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  1. #1

    95 slt 750 with 780 carb.

    just bought machine, did compression check, mag was 160 center 150 and pto was around 145. Has sbt pistons and hot seat exh plus the performance reeds. All fuel upgrades had been done. Has a 780 carb, which concerns me, only because im not sure how to dial it in for this motor. Ran really good for about five min. then holed out mag piston. Removed head and noticed that the gasket was not properly indexed. "UP" was at the 1 o clock position. removed other heads, same thing. Mag piston had huge hole and was pitted. light grey in color. Other pistons are dull grey and pitted. All three heads seem fine. Smooth and clean, as do all three jugs. Cross thatch is still present on all three jugs. My question is weather or not it simply overheated and fried the piston or if its a carb issue. The compression on pto and center are still as they were before i rode it. but it seems really high too me. Other 750 i have is 130 across the board. I do run a 93 octane premixed fuel. Oil pumps are out.


  2. #2
    BlueFishCrisis's Avatar
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    Welcome!

    That is a high compression for a 750. 780's will top out around 140 with 750s typically around 120. Someone may have modified the cylinders/heads to increase it. If you don't know what is inside the carbs, you need to pull them apart and have a look. 780 jetting is much different from 750 jetting, but towards the rich side vs lean. Either way, sounds like you have a fuel issue. Whether it be not enough fuel, or not high enough octane. I am sure someone more experienced with high compression/high octane fuels will chime in.

  3. #3
    Thanks for the reply. Not sure if i could tell the dif in jetting to know whats what. I will research that a little more. What effect would the head gasket being off index that much have?

  4. #4
    BlueFishCrisis's Avatar
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    Depends on what the gaskets look like. I have seen some with small holes, and some with much larger holes. If anything it may cause incorrect water flow resulting in overheating. If your temp sensor is working properly, you should have received a warning on your MFD. Typically, overheating causes the pistons to expand and seize in the bore. If you holed the piston, I believe it is likely due to a lean condition, not an overheat.

    You will need to take the carbs apart and pull the jets out to see what numbers are in there. A stock 750 has 90 mains while a stock 780 has 110 mains. There really is no other difference in the "780" carbs other than jetting and some fuel routing upgrades. Are they the black carbs?

  5. #5
    yes they are black .

  6. #6
    I just have really bad luck with carbs, its never as easy as putting a new set of gaskets and seats. Im sure the factory settings would not apply for this, so any advice would be extremly helpful.

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