Rochester M4ME Carb problems

[stox258]

I have a problem, I went to take out my 80 Vette for a run yesterday in the beautifull sunshine. When it started it sounded like it was only running on 1 bank of cylinders and wouldn't rev. When it warmed up and the choke appeared to release, it sounded like it was running fine. Backed it off the drive put it into forward and as soon as I pressed the accelerator it just completely bogged down and sound like it was running on one bank again. As soon as you lifted of the accelerator it sound like it was running on all 8 again. I am assumming it is a vacuum issue but I have looked over the engine and it all appears to be connected. It's an ex California car so has the full smog kit fitted.

Any ideas of an easy check or what might be the issue?

 

[Oneball]

I’d try to determine if it’s running rich or lean. When it’s warm does the exhaust look sooty or smell of petrol. If you’ve got a dual exhaust
check both pipes.

I’m guessing it might be rich as it’s better without the choke.

Float level in the carb would be my first guess. From memory you can do it on a QJet through the vent hole. Have a google.

Another option is you’ve got a very week spark, so you can pull an HT lead and check how good the spark looks. Don’t electrocute yourself!

You can easily check for a cylinder specific misfire by checking the exhaust manifold temps right next to each cylinder not long after start up (a few seconds and you can do it by hand) again health and safety don’t burn yourself! I’m amazed I’m still in one piece, almost!

 

[stox258]

Before yesterday it ran fine. When I have it MOT'd they do a exhaust emmisions check and it is always fine. There is no sooty exhaust and when you put your hand over the outlets they are both blowing out warm gasses.

 

[Oneball]

If it’s not rich then I’d be looking for a blocked jet in the carb or an ignition fault.
It could be a vac leak too but they’re usually a bit more subtle.

 

[stox258]

I take it that if it was a vac leak you would hear some hissing?

Who are the best people to diagnose and rectify these carbs?

 

[Oneball]

You’d be pretty lucky to hear a vac leak over the engine.

 

[Chuffer]

I take it that if it was a vac leak you would hear some hissing?

Who are the best people to diagnose and rectify these carbs?

Where are you based ?

 

[Chuffer]

You’d be pretty lucky to hear a vac leak over the engine.

Totally agree with you there Oneball . It could be anywhere in the system . When I was tracing faults in the vac sytem on my car to get the lazy headlights sorted , I found the non return valve not working on the line to inlet manifold and no in line filter fitted either . Once the headlights were refurbed with new seals etc. and return valve and filter sorted too , the engine ran much sweeter too .

 

[teamzr1]

It is really worth either making or buying a smoker for less than $200 US

This one I have, generates pressure and smoke flow and all it needs is like small amount of cheap baby oil
Pressure is about 15 PSI so will not harm anything as to pressure
This allows finding the smallest of leaks if EVAP, A/C system, exhaust, vacuum lines, etc
This unit is only about 1 foot tall so easy to place in engine bay area and using 12 volts, even from vehicles own battery

I have found small crack in welds of exhaust they could not easily see under the car but with smoker can smell and see the smoke to
locate the leak.
Since it maintains 15 PSI you can connect this and then watch the PSI meter to see if what you're testing holds that pressure or is informing there is a leak in what you're testing

 

[stox258]

I'm in North Warwickshire

 

[Chuffer]

It is really worth either making or buying a smoker for less than $200 US

This one I have, generates pressure and smoke flow and all it needs is like small amount of cheap baby oil
Pressure is about 15 PSI so will not harm anything as to pressure
This allows finding the smallest of leaks if EVAP, A/C system, exhaust, vacuum lines, etc
This unit is only about 1 foot tall so easy to place in engine bay area and using 12 volts, even from vehicles own battery

I have found small crack in welds of exhaust they could not easily see under the car but with smoker can smell and see the smoke to
locate the leak.
Since it maintains 15 PSI you can connect this and then watch the PSI meter to see if what you're testing holds that pressure or is informing there is a leak in what you're testing

Ingenious ! I never realised that you could have so many varied uses for a bottle of baby oil !!

 

[Chuffer]

I'm in North Warwickshire

You could try Williams Brothers Racing in Bugnrooke , Northamptonshire if you cannot find someone closer to home . They are great guys who really know their stuff from and Chevy engines are their forte` with a life time in drag racing . They set my C3 up a real treat and at £55 per hour labour not mega expensive either .

 

[teamzr1]

I have a problem, I went to take out my 80 Vette for a run yesterday in the beautifull sunshine. When it started it sounded like it was only running on 1 bank of cylinders and wouldn't rev. When it warmed up and the choke appeared to release, it sounded like it was running fine. Backed it off the drive put it into forward and as soon as I pressed the accelerator it just completely bogged down and sound like it was running on one bank again. As soon as you lifted of the accelerator it sound like it was running on all 8 again. I am assumming it is a vacuum issue but I have looked over the engine and it all appears to be connected. It's an ex California car so has the full smog kit fitted.

Any ideas of an easy check or what might be the issue?

How long since the car was driven before this happened ?
How old is the gas and about how much gas in tank ?

Since this happened, have you driven it more and does it still happen ?

Need a vacuum gauge hooked up and rigged, so it can be seen when driving to see how it looks under different RPM/load
With engine warmed up and no vacuum leak that at idle gauge should read like around 16 (53 KPA) if around sealevel

Have you checked fuel pressure ?

 

[Chuffer]

You could try Williams Brothers Racing in Bugnrooke , Northamptonshire if you cannot find someone closer to home . They are great guys who really know their stuff from and Chevy engines are their forte` with a life time in drag racing . They set my C3 up a real treat and at £55 per hour labour not mega expensive either .

Sorry ! typo error - should read Bugbrooke . They are on the Corn Hill road .