Bogging down on hard accel

[Adam Wartell]

I had my '79 into a shop a couple of months ago. Paid a boatload of money to have them tune-up the engine including rebuilding the carb.

It was running great when I brought it home. A day or two later, I noticed the performance slipping. It was running real rough at idle when I first started up in the morning.

The most annoying problem is that when I would leave my house in the morning, and make the turn out of my neighborhood... the engine would completely bog down when I accelerated. I noticed that the harder I accelerated, the worse it was. So I learned that if I accelerate slowly, there would be no problem. This problem mostly goes away after it is warm.

The thing that bugged me was that this never was a problem before I brought it to the shop for the tune-up. So I took it back to them, telling them my complaints.

I was told that the bogging down is expected when cold because the rear 2 carb chambers are blocked off until the engine warms up and the choke opens.

First question... is the shop right?

I asked why this didn't happen before and he couldn't give me an answer.

Second question... is it possible this wasn't a problem before because the choke was stuck open?

I picked it back up from them today and it is still doing the same thing!

Any help you can offer will be great!

Thanks!

 

[blacksharkL82]

Bogging?

I had my '79 into a shop a couple of months ago. Paid a boatload of money to have them tune-up the engine including rebuilding the carb.

It was running great when I brought it home. A day or two later, I noticed the performance slipping. It was running real rough at idle when I first started up in the morning.

The most annoying problem is that when I would leave my house in the morning, and make the turn out of my neighborhood... the engine would completely bog down when I accelerated. I noticed that the harder I accelerated, the worse it was. So I learned that if I accelerate slowly, there would be no problem. This problem mostly goes away after it is warm.

The thing that bugged me was that this never was a problem before I brought it to the shop for the tune-up. So I took it back to them, telling them my complaints.

I was told that the bogging down is expected when cold because the rear 2 carb chambers are blocked off until the engine warms up and the choke opens.

First question... is the shop right?

I asked why this didn't happen before and he couldn't give me an answer.

Second question... is it possible this wasn't a problem before because the choke was stuck open?

I picked it back up from them today and it is still doing the same thing!

Any help you can offer will be great!

Thanks!

Yes, the choke has to pull off befor the secondaries will open. It aint a fulie!
Your wrench is right.

 

[Bob Chadwick]

Your mechanic is right. The secondaries don't open until the choke comes off.

Mine definitely bogs until it gets up to operating temperature. Normal for an old car running a carb with a choke.

It may not have occured before if the choke was not set up properly.

 

[Adam Wartell]

Mine definitely bogs until it gets up to operating temperature. Normal for an old car running a carb with choke.

It may not have occured before if the choke was not set up properly.

REALLY!? I've had the car for 3 years and up until I took it to them for a tune-up, it never had this issue!... before OR after the engine was replaced. (Same carb).

So, you're telling me when it wasn't bogging down, the choke was NOT adjusted correctly, yet it felt like it was running ok?

It seems almost unsafe. If I had to accelerate quickly to get out of a sticky situation, I wouldn't be able to (until the engine warmed up).

 

[magoo]

If you're trying to accelerate hard when cold this could be normal. But there is no reason the car shouldn't accelerate normally on the front 2 barrels. If it's a q-jet, I suspect the vaccuum choke pull off is inop or misadjusted. But unless you're asking too much power when cold, this is not normal. Hope this helps.

Mike

 

[Bob Chadwick]

I need to clarify my prior post. I wanted to confirm that the statement about the carb was true.

My car accelerates fine if I give it about 1/2 throttle quickly when cold. If I take it quickly to the floor it bogs. Slowly going past halfway is not as snappy when it is hot but accelerates ok. Once the temp reaches a certain point, then full throttle acceleration is fine.

I don't know if it is because the secondaries don't open ( I don't think this is the case) or if it is because the fuel is not vaporizing as it would when hot creating a difficult air fuel mixture situation for the carb when you give it that sudden shot of gas.

It's hard to know what changes are going on in your carb without knowing what your mechanic did to it. Also, considering a full tune-up it could be non-carb related.

 

[Adam Wartell]

It is a QJet.
The expensive bill I got from them included a full tune-up, new plug wire (routing them correctly) and a carb rebuild.

 

[magoo]

Bob is right on. There are many variables you did not tell us. I was thinking you were talking normal accel. and he was thinking hard accel. The secondarys are locked out when cold. If they weren't you'd really see a bog. But normal pulling out at a safe rate should be possible at any temp. One more thing, no it is not possible this wasn't a problem because the choke was stuck open. That too would have caused a bog when cold. My guess, it was working fine before, and now just needs a tweak of the choke or the vac pull off. If that pull off isn't working, the choke will stay closed too tightly not allowing enough air into the primarys. This would also cause the rough and rich idle. Just my $.02. Hope we can help ya. Thanks

Mike

 

[Adam Wartell]

If you're trying to accelerate hard when cold this could be normal. But there is no reason the car shouldn't accelerate normally on the front 2 barrels. If it's a q-jet, I suspect the vaccuum choke pull off is inop or misadjusted. But unless you're asking too much power when cold, this is not normal. Hope this helps.

Mike

This is supposed to be the best Corvette shop in my area. So I'm going on the assumption that they adjusted everything correctly. It is a QJet.

I'm glad to hear that I am not the only person that experiences this issue.

What is interesting is that the responses I am getting here on CAC are mostly saying that the shop is correct and this is proper operation of my Vette. Yet, I asked the same exact question over on CF and the majority are saying that it is not right and the shop should fix it.

 

[Bob Chadwick]

Yet, I asked the same exact question over on CF and the majority are saying that it is not right and the shop should fix it.

Ask us an easy question. Who do we think is right???

 

[magoo]

Adam
I stick by my other posts. Remember your title said "bogging under HARD acceleration".The other guys think you expect a cold engine to take full acceleration. You could say the shop should fix it. I chose not to point fingers. It is very hard to diagnose a choke problem if the car drives into the shop already warm. If that choke pull off opens too much it will also idle rough but lean when cold and will bog because of too much air getting into the cold engine. Check for a vac hose off at the carb which could be taking vac away from this pull off. Either way, safe acceleration should be possible at any temp. I think everyone here agrees. Thanks,

Mike

 

[JohnZ]

The accelerator pump is either mis-adjusted, not connected, or the pump piston is shot and wasn't replaced during the rebuild. Has nothing to do with the secondaries or the choke.

 

[magoo]

Again, I stick with my other posts. It works fine when it's warm.

Mike

 

[Adam Wartell]

Maybe "hard accel" was a little to vague. I didn't mean that I am slamming it to the floor. I meant hard as opposed to soft, really soft. In order to keep it from bogging down... I have to push the accelerator VERY slowly. If I push it as quickly as I would consider a "normal" acceleration from a stop... that is when I have the problem. I wish one of you could drive it to tell me what you think!

Oh, and as far as the shop being able to check it when it is cold... they had it for TWO WEEKS! They had about 10 chances to check it cold in the morning!

Sorry if I was too vague in my descriptions before. If you need more info to help your diagnosis, let me know.

Thanks for all your help!

 

[magoo]

It does work fine when it's warm? If not John Z has another possibility. I've already installed new accel pumps that didn't seat right and wouldn't work. But I don't think the accel pump would make it idle rough. And it should still stumble when it's warm. If they had it for 2 weeks for the original job, that doesn't mean they were looking for a problem for 2 weeks. Honestly you can only check some choke problems once a day. Did they have it 2 weeks when you took it back for the problem? Sorry man that is an awfully annoying problem. CAC will get it, there is a lot of knowledge here. Thanks

Mike

 

[Adam Wartell]

It does work fine when it's warm?

For the most part, yes. When it is warm, it never bogs down to the point of it seeming like the engine will stall, but I never get any really power until 2nd gear... but that I'm sure is just the way a 1979 Corvette L48 w/Lousy Trans is.

... If they had it for 2 weeks for the original job, that doesn't mean they were looking for a problem for 2 weeks. Honestly you can only check some choke problems once a day. Did they have it 2 weeks when you took it back for the problem? ....

They had it for 2 weeks the time I brought it back. I think they had it a week or two for the original work. But they had it for 2 weeks strictly to look into the boggind down issue.

I'm starting to feel like there is still something the mechanic is missing. I also get the feeling that only one guy in the shop looked at it (other than the manager or the shop). I think this shop has many Vette mechanics, but apparently only one guy that specializes in my year/generation.

 

[4 webers]

Adam has every right to complain , a carb should not bog because the choke is not fully open . I agree that a Qjet will not perform as well with just the front butterflys opening ,,,,,,, it under no conditions bog or hesitate on acceleration . Obviously there is a problem with the tune up or something that happened after leaving the shop . A good shop employing a good diagnostics tech should have no problems getting to the root of the problem . If anyone here drives a carbed vette with similar problems you need a good tuner !

 

[magoo]

I agree completely with 4webers. But a carb will bog if the choke is too far "open" when the engine is still cold.

 

[4 webers]

You are quite right about that ,,,,,,,



Geo

 

[MMM]

Magoo is correct, the choke pull off is either inoperable or mis-adjusted. A bad diaphram in the choke pull off is a common carb problem, any kind of carb. And most mechanics today neither understand how it is supposed to work or how to adjust it.

Mike