Revs don't fall during shifts

[artmck]

Everything is hooked up correctly, throttle return springs are working, but especially when cold, engine either seems to rev up or does not come down during shifts. I must have some kind of vacuum leak through the emmisions system, but I thought maybe this was a common problem that has an easy fix.

Thanks--

 

[jlaird]

Weeeeell, you do take you foot off the gas when shifting, right? /ubbthreads/images/graemlins/laugh.gif

 

[AlanT]

Really need to know what model MG and if it is stock or modified.

Alan T

 

[artmck]

Sorry, it's a '74 B totally stock including all emmissions stuff.

 

[AlanT]

If you suspect a vacuum leak then either pinch off or disconnect and plug the line to the gulp valve from the intake manifold and see what happens. Also with engine off push the pedal to the floor and let off and see if any of the linkage is binding.

Alan T

 

[Nunyas]

It's most likely the gulp valve, IMO. It's designed to prevent backfires through the exhaust when you let off the accelorater. The Gulp Valve should only open when you let off the accelorator. So, it shouldn't affect the idling of the car, and it shouldn't affect the performance during acceloration or cruising. The Gulp Valve opens up when you let off the accelorater and dumps fresh air into the intake effectively leaning out the mixture. Once the valve closes, the engine speed comes back down. Between shifts, there isn't enough time for the engine speed to come down after the Gulp Valve opens and closes.

My car ('76 MGB with full emissions) seems to do the same when cold. Once it warms up, the engine speed seems to drop off easier between shifts.

Hope this info helps.

 

[PAUL161]

Probably the gulp valve. If it's the original, I'd replace it with a new one. /ubbthreads/images/graemlins/cheers.gif PJ

 

[artmck]

Thanks very much, I'll check it out. I had it diconnected it and did not have the problem, but I'll have to test it to see what is going on.

Maybe this is related to why I always seem to be lean, I have the carbs as rich as they will go and still need a fair amount of choke until really warm. Even then it seems to want to be richer.

 

[Nunyas]

I was about to add another comment, about revs not falling while the engine is cold. It's a lot more noticeable when the engine is cold because the choke is normally on creating an even richer mix. There's a whole theory behind the need of the Gulp Valve, and at least some of it seems to be supported in practical application.

If your Gulp Valve is sticking even partially open, the front two cylinders will appear to be much leaner than the back two (actually, any time you rev then engine and allow it to fall back to idle "naturally" the plugs will look leaner up front than in the rear). The idle will be really rough no matter how warm the engine is; how rough is dependant on how open the Gulp Valve is. I've had old Gulp Valves stick open on me in the past, and the car ran like it was only on 2 cylinders.

If you can't get the carb rich enough, then you might have a problem in the charcoal canister area. Check to make sure you have good air flow in the line between the charcoal canister and carb. Or possibly leaks between the carb & manifold, or between the manifold & head, or in any number of the vacuum lines.

Also, don't forget that any number of items in the ignition system can create the illusion of running lean.