# 67 Grand Prix convert with 400 engine,, hesitation at or near takeoff



## Picasso (6 mo ago)

Hi,, new here. I've been in the caddyshere (63 deville and 73 eldo convert) for about 7 years so a Pontiac is new ground. still learning however...lots to learn in both the cad world and over here.. recently picked up the above 67. 70 k miles. looks pretty much stock,,new plug wires, plugs, regular distributor, dual exhaust. 
overall looks good, made the 3 hours back from purchase with no problems but after returning, noted the following.

the problem is at takeoff or after slowing down and then accelerating, there is a slight hesitation or sag in the acceleration. doesnt happen every time but is a noticeable hesitation. I thought it may be timing, so I checked that, it was at about 4 so I checked manual and it stated should be around 6 or so.. so I made it and set it about what appears to be about 7 degrees btdc.

set out again after timing change and it didn't solve the problem. same symptom. I was looking towards vacuum advance but sheer conjecture.

What would be recommended in search for cause of this hesitation.

thanks for looking,,
Sincerley

Kim


----------



## BLK69JUDGE (Jun 10, 2010)

welcome !
carbs accelerator pump rubber
would be my guess ,,,,

Scott



quadrajet accelerator pump - Google Search


----------



## BearGFR (Aug 25, 2008)

Accelerator pump is a likely cause, also the possibility that a previous owner may have "messed with" the carb. Quadrajets have a reputation for problems like this, mostly undeserved, usually caused by someone who didn't really know what they were doing having tried various tuning "tricks" that they got from equally unqualified sources. 

Bear


----------



## LATECH (Jul 18, 2018)

Look for the "shot" of fuel from the accelrator pump when you open the throttle. If it is good then check the vacuum advance and the mechanical as well. Both are likely culprits
Also the vacuum supply to the vac advance is an issue. I like ported as it pulls vacuum as you open the throttle and it doesnt mess with base timing.Put a vacuum gauge on it and open the throttle with the engine runnig to see if it deflects and shows vacuum. If vacuum is low an adjustable vac advance can be used as you can soften the spring tension to achieve more timing 
IMHO you could run 10 degrees base timing too.


----------



## Picasso (6 mo ago)

thanks for the ideas.. I sholdhave noted in my intro. the carb is a Carter, and it was recently rebuilt,, doest exclude the pump, but I should have noted that in the description of the engine... I will give the ideas a try, see what pops..
thanks again


----------



## RMTZ67 (Mar 12, 2011)

Since it's new to you, check for vacuum leaks with brake cleaner,starter fluid. Check dwell, check fuel hoses for leaks. Maybe set idle mixture screws with vacuum Guage. Start by taking a look at plug color for reference. All above comments are likely culprits. Still, bog and hesitations are a pain in the as_.


----------



## Picasso (6 mo ago)

following up on the timing suggestion. my location is at about 5600 feet. manual states 6 degrees on a 400 engine. I have it at about 7 or 8,,,, one of the post suggested more, like 10 or 12, would that be a reasonable suggestion? to accommodate less air in the atmosphere? I will be getting a hold to a vacuum pump to see if the vacuum is holding at the canister today,, then go from there,,


----------



## geeteeohguy (Feb 2, 2008)

If it's the original engine, stay with the factory specs at idle. Your car wants 100 octane fuel as it was designed, today's fuel is inferior. Advancing the timing is a recipe for broken piston lands. What Bear and Latech said. Remove the air cleaner and you should see a shot of fuel when you goose the throttle. First thing to check, takes 30 seconds including removing the air cleaner lid. If no shot, try doing it while you shoot carb cleaner down the car. If the problem goes away, it's the accel pump or circuit. (most likely) It could be ignition, but doubtful if it otherwise runs fine. Be advised that Pontiacs of this era ran full intake manifold advance at idle, not ported. This means, with the engine idling, when you pull the advance hose the idle will drop and you will feel vacuum at the hose. Set the timing to 6 BTDC with this hose disconnected and plugged. Then, re-connect the hose and you can set your idle speed and mixture.


----------



## RMTZ67 (Mar 12, 2011)

Picasso said:


> following up on the timing suggestion. my location is at about 5600 feet. manual states 6 degrees on a 400 engine. I have it at about 7 or 8,,,, one of the post suggested more, like 10 or 12, would that be a reasonable suggestion? to accommodate less air in the atmosphere? I will be getting a hold to a vacuum pump to see if the vacuum is holding at the canister today,, then go from there,,


I am at 5300 ft and have mine at 12 degrees with no pinging or issues.


----------



## geeteeohguy (Feb 2, 2008)

RMTZ67 said:


> I am at 5300 ft and have mine at 12 degrees with no pinging or issues.


Stock engine with stock pistons and no mods? 670 heads?
Just asking because I used to have a 428 with the stock pistons and I never heard it ping, ever. When I took it apart to overhaul it, it had 4 or 5 broken pistons. From pinging. That said, lots of guys seem to run a lot of initial advance and 'get away' with it.


----------



## RMTZ67 (Mar 12, 2011)

geeteeohguy said:


> Stock engine with stock pistons and no mods? 670 heads?
> Just asking because I used to have a 428 with the stock pistons and I never heard it ping, ever. When I took it apart to overhaul it, it had 4 or 5 broken pistons. From pinging. That said, lots of guys seem to run a lot of initial advance and 'get away' with it.


mine 9.6 CR etc. Good call, his may be stock.


----------



## geeteeohguy (Feb 2, 2008)

He stated 70k original engine/miles. So that means closed chamber heads and cast pistons, likely with carbon on them...which means high compression and detonation prone.


----------



## Picasso (6 mo ago)

to my knowledge, its all stock.. Per the prior owner. 
I got a hold to a vacuum pump and was setting it up. When I was pulling the vacuum hose from the vacuum can and the carb, I noticed the end on the carb side was split on the connection,, it goes on vertical, then bends down and over to the vacuum can.

I am wondering if the split in the hose there and not having a good seal could be the culprit as it probably hasnt got a good seal with the split. I tried to repost it on the carb and it split and cracked again. so im guessing probably things are dry under there and inline with original equip and hoses, at least some, so I can probably look forward to replacing others in the future,, I didnt have any extra vacuum hose, so its off the the part store tomorrow,,,

DO any of you think that split could be enough of a leak to cause that sputter, or just a chance find with no consequence

I also did the acceleration pump test,, it appears that both jets are functioning. they both squirt a visually similar amount of fuel.


----------



## 1969GPSJ (Feb 26, 2020)

maybe this will be helpful


----------



## geeteeohguy (Feb 2, 2008)

Yes, fix the vacuum leaks and old hoses. With any car or equipment repair, always fix the known broken/defective items first.


----------



## Picasso (6 mo ago)

I have gotten the hose replaced and nothing else seems to be leaking. did the vacuum pot test, that held up just fine. check squirt on the acell pump and both seem to be shooting equal gas, at least a visual look at it,

car is still coughing, not every time. but it seems to get off initially, then the cough then off it goes with no problem.
I can feather the accelerator and you cant notice or feel it, but I dont always feather, just normally drive it and it sometimes sputters.

I dropped the timing down to about 9, same result as 12 and at 5, but it feels like the engine is smoother at 9 than at 5 so that's where it is.


So what should I go to next,,fuel pump?? cant find any online articles to reference after this point in the search.

just to update, its a standard distributor, no petronix, it has a rebuilt Carter four barrel. it happens typically at take off and at a slower speed, and then accelerate, like slowing at a yellow light to about 5 or 10, then accelerating..,,
sincerely


----------



## mattcraw (Aug 9, 2021)

Mixture.


----------



## geeteeohguy (Feb 2, 2008)

Yes. Too lean due to weak/worn accelerator pump, clog in the circuit, or metering rod issues. Easy to check if you know carbs. If not, not so easy.
Not a fuel pump. Fuel pumps cause issues under load and full throttle or just crap out entirely. Not flat spots on take off that you can drive through.


----------



## mattcraw (Aug 9, 2021)

Or mixture screws not set right


----------



## Picasso (6 mo ago)

had someone help me,, turned out it was the minute in the carb, seems to be much better now...
thanks for the suggestions.


----------



## geeteeohguy (Feb 2, 2008)

Picasso said:


> had someone help me,, turned out it was the minute in the carb, seems to be much better now...
> thanks for the suggestions.


I've been at this game since the '70's, and have no idea where the minute is in a carburetor.


----------



## Picasso (6 mo ago)

sorry,, mixture


----------

