Rebuilding top end and need some tech info please!

SR-powered-240

SR-powered-240

New Member
Dec 8, 2008
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Indian Trail, NC
Hello there, I am new to two strokes but am no stranger to engines.

After a couple weeks of riding my new/used blaster I noticed the compression getting less and less until it stoped running. I ripped the head off and found the top ring blew out and gouged the cylinder wall so now I have to bore it out. After takeing the cylinder and piston to the machine shop he told me it was already .030 over and the best way to fix it is to take it .060 over stock (67.5mm).

Since I have only had my blaster for a couple weeks I have not found a service manual for it yet. I was wondering if anyone could tell me the clearance the piston needs?


My next question is will I have to rejet the carb to make up for the overbore?

If so, what size?

All I have on the blaster is a stock exhaust and boycen reeds.

Thanks for any help you guys can give me.
 
depends on what your jetting is at. do a plug chop after you get it back together to be possitive. the piston clearance should be approxmitly .003-.0035". I would also see if it can go .040 instead of .060 it will save bores.
 
just go with the smallest bore u can is really all u can do and i dont think ull have to rejet ive never had to unless u go with a really big bore
 
i dont think u will but after u get it running again put a new plug and check the color to see if its lein or rich and just go from there i know theres a chart somewhere on here on how to read your plug
 
also sounds like sumthin was causing ur motor to seize. do u still have ur oil injection installed b/c if u do i bet its bad and u need to remove it and start premixing.
 
yeah, if the compression decreased noticably over only a few weeks, something is up!! Too bad you didn't check the seals on the lower end while you had it apart, but if a ring was gouging the cylinder wall, that definitley would explain a drop in compression. Obviously you want to bore it the least amount possible, but it seems like the shop understands and if he thinks it's going to have to go out another .03 to .06 over stock, that's what's gotta happen. I would definitley eliminate the oil injection and make sure you're running the correct ratio when you mix!!

If you've never re-jetted and you have the aribox cover off or a new exhaust, I would re-jet. It's way easier than you think. Search in the DIY section. Also, you'll want to check the connecting rod play...hopefully everything in the bottom end is fine.

You can find a service manual in the DIY section here...or one of the sub forums here!!
 
Ok, when I bought the blaster the guy before him removed the oil injection. He didn't have it for very long before selling it to me, probally because he saw it had low compression. When I ran through the gas that was in it and I called him asking for the gas oil ratio he told me 50:1 which sounded a little lean but I ran it hard for about one gallon at 50:1. I swear after that it felt a little low on power and when asking a friend that sorta works on small engines he told me 32:1.

I still have the engine apart but the machine shop has the clyndier waiting on the new piston which is comming in this week.

What I forgot to mention before was that a third of the top ring was missing and thats probally when I noticed the most compression drop before it shutoff on me.
 
def check in the pipe for your chunk. also, maybe pull the motor out and see if you can get it fall out of the crank....maybe use a magnet or something.
it's possible that the chuck flew out the exhaust, but it's also possible that you've got chunks of ring and bits of cylinder shavings sitting in your crank area. in the bearings and sh*t.
at least your piston didn't blow up.
but you really do need to consider what it is that caused the ring to blow. most likely it's the lack of oil.
 
Wow, I guess I didn't think of that. I have the motor out and sitting on my bench because I'm going to repaint the frame while the engines out. I will check the bottem end and see if theres anything down in there. The last guy also removed the spark flame thingy in the muffler so I would think if it was in the exhaust it would have blown out, but I will check that too.

To answer the question of noise, I did hear what sounded to be like a pop and then the engine died. I kicked it a dozen times (not thinking of the worst) until I decided to do a compression check which yeilded 25psi at wot if I kicked it a few times.

It was then that I relized that I might have caused the deep gouge by trying to start it not really thinking, about it.


Oh yeah I only ran 50:1 for about one gallon until I talked to my friend. After that I removed the gas and caculated to make it 32:1, and then a little extra for grinns.
 
I just got home from working all day and am planning on going out into the garage and preparing the frame for paint. While I'm out there I am going to check the bottom end out.

I'll update you guys.
 
Update.

So I checked everything out and the ring peice seems to have blown out.

My cylinder should be coming back maybe tomorrow so I will have it reassembled withing the next few days. Right before it died I was messing with the air idle screw trying to make it run better but now it looks like I had a air leak at the reed boot on the engine(nuts weren't that tight). Think its a possibility thats what made the engine go out?

Does anybody know how far in the air screw should be to start with when I crank up the engine?
 
the air fuel screw should be 1 and 1half turns out to start with
it can be anywheres from 1 .5 to 2.5 turns out
anything other than that and you will need different jets
 
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