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MG MGA - Mis-firing woes

I actually live in Malaysia, but the drop down list doesn't include it.....
I have recently rebuilt the 2 SUs on my 1957 MGA with new parts from Burlen UK. Previously, the car ran okay but could not be balanced (one carb had different needle, piston etc to the other one. Now both the same). After tuning and balancing, the engine seemed to run well and idle/rev smoothly whilst in the garage, but once on the road it started to misfire occasionally. Checked plugs and they looked sooty, so I made the same small jet adjustment on both to lean the mixture a little (turned clockwise to allow jet up, hope that was correct).Took it on a longer run, still missing, still running smoothly at idle and revving up, but only when not under load. After driving around on an event today, when the engine got good and hot, the misfiring worsened, to the stage that on overrun down hills it was popping and banging loudly (presumably mixture charges that were not being properly ignited?). I manged to get home, but it sounded like mostly running on 2 - 3 cylinders. Still idling okay. Haven't looked at plugs yet. I thought, once an SU was set up at idle, it would work okay thoughout the range of throttle openings. So before I blame this on the carbs (this being the recent disturbance to the engine), should I be considering something else like the plugs (several of them failing?!) or the ignition such as condenser (not disturbed, seemed okay before, but could just be a coincidence that it should start to fail just now?).
Appreciate any comments....

Andy Drinkwater
Kuala Lumpur
A R S Drinkwater

90% of all carburetor problems are electrical. I would check in order, broken flex wire in the distributor, bad ground connection distributor to engine block, loose primary wires, bad condenser, bad coil. Very far down the list, leaky vacuun advance unit, bad ignition switch. Bottom of list, vacuum leak at manifold, deteriorated rubber grommet between float chamber and carburetor body, intermittent sticky float valve.

Start with a systematic approach to inspection and diagnosis of the ignition system. Assume nothing. See here: http://mgaguru.com/mgtech/ignition/ig106.htm
Barney Gaylord

Hi sounds like dirt in carbs to me I had same recurring problem every time I checked bowls they were full of a fine red rust, so I put an in line fuel filter which worked well until it clogged up, cleaned it out and everything was fine until it repeated, ended up having a new tank fitted problem solved, although the old tank was not that old 1996 I think it was covered in rust inside, I guess one should keep the tank full, of course one would need to know when to fill up, oh thats another thread lol


gordon
g c pugh

Hi Andy. It sounds like your car was running OK before the carb overhauls. You mention that the plugs looked sooty. Sooty plugs can cause extremely poor running and extreme misfire. How do I know this? After overhauling my engine, I got the ignition timing screwed up. (distributor 180 degrees out) The car backfired and sputtered and would not even run, even after I got the timing set correctly. When I pulled the plugs, they were sooty. I tried cleaning them, nd the engine at least started but ran extremely poorly. As soon as new plugs were installed, the engine ran perfecly! I would recommend that you try leaning out the carbs, install new plugs, and then see if it runs properly. Good luck, GLenn
Glenn

I used to have similar problems, and cleaning sooty plugs is only a temporary fix. New plugs are the only way.

You say that you are turning the nut clockwise to lean the mixture. I take it that you mean clockwise looking up from underneath the car.
Dan Smithers

Andy
All good advice above, but do check the dash pot and piston are clean. Any grit in there can cause back-firing and missing at high throttle settings, (giving it one!) if the piston is stuck up. An aerosol carburetter cleaner clears the gunge nicely. Also check the section between the float chamber and jet is clear of gunge, which can occur after the car has been idle for a long time. The cleaner can be squirted down through the jet to clear. Hope you reassembled the bottom 'jet assembly' correctly to ensure the needles are central in their jets. Am sure you will have instructions from Burlen for this.
HTH
Pete
Pete Tipping

This thread was discussed between 19/11/2006 and 20/11/2006

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