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Running lean

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W

William

Darth Vader
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Okay, so we finally got the B back....carbs re-rebuilt (apparently they'd been monkeyed with and needed remachining), electrical faults fixed, and a new head with gasket. That's right, a new head-apparently the old one had developed some cracks (saw 'em. Very nasty), and had burned through the copper head gasket between two cylinders.

Now, my question is-the car (which is overbored 40 thou and has a warmer cam than standard-which kind of cam we don't quite know), has a bit of a hesitant spot, right around 2000 rpms. Youll be cruising along in fourth, say, and hit a hill and the revs will drop down to around 2500 or less, and it sputters a bit. It only happens on part throttle like that, and has always happened a bit. Its a little more noticeable now, probably because of the complete, leak free exhaust system we now have. I figure this means the car is running a bit lean at this point, and I usually just rev through it. Dad had the tendency to drive around in fourth gear at thirty mph sometimes, and now that sputtering is quite noticeable. I'm trying to get him to stop that. Anyway, could this sort of sputtering/lean condition have cooked our old head and gasket? I'm trying to not have this happen again....

-Wm (Boy I ramble on when I can't come here every day!)
 
My midget does that (with webers). As I believe that I have problems with my carbs, its most likely mixture adjustment. Also, the hot cam probably doesn't help ...
 
There is oil in the carbs (I checked!), and we had everything set up by our local Olde Car Shoppe. Tuning this car has always been a bit of an experiment, because of that unknown cam. (When we bought the car the PO had worked around this by having the ignition timing set to some absurd degree, almost opposite of what it's supposed to be). The car is actually set up to run a bit rich, because of the hotter cam, but I just figured that the little hesitant spot was due to not enough air and fuel getting into the ports during part throttle driving. Does lean running cause extra heat in the engine/head, or does rich running?
-Wm.
 
Lean conditon causes higher temps in the combustion chamber (until the car is so lean that it misfires......then the temps drop).

I can't recall what year MGB you have. If it has a manual choke, you should be able to pull the choke out just a bit...if this improves things, the car is likely to be lean. If it doesn't help, the problem lies elsewhere.

Be sure that you have a clean fuel filter and that the float level in the carb(s) is set correctly. Either of these can cause a misfire and stumble.

A common problem in these cars is a leaky manifold gasket due to loose intake manifold bolts. This can create a vacuum leak that can generate a stumble. Since you've just had work done on the head, this seems unlikely....but not impossible. An easy test of this is to start the car and allow it to idle. Spray WD-40 in the area of the intake manifold gasket (where the intake manifold is attached to the head). If there's a vacuum leak, the WD-40 will be sucked in and the car will slow down a bit (the WD-40 won't hurt anything).
 
Hey adam, thanks for putting me on to that link on tony's website! I'll most likely be examining other carb issues soon, so I'll definitely include that little glitch in the analysis!
 
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