View Full Version : Engine misfire - Help!

08-27-2007, 02:19 PM
I just returned from a weekend on the water. I have a PS 190-1991 Indmar. All weekend I have had a really bad misfire when pulling skiers/boarders out the water. Once you get past 2500rpm it clears. Goes fine at high speed. I can also make it clear at 2000rpm by going WOT (this could be because it accels to 200rpm so quickly even with a misfire!). Once it's cleared I can come back down to 2000rpm again for boarding but it misses again going below this speed. I checked all the plugs and leads - all seems OK. The inside of the distributor cap was very slightly corrosed so I cleaned it up with a screwdriver (didn't have any wire wool and will get a new one asap!) but it was the same afterwards. Starts and idles perfectly but misses as soon as load is applied. I also blew out the fuel filter - it's stained brown but no lumps of crap in there. Again was the same afterwards. Removed the flame trap, still the same; maybe a touch worse but could just be my imagination!
Only other info is a few weeks ago the drive belt went and coated everytning in powdered rubber, then the next weekend it had a slight misfire at c.3000 rpm when cruising (very slight) - turned out the flame trap was absolutely covered in black powder so I cleaned that with solvent and it seemed to be OK after than, choke flaps were also coated so I have cleaned them up a bit but was fearful of dropping rubber into the manifold. - can it be related - any ideas? I really don't want to strip the carb and there are only about 2 MC specialists in the UK, none anywhere near me!!!

08-27-2007, 02:32 PM
I would get a spray can of carburetor cleaner and see if that helps.......Sounds like you might have some trash in it...........
Good Luck

08-27-2007, 02:33 PM
If you have, or can get, a timing light, make sure the advance is stable with no jumping around and make sure it advances when you increase the RPM. If it doesn't advance, I would suspect the centrifugal advance weights or springs. Cheap to replace the springs and easy to free up sticky weights. The rust under the cap is one reason I mention this. I would also check the points gap. If it has been a long time since the points were set, they're probably not opening as far as they need, in order for the coil's field to build enough to give you an intense spark.

If you can get a dwell meter, use that instead of just gapping the points. It should have a tach function, as well.

08-27-2007, 02:50 PM
It's electronic - At least I think it is - looks like a small chopper wheel and pickup but the only mechanical ignition I saw is on my 1978 Triumph Spitfire. The MC definately does not have this type of points. I assumed it was electrinic as has 2 wires to the distributor and inside is a small plastic wheel and what looke like a couple of large magnets. Any photos?
It runs fine at higher speeds and accels well at speed but I will check the advance in a few weeks when I get back out again.

08-27-2007, 03:06 PM
OK, not having points is good. The centrifugal advance weights are under the plate beneath the ignition.

08-27-2007, 07:43 PM
Check for a spark plug that has a crack or is broken in the porcelin area.