heat dumped in the engine compartment from the AC Condenser. So as I mentioned 1/4 return fuel line,4 core radiator, seven blade fan.

But they also put a thermostatic valve in the back of the carburatur that would open an air vent at idle when the percolation of fuel causes poor idle. It sat on the back of carb. 

There was an after market version as well called "carbaire" that would work the same it would open when the carb temp was hot and vent some of the mixture, so slight, but would prevent the boiling, and that was on straight gas.

It was a problem even then. But don't forget timing as retarded timing at idle was only a crude emissions fix, if you don't have proper advance at idle from a good vacumn cannister car will run hooter at idle.

Of course this is all after your total cooling system is A OK:eek2:


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## trainman115 (Jul 31, 2014)

*Thanks once Again guys*

Ok Thanks again for the reply I ordered the right carb gasket yesterday and also 2 new mufflers hope they are in today I will let you know if that helps everything else that you guys said to do all seem fine with my car the temp runs right on the money when the engine is ran up to tempture no problem with that like I said I did find two different size mufflers on my car which I was told that the exchaust has to be equal on both sides I am not sure if that is true or not the only other problem I have is when the car does idiel and I do to pput it in drive it stalls right out not sure if that has to do with the carb problem or if I have a different one The trans is a stock 400 turbo (At least I think) but I guess the first thing to do is get the motor running right then take it from there
Thanks once again I really appreciate all your help Ed


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## Lemans guy (Oct 14, 2014)

Those are good moves and lots of guys use those phenolic carb spacers with great effect. On the stall start with right carb and choke adjustment and right timing.

That should get you idling if ignition and compression are OK....

Your passion for it shows you will get it!


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## trainman115 (Jul 31, 2014)

*Thanks*

Thanks with out you guys I would never get it I use to have a 69 Judge but had to sell it many years ago due to fixing up a house for our family now that I am older and bought another GTO I just hope to make it for a ride in my GTO now would be a dream come true but she need so much if the car was suppose to be in the condition it was stated to be in I would not have all theses problems I could have put the money into the body and interior but it is what it is I will never buy another car of Ebay thats for sure but its my baby and I want to bring her back to life for this ca means the world to me and to my one son too he is only 12 but got his heart into it selling off some of his treasures so we can put the money into the GTO me as well selling off my trains when I can go for a ride with him in it it would be like hitting the lottery or the day my sons were born we will keep working on it if we can but again I think its just a dream come true for me Thanks again so much for all your help I could not have got this far with out the form and U guys Ed


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## Lemans guy (Oct 14, 2014)

Train you are on the right track, body and interior, are not necessary for fun! The well worn look is now a full category at the Pebble Beach concours with all the high dollar cars. Original Patina can be cool as well, just make it run smooth and clean it. A little rust and fabric tears don't erase fun...the sense of accomplishment for your son at that age is cool as well, just keep his hands away from the fan when your tinkering!

It may not be as far off as you think and PS the Rat rodders that I see at the shows are always having fun and laughing! Talk about body work and paint! 

Make it run and have Fun......


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## trainman115 (Jul 31, 2014)

*carb trouble*

Well I changed the mufflers on my 69 car runs a lot better ideling now but I still have the same problem when car is shut down I have lots of white vapor coming from the top of the carb and still a little smell of raw gas from the back I have tryied 3 different carbs now all 3 the same the only thing I did not change is the intake which I dont see a problem the float or the fuel pump I did take notice that when the car sits over night I have to fill the the filter up and the carb is empty I dont know if the filter (I have a clear one in the line) should stay full or not. I have the timing set at 9 degrees TDC as the book stated I hate to invest in a new carb thinking I will still have the same thing I have no pressure in the fuel line after shut down I can squeez the rubber line together with my fingers I have no clue what to do next buy a new float?? Fuel pump?? or junk the car no only kidding any thoughts Ed


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## chuckha62 (Apr 5, 2010)

So, there are a lot of things you can do to tighten up your entire fuel system... 

Starting at the tank and moving forward you should replace every rubber line in your fuel system. If someone has replaced the steel lines with rubber, the correct lines are available and should be used. That will help with the empty fuel filter. Next, do not use a glass or plastic fuel filter. That's a fire waiting to happen.

You say you get a trail of smoke or steam after you shut down the engine? This is a sign of running rich. It may NOT be your carburetor. When replacing the fuel pump, most shadetree guys and MANY "professionals" just go to the parts store and get whatever they offer as a replacement fuel pump. This has created as many problems as it has solved. The factory AC Delco fuel pump was calibrated to approx. 4.5 PSI of fuel pressure. The aftermarket pumps are routinely in the 7.5 PSI (and up) range. THIS is enough pressure to unseat your needle and seat in the float bowl and will make your car run rich, idle poorly and cause all sorts of driveability issues. Do yourself a favor and get an AC Delco pump. You won't be sorry!

Next, do a bench setting of your carburetor before you start your car. There are all sorts of instructions available on the interweb, but the basics are this... Your choke plate should be set to around 3/16" clearance in the choke horn, when cold. Your idle mixture screws should be indexed (lightly bottomed out) then turned approx. 1-1/2 turns out as a starting point. This will usually get you VERY close. Then, with the choke fully open and the fast idle cam fully disengaged, the idle speed screw should be backed off so that the throttle blades are fully closed. Then carefully turn the idle speed screw in until it begins to touch the arm. Once it has just touched, turn it in approx. 1-1/2 to 2 turns. 

This should be a base setting that is close enough to allow it to run and idle. From there, set your timing and finish with fine adjustments to idle speed, etc. Don't set the timing by ear! Use a timing light and set it correctly with the vacuum advance blocked off. Setting it by ear is asking for problems. My '67 runs the best (no stumble) at around 10 degrees initial, but this is aggressive given the quality of today's fuel. (I mix in 110 octane racing fuel).

I hope I've helped. I do suspect your fuel pump more than anything, but it's also sort of a soap box issue for me, so YMMV.

Good luck!

Chuck


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## trainman115 (Jul 31, 2014)

*carb*

Thanks Chuck the fuel pump is the only thing I did not change yet I thought about it but wasnt sure I am going to order a AC pump like you said and see what happeneds Thanks again for your help Ed


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## trainman115 (Jul 31, 2014)

Chuck I been looking all day for the AC fuel pump I found the number 40679 but How can I find out the pressure seems no one knows and are you talking about the new AC or the NOS AC Thanks Ed


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## chuckha62 (Apr 5, 2010)

They don't advertise the fuel pressure, unfortunately. I believe Ames sells the correctly calibrated pumps. If you don't have an Ames Performance Engineering or Performance Years catalog, get one. 

Ames Lists two, one with the vapor return vent (N128PJ) tube and one without (N128PH). They show them to no longer be AC Delco, but call and ask if they can guarantee the AC Delco fuel pressures. It does make a difference.

Good luck! Chuck

edit. I checked the standard Airtek replacement and they state a maximum pressure of 6.5 psi. This MAY work just fine, but you wouldn't know for sure until you try. I've never had good luck with Mr. Gasket pressure regulators but other, more expensive regulators may work just fine.


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## trainman115 (Jul 31, 2014)

Thanks Chuck ya I did put a fuel pressure gauge on my pump it says about 10 lbs maybe way to much I will try to get another pump Thanks again Ed


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## chuckha62 (Apr 5, 2010)

trainman115 said:


> Thanks Chuck ya I did put a fuel pressure gauge on my pump it says about 10 lbs maybe way to much I will try to get another pump Thanks again Ed


There ya go... That's way too much. Holley Carburetors tend to be very fuel pressure tolerant, but AFBs (Edelbrock) and Quadrajets are very sensitive to excess fuel pressure.


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## Matthew (Feb 11, 2011)

I agree that there's too much pressure from that pump and changing it out for the right one is the thing to do; however, there should be zero pressure when the car is shut off. The problem sounds to be continued leaking of fuel into the venturi after shut down. Apologize if I misunderstand the problem here, but not sure this is going to fix it. Matt


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## chuckha62 (Apr 5, 2010)

I suspect the fuel after shutdown is just a residual from being over rich at idle. Also, his well plugs are probably leaking and need to be sealed (assuming he's running an early quadrajet).


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## trainman115 (Jul 31, 2014)

*car*

Hi guys I changed the fuel pump still same problem vapors coming from top of carb when motor is shut down I did seal the wells on the carb. my timing is set a 9 degress no clue what else I can do any one have any idieas ?? Thanks Ed


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## chuckha62 (Apr 5, 2010)

trainman115 said:


> Hi guys I changed the fuel pump still same problem vapors coming from top of carb when motor is shut down I did seal the wells on the carb. my timing is set a 9 degress no clue what else I can do any one have any idieas ?? Thanks Ed


Where's your fuel pressure with the new pump? It could still be too high. You also could have a heavy float, or it is simply set too high.


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## torrid red sap (Sep 27, 2014)

short answer= too much heat!


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## eeyore (Oct 23, 2014)

Under hood temps could be a factor especially with raw steel headers. 
The only real advantage to the ram air car's is cool air induction.
ALL auto manufactors pull cool air from voids between fender and wheel well or some where at the front of the car.


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## trainman115 (Jul 31, 2014)

*carb*

My pressure is around 6 or maybe just a littler lower I cant find any pumps that go any lower I was told to buy a AC delco and thats what I did same problem the carb is off a 1975 chevy truck I cant find the right carb for the right price so I thought this would work for now the only thing I did not change yet is the float could that be the problem Thanks Ed


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## Matthew (Feb 11, 2011)

Please confirm you have a fiber and metal gasket set under your Qjet, or you are using a phenolic spacer. Also, confirm you have no more ethanol fuel in your tank. Thanks, Matt


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## chuckha62 (Apr 5, 2010)

Matthew said:


> Please confirm you have a fiber and metal gasket set under your Qjet, or you are using a phenolic spacer. Also, confirm you have no more ethanol fuel in your tank. Thanks, Matt



...and if this is a '67, that you have the gaskets on right side up. It does make a difference. Upside down will open up a small passage at the crossover and create a nasty vacuum leak.


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## trainman115 (Jul 31, 2014)

Hi guys its a 1969 GTO I have the fiber but no metal gasket under the carb and no spacer I dont have any other fuel to use should I get a metal gasket ?? wont that add more heat?? Thanks Ed


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## chuckha62 (Apr 5, 2010)

trainman115 said:


> Hi guys its a 1969 GTO I have the fiber but no metal gasket under the carb and no spacer I dont have any other fuel to use should I get a metal gasket ?? wont that add more heat?? Thanks Ed


The metal gasket acts as a heat shield. 

I wish we were better able to help you out here, it's nearly impossible to figure this stuff out remotely. Have you tried to buy a good pressure regulator? Have you checked your float height? Do you have access to an emissions tester to see if you can nail down the air/fuel ratio? It seems to me to still be an over rich condition.


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## Matthew (Feb 11, 2011)

Ed, yes, you need that metal gasket. Like Chuckha said, it acts as a heat shield. The ethanol is working against you. As pointed out, it has a lower boiling point. The metal gasket you are missing might fix the problem. As Roger That points out, you might try a phenolic spacer. The best way I know to isolate whether this is a heat soak problem is to replace the fuel in your tank with high octane (93+, 100 optimal) non-ethanol fuel. Matt


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## trainman115 (Jul 31, 2014)

*metal gasket*

Thanks guys I will try that do I have to put regular gaskests between the metal or how does that work Thanks again Ed


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## geeteeohguy (Feb 2, 2008)

Trainman: it is factory to have one muffler longer than the other. These cars came that way. 6PSI fuel pressure is too much. You need about 3-4 psi, that's it. I would install a regulator or get a pump that is within factory specifications. The metal gasket was only used with '67 intakes, to block off the exhaust port...a one-year-only design. You'd be better off using a couple of regular gaskets. Heck, even three of them, stacked up.


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## Matthew (Feb 11, 2011)

Ed, no matter how many gaskets you use, put the thin metal gasket on top. And they are usually stamped "TOP." Matt


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## trainman115 (Jul 31, 2014)

*Thanks*

Thanks guys I will give that a try all I know is the timing is set new fuel pump, new mufflers, new plugs, wires cap rotor, new rebuilt carb, and still have the same problem very strong raw gas smell from back tail pipes and white vapor coming from to of front carb, plugs are black like she no buring right I wonder if something is wrong with my intake dont know what else to do points set with dwell meter and timing with light I know she is not sounding right thats for sure good vacum good compression new timing chain and gears, everything is the way it suppose too but yet not right never in my life I had this much trouble Thanks again Ed


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## rickm (Feb 8, 2012)

on my '65 389 carter afb, I used two thin base gaskets that came with the rebuild kit and a steel spacer off a '59 Pontiac instead of the thick "stock" replacement gasket felpro makes. the felpro gasket has metal rings around the stud holes that will do nothing but distort the base of your carb if you try to mount it. W.T.F.? I have an original base gasket off a '65 Bonneville that does NOT have the metal rings. my sandwich base gasket set-up seems to keep too much heat from effecting carb. performance n makes a good seal on the manifold. don't know why felpro has those metal rings on the stud holes.


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## trainman115 (Jul 31, 2014)

Ok Thanks for the info I will try that and see what happeneds I really think I got other problems but just cant nail it down I should nothave that bad gas smell I tryied three different carbs and all do the same thing I also belive it something simple that I am over seeing when I got the car it had a 400 in it that I would like to get rebuilt since its a matching number engine this is the same thing that happend to the 400 same bull crap so I went and bought a 350 for it just until I get the 400 back in the only thing left I use off the 400 is the intake everything else new the guy I bought the car from had everything shitted up on this car why not the intake but she is coming back to life Ed


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## chuckha62 (Apr 5, 2010)

Your plugs are telling you everything you need to know. If they're black, it's because it is running rich! Sorry to keep harping on this, but more than likely you still have too much fuel pressure. GeeTee and I are on the same page, here. Until you correct that issue, swapping the carbs that you have back and forth is not going to fix it, but instead is only reinforcing the fact that the problem is somewhere else.


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## trainman115 (Jul 31, 2014)

*carb*

Thanks Guys but you will never belive this I put a vacum gauge on the motor found out I had low reading but steady took compression test all cyc read 150 next I did a smoke test looking for leaks had none next I took off the carb I really wanted to see where the gas was leaking making the vapors and when i pull off the carb and look in there, there it was 2 cracks in the intake right under the carb how I miss it I have no clue what caused it to crack I have no clue so I got another intake and am going to put it on I just hope this is my problem I thought it might be the intake as I changed everything else I never thought to look inside and could not belive this could happen either but it did I belive thats why the guy sold the car he could not figure it out either I will let you all know when I get it on and guys Thanks for all the help and replys I really appreciated it Ed


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## ALKYGTO (Mar 29, 2010)

Amazing find. What a crazy thing, is this a cast iron or aluminum intake? :banghead: Good job! :thumbsup:


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## trainman115 (Jul 31, 2014)

*carb intake*

Thanks again guys I hope theses pictures come out just in case anyone would have the same problem you would know where to look she purrrrs like a kitten now I still cant belive it and all that trouble and money spent should have had the vacumm gauge 1,000 dollars ago LOL Ed


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## geeteeohguy (Feb 2, 2008)

Wow! Looks like a cast iron intake to me. _Very _rare occurence. Yes, a vacuum gauge is one of the most useful tune up tools there is. Cheap, too. You can verify valve condtion, late timing, restricted exhaust, etc. All in a minute. I'm an old school guy, and a vacuum gauge was in my first toolkit.


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## trainman115 (Jul 31, 2014)

Hi ya it was cast iron now I have a miss vacumm still only at 15 -16 set dwell at 30 my timing marks are way off not even on the scale wondering now if I put the timming gears in wrong I put the dots one and then the other one right above it I am really digested I thought I had it for sure each cyc has 140-145 I dont know if i would have that id the gears are wrong never in my like did I ever have this problems I give up Thanks Ed


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## geeteeohguy (Feb 2, 2008)

Digested? Man, I hope not! Some timing sets install differently. Check the instructions, or find out from the manufacturer. If your dwell is ok, you can static time the engine, with it off. Get #1 piston at TDC compression stroke, pull the dist cap, and verify that the rotor is pointing at #1 cyl on the cap. Then look at the harmonic balancer. Should be at TDC, right on the mark. If not, the distributor is in wrong, or your timing gears are mis-installed. My gut tells me your timing chain is off. You should have 18-21" hg at idle with a stock cam. As a side note, timing gears being out of whack will cause sluggishness and other issues, but usually not a misfire. How are your ignition wires?


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## Goat Farmer (Sep 3, 2014)

If the gears were installed incorrectly you would have many more issues ... need to get back to basics ..... as the expert above mentioned .. Good luck


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## chuckha62 (Apr 5, 2010)

geeteeohguy said:


> Wow! Looks like a cast iron intake to me. _Very _rare occurence. Yes, a vacuum gauge is one of the most useful tune up tools there is. Cheap, too. You can verify valve condtion, late timing, restricted exhaust, etc. All in a minute. I'm an old school guy, and a vacuum gauge was in my first toolkit.


I'd respectfully disagree. When I picked up my date code correct manifold, I had looked at three manifolds in the date range I was needed. Of the three I looked at two had cracks exactly like that. I specifically asked my machinist to magnaflux for exactly that issue because I had heard it was fairtly common. He said it is very common and while fixable, he sees more cracked than not.

Good catch.


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