98 Srx 700

[tntx3]

I just bought this sled and the motor was shot. the center cylinder was totally cratered. He had a replacement motor that he rebuilt and we swapped it. We used the same carbs, and exhaust valve from first motor. took it out yesterday let it warm up good and did a few quick runs couple hundred feet or so. Pulled plugs and all looked good so rode it home,about 5 miles, ran awesome.
Later that day went for about a 30 mile ride, ran great all the way, shut it down and let it sit for about 6 hours. Went to leave and it was hard starting and once it did it was not idleing very well and backfiring a bit. Loaded it up and hauled it home. Pulled plugs and had no compression on #2. Pulled head and the piston was totally fried around outside edge. Cylinder was scored real bad. What could have caused this.

 

[Whammy]

Inspect that carburetor, boot and reeds on that cylinder........ Others also.......

 

[sideshowBob]

-post pics if you can

Sounds like detonation which can be caused by:
-low octane fuel
-dirty carbs and/or fuel filter[causing a lean condition]
-modified heads that have too high compression
-lean jetting[ensure you put all jets back to stock or slightly richer, paying special attention to the needle clip positions]
-cracked or leaking carb boots or reed cages[causing a lean condition]


Some other possibilities:

-old stale fuel
-There could have been shrapnel left in the exhaust pipes.
-Are the the cylinder heads stock?

If everything is stock...It obviously sounds like the problem that caused the first engine damage was transfered to the second engine....Carbs?
The carbs must be thoroughly cleaned including the top hat filters and fuel tank filter.

Just some ideas.

Bob