what is causing timing mark jitter at part throttle?

[77jeff87]

My '91 Merc 25 hp has a jittery timing mark. At idle, it as at about 0?, and mostly steady except for a few pops or stumbles (motor jerks side to side slightly), but when I increase the rpm's slightly, it pops, backfires ocaissonily, and the mark jitters in the advance and retard directions. I am testing it on the trailer with the water hose muff, and I don't know if I will damage something running up to WOT, so I didn't. So I didn't adjust the high speed timing. I put in new spark plugs before I did this. Before timing, I tried to set the idle mixture, but the few pops or stumbles, I am not sure it is correctly adjusted.
Please help me with fixing this problem.
Thanks in advance.

 

[Laddies]

Re: what is causing timing mark jitter at part throttle?

Don't try to run the engine anywheren close to WOT on muffs you won't have a engine left to try to repair about 1500 to 2000 RPM max on muffs. The timing will move around alittle at idle speed, the trigger is not held tight on anything fixed till it hits the advance screw. You will never get the idle right without the engine in the water in gear it takes both exhaust back pressure and the engine being loaded in gear to make proper idle adjustments.