Jetmarine Miss fire/Jolting

[rob peters]

Hi,

I have a 15hp 2S jetmarine and 3.5m suzumar air deck. The engine starts fine on second pull and at neutral runs about 1300rpm. When i accelerate away after 3000rpm the engine starts to jolt or missfire. When it gets to WOT it happens about every three seconds. I am running a 10 pitch yamaha propellor 9 1/4 diameter. This runs up to about 4600rpm giving 19mph 1 person light load. I have tried a smaller prop 8 pitch this runs up to about 5300rpm 15mph jolting is still there but not as forceful.

I have undertaken the following

1) Spark plug ignition test all fine but replaced anyway
2) HT leads checked for looseness all fine
3) Emptied fuel contents replaced with new fuel and 2S oil fuel stabiliser added
4) Fuel lines and fuel pump inspected
5) Quicksilver powertune no improvment
6) Compression test 110psi on both cylinders
7) Run each cylinder by itself with a load No jolting (RPM only went to 2500)
8) Kill Switch replaced not for the above reason but because the old one was dodgy

I am not sure what to try now. Has anyone else had a similar problem. Don t really want to take into the local repair shop as it has been in before and they havn't identified it

Any help would be greatly appreciated

regards

Rob Peters

[jyasaki]

1300 sounds a little high for an idle speed.

Doesn't sound like a fuel problem, as that usually is more a bogging down feeling than a momentary cutout.

Is it a magneto fired ignition? It sounds to me like an electrical problem with shatever is driving your spark. HV Coil? Induction pickup? Don't know what you've got on your motor, so it's kind of hard to say...

jky

[9D280]

How did tyou test the HT leads?

If you can find a spark, ask him or her to Megger them - what your "standard" multimeter says is good insulation can break down when you show it high frequency at a few KV.......


Also how regular is it? - like a clock, or "very roughly" 3 seconds

[rob peters]

HT Leads were tested in a tank using standard multimeter. This is half the battle as it only shows up under load when out in it so wondering if it is vibration related. I have checked most of things in the yamaha book under ignition trouble shooting and they are what they should be. Havn't done the peak voltage parts as i don't have the equipment to do this. I am sure it is something to do with ignition because beside the jolt it seems to run fine no bog down during acceleration etc. The 3 seconds was roughly timed whilst at WOT

[rob peters]

After all that it turns out it was issues with gearbox and slipping out of gear. Now repaired finally with successful first outing of the year yesterday.