MG-Cars.net

Welcome to our resource for MG Car Information.

Recommendations

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGB Technical - SU Carbs

I've been trying to work my way through the problem listed in message Engine skipping at 3000 rpm (although it's really 3300, but who's counting<G>).

I've switched to AAA needles, and that seemed to help a bit, but the stumble still occurs and remains constant up through the rpm range to at least 4000 (I've not bothered to run it much higher under normal driving conditions...Any higher and I'll get a ticket for sure).

Question. On SU's (I'm running HIF4's on a non-stock 73B engine) is it reasonable to assume the dashpot piston would be in the same relative place re: rise and fall, at a given rpm whether driving under a load or sitting still? The reason I'm asking is that the stumble occurs sitting or driving, but not under "hard" acceleration. The one thing I have not done is to thoroughly, completely, absolutely clean the insides of the dashpot covers to ensure there are no possible sticking points. I have used the SU tool to ensure the pistons are rising and falling at the same rate, and they are.

Any thoughts??

Thanks,

John
John Staub

John-
No, the position of the piston at any given rpm is dependent upon load. Certainly the dashpot/piston interface should be chemically cleaned (never use abrasive cleaners as the clearances are critical!). It may be something as simple as insufficient or the wrong oil in the piston damper tube (use 20W/50). You may have a "compound problem", that is, a problem that's the result of several small things that taken in isolation would mean little, but when they occur at the same time, all total up to one problem. In order to isolate any major contributing factor you need to eliminate the variables in the equation. You need to go back and check everything: valve adjustment first, then camshaft timing (use a degree wheel and a magnetic stand with a dial indicator at the rocker arm), then check both static and dynamic ignition timing before trying to sort out the carburetion.
Steve S.

Not sure it will help (sure can't hurt) but have you checked the little vent holes for the dampners are clear. Either on the cap or in the small neck of the dashpots.

Mike
'79B
Mike Janacek

Hi All,

I seem to have fixed it. Hmmmmm.

The AAA needles made a world of difference when I put the air filters back on.

I worked this morning on balancing the carbs better (they were still a bit rich), and after balancing them, put the air filters on and prepared to take it to a shop for a 'scope job.

Runs pretty darn well. I've still got a bit of what seems to be a miss at "speed", but will wait until I can get it on a scope to see what may be causing that.

Bottom line....The car is "very" driveable again.

A hearth thanks to all for their suggestions and assistance.

John
John Staub

This thread was discussed on 28/09/2002

MG MGB Technical index

This thread is from the archive. The Live MG MGB Technical BBS is active now.