10 replies to this topic

[miniruby]

Would appreciate any advice on this. My daughter's 1990 mini has started misfiring. I replaced the condensor, coil, points and stripped down and checked the SU but still no joy.

The symptoms: Runs fine for 5 - 10 mins then the tick-over revs drop a couple of hundred and it starts to misfire and backfire when I try to rev it. Once cooled down, it struggles to start, hardly firing, then bursts into life. This then lasts until then engine has been running again for 5- 10 mins when it all repeats.

Valve clearances are all OK. Good pressure from fuel pump. Float chamber is full when the misfiring starts.

I was thinking that maybe it is the head/gasket. When the engine gets hot, maybe it develops a leak and draws in water. The initial bad starting might be clearing the water out? Then when it cools, it seals again.

The chamber of the SU was a dark orange colour (rusty water?). Don't know about loss of water as my daughter filled the rad (after many months) just before she gave me the car. No apparent oil in the water.

What do you think? Any other ideas before I remove the head?

[leonie]

i had a problem like this and found that the jet assembly underneath the carb was getting stuck. It would lower when i pulled the choke out and then not close up again when i pushed the choke in so the engine was flooding !!!! Worth a look !!!

[miniruby]

Thanks Leonie, I will certainly double check this.

[minislapper]

Air leaks?

Has it got the correct coil fitted for the ignition system?

[miniruby]

It's got a new 12 volt ballast coil which minispares tell me is the correct one. Still no improvement. I fitted new gaskets where the carb mounts to the manifold. There is VERY slight play in the butterfly spindle .... not sure how much is acceptable but car has only done 48,000 miles.

[minislapper]

I replaced the condensor, coil, points

Just because they're new, doesn't mean they aren't faulty! Condensors especially are renowned for this.

If it was running fine before you changed all these things, put the old ones back on and change one thing in turn to eliminate any possible problems from the other items. By changing lots of things at the same time you have no idea which bit, if any, has fixed a problem or made it worse.

[miniruby]

Thanks. maybe the new condensor is faulty then.

I had the problem before I changed anything ... and I changed the items one by one but it hasn't fixed the problem yet.

Have also checked the mixture now with Colourtune and it seems to be OK. I will invest in yet another condensor tomorrow and see what happens.

[minislapper]

Worth a try for what they cost. Good luck

[dklawson]

Minislapper has given you excellent advice. When you're not sure what's causing the problem and your casting a wide net, change one thing at a time and monitor the result before making another change.

While it could be the choke adjustment as mentioned, it sure sounds like an overheating coil and/or bad condenser. You've got that new coil and the supplier told you it was "right". However, you MUST confirm that you have a ballast resistor or resistor wire feeding the coil. If you don't, you're putting too much current through your new coil which will cause it to overheat and burn out prematurely. If you're not the original owner of the car there is no telling what a previous owner may have changed.

[miniruby]

Thanks Doug. Yes, I do have the ballast coil wiring.

Well, I discovered the problem. The timing had been set with the vacuum advance attached! It was spot on with the strobe when it was attached but about 16 degrees retarded when the vacuum pipe was removed. What tipped it over the edge was that the diaphram of the vacuum mechanism had gone and without that extra bit of advance to the ignition, the misfiring / backfiring started at any revs over tick-over.

The other good news is that the occasional running-on has also stopped. I assume because the engine isn't getting as hot now that the timing is correct.

[dklawson]

I'm sorry about the vacuum advance unit but I'm glad you sorted it out.