Thread: Piston Carnage again...
08-19-2007, 05:20 PM #1
Piston Carnage again...
Well I just got the motor back together last week. Three new pistons bored to 5 thou clearance etc... I got out to the lake this weekend, broke it in real easy with the first tank of gas, then decided to have some fun. The first day went great, probably went through a tank and a half. Second day it started bogging and would actually die after being runt at WOT for a few seconds. Initially I though it was starving for fuel. So I checked the lines to make sure there were no leaks and that the tank had pressure (remove the cap) and everything seemed alright. Then about an hour later it won't rev past 5,000 rpm?
So I checked the compression when I got home and the front and middle cylinders were good at about 145-150, but the back only had like 40. So I pulled the head and here's what I see. Like I said I just went through this same issue and have attached the pic of that piston. I'm thinking maybe the rod is a bit bent and causing the piston to get caught on the exhaust port?
08-19-2007, 05:44 PM #2
I am having the same issue with the middle cylinder. I bought it with the middle cylinder low on compression after the PV dropped and took a chunk out of the piston but still ran. Then rebuilt it but happened again to the same cylinder but I am assuming mabye a ring spun cause it looks like the ring caught caught the exhaust port. Is this the result of a bent rod??? I was gonna rebuild again this time with pro x pistons and new middle cylinder but with same crank since it only has like 50hrs in it.
08-19-2007, 05:48 PM #3
lots of work but its probably best to crack the case and get the crank and rods checked...go ahead true and weld crank then modd the hell out of it..
08-19-2007, 06:17 PM #4
not knowing the history of your last rebuild
left over bits in crankcase from last failure.
the way the piston has melted may of had a big lean out ( blocked fuel system somewhere)
timing way out ( thats if you had that part of engine down)
crankcase air leak causing lean mixture again
08-19-2007, 06:17 PM #5
where can I send the crank to have it checked that has quick turn around
08-19-2007, 06:41 PM #6
08-19-2007, 11:59 PM #7
From my experience as aengine builder it looks like from the photos
it has burnt a piston from a lean mixture!
how long after and how long where you at WOT ( full throttle)
08-20-2007, 12:07 AM #8
if you look under the piston on the underside of the crown, and you can see a burn residue stain. normally means it has been running to hot and burnt the oil to make a stain.
in saying that from your last repair what happened , why did it fail ,
did you find the reason it failed.
was this the same result
bit more info will go a long way
08-20-2007, 12:37 AM #9
Bainer it does look like Deto form a lean condition, did you pressure test the rework? you have an air leak somewhere if it happened on the rear last time it could be a crank seal.
08-20-2007, 01:54 AM #10
+1 on the crank seal. Had a 99 gp that did the same thing.
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