Slt 750 Burning Holes In Front Piston

[2sl750]

New to this site, need help. I have an slt750 and an sl750. the sl runs great. the slt came with a burned front piston. i rebuilt all three cylinders and took off oil pump. ran great for about 20 hrs then, yup, burned front piston again. I rebuilt cylinder,carbs and now replaced fuel pump. The front plug stays dry and looks like ash, the other two plugs are very wet. I set carbs to factory specs.

What am i doing wrong. i dont want to rebuild this thing again, why is the front plug dry. any ideas are appreciated. thanks

[ph2ocraft]

Those early years were also famous for bad fuel selector switches and hoses kinking, swelling etc. When they are this old and have this many hours it's important to do a complete fuel supply work-over.
Have you verified there are no air leaks in the block (engine seals)
What's compression after the top end?
How does the piston wash look?
Have photo's to help us out?
But most important have you verified the crank index is good? One of the first indicators of a bad crank is the unexplained holing of a piston.

Oh and WELCOME to the Green Hulk Forums!!

[2sl750]

compression is about 145 all three cylinders, i had a guy check the index he said it was about 2 deg. off. said it would not matter much. Sorry I dont know what wash is or how to check it. thanks for responding, any ideas??

[ph2ocraft]

Symptoms of an out of index crank.

Difficulty calibrating carburetor (repetitive plug fouling on one cylinder with no other cause).
Unexplained piston failure (ie. severe detonation, broken ring lands, piston holing)
Excessive vibration of engine, backfiring etc.
Rough idle or poor top speed.

Have the crank index verified!!

[ph2ocraft]

After you know the crank is good, I'd start chasing down the lean running condition. Hoses, valve etc.
If you go to the tech section you can find the photos of rich/good/lean on piston wash, it's important you know how the boat is actually running but it sounds lean to me.

[xlint89]

Also pressure test the cases like he said ^^^^. If the fuel lines were ever replaced, was the restrictor reinstalled into the return line? Have you checked the fuel filters inside the carbs? It's possible that one is partially clogged and leaning the cylinder out.

[2sl750]

where is the restrictor, i changed all internal parts, including those small screens inside the carbs.

[2sl750]

i will check the index again. ill try to find that link for the wash you talked about. thanks ill let you know.

[xlint89]

where is the restrictor, i changed all internal parts, including those small screens inside the carbs.

Most are found in the return line about 3/4" away from the carb rack. Some however had them built in to the carb itself. Delegem's post ahs a pic of the carb restrictor built in.

[2sl750]

thx for your advice