Two stroke compression

Wed November 02 2011, 05:50 PM

TRA-742

Anybody know how much compression a 09 yz125 should have? I am finding a lot of varied answers online and my manuel doesn't say.. I was told that you need to hold the throttle wide open when kicking/checking compression but I don't understand why

Wed November 02 2011, 07:36 PM

Rays Dad

I never checked compression on my 2-stroke. Removed pipe and looked up exhaust port to check piston condition every two or three rides. With a good flash lite I could see piston, rings and most of the cylinder.

Wed November 02 2011, 10:45 PM

17dad

150 to 160 is normal. Below 150 time for at least a ring for most stock 2 strokes.
Hold the throttle wide open and kick it untill the gauge stops inching up. Usually about ten kicks and this is what your compression reading is.

Thu November 03 2011, 11:27 AM

TRA-742

Thank you

Thu November 03 2011, 09:28 PM

TRA-742

Gage read 85-90lb..I thought it felt alitttle slow coming out of corners generaly its a rocket---> I pulled the pipe looked up in there looks brand new, I was expecting it to be all brown black burnt on the front side of piston, so I'm thinkin just needs rings? Proly should do the piston/pin while I'm in there and go oem kit-
I was told go up one size on main since its colder out

Thu November 03 2011, 09:47 PM

TRA-742

FYI on a side note; Prior to needing a new top end I ran some high octane VP mixed 50/50 with 91 pump gas and the bike didn't run any better it ran worse, then I read on VP site you need to go up 2 size main jets on two strokes when runing race gas, I drained it went back to straight pump but I think it was to late I lost a lot of power...awesome so pumped now I need new top end! lol

Fri November 04 2011, 11:11 PM

TRA-742

I ordered complete new topend kit including piston, rings, gaskets, wrist pin an bearing.. With sales tax and shipping my cost $77!!!

Sat November 05 2011, 07:56 AM

Paul 061

I wouldn't go up any on the jetting until it starts to knock, then change it. Running high octane fuel in a 2 stroke wont buy you anything unless you are pinging. The good gas only makes jetting more even but if the jetting is already right and doesn't seem to vary from tank to tank, save your money. If you really want to have some fun, do a google search on wetline jetting. It takes a while to do but once you get it right you will really be impressed with the performance.

Sat November 05 2011, 12:52 PM

spin05

We have been mixing 1 gal of race gas to 3gal of regular for a year two.It eliminates ping and keeps the cylinder from getting pitted.Never did re-jet for it

Sat November 05 2011, 06:27 PM

TRA-742

Idk maybe it wasn't the race gas, probaly just time for new topend. I was just surprised on VP site it read if your runnin race gas in two stroke need to go up two on the main, I've never heard that before and have ran 50/50 on many two strokes with no problems. Paul I looked up that wetline jetting, I've never heard of it, thats was very interesting to read, I might try that once I get the fresh topend.

Sat November 05 2011, 08:04 PM

Paul 061

I went to a motor building seminar with Rod Falkner, the owner of Dumonde Tech. It was a very interesting evening. He explained wetline jetting and the theory of jetting over and above above the stoichiometry parts. I did the process when I had my KX250. Took me about 2 hours and I got really good at pulling the springs off the pipe. It made a huge difference and really helped me understand and feel jetting changes.

Sat November 05 2011, 10:28 PM

David Rose SR.

On my YZ125 I run half C12 and half ethanol free unleaded premium fuel. Its a stock 2005 with a Pro Circuit pipe, silencer and V-Force 3. I don't have the jetting specs in front of me, but I called Pro Circuit and they gave me there recommendation. And of course EVO did the suspension. It is the best bike I have have owned. Ride 2 strokes be happy.

Sun November 06 2011, 01:31 PM

TRA-742

Good idea..the bike came with FMF so I will call them for jet specs and might even call PC for second opinion.
Yes I agree the best bike I have ever owned and I have owned a lot of them, so much fun it could be illegal !

Sun November 06 2011, 06:17 PM

moderbike x

pro circuit has all jetting on there web sight under support and jetting its a good start

Tue November 08 2011, 05:49 PM

blaze

Nothing is more fun than a 125. AS I got older the more fun they got.

Thu November 24 2011, 11:47 PM

TRA-742

I put the new 125 topend in. The old piston was severely eroded around the top edge all the way around the most severe part was the back half. Something I've never seen before. The bike is stock so I would think 91 pump gas with 927 mixd 32:1 should be good but people are tellin me I need race gas..? I would rather not$-- I went up one on the main from 410 to 420 haven't started yet thinkin about up one more to 430.. ugliest piston I have ever seen- I had 80psi now have over 160

Fri November 25 2011, 09:23 AM

E.W. #849

TRA, you have to run 95 or higher octane fuel in a stock YZ125 (On page 1-12 of the manual). They will detinate and ruin the cylinder. 2 gallons of race gas to 3 gallons of premium pump gas is what I run.

quote:
Originally posted by TRA-742:
I put the new 125 topend in. The old piston was severely eroded around the top edge all the way around the most severe part was the back half. Something I've never seen before. The bike is stock so I would think 91 pump gas with 927 mixd 32:1 should be good but people are tellin me I need race gas..? I would rather not$-- I went up one on the main from 410 to 420 haven't started yet thinkin about up one more to 430.. ugliest piston I have ever seen- I had 80psi now have over 160