Why would my top end be shot after 20 min riding after a rebuild.

[Blown 97 GT]

Ok, well i'll start with when i first got this blaster it bogged at the low end of about every gear and didnt have great compression so i did a top end rebuild. Took it to yamaha and they bored it (found out later it was on the last bore which i believe is .90 over) and they ordered my piston kit, gaskets, etc. Me and a buddy who has 2 blasters put it back together and rode it a little to break it in. The next week we go riding and we ride for 20-30 minutes and my blaster cuts off in the middle of 3rd gear. Wont crank back up, the kickstart locks up sometimes but then breaks loose, then it wont have good compression. So we take it apart and the piston has had one top side shaved off a little. What in the hell caused this? I was running a 310 mian jet with airbox lid off and a full pro circuit exhaust. Everything else is stock motor wise.

 

[Blown 97 GT]

Sorry for the long post, im just stumped. Is it because it ran to hot being .90 over or what not or do you think i have an air leak or something somehwere and it made it suddenly run to lean?

 

[specialized7]

did you break it in good?

 

[stephen12091985]

i think you have to big of a jet but being on that more creates more heat.have you checked to see if its got spark?

 

[CC&Fabb]

Did you do a proper break in process? Starting and letting the bike run for 15 minutes, varying the RPMs not letting it idle? Not riding it, just sitting there reving it? Then let it cool down 100% to room temp and start the process all over again til you have done the process 4 times. Then, you need to take the bike out and run it gently for about 30 minutes getting into all of the RPM ranges and all of the gears. After that you can open her up. You then need to do a plug chop to see how the bike is running.

From the mods that I see on your sig you only have a filter and the pro circuit pipe. A 310 main jet seems way to big if you ask me. Also, your bike is not going to over heat from being bored .90 over.

 

[Blown 97 GT]

The piston is shot so i didnt even check to see about spark. I broke it in correctly, so idk...

 

[wrench man]

I don't consider my Blaster builds broke in until at least two tanks of fuel have been run thrue them.

 

[blaster biotch]

if the crown of the piston is "shaved" then there must be way to much piston rock....are u sure u have the correct size piston for the bore? and yes u should break in good before romping it.. heat cycle is best way....then retorque everything...i say mic the gap make sure ur rings and piston are for the bore u have

 

[jeepinncreepin]

310 way to big. maybe a 280 or little smaller.follow the breakin process on here.it takes time to break in even a stock motor dont rush it.

 

[mystahagy]

I don't think the jet size is a concern here unless it was too small and leaned out. My blaster has been happy as can be with a 310 for about a year now.

 

[custom2stroke]

What do you mix your gas at?

 

[yfz450bence]

^^^ wow someone actually asked a question related to the topic, a 310 main jet has nothing to do with what happened, the worst thing that could happen is fouled plug, improper break in is just someones opinion, do it how ever you want, that wont cause the pistion to get shaved of one side,

like mentioned, #1 for getting to pre mix can cause this
#2 not warming it up and going full throttle can cause this
#3 yamaha didnt chamfer the ports, so the rings started catching and lost your compression, can cause this, very important to have smooth port edges, very important. also improper piston to bore clearence, um once its at .090 i just have this feeling it wont be very reliable anymore, but thats an opinion, the rest are facts

 

[Mert]

do you pre mix or do you use the auto lube system, if so are you sure you reconnected it properly, I have a friend who did that

 

[Blown 97 GT]

I premix my gas, 36:1. yfz450bence are you talking about letting it warm up before every ride? I have a bad feeling it is becuase we were on a long straightaway on the main road (probably a mile or mile and a half long) and i was crusing in 5th gear the whole time. Should i let my blaster warm up before every ride and if so how long? I have an idle problem so i would have to be giving it a little gas to keep it running...

And also, i really think yamaha messed this up. I looked at the pro x piston that was in before the bore and rebuild, and they look the same size and have the same numbers/letters on the inside? Does that mean its the same damn piston?!

 

[custom2stroke]

When they bored it up a size it's most likely about 20 thousandths bigger which is hard to see by eyesight. use a caliper to check more accurately then you will know exactly what sizes the 2 are. So your saying one side of the piston is totally scuffed up? is it exhaust side of piston? if it idles high it could of been a leaking lower gasket so you would be running lean and it blew up no matter what the main jet was or is. Yamaha could of forgot to clean the ports after boring and the rings were dragging on them. could also have been an improper ring gap being to small and as soon as the rings warmed up they got bigger and then started flaking off and scuffing up the cylinder and piston and kaboom bye bye motor. can you get pics at all?

 

[Chevyssimpala]

It probley happend cause of the bore size .90 over is too much and the cyclinder walls are too this so it most likely overheated u should have sleeved it or got a new cyclinder