Compression goes from 200 to 50 on all cyclinders

[jackmanjls]

I can't get this F90 to run at idle. I"ve had several posts to this forum and got some great ideas but no "ahh moments".

As the title says the compression on all cyclinders went from ~200 last year to ~50 this yr.

The test was performed with the throttle plate open and the engine warm.

This doesn't make sense to me but might explain my rough idle issue.

I'm looking for suggestions.

[walleye1]

Do a leak down test
no way it was 200 psi

[boscoe99]

I am guessing that the cam shaft may no longer be corrected phased to the crank shaft.

[walleye1]

[jackmanjls]

walleye1...you're right I went back and looked at previous comp test and it was ~150 not 200 like I stated, my bad.
Your comment on leak down sounds like something RB17 would say, he hates comp tests

B99...the SM says the following:

Turn the flywheel magnet clockwise and
align the “TDC” mark on the flywheel
magnet with the pointer, and check that
the “arrow ” marks on the driven sprockets
are aligned.

I check the flywheel for TDC. When the arrows on the 2-cams (sprockets) line up the TDC mark on the flywheel is pointed to the back or 180degs out of phase from where it's suppose to be.

So, in other words, the TDC mark on the flywheel is suppose to face the front of the motor when the arrows line up. On my motor, when the arrows line up, the TDC mark faces the back of the motor, ie, 180degs out.

Is that possible that it would jump exactly 180 out and still run? It runs rough but it runs.

[rodbolt17]

nope, your checking something wrong.if the timing was 180 out it would not run at all and most likely would have some bent valves. remember in a complete 720* cycle the TDC mark is going to line up on the flywheel twice.
pull it around another 360 and you will see the marks now line up.
the cams run at 1/2 crankshaft speed and it takes 720* of crankshaft rotation to fire all 4 pistons.
and you are correct,a compression test on a four stroke is about a teat on a boar hog.