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Testing compression??

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So I wanted to test the compression on my 09 300 and so I went to Autozone and got one of those cheapo testers that they loan out.

With the throttle wide open I only got around 135psi...!! That seems really low to me but I would expect a bike with only 50hrs on it to be much higher. Then again could it just be that the tester is a POS? The one I used was very tricky to even get on and I can't be 100% certain it was on properly.

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Problem with many testers is the trap valve is on the gauge side of the hose. So whatever the volume in cc's in the hose lowers the actual reading. Maybe fine for a big V8 but not very accurate for a small engine.

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Problem with many testers is the trap valve is on the gauge side of the hose. So whatever the volume in cc's in the hose lowers the actual reading. Maybe fine for a big V8 but not very accurate for a small engine.

Ya I was thinking these were likely dialed more for large engines rather than little bike motors...

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Ya I was thinking these were likely dialed more for large engines rather than little bike motors...

WR, I wouldn't worry about it. I couldn't get a consistent reading with a Snap on tester. You have plenty of life left in that 300. I can't believe how well my 200 ran after seeing what it looked like when I pulled it apart.

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WR, I wouldn't worry about it. I couldn't get a consistent reading with a Snap on tester. You have plenty of life left in that 300. I can't believe how well my 200 ran after seeing what it looked like when I pulled it apart.

I will say that it fires up on first kick almost every single time and never needs more than 2 kicks. Don't know if that's much of a real indicator though.

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I will say that it fires up on first kick almost every single time and never needs more than 2 kicks. Don't know if that's much of a real indicator though.

my 200 still ran and fired up first time with 200+ hrs on the piston. 300's are well known for going this far + some. I really think you are good and just need to enjoy that bugger. :moon:

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I got my tester from Pit Posse and it works pretty well... My 250 reached 200 psi easily after a few kicks. I've read before that the engine must be stone cold to get an accurate reading.

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When the engine is cold the oil on the rings is thicker so is likely to give a better seal and slightly higher reading. On multi cylinders (twins and triples) I've found the 1st hole tested reads higher cold. Because of this a procedure of starting the engine then shutting it right off yields a more consistant reading hole to hole. A single? just do it the same way every time for comparisons purposes.

Fwiw a twostroke may have a compression ratio range depending on the port height or whether the exhaust valve is open or closed. A engine with a taller port height may trap less air at compression gauge testing rpm than one with a lower port height, though the compresion ratios are identical. So the gauge reading only tells a small story when trying to compare with another engine of different design.

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Did you kick it through 5-10 times? Kick it till it max's out? I had a reading of 208psi on a harbour freight gauge and 192 on a good sears gauge. 6 year old 200exc never been rebuilt. I don't believe either reading. But it's a relative number. I just ordered the parts to redo the topend and will test for a beginning baseline number once I get it back together.

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I kicked this one to over 200 psi. Don't put much faith in it, as I couldn't get a consistent reading, and likely just didn't do it correctly. I will just run it, keep track of time and do a top end when necessary.

ACF835A.jpg

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Also the 300's have a decomp hole that is uncovered until running.

Fer real? I didnt know about this. Where is it located? How does it work? Why doesnt my 250 have one?

My bike is alot harder to start with the new piston and head cut. It use to be a start with your hand bike, now it takes a good bit a leg and a few good kicks to get her goin.

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Fer real? I didnt know about this. Where is it located? How does it work? Why doesnt my 250 have one?

My bike is alot harder to start with the new piston and head cut. It use to be a start with your hand bike, now it takes a good bit a leg and a few good kicks to get her goin.

Tiny hole in the top of the cyl. above the power valve. From my understanding, it relieves PSI on starting, when the PV moves, it blocks the hole off eliminating its effect.

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Talking to Jeff Slaven I guess 135-140psi is correct for my elevation on this bike. He mentioned that his $50 head mod will bump that to 170ish and give me all kinds of torque off the bottom through mid range. I'm thinking that's more than worth $50 so I'm sending my head off!

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Talking to Jeff Slaven I guess 135-140psi is correct for my elevation on this bike. He mentioned that his $50 head mod will bump that to 170ish and give me all kinds of torque off the bottom through mid range. I'm thinking that's more than worth $50 so I'm sending my head off!

Ive got his mule port. Which is his head mod and some porting.

Could not be happier.... My compression was 135 to 140 before at 5500 ft elv.

Now i am @ about 180

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Talking to Jeff Slaven I guess 135-140psi is correct for my elevation on this bike. He mentioned that his $50 head mod will bump that to 170ish and give me all kinds of torque off the bottom through mid range. I'm thinking that's more than worth $50 so I'm sending my head off!

Good choice, he does great work. He was also very kind and put up with my stupidity. When I get some more bike money he will get my jug for porting. I need to ride a 300 to see if that is the route I need to go.:moon:

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