21 replies to this topic

[mini_mission]

hi all,

for some reason my mini has developed a misfire and i cant work it out the problem. it occurs in the top end of 4000rpm ,more like 5000. also the popping is more noticable at higher speeds or if the wind is against me- 1st gear isnt so bad, 2 is worse, 3 is even worse etc..

up to 4700-5000 she revs clean and smoothly- luuuverly...

i have checked the jets in the weber- all clear.

plugs, leads, coil, dizzy and eleccy ignition is all new.

earths are fine.

the fuel pump is a big SU double sided electonic one, so i doubt its fuel related.

no luck on the search either

so i am thinking its either the coil, (doubt it but i have a spare lucas gold that i havent tried on yet so i dont mind it that could be the prob) or its the petronix leccy ignition as apparently they suck anyway (its my second one as first was faulty and steve from MED said they were C**p too)

anyway ideas guys? its started to really bug me as its so annoying driving within such a tiny power band (3K- to just under 5!!)

[ginigwunkle]

mixture? getting rich top end and getting through and ignigting in the exhaust....

[mini_mission]

hmm maybe but i doubt it.. its not an exhaust popping- not like the pop you get on overrun anyway. its sounds like an ignition thing plus you can feel the power come and go as it occurs.

i think it is on the rich side as it was rolling roaded by a hillclimber chap (as race cars usually are on the rich side) as the sparks are a bit sooty but not so much to cause this surely? (besides it hasnt always happened since i got it tuned)

[ginigwunkle]

do you have a gas analyser to check the mixure now, could have gone a bit more out that when it was set...

im running at 4.5-5% co2 and my plugs are golden brown!

[minimender]

I'd look for a bad connection on ignition side or maybe damp in something to do with sparks.

Or small plug gaps

Edited by minimender, 10 February 2008 - 10:21 PM.

[dklawson]

I typically discourage people from replacing coils but a high speed misfire is consistent with a failing coil AND you already have a spare one to try. Take a test run with the existing coil first and find a condition that WILL cause the misfire. Then fit the backup coil and repeat the same course and conditions to see if the problem still exists.

[mini_mission]

ok well i have put on the new coil- the problem still persists.

i have checked the connections- they appear to be fine.

i will readjust the spark plug gaps- what tho is best?

and DK, im not sure exactly what you mean, find a condition that will cause the misfire... think i am being stupid- but if i could do that then i wouldnt be stuck?

its seems that this misfire has got worse it occurs at lower revs now, about 4500 rpm

could it be a faulty electronic ignition?

[dklawson]

When I suggested finding conditions that would make the misfire happen... I was only suggesting that you find a location and diving technique that would make the car misfire. Once you know how to make the car misfire you can repeat those driving conditions each time you make a change to the car and know when you've improved or hurt the situation. It sounds like you're not having problems creating the situation.

Do you have a tach fitted? I assume you do. When the misfire happens do you notice any weird behavior on the tach (bouncing or jumping needle, a flutter perhaps)?

When this misfire happens you said that you could feel a change in the car. If you try and accelerate through the misfire, does the car loose power and hit a wall where you can't go any faster? Does the car feel like it's surging forward and back when the misfire occurs?

[mini_mission]

ooh right- with you now!

yes i do have a tach, but its a digidash one and it displays e.g 2.3.. 2.4.. etc or a more detail one is available to, but to answer your question- i havent noticed the revs bouncing or fluttering

its is possible to rev through the misfire (sounds bloody awful) and it doesnt appear to hit a wall. but your second point is correct- i can feel the car surge back and forth when this occurs. feathering the throttle helps this.

I dont know why, but for some reason i think it has something to do with the dizzy and or electronic ignition- but it all looks fine.

[Ethel]

If it just ran out of puff you might think that the advance curve is wrong but if you can drive past it it may be a faulty dizzy or something else causing it to flutter.

Is it an aftermarket electronic kit or a genuine Lucas electronic unit?

[dklawson]

If you can borrow and temporarily connect an analog tach (with needle) try that and observe what the needle is doing when you feel the misfire. If you don't notice any change in the tach's behavior (and since you've tried the second coil), any ignition related problems are probably not in the ignition module but something in the secondary (high voltage) side of the ignition system. This could be anything from the rotor and cap to the plug wires and plugs themselves.

If the analog tach does flutter and dance when the problem occurs, this would indicate some form of problem on the primary side of the ignition system. Again, since you've tried the second coil, the tach behavior would point to the ignition module.

Once you feather through the misfire how's the power? Does the car feel sluggish like it won't go any faster if you try to accelerate? You reported the misfire now happens at about 4500 RPM. Is that true in all gears... or just 4th?

[mini_mission]

If it just ran out of puff you might think that the advance curve is wrong but if you can drive past it it may be a faulty dizzy or something else causing it to flutter.

Is it an aftermarket electronic kit or a genuine Lucas electronic unit?

the dizzy was supplied to me by MED when they built the engine. therefore i doubt the curve is wrong.

the electronic ignition is a petronix one, which from what i have heard from MED is that they are carp.. hence why he has stopped supplying them now.


I have a spare dizzy cap and rotor that i will try tomo. the sparks are very new ( i have taken them out and they look fine, if a bit black) as are the leads, so i really doubt it is that.

its a bit difficult to describe what the power is like when feathering it.. but i would say it is a bit sluggish- but it would i guess as im feathering it to stop the misfire a bit- then i change down as soon as i am out of the corner.. (n.b. we are talking 6000-7500rpm here).. the misfire occurs in all 4 gears.. less so in 1st (not sure if thats because there is less air resistance)


thanks for all your help so far by the way guys... hopefully we can find the culprit soon!

[dklawson]

Pertronix is not bad. There are some areas where you want to be careful with them... such as making sure you're supplying them with a full +12v supply on the red wire. Not knowing what vintage your engine/car is, have you considered temporarily hooking the Pertronix red wire up to a different connection point that you KNOW gets a full switched +12V? It wouldn't hurt to try. Operating at less than a full +12V can affect the switching of the module.

MED built the engine, but did they install it or did the installer make sure the right coil was fitted? This goes back to my first paragraph. Since I'm not familiar with your car, let me suggest taking a close look at the ignition wiring. You MAY have a car that was built for a ballast ignition coil while you have standard coil and an underpowered Pertronix. Pictures of the coil wiring might help.

Since you have Pertonix fitted, once you have confirmed that you have the right coil for the right type of ignition wiring, you could always remove the Pertronix and fit points. If fitting the points addresses the problem you've got an inexpensive way of identifying and temporarily fixing the problem.

Plugs can be deceiving. Sooty plugs aren't just an indication of a rich mixture, the carbon can also cause the plug to misfire. Again, make only one change at a time but... if the alternate cap and rotor don't address the problem, throw in a clean set of plugs and try that.

[VulcanBomber]

My car had a strange missfire whilst cruising on the motorway, about 4500rpm. It sounded like it was coming through the carb and would hesitate a little.

It turned out to be the valve seats burning out, caused by running a leaded head with unleaded fuel. Check your valve clearances and see if your valves are moving in to the head.

[mini_mission]

deffo not the case- i have recently taken the head off as i broke a valve spring. replaced all the springs, the valves, tappets clearances are absolutely spot on. i very anal about that as its a new engine (just been broken in with a bout 600 miles on the clock)


dklawson, im going on holiday tomo so unfortunately i dont have time to sort the prob out today. hope you dont mind if we pick this thread up again in a fortnight.