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MG MGB Technical - Warm engine misfire...

My car seems to have developed a couple of strange(?) symptoms lately... 4main, dual SUs, headers...

First off... its seems to take FOREVER to warm up, even when driving in 60-70 degree weather. But it seems that after about 10-20 mins of driving, it will warm up very rapidly as soon as the car is sitting still (all cooling fans are working, thermostat checks out)

When it is below 1/4 on the temp gauge, the car will stall if you go too deep into the trottle at low RPM. It wont even move in the first few mins after startup without stalling (choke activated).

When the temp guage gets to about 1/2 (or a little above, which I normally consider the normal operating tempo, and is before the electric fans switch on) it misfires like crazy...


Do these symptoms mean anything to anyone? is this a mxture adjustment problem? (too lean?)

Thanks in advance for the advice.
Jameson

The slow warm up could be caused by a thermostat that is stuck open. From your description it sounds as if the mixture may be running lean. Acting up when hot is a common failure mode of ignition coils, and electronic ignitions.
John H

You don't say what year your car is but with the electronic tach (as opposed to the mechanical rev counter) problems in the ignition LT usually show up on the tach as the needle dipping down in time with the misfire.

There should be enough enrichment from the choke to avoid stumbling, particularly with HSs which I find can be reduced to half-choke immediately after starting, although I find HIFs do need full choke for a few seconds. Can't speak for Stromberg.

You can check the thermostat warming up from cold at a standstill. Keep feeling the header tank, at about half-way from C to N on the gauge there should be a sudden rush of hot coolant into the rad, and that means the thermostat is OK. If the radiator starts warming up gradually from cold, same as the head, it is stuck open. That will need more choke than it should when driving off.
Paul Hunt 2

Its a 79.... The tach does not dip when it misfires... Ill check on the other suggestions so far... thanks!
Jameson

Something else I thought Id mention...

Its an over-bore to 1950cc, with cams, and a custom head and pistons. When I tune it much richer than it is, it is running very rich... Large clouds of un-burned gas, and very smelly. Asuming that the stammer / misfire at "normal operating temp" is caused by a lean mixture, what would be causing the car to need to run so rich?
Jameson

With those modifications you may need to change to carb needles with a different taper to get the mixture correct throughout the rpm range. From your post it appears this has just started lately which would rule out the needles.
John H

Jameson,

you should select the right needles, as Jhon allready mentioned and you also shoud have your dizzy recurved to fit your engine and cam or go for EDIS or Megajolt and do your own ignition settings.

Ralph
Ralph

If it ha sstarted recently as John H suggests, definitely find what has changed/gone wrong first and put that right before spending any money on new stuff.

What carb(s) so you have? With variable jet carbs if there is a vacuum leak round the manifold it is possible to tune the effects of that out and get a reasonable mixture at idle, but then it can become over-rich at larger throttle openings. A grossly rich mixture can be caused by a blocked float chamber vent, and in some case by a faulty float valve. Although in that case with SUs and Stromberg at least it usually runs out of the overflow onto the ground, although with a charcoal canister attached it can be some time before it is noticed.
Paul Hunt 2

This thread was discussed between 17/01/2008 and 19/01/2008

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