Carb maladies

BX Tech talk
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Meteor Man
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Carb maladies

Post by mnde »

Hi all,

My BX has recently developed a couple of niggles.

1) When I start the car, the engine will run for about 5 seconds, then die - even if I rev up to 2500 and hold it there. It's then difficult to restart. But once I get it started it runs fine.

Coincidentally, sometimes when I turn on the ignition I hear what sounds like a "spit" from the exhaust manifold, like the noises it would make on the over-run if it was blowing badly. How is that possible, just by turning on the ignition (not operating the starter)?

2) When accelerating hard over 4000 revs in 4th gear, e.g. when joining the motorway, the car begins to misfire and tries to cut out - it only settles down when I change up to 5th.

It looks like I'm going to have to get on with prepping that carb you sent me, Cav! It's pristine clean having been dunked in white spirit for a week and paintbrush cleaned a while back, now I just need to get a service kit and change all the o-rings, gaskets etc. and set it up according to the Haynes carby manual.


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Post by Way2go »

I had this 'spit' on a car years ago. Three out of the four exhaust valves had burnt holes in the edges and the spit was unburnt fuel being combusted in the exhaust manifold. Starting problem fits too because the engine is happier when warm and of course when in 5th gear presumably the load is light so again the engine is happier.
In my case it was 3 new valves and regrinding the seats. (This was after I had cursed and fiddled with a blameless carburetter!)
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Post by tom »

Sounds like a compression test first would be a good idea.

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Post by jeremy »

The spit at standstill is interesting. What is happening I think is that a plug is sparking and lighting mixture left in that cylinder - and the noise is because an exhaust valve is open.

i think it was possible on some ancient engines like RR Silver Ghosts to start them by manualy varying the spark advance. By doing this you could separate the contact points which produced a spark - and if the engine had been running recently - it would burst into life!

What is surprising is that the spark is being produced and this might be a clue as to your problems. Normally turning on the ignition would simply activate the circuit and no spark would be produced - so here something must be activating it here. It may be the function of the electronics as they come to life a spark is thrown out - but I think it more likely that the ignition switch does not connect cleanly and sparks internaly a bit - hence the spark production. A poor connection somewhere in the primary circuit might produce the same result.

I'd look very carefully at the circuit and see how close to the business end I could hot wire it (ie how much of the primary can I eliminate - switch and its wiring etc) then hot wire it and see if there is any difference.

I agree a compression test will eliminate valve and similar problems (no pressures stated by Citroen) - bt compressions must be nearly equal and you'd be looking for between 130 and 180 psi as a guess.