Finally got my ski running, drop in compression?

[yamanube]

Got the 790 big bore in, Jetinetics TL ignition setup and everything running this weekend. Feels good to have it all together, but Ive got a couple problems.

I took it out for the first ride of the season, gathered jetting specs from a ton of different setups and ended up getting info from Ross C to get a good starting point. Got it on the water and it was running okay, messed with the screws a bit and fattened up the low end to get it running pretty damn good and lightly burned up a tank of fuel. Before putting it on the water the compression was 175/175 on a cheap guage, after I got it home (it cooled down quite a bit) it read 165/175. I was a little scared by that so I pulled the head, it looks to be running a bit rich but the cylinder walls had slight scoring on them. Each one had vertical lines worn in the cylinder walls, nothing you could really feel with your finger nails but they where visible and wore away the crosshatch marks.
My questions is, if I go back through it and get my carb totally dialed will the scoring get worse or stay the same? I really dont have the coin to do a topend or even a hone and rings right now (or the motivation to pull my topend off) Part of me says, tune the carb in better, and rip it, shouldn't see any drop in performance and hone/rering this winter and another part is saying to pull it apart now and get it done.

 

[yamanube]

The first two are the mag cylinder the second two are the PTO cylinder.

 

[astral]

Looks way lean to me..I would fatten up the mix and run it..Whats done is done but ..
You need to know what caused it.Carbs or water ingestion...I ran a motor last year that started with 180psi front and back and sucked water in doing subs and lost compression down to 140 f and 130 r .I ran that motor for the rest of the year with no other problems....If you are broke, fix the problem and run it.If you have the money get it honed and ringed.

 

[SUPERTUNE]

IMO... the piston tops look way to hot for my liking, Try richening the jetting or check the ignition timing.
Too much timing will also cause a hot piston tops right before detonation starts.
My advice is pull the cylinder and check for any slight piston seizures, recheck piston clearances and remove the rings and look at the ends to see if you butted them (they will be shinny) from too much piston top heat or too tight on piston to wall clearance. Cylinder wall and piston damage can be caused from not enough end gap.

 

[SUPERTUNE]

Is this the engine you said you were running 33 cc domes with Yamaha Wiseco 757mo pistons? For a freestyle engine I would think that the measured compression ratio would be pretty high...

QUOTED from the 760cc dome thread: I will hopefully be testing a 790 with 33cc domes this weekend. Im a little scared the comp. will be too high....we will see.

 

[yamanube]

Is this the engine you said you were running 33 cc domes with Yamaha Wiseco 757mo pistons? For a freestyle engine I would think that the measured compression ratio would be pretty high...

QUOTED from the 760cc dome thread: I will hopefully be testing a 790 with 33cc domes this weekend. Im a little scared the comp. will be too high....we will see.

Yeah thats the one, measure 42thou squish.

 

[yamanube]

Im leaning more towards pulling the damn thing apart and having it honed and put new rings in it today, gotta check some prices and see how long it will take to get done.

 

[yamanube]

If I leave the engine together and adjust my fuel mixture correctly will the problem get worse or will I just notice maybe a slight performance loss and be okay with pulling it apart over winter and honing/rering?
or
Should I pull it now to prevent further damage from happening, have it honed and re-reing?
Our season is so short and money is tight lately so pulling it apart if not necessary is not the most desireable option but being at 86mm bore doing any more damage to a max bore cylinder could be a very costly mistake.

 

[SUPERTUNE]

It's always cheaper now to fix it than if you wait until you burn it up...(BLOW IT UP).

 

[yamanube]

It's always cheaper now to fix it than if you wait until you burn it up...(BLOW IT UP).

Very good point but as of right now my budget allows 3 options:

a) run it as is, as long as it lasts, if it blows put it in the corner of the garage and do a topend over winter
b) hone and ring, run as long as it lasts maybe do a topend over winter, if it blows put it in the corner of the garage
c) send it out to Team Scream to be checked over, if its just a hone/ring see option B, if its a topend, put it in the corner of the garage and have TS send me back the parts or hold on to them till I can afford the work (ie put it in the corner of the garage till winter)