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Engine Misfire
Brook_lands - 30/6/14 at 06:22 PM

One for the collective.

I have a Suzuki Jeep 1.3 with carb. The engine is misfiring on cylinder number 1 (front of engine). But the strange thing is as you pull the spark plug lead off at the point where the lead is off the plug but still close enough for the spark to jump across to the end of the plug lead to the plug suddenly the engine speed increases and it runs much more smoothly.

So far - new plugs fitted and plugs swapped between cylinders 1 and 2 to show plugs working ok.
- new ignition leads fitted and leads for cylinders 1 and 2 swapped over to check leads OK.
- new dizzy cap and rotor.
- checked and re - set timing.

Removing ignition leads from cylinders 2, 3 and 4 in turn causes engine revs to drop.

Removal of ignition lead from cylinder 1 causes engine revs to rise (see above) and then drop back to what they were before the lead was removed.

Next steps will be compression test and (if it will fit) a look with the colourtune.

At the moment I'm at a loss as to why partially removing the plug lead appears to make it run properly anyone got any suggestions?

Dick - 30/6/14 at 06:42 PM

Sounds like a burnt valve or bad seated valve to me

K13JOB - 30/6/14 at 06:49 PM

making the spark cross the air gap would slightly delay it i.e. retard the ignition on that cylinder. If you have lost some compression on the cylinder, would the retardation make it run better?

britishtrident - 30/6/14 at 07:01 PM

quote:
Originally posted by K13JOB
making the spark cross the air gap would slightly delay it i.e. retard the ignition on that cylinder. If you have lost some compression on the cylinder, would the retardation make it run better?


Actually no it has no effect at all on ignition timing as the plug voltage rises so fast it is to all intents an purposes a step change.

The effect of making the spark jump a gap is to increase the ht voltage, this can enable the spark voltage to be high enough to fire a wet or oiled fouled spark plug or a weak mixture.
This was the principle of the spark booster devices fitted into the coil lead that used to be sold old through adverts in magazines like Car Mechanics.

rusty nuts - 30/6/14 at 07:12 PM

It's probably worth checking the valve clearances, a valve with no clearance may well cause similar symptoms . after checking the clearances check for air leaks using something like WD 40 spray around the inlet manifold area,any leaks will make the engine speed increase.Also check for vacuum pipe leaks

britishtrident - 30/6/14 at 07:14 PM

quote:
Originally posted by Brook_lands
One for the collective.

I have a Suzuki Jeep 1.3 with carb. The engine is misfiring on cylinder number 1 (front of engine). But the strange thing is as you pull the spark plug lead off at the point where the lead is off the plug but still close enough for the spark to jump across to the end of the plug lead to the plug suddenly the engine speed increases and it runs much more smoothly.

So far - new plugs fitted and plugs swapped between cylinders 1 and 2 to show plugs working ok.
- new ignition leads fitted and leads for cylinders 1 and 2 swapped over to check leads OK.
- new dizzy cap and rotor.
- checked and re - set timing.

Removing ignition leads from cylinders 2, 3 and 4 in turn causes engine revs to drop.

Removal of ignition lead from cylinder 1 causes engine revs to rise (see above) and then drop back to what they were before the lead was removed.

Next steps will be compression test and (if it will fit) a look with the colourtune.

At the moment I'm at a loss as to why partially removing the plug lead appears to make it run properly anyone got any suggestions?


Oil fouling due to broken piston ring or low compression due to burnt exhaust valve or weak mixture ( loose inlet manifold, duff injector)?

The effect of making the the ht current jump ah air gap is to boost the plug voltage which can often be enough to fire a cylinder when cylinder has problems.

Back in pre-WW2 days a common trick to make a worn out cylinder to fire was to make a spark gap out of a button, later these devices were marketed as spark boosters.

Brook_lands - 30/6/14 at 10:51 PM

Thanks Guys great info - compression test followed by valve clearance check I think to start with and a leak hunt to follow.