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Heat in engine bay causes idle to drop?

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Astroguy

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Hello all,

I've got a 71 MGB, with a freshly rebuilt 18GK engine that has about 750 miles on it, running HS4 carbs. The engine has been running great in the past winter months, usually in temperaturs around 50-60 degrees. With the warmer weather, and especially in traffic where there isn't much airflow the idle will, over a period of 5-10 seconds, drop down until the engine finally dies. At stop lights, I have to keep a foot on the gas to keep it from stalling. Afterwards, an extended run with low stress on the engine and some good airflow will the get the idle back up to normal, so it looks like something under the hood is overheating enough to cause this.

I've thought I may be having fuel may be vaporizing in the line, but I am having trouble diagnosing if that is the case. And if it was the case, why would the engine come back to life when I give it a little throttle?

I've just tuned the thing up, adjusted the valves, set timing, adjusted carbs. Any suggestions on where to begin looking would be appreciated. Any suggestions on where to begin looking would be appreciated.
 
For some reason, I have the impression that the members of the forum whom are more experienced than myself will say that it's unlikely to be vapor lock..I'm not certain of that, but I thought that was a relatively rare problem which generally doesn't occur while the engine is running. Nonetheless, is the heat shield in place between the manifold and the carbs?

Perhaps it's dirty air filter elements? I wonder if the car, on a daily drive, would run better with the air filters removed, to check.

I have H4s on my Healey, which I tend to run rich; sometimes it'd have symptoms similar to this at stoplights (though I didn't notice a dependency on airflow), and it felt like the car would want to die while idling, especially when warm. It'd need gas to keep it alive, at points. I attributed it to "run up," which I think is basically the mixture being too rich at idle.

Is it possible that the mixture is wrong, or perhaps there's another problem, maybe a dirty needle seat in the float bowl, which is causing the float level to rise too high and flow out the top of the jet? If that's a valid potential problem, it's easy to check, but I'd wait for someone else to post, who has more experience.
 
From the description it sounds as if it's too "fat" (rich) at idle (and therefore throughout the range) for sure. Do you have a "sooty" exhaust at the end? That's another seat-of-the-pants diagnostic check. Should be a "light cocoa brown". Plug color check should be the same.

It would be odd for it to be a vapour lock. We'll assume for the sake of "common" causes that the heat shield is there unless otherwise stated...(good thought, 100-6!) Some other things which can affect idle mix are worn jets and needles (from jet mis-alignment, over time) or needles rubbing the jet orifice and not allowing the pistons to fall to the idle position smoothly. Float levels adjusted? Have the carbs been gone thru as well as the engine?

Oh! And welcome to the forum!!! Hope you find it helpful AND fun!
 
Welcome to the forum.

I'd have to agree. I had the same problem and all my stuff is new. Black sooty exhaust as well. I leaned it out a bit and it got better but still with stalling at traffic lights. Winter hit and too cold to continue with the experiment but so far results lead me in the right direction.
 
If all else is right, you should be able to lean 'em out too much. A suggestion for a "first consideration" is to check the float needle/seat. I would HIGHLY recommend the Viton tipped sets for peace of mind. Curto has 'em and they last a LONG time. THEN go on to start tweaking the adjustments.

Just a suggestion. /bcforum/images/%%GRAEMLIN_URL%%/smirk.gif
 
Thanks for suggestions everyone.

Carbs were rebuilt at the same time as the engine (we had them sent away to be rebuilt). I checked the float needles, and they operate smoothly.


I checked the plugs, cylinders 1, 3, 4 were black, but not enough that any soot came off on my finger. Plug #2 was the light cocoa brown DrEntropy spoke of. In addition, the exhaust does have a lot of soot in the pipe, when I rubbed the inside my fingertip was black.

Removing the air filters has made no difference, the car acted the same with or without them.

Yes, I don't think it's vapor lock. The heat shield is in place, as well as the heat shield insulation pads.

I think I will go out and try leaning out the carbs a bit and see what that does for me. I'll check back in to post the results!
 
Too rich, loading up.
 
If it starts doing that at a stop sign, you can blip the throttle to clean your plugs.

Then it will tick-over fine for 30 seconds & you'll have to do it again.

I moved the needles down 1/32 on my H4s and it cleared most of it up while still giving me a good off idle AFR.

People don't understand blipping when stopped.
(Or charging Accusumps before shut off)

So you might get some nasty looks.
(especially if you have a performance exhaust) /bcforum/images/%%GRAEMLIN_URL%%/grin.gif
 
Well, I also think your mixture is slightly rich, but, no mention has been made about your distributor. Has it been rebuilt? Is it set properly? Is the shaft loose? Improperly set ignition can cause many problems that seem to be else ware. Double check it and make sure it's right. Just another thought. PJ
 
Distributor is new, and I've had the points properly gapped. I'm pretty confident the timing and valves are all OK, 200 miles ago I had the car professionally tuned because of this problem. Only trouble is, I couldn't get it to reproduce this problem in front of the mechanic (isn't that typical).

I've started from scratch with the carbs, and gone back and did a tune. I have them set a bit lean now, when I raise the piston the rpms drops a bit but remains steady. So far after this, the car has been behaving, but today was only about 70 degrees so it's hard to judge. But it did do fine in traffic and idling for long periods of time. So I think the problem does lie in the car being too rich.

What puzzles me is, and hopefully someone can explain, if all this was from the car running rich, why does engine bay temperature set it off? Shouldn't it happen all the time, independent of what the temperature is?
 
The hot air in the engine bay expands the air and thus thinning it out causes the mixture to change to a rich setting.

I experienced something similar for a while and had to adjust my idle up higher than I would like to keep the car running in traffic once hot. Ran fine when cool but when hot the idle would go from being set at 900 to 1100 then down to 600 and points in-between. At the time I had a new eurospec 45d4 installed. I recently reinstalled my original 45d4 that had been recurved and rebuilt by Jeff at advance distributors https://www.advanceddistributors.com/ and I'm not exactly sure what he did but he cured all my running problems and the car now run's like its computer controlled. I reset the idle to 750 and it stays there all day. The B now runs smoother than I thought possible at idle and is much smoother overall and has more power. The rebuild and recurve was well under 100 bucks and has been the best money I have spent on the B.
 
Thanks everyone for the very helpful replies! It seems pretty obvious now what the problem is. I'll also look into getting the distributor recurved.
 
I had the exact same problem on my Healey 3000. On a cool day, it could / would idle steady . The temp gauge showed a steady 185, even after sitting at a stop light for a long time. Then I drove it on a freak 80 degree day. Got to a long stop light and the temp gauge climbed to 190 and at the same time the idle was dropping off until I had to push down on the throttle a little to keep it running. I didn't have any signs of richness, soot in the exhaust, etc. so I readjusted my idle mixture screws . I tried screwing them in about a 1/2 turn but the idle got worse, so I backed them out 1/2 to where they were and then another 1/2 and that seems to have fixed it. In my opinion, there is a definite relationship between under the bonnet heat buildup and idle mixture adjustment. Once the real hot weather gets here, perhaps they will need further tweaking. I'll know soon enough. I don't know if the MG carbs are the exact same as my Healey, but my owner's manual says to expect a slight increase , then drop in RPM when the carb pistons are raised with the pins, if the adjustments are right. I think you said yours slowed down a little which would indicate that your carbs may be too lean instead of rich. It's easy enough to experiment, just keep track of the turns that you are making and do the exact same thing to both carbs.
 
Hi AstroGuy, When the engine is cold and under hood temperatures are low the air supplied to the carbs is dense. As the engine bay warms up the air becomes less dense causing the carbs to become rich relative to their open bonnet settings. Most Big Healeys require that the mixture at idle be set so just a bit of choke is required until she fully warms up. Then the Idle speed will increase as the under hood temps rise and the choke can be set fully off. Consequently, it depends on which side of the ideal curve the carbs are set as to whether the idle increases or decreases with rising underhood temperature--Fwiw---Keoke
 
Keoke said:
Consequently, it depends on which side of the ideal curve the carbs are set as to whether the idle increases or decreases with rising underhood temperature--Fwiw---Keoke
Click to expand...
... I thought that is the way it was working. Thanks for the explaination Keoke.
/bcforum/images/%%GRAEMLIN_URL%%/thumbsup.gif
 
I think my MGB knows that I've been reading this thread, and I am beginning to think my B is also a hypochondriac. This morning she stalled at a couple of lights and I ended up having to hold my foot on the accelerator at all the remaining lights to get to the office. I also had to rev her up to 3K RPM just to have enough power to take off.

Definitely have to do some trouble shooting to do this weekend.
 
Iggy first, Rob. You know th' drill. /bcforum/images/%%GRAEMLIN_URL%%/wink.gif

...then check that poxy rubber thing in the Stromberg.
 
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