MGB & GT Forum

Okay, what the heck is going on here???

When the GT is cold to moderately warm it idles around 1,000 RPM. After the engine warms up, it is hard to start once I stop and when I do start it, the idle drops to 500 RPM. The weather has been quite warm here for the past few days.

I have noticed this issue getting a little worse over the past few weeks. Maybe it coincides with the sunny, hot weather we've been having (85 degrees today)??

Here is a bit of info on the car:

- The carbs are rebuilt a la Jimmy Hilton.
- The distributor was schlemerized.
- The engine has 1,000 miles on it.
- I had an exhaust leak at the header that I fixed over the weekend (the flanges on the Tourist Trophy SS header are slimmer than my intake manifold flanges. I wish I knew that before I purchased the system!!!).
- I probably need to adjust the valves because I haven't done it since setting up the engine this spring (they sound like okay).
- The car is not overheating even when idling in traffic.
- The spark plugs look fairly good, but it may be running a bit rich.

Could it be that the hotter weather is making the car run a bit rich, which is resulting in the change in idle speed and hard starting?

I didn't have this issue earlier this spring.

Any input is appreciated!

Cheers,
Chris

vacuum leak maybe?

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RIP Christopher Allan Roop
RIP Jack Austin
RIP P.J. Lenihan
RIP Peter Cummins
RIP John D. Weimer
RIP Mick McGuire

It wasn't doing this a few weeks ago when the weather was warmer.

About a month ago I did a serious once over to look for any vacuum leaks and didn't find any. I'll check again just in case but don't think that's my problem because it runs fine on the highway and accellerates like a scalded cat. It just doesn't like being warm and idling.

When the air is cool it is more dense and when it is hot the air is thinner it is called pressure altitude. If you are running rich when it is cool it will run way more rich when it is hot.


Dwight

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1979 MGB

hunter1951 Wrote:
You've described density altitude (altitude corrected for barometric pressure = PA, and PA corrected for ambient temp =DA).

Should have very little effect on idle rpm. Certainly wouldn't cut it in half.
Besides, OP described cold engine vs hot engine.

The ambient temp would produce rpm that would effect both cold and hot engine.

Cold engine may have leaks that aren't evident when engine warms up due to metal expansion.

There's no automatic choke on that model...is there?

Come on fellas...throw us a bone.

Bob



Edited 3 time(s). Last edit at 2009-06-04 04:31 AM by crustyoldfe.

You could spend lots more time trying to pin point the cause but I would suggest going back through a full tune procedure, forgetting and ignoring nothing, all the way out to the proper setting of the carburation. Play like it is an old million mile beater and don't skip anything.

Most probably you will find and fix/adjust what ever is going wrong, perhaps without even knowing that you did the trick.

If the problem still exists after doing this you can go hunting, but at least you know that this or that isn't in the equation.

Having said all of this, and if you chose to ignore it, I would start looking at the fuel tank/carb venting system and/or fuel delivery. It sounds to me like the mix becomes lean over a short time. I would disconnect all of the lines at the canister and see what happens. If the problem goes away, you can start reconnecting them one by one and identify the culprit by a process of elimination.

Jack

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--------------------------------------- Services to Interesting British Automobiles-----------------------------------
----------------------------------------------------www.jackscars.net ----------------------------------------------------

-------------I like to think that I don't service automobiles so much as I provide entertainment, fulfill dreams and revive wonderful memories --------------

I am in the northwestern North Carolina mountains, right up against the Blue Ridge Parkway. Lawn chairs are in the shop and always time to talk. Drop in any time.

Thanks Guys!!

I will try the full tune again but will start with the carbon cannister lines. I think that might have caused me some issues last time. I will update once I've had a chance to tinker.

My question though, is why would the car run really well once I'm back on the highway or any place that I can get the revs up above 2500. In town after it's warmed up, it isn't a happy camper....

I only mention vacuum leak cuz I finally found mine - gasket between the MC & Servo booster. No more leak and idle is not dropping anymore.

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RIP Christopher Allan Roop
RIP Jack Austin
RIP P.J. Lenihan
RIP Peter Cummins
RIP John D. Weimer
RIP Mick McGuire

Thanks Lou. I'll take another look for a vacuum leak today.

cgill Wrote:
well, my issue won't help you cuz I don't think you have the Servo (brake) Booster on yours... but the concept is there LOL!!

I'm going to pull my carb and exh manifold this weekend and double check all those fittins too. I just realized the other day, my carb-to-manifold nuts do not have lock washers on 'em (didn't when I pulled it either) so I'm gonna put some on. I figure the more little things like this I do, will get me all the better in the long run.

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RIP Christopher Allan Roop
RIP Jack Austin
RIP P.J. Lenihan
RIP Peter Cummins
RIP John D. Weimer
RIP Mick McGuire

Check with Hillary for the correct size lock washers for your nuts.

Jack

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--------------------------------------- Services to Interesting British Automobiles-----------------------------------
----------------------------------------------------www.jackscars.net ----------------------------------------------------

-------------I like to think that I don't service automobiles so much as I provide entertainment, fulfill dreams and revive wonderful memories --------------

I am in the northwestern North Carolina mountains, right up against the Blue Ridge Parkway. Lawn chairs are in the shop and always time to talk. Drop in any time.

lean it out 1 flat and see what happens .I had the same thing happen with the hot weather . I leaned it out a bit and problem gone .

Thanks Derek. That is my plan for today as well as looking for a vacuum leak.

Cheers,
Chris

twigworker Wrote:
Well said!
This is true of so much of the questions asked here.
Set everything to spec, and it will most likely run right.
Does it have 12 volts and a good ground, it will probably work right.

THEN, "you can go hunting"

If you get rough running at high temps, the heat could be impacting the fuel delivery from the float bowl to the carb. I have had this problem sometimes in hot weather especially when I switched to a Peco header. The thinner metal radiated more heat towards the carbs. It only showed up at idle, not when moving due to cooling impacts.
you can check your heat shield, wrap the header with heat tape.

tc