08-01-2010, 08:06 PM #1
- Join Date
- Jul 2010
First Ride on Rebuild....need advice.
I'm an experienced "shade tree" mechanic and have done rebuilds on automotive engines etc. I know virtually nothing about 2 cycle engines...at least in terms of working on them. I bought a 1997 SLT 780 that had melted the front piston and messed up the head. I bought a .5mm overbore kit, had all 3 cylinders bored and honed and put it all back together. I rebuilt the fuel pump and all 3 carburetors and replaced all of the fuel line. I pulled the oil injection and switched over to premix. I never really identified why the front cylinder burned down. It was black with carbon and had the edge chipped off all the way down to the top ring. Anyway....I took it for it's first ride today and it ran perfect. I was gentle with it and did not exceed 30MPH. It idled well and accelerated fine. I used light throttle and I noticed that there was a big burst of power if I gave it any more gas past where I was running. I want to verify that it is not running lean before I give it hell (after break in of course). How do I do that???? Today I ran it gently for a while, pulled up to the dock and pulled the plugs. All three looked black. I used a laser/infrared temperature sensor and pointed it down the sparkplug holes. The three cylinders all read about 130 degrees (I must have waited too long to check them because I know that can't be hot enough). So....is there a way to verify the mixture BEFORE I burn a piston again? It runs great but I assume that's not enough evidence that the mixture is correct.
08-01-2010, 08:39 PM #2
You have to check your piston wash. I don't have a recent picture of mine because I used my scope yesterday that doesn't take pictures. I can only see it on the lcd screen, but the picture below shows a piston that would be considered a little lean but it is the way I like to keep mine if possible.
08-02-2010, 02:07 AM #3
That picture above is great wash!, its NOT lean.. its fine..
the pump you rebuilt.. is it a pentagon pump or a triple or a single? I'm guess since you have a 780, it has the pentagon pump..
did you clean the screens inside the carbs and or replace them?
you want warm up the ski first.. Then,, go for wot runs for a 50 yards or so, and then let it cool down.. then another wot run (50 yards) and then let it cool.
that will set the rings for you.. otherwise , just crusing around lightly may glaze the cylinders ans keep it from seating the rings.. gentle is not good for seating rings. letting it cool down between runs is a must..
130 degrees is fine.. 143 degrees is where the thermostat is set at.. so your good there.
you can eeither take the heads off and look. or... use a led flexible light to look down the bore thru the spark plug hole or.. buy a boroscope with a optic fiber cable light on the tip.
it takes a little time while to devolop wash on a piston. usually a half tank or so should start showing signs of wash patterns.. all black is bad.. way lean,, big clean spots are rich, and little spots of clean areas are decent.. a little more than that is ok too.. you just dont want a clean ring all the way around the piston.. thats a little too rich.. small fingernails spots are what ya want or else you sacrafice power for a rich piston..
get a compression gauge and check the psi for each cylinder. all plugs out, wide open throttle, ground the plug wires and hit the starter for a few seconds for each one.. then do it again to re check.. they should be all even and about 120 psi for each cylinder.. 5% difference is fine,, anything over 10% difference is bad.. even try another comp gauge if you can get another type to recheck if possible.. some gauges are not that great..
you need to find out why the piston failed in the first place.. or else it could happen again.. a leakdown test may indicate a air leak.. .. otherwise, it could of been debris in the carbs, clogged screens in the carbs, a weak pump. ete..... sounds like you have your bases covered so far,, i would do a leakdown test to be sure it can hold 10 psi for 10 mins.. if thats fine......carbs and pump would be next.. check all your lines for tightness and dont overtighten them. somes.. look for air bubbles in the lines .
did you check the end gap on the rings before you installed them?
pre-mix is 40/1 which is 3.2 oz per gallon..... 5 gallons gas and 16 ounces of 2-stroke oil.. Use a good brand.. dont be cheap on oil.. its what keeps your engine happy!..
look up in the manual for the settings for the high speed adjuster and the low speed adjusters.. set them to factory specs... the top screw is for the high speed circuit 30-100% and the lower screw is for the low. 0-30% throttle.
gently seat the needle and turn them out the specified turns. take the adjuster out and inspect the tip for debris, bent, broken, chipped needle. that way you know whats going on..
do you know the jet sizes in the carbs/? there is a main jet and a pilot jet in each carbs..
exlint89 would be a great guy to ask.. his 780 runs awesome..
08-02-2010, 12:34 PM #4
08-02-2010, 04:11 PM #5
It really depends on the ski. A pro can have less than that and be spot on.. if your a little lean. I would go a little richer to be safe. 1 or 2 mph isnt that much unless your racing. then .1 mph gain is a lot!. that would be about 520 feet in a mile.. you would loose the race..lol But even still, it's the rider that makes or breaks the race.. just like in drag racing, if you sleep on the tree, you'll loose every time.
you should see 5 nicks or spots on the piston(small fingernail spots). some of the nicks by the tranfer ports will be longer..A chocalate brown color on the dome is perfect. On a Pro. it will be black usually.
(just my opinion!)
Ryan's ski (97sltx)1050 had great wash on his Mag and Center and the Pto was a little rich. Maybe he can post some pics of his. His ski ran great. Just a tad boggy out of the hole but I think thats because of the Pto being too rich.. Or possibly the low adjusters need a 1/8 turn in or less probably. Up to him!.
08-02-2010, 04:15 PM #6
08-02-2010, 04:16 PM #7
+1 for lean, looks waaaay to lean for my liking
08-02-2010, 04:20 PM #8
08-02-2010, 04:23 PM #9
- Join Date
- Aug 2006
- Homer Glen Il.
i like it that looks good to me but i like to run on the edge but thats pretty safe to me
08-02-2010, 04:31 PM #10
That is perfect wash for the GPR like the one in the pic. With power valves the wash is much less on the exhaust ports. A Fuji 780 should have more wash around the intake and exhaust like a thumb nail size.
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