Posts
48
Joined
10/3/2017
Location
Rancho Cucamonga, CA
US
Edited Date/Time
6/10/2021 11:09am
Obviously it’ll pretty damn hard to determine at this point but I’m stumped and curious what you guys think.
2019 FC 450 supermoto bike 46 hours
Sucked in some dirt 2 weeks ago. Bike would run but was hard to start. Did a leak down and determined it needed a piston. Replaced the piston with a vertex piston.
I broke the motor in on Maxima break in oil by riding it around my neighborhood for a little over 1 hour. I drained the oil and checked the valves because I hadn’t before the tear down. Valves were in spec.
I took the bike to the track and it ran great for two 20 min sessions. 3rd session the bike burped like it was almost out of gas. Went back the pits a filled it up with VP U4.4 and went back on track. Bike cranked more than usual before starting but once started felt strong as ever. About 3 laps into the session it blew up. Piston to valve contact.
The rod is also bent with two chunks missing from the cam chain side.
1.7 total hours since the rebuild.
It’s clear at this point that both exhaust valves were hitting the piston. But what would cause this? My first though was timing but with the crank locked at TDC like you do on the ktm motors and the single cam it seems pretty hard to do. It’s not my first go around with these motors so I know even if I was drunk I wouldn’t be more than 1 tooth off.
Is one tooth retarded enough to cause this? I don’t think advanced timing could do it.
My only other guess is that I got the wrong piston. I didn’t check piston to valve clearance as this was a relatively simple rebuild.
I don’t really believe in “random” failures but I’m having a hard time believing I screwed up the rebuild.
Anyone care to share their 2 cents as to what they think happened?
2019 FC 450 supermoto bike 46 hours
Sucked in some dirt 2 weeks ago. Bike would run but was hard to start. Did a leak down and determined it needed a piston. Replaced the piston with a vertex piston.
I broke the motor in on Maxima break in oil by riding it around my neighborhood for a little over 1 hour. I drained the oil and checked the valves because I hadn’t before the tear down. Valves were in spec.
I took the bike to the track and it ran great for two 20 min sessions. 3rd session the bike burped like it was almost out of gas. Went back the pits a filled it up with VP U4.4 and went back on track. Bike cranked more than usual before starting but once started felt strong as ever. About 3 laps into the session it blew up. Piston to valve contact.
The rod is also bent with two chunks missing from the cam chain side.
1.7 total hours since the rebuild.
It’s clear at this point that both exhaust valves were hitting the piston. But what would cause this? My first though was timing but with the crank locked at TDC like you do on the ktm motors and the single cam it seems pretty hard to do. It’s not my first go around with these motors so I know even if I was drunk I wouldn’t be more than 1 tooth off.
Is one tooth retarded enough to cause this? I don’t think advanced timing could do it.
My only other guess is that I got the wrong piston. I didn’t check piston to valve clearance as this was a relatively simple rebuild.
I don’t really believe in “random” failures but I’m having a hard time believing I screwed up the rebuild.
Anyone care to share their 2 cents as to what they think happened?
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Sounds a tad batshit, but I’ve personally seen it before.
Did the bike happen to go boom on decel/decompression braking or initial accel?
Assuming the chain isn’t broken, piston isn’t wrong, cam gear stripped or spun/broken, rod let loose or valves dropped (obviously none of which happened, sans piston which is tbd) there’s literally only two ways it’s possible - chain jumped the teeth somewhere or you done goofed.
If the piston is good, split the cases and check your oil passages along with the tensioner.
Anybody know if one tooth off is enough to crash the exhaust valves? I’ve seen bikes with the timing off before but usually they even won’t start let alone run great for 2 hours.
Paw Paw
What made you say all 4 valves were touching? I don’t think the intakes valves were hitting the piston. Looks to me that broken exhaust valve is what hit them.
The cutouts (red) appear to be shaped differently (longer circumference on the new piston) and the cutout appears deeper on the old (blue).
Again, could just be an illusion.
Definitely could be cam chain and/or tensioner that caused it.
It was hard to Start the chain jumped 2 Teeth but it was not hitting the Valves
Post a reply to: Why’d it blow up?