Mercury Sport Jet 90 (Force) Ignition Timing.

[SB]

I set my timing according to the book, remove plugs and crank engine, set to 32 degrees at WOT. It now runs terrible with 8mph off of top speed. I can tell that the timing is retarded. Before I changed anything I checked the timing and it was at 45 degrees.I will try setting it back to 45 degrees but this doesn't seem right either. I have read and reread the manual and I am sure that I am following the instructions. I will try the dynamic timing procedure this weekend. That is with the boat in the water and running at WOT.Anyone else have this problem or can offer any suggestions?

[SB]

Surely someone has timed a Force 90 engine?

[clanton]

I only have manual for 120, 4 cylinder, which call for 34 degrees at cranking speed with sparkplugs out. Timing set at cranking speed should be verifyed with engine running. Full throttle timing engine running loaded is 30 degrees. If the timing is realy at 45 degrees, the pistons would have holes in them.

[SB]

Yes, exactly my point. Which is why I am confused. However, after banging my head against the wall I suddenly wondered if maybe my advance timing light, which I have used on multiple 8 cylinder 4 stroke cars for 15 years, wasn't working on a 2 cycle 3 cylinder engine. So I marked 32 degrees on the flywheel and timed it with the light advance set at zero. Now when I check the timing using the advance and the 0 degree mark on the flywheel it reads 60+ degrees advance. I may have found the problem. Except now it won't idle below 1400, I will deal with that later. I am off to the lake to check it at WOT.Thanks for the response.Scott

[alounsbury]

Scott,I have a Force 90 and do my timing by the static method. I would be curious if you noticed a radical difference in the reading on the lake. The book says allow for a 2 degree difference - is that what you saw?Thanks

[SB]

I was able to check it at WOT at 5000 rpm. The timing was at about 31 degrees. I dropped it to 30 as the manual says but it was very close to being correct.I haven't figured out why the timing light was incorrect, but right now I don't care. The boat is finally running after being down a month due to a succession of problems.