ATV Florida Forum

Question: Will increasing compression ratio and/or changing port timing increase cylinder noise in a 2 stroke?

Background:
The oil injection pump on my son’s Blaster recently failed, nearly causing the engine to seize. The damage appeared to be confined to the piston and cylinder bore. The bottom end spun smoothly and there was no play was apparent when I pulled on the rod so I figured the bottom end was good to go. I honed the cylinder to remove the score marks and it checked out ok with an inside micrometer. I bought a new piston kit from Vito’s that included the piston, rings, wrist pin, wrist pin bearing and clips. The piston is a Wiseco “super stock” that has bigger intake ports and a chamfer cut in the top to expose the exhaust port sooner. I also installed a thinner head gasket (.018” versus .032” stock) to increase compression ratio and added a Vforce reed valve for good measure and bumped the main jet up to a 330. Oh yeah, I also installed an oil pump block-off kit so this won’t happen again.

Well, we finished reassembly today and it fired right up. I noticed that there was more top end noise than before. We ran it for about an hour in break-in mode and it ran great. The noise did not get worse but it did not go away either. It is definitely coming from the top end so I doubt that it’s a crank or rod bearing issue. It sounds like the same noise the top end used to make (tapping), just louder. I am hoping that it’s only the higher compression and/or change in port timing (from the new piston) but I have very little experience with 2-strokes. Any thoughts?       

on my wifes blaster, when i built heres it seemed pretty noisey to me to, check the squish. or better yet pull off the head and see if you see the piston hitting anything. the 330 main souds way big.. i would think a 290-300 would be about right. i put the 310 jet toomey gives you with the pipe and it would bearly run at WOT. talk to Paul over at Vitos,,,

Thanks. I will pull head today and check it out. We have Toomey pipe also and it ran great with the 310 main jet but the new piston has bigger ports and with the higher compression I figured I should bump the main up a little more. We'll see how it runs after I check things over. Thanks again. 

What is the piston to cylinder wall clearance ?

OK I pulled the head off and stuck my finger up the exhaust port and noticed that I could make the piston rock visibly. I also noticed polished areas on the forward and aft sides of the bore above the ports (see photos). I pulled the cylinder and checked clearance (bore diameter minus piston skirt diameter) and got .0035”. The nominal clearance is supposed to be .0015” and the limit is .004” so technically I am ok. Plus, I think you are supposed to run forged pistons a little looser so maybe this is good. What is weird is that the piston diameter is much smaller at the top than at the skirt (2.578” between rings versus 2.596” at skirt). I am now convinced that the cylinder noise I hear is the piston rocking in the bore. Is this ok? Is the piston supposed to be tapered that much?

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Cylinder view forward-looking-aft:

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TO LOOSE, piston must be warp.

If your running .0035 clearance, on basically a stock bore, it is set up way too loose. You should be between .002 and .0025 on a bore of that size. The only time you would run .0035 or more clearance is on a 68.50mm or bigger piston. Your ramping the piston in the bore, hence the polished markings on the one side.

Mike

mike beat me to the punch on this one!   I Agree!

Thanks for the input guys...I really appreciate it. The .0035" clearance is at the skirt where yamaha says to measure it. It gets worse further up the piston (.0075" at pin and .0215" between the rings). These are diametral clearances (bore diameter - piston diameter). Putting this another way, the piston diameter is 2.596" at the skirt, 2.592" at the pin, 2.578" between the rings, and 2.576" above the rings. Should a new forged piston taper this much?

All pistons have tapper. Go only with a measurement 90deg from the pin bore, just below the pin. This tapper is there for thermal expansion.

You will need the next oversize piston, a get your cylinder bored and honed.


Mike

Yes that makes sense...top of piston gets hotter and grows more than bottom. Anyway, I just ordered a .020 over piston. I will get it right this time.  ;D ;D ;D

Thanks for the expert advice!

Set the new piston up at .0022 to .0024 Break it in slowly, and properly, and you should have no problems.

Mike

Thanks again.

Should I run more oil during break-in (say 20:1 versus 32:1) or will that prevent the rings from seating?

Break it in the same as your going to ride it. 32:1 is fine. If you run more oil during break-in, your actually leaning out the fuel mixture.

Mike