Ultra 150 no start plus backfire

[FL_Rider]

I am interested in ideas on confirming correct timing. Only option I see is to fab a pointer and then find and mark TDC for all three cylinders on the PTO coupler. If I am overlooking something simpler and quicker please advise.

That's exactly how you do it. You don't need to really fab a pointer though, just put a piece of tape on the engine next to the pto and draw a line on it that lines up with the mark on the pto for cylinder 1 TDC. Steve is right, you only have to mark 1 cylinder to verify timing unless you think the crank is twisted. A couple more marks for the other cylinders is easy enough to do though.

[Trimup]

So I marked TDC for all three. I initially used a fixed stop rotating to it from both directions and then double checked my marks were in the center of the dwell period with a dial indicator. All three marks are also precisely equidistant from each other, within the limits of my measuring capability on the coupler in the hull of course. All three cylinders are sparking a few degrees BTDC. Does anybody know what the base starting timing is supposed to be on an Ultra 150?

[steve45]

From the service manual:

15° BTDC @1 250 r/min
∼ 22° BTDC @3 500 r/min (rpm)

Guess you could use a timing light to check it while it's running.