truck stalling, possibly carb issue?

[slam59]

This is the second time my wifes 77 c20 has done this. Has a 454 motor in it. While taking off from a start and accelerating it stumbles and as i press down on gas it goes nowhere just coast a bit. Then I would pump gas slightly and then it finally would accelerate. It almost would feel like I was out of gas or not getting gas into carb. Well after that a few days later I gave it a much needed tune up. New ac delco plugs with some good msd wires timing was set and even sea foamed it for the first time and man oh man that sea foam really made a difference in acceleration and a good smooth idle. Kid you not this truck has NEVER ran so good ever. Also put a new fuel pump and fuel filter. That was 2 weeks ago. Well just this morning went to take stuff to the dump and upon leaving as i drove out the gate on flat land it did it again. as i was slowly pulling out as i accelerated it would not respond. It just coasted so I pumped the gas and it did not help. so i stopped put it in park, revved up the motor off and on for about 30 seconds and it seemed to respond normaly upon pushing down on the gas pedal so then i put it in gear and drove off. I had about a 4-5 mile drive home and it drove like a champ the way home. Since i am getting plenty of fuel to carb with new pump and filter, and such a good tune up done, can it be the floats in the carb getting stuck and or needing adjustment? It is an Edelbrock carb. Just saw a show where it happened to these guys and they opened the carb and moved the floats and linkage around and it drove after that. What you guys think?

[carguy94]

what psi is that pump you are running. try closing the choke manually or placing your hand over the carburetor what is the engines first reaction does it speed up or slow down. if you have a vacuum gauge it would be good to check from a manifold vacuum source. I take it this happens more when the engine is cold than hot. what did the old plugs look like all wire routed and spaced properly. did you have the cap off to take a look at things.

[slam59]

No vehicle was not cold when it happened both times. Motor was good and hot. Has only happened those two times I spoke of. Plugs did not look in bad shape at all according to my mechanic

[johnwaynejeep]

I have a theory

It sounds like it might be a bad vacuum leak in the carb.

Your carb is a vacuum pump, when you hit the gas the butterflies open up and vacuum generated from the engine sucks fuel through the carb. If you hook a vacuum gauge to the manifold you should see around 20psi. When you hit the gas you'll see a momentary drop in vacuum. If you nail the gas it may drop to the single digits as air floods through the carb and satisfies the engines demand for air. the bigger the engine the bigger the effect. Very shortly the engine will speed up and the manifold vacuum will increase again. If you have a vacuum leak in your carb then the drop in vacuum pressure is amplified and your carb may not be able to draw fuel the way it's supposed to. A slow increase in engine speed will prevent the drop in vacuum pressure. The tranny being in gear and the engine trying to move all that weight will amplify the problem and make it more visible.

This is all theory, but I think if you listen, you'll be able to hear a hiss or a constant jet engine whoosh sound from your engine that may indicate a line has popped off your carb somewhere.

Good luck

[slam59]

John Wayne jeep. He checked the carb good but I will have him look it over again. If the floats in the carb are sticking that can be what causes this also don't you think?

[carguy94]

it still is possible you have a fuel delivery issue so do not rule it out as it may be an issue. go ahead an do your basic trouble shooting with the engine running fine check all carb adjustment look for anything out of place or specification. what type of edelbrock do you have and what tools do you have access to as far as vacuum compression and things of the like. and if you get the thing to act up that is the best time to trouble shoot. and make sure you have good amounts of fuel being squirted in to the carburetor.

[slam59]

It is an edelbrock 1400 performer series. I believe my mechanic might have things he can check compression with. Hopefully he does. It would be hard to get it to act up in the shop. It has only happened twice and both times while driving. That's why I keep thinking maybe it is the floats in the carb sticking.

[carguy94]

is the carburetor dirty on the outside how long has it been on. do you make seasonal adjustments on it. you need to duplicate the symptoms if you can not do this then your basic tune up inspection along with a possible overhaul is what you have to deal with. so check over the basics have you had a look see in under the distributor cap and what about pressure testing and checking for leaks. i you have no proof so until that changes you need to go over basic things and keep in mind its intermittent. that's one thing about a carburetor its mechanical no diagnostic port to analyze air fuel spark and all that good stuff. and you can check for vacuum leaks with propane or either or something that will disrupt an air leak.

[K-Ray]

A few things to think about.
Is the carb old? If so the accelerator pump could be bad..
One member said vacuum leak, good possibility. Leaks around the base of the carb are sneaky. A bad pcv valve may be the culprit to. Check bolts of carb for tightness. If vacuum tubs are old just replace them and save yourself some heartache their cheap. Good luck!

[slam59]

Carb is only like 3 maybe 4 years old. Very clean. Mechanic just put a spacer on it last weekend due to carb boil and hard starts when hot and he tightened everything good. And it did this before spacer was put on. Will replace PVC and see if that helps. Upon fuel tune up 2 weeks ago distributor cap was taken off and looked very good nice and clean.

[rich weyand]

Is the vacuum advance connected to the driver side port on the front of the carb, or the passenger side port?

How old is the distributor? What kind is it? Stock GM HEI?

Are you sure the distributor is clamped down tight?

What is your base timing set at?

I'm thinking the vacuum advance may be sticking retarded. If the timing is retarded enough, and it is on ported vacuum, if the vacuum advance sticks, you will have zip power.

[slam59]

It is a accel distributor. Bout 5 years old. Will have to ask mechanic what the timing was set at. It runs like a beast. Really good. Accelerates better than it ever has since tune up and all It has only done it two times. First time before tune was done and second time after tune up was done. That's what is weird that it runs so darn good.

[johnwaynejeep]

Slam59, I imagine a sticky float bowl would act very similar to a vacuum leak. Both conditions starve your carb. I would be concerned that you had carb boil issues in the past. My first thought is that they might still be connected.

Do you have an electric fan or a belt driven fan? Belt driven fans are good as long as you are moving, but at idle they push very little air and can allow the engine to heat up, sometimes even overheat, this could contribute to a potential carb boil issue. The fan shroud is a big part of overheating as well, the fan shroud helps direct air through the radiator fins instead of allowing air to get sucked from around the radiator. If your fan shroud is missing then I would look at getting a new one. An electric fan would push more air at idle and would help disipate that heat and would eliminate the need for a replacement fan shroud. But I wouldn't put the money or time into any of this until you nail down what the problem is. If it is carb boil then addressing the weaknesses in your cooling system will help, you could also put those nifty louvers in your hood to help draw in more air and push more heat out of the engine bay...they also look cool

[slam59]

It's a belt driven fan and perfect shroud is on it. I think we will check for vacuum leaks and have the floats reset and checked. Have not checked them in 2 years so could be need adjustment and sprayed and cleaned in case it is sticking.

[geezer#99]

Are you using a fuel regulator set to 5 psi.
Edelbrock carbs are fuel pressure sensitive.

[Jake1971]

An easy way to check for vacuum leaks is using carb cleaner. Spray it anyplace you suspect leaks while the engine is idling. If the engine idle speed increases it is from burning the carb cleaner which leaked thru to the intake manifold.

[dfrank]

Originally Posted by slam59

Carb is only like 3 maybe 4 years old. Very clean. Mechanic just put a spacer on it last weekend due to carb boil and hard starts when hot and he tightened everything good. And it did this before spacer was put on. Will replace PVC and see if that helps. Upon fuel tune up 2 weeks ago distributor cap was taken off and looked very good nice and clean.

I am thinking it may be the accelerator pump. Did it get worse after he put a carb spacer on? When you put a carb spacer on you increase the volume of the plenum in the intake so sometimes you may need a bigger shot of fuel off the start. One thing you can do to test this is let the truck idle in neutral (or park for an automatic) and punch the gas real quick. If you get any kind of stumble than you know the accelerator pump circut needs some tuning. Try increasing the accelerator pump nozzle to the next size from the one you have now. I am not sure what else you can do to the accelerator pump circut in and edlebrock as I am more familiar and fond of holley's. It could also be the timing as well. I would get the timing right before messing with the carb at all or you are just chasing your tail if your timing isn't right.

[slam59]

Dfrank. No it did not get worse. It only happened 2 times. Once before tune up and spacer being put on and once after. It only lasted like 10 seconds or so and engine never died it just did not acellerate when I was pressing on pedal. Both times truck was already moving and not like I was taking off from a stand still

[rich weyand]

Pull the step-up pistons on the Edelbrock and clean them up, and clean the step-up cylinders with WD-40 on Q-tips. They gum up, and will stick. When they do, the carb fails to adjust the mixture properly when the vacuum changes, such as when you step on the throttle.

[slam59]

Well had a guy from a local shop that builds motors tell me to bring it by when we were talking on phone about other stuff. He waited about 20 minutes to let it cool a bit so he could take off air cleaner and filter and look down into carb. With truck off he moved the throttle cable as he looked into carb. He said there was a hesitation between when he moved the throttle and when the gas squirted into the carb indicating to him that the accelerator pump was needing replacing. Said once carb is out and he is replacing accelerator pump he could look and see if it needs rebuild or other parts replaced. I would not think other parts would be bad it's only like 3 years old but u never know. Said that this could be the cause of my issue.

[rich weyand]

Carb doesn't have to come out to replace the accelerator pump, you just have to take the cover off. Pull the step-up pistons and metering rods first -- it's much harder to align the cover when reinstalling if the rods are in place.

[johnwaynejeep]

Slam59. Thanks for posting the suspected fix. I'm sure multiple people on this site would like to hear if the accelerator pump really fixes the issue. It's always nice to hear the final fix so you can make a mental note and learn from the experience of others.

Thanks
Robert