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winners

Misfire

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Typical I get excellent advice on one problem then another larger problem looms.

Took the S1 out for a good hoon on Sunday, really enjoying myself for about an hour, chucking round corners etc, then it starts to misfire. At first I thought it may be lack of fuel as it was low and the gauges aren't that accurate. But no topped up and no different.

I then checked the points(OK gap), plugs (all sparking) so leads should be OK.

The car starts and idles fine but under accelaration misfires. It appears to be getting alot worse now, proper jolting the car.

Whats the next step? I'm churning over; condenser, fuel filter blockage (caused by debris dislodged my hard cornering), fuel pump & carbs. Last three not really my thing and the condenser I'll change as it's easy although I personally think it's unlikely.

Over to you guys again please.

Many thanks in advance.

Mark

Edited by winners

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Hello Mark

The usual things are always work checking but dont overlook the dizzy cap........sounds as if you have and removal and holding up to the light you may see a crack or evidence of tracking out.

Hope this helps

Cheers

paul

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I diagnosed mine the other night (Excel, which then went on to cure itself) by seeing if it mis-fires under load accelerating hard if you pump the throttle a bit. As Dellortos have an accelerator pump you can throw a bit of extra fuel into the engine by pumping the throttle. Mine cleared when I did this, then returned as soon as the throttle was held constant, so it must be fuel, but not supply to the carbs.

Try that, if that clears it momentarily, then think dirt in a jet or something has gone really wrong with float heights without anybody touching them (curse put on by local witch would seem most likely for the latter).

If that doesn't reduce the mis-fire then think electrical.

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Had an annoying misfire on my S3 NA which went when I changed the coil. If I accelerated hard then lifted off, it would miss for a few seconds. I was convinced it was fuel related and swapped the coil only as I happened to have a spare and it's only a few minutes to swap. That was a few thousand miles ago and fine since.

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The slow running jets on Dell'orto carbs also provide the fuel during the progression phase..quite a long way up the rev range. If it misfires on part throttle, but pulls well if you floor it, think slow running jet. Modern petrols evaporate and leave varnish behind...this blocks the jets a bit; since they are only marginally big enough anyway, you get misfiring...typically around 2,500 to 3,500 rpm. I finally had to clear the jets with a short piece of guitar string, calibrated with my digital vernier caliper to ensure the diameter was correct. After that...no more misfire!!

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Mark, How did you check the plugs were all sparking? Did you take them out of the head and simply ground them and crank the engine over. I have had numerous cars (older generally) that developed misfires over time. In most cases, it was one of the plugs that I tested as ok. Under load in an engine, a plug can fail to spark as the cylinder pressure increases with increased rpm. If you haven't already, try new plugs in the car. Chasing fuel issues can drive you nuts, but plugs are easy and not expensive. If it doesn't work then you've simply got a spare set for later on.

Michael

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