Compression Test

junglepig

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Oct 30, 2018
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#21
Update: Got both bikes running again. It seems the CDI went bad on the GT bike. I put a new CDI on the GT bike and it immediately got the spark back. I'm not sure if the compression test could have shorted out the CDI but I don't know how it could have. I think my problems were just a coincidence that they happened right after I did the compression tests. The Peugeot CDI didn't short out after the test, it was just a carb problem.
That's possible. It would be best to ground the plug lead while doing the compression test so that the high voltage generated in the secondary doesn't discharge within the CDI to ground. That could kill several of the semiconductor components in the CDI.
 


Joined
Dec 13, 2015
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#22
Update: Okay, I have done compression tests on both of my bike engines. They are both China Girl PK80, 69cc/47mm cylinder/40 stroke with the short connection rod and the low wrist pin on the piston. However, the bikes have different set ups.

GT Bike--> Ported by my friend who works at a machine shop. He ported the intake/exhaust/transfer ports and decked the cylinder to .03' squish gap while using a mill. The bike is also set up with a .6cc Clone FredHead, CHD-G2 4 peddle reed valve with windowed piston, 15mm Dellorto Clone Carb (HT/RT), MZ Miami expansion pipe, FMF Powercore 2 sliencer. This engine has about 400 miles on it.

Peugeot Bike----> Purchased from Dax about 6 years ago also Duane from Dax ported the engine for me. I decked the cylinder myself using a piece of glass and sandpaper method attempting to get it to .03" squish gap. The bike has a .6cc Real Fredhead, a 15mm shorty intake, stock piston, 15mm Dellorto Clone Carb (HT/RT), Sick Bike Parts Expansion pipe. This engine has over 2000 miles on it.

View attachment 81881 View attachment 81882 View attachment 81884


The results of the test came out:

GT--------> 158 psi
Peugeot--> 195 psi

I gotta say that the Dax engine on the Peugeot has always been just an awesome engine, once I found the right jet (#60) the thing just took off. Not only did it have awesome throttle response and a steady low idle, it has almost no chain drag, the engine sprocket seems to spin endlessly. It has got to be the best engine I have ever gotten. I had the bike running so well that I didn't want to change anything and, you know, screw it up, haha. But perhaps it's just the luck of the draw? I don't know. But I was very surprised to see a compression of 195 psi, a full 37 psi higher than the GT one and 30 psi higher than the top range that you gave me. Is that unusual?
Does those fancy freak exhaust really make a noticeable difference?
 

CrazyDan

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Jul 9, 2016
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#23
Does those fancy freak exhaust really make a noticeable difference?
There was not a single 2 stroke at the race without a proper tuned expansion chamber. If it didn't make a noticeable difference then I'm sure there would have been at least a couple with the crap banana or even stock muffler pipes.
 

Tyler6357

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Sep 28, 2011
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#24
Does those fancy freak exhaust really make a noticeable difference?
Yes, that's an MZ Miami expansion pipe, it's the same the Zeda 65, it helps along the entire throttle range. I have an FMF powercore 2 dirt bike silencer. They call it a spark arrestor-silencer. They sell long ones like I got and shorter ones, the shorter ones provide power on mostly the low end while the long ones give more power along the entire throttle range although I'm sure it works better on a bigger dirt bike engine. I wouldn't have put it on my bike if I hadn't got such an awesome deal on a brand new one, It only cost me $80. The silencer makes the thing purr like a kitten, it's so quiet that sometimes at stop lights I have to hit the throttle just to make sure the engine hasn't died, It also reduces vibration quite a bit. They don't make the one I have anymore, they have new generation ones called the turbinecore but they are very similar. I would have gotten the "shorty" one but, like I said, I had one chance to get a brand new long one for $80, free shipping, so I went for it, the newest ones cost $206. See links
https://www.motosport.com/fmf-turbinecore-21-spark-arrestor-silencer
https://www.motosport.com/fmf-powercore-2-shorty-silencer-2stroke
 
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