kiyoung

I have a Zenoah g260rc. I drove it about 20 liters.
Today, I bought "Zenoah 34mm Big-Bore Kit" from DDM and changed a cylinder, piston, piston and etc.
I measured compression with a gauge.
I got somewhere between 48psi and 40psi.
Is this a good number? Or am I supposed to have better number?
Maybe, my compression gauge is too long?
The total length of the gauge is 30cm or 12inch. I don't know.
Anyone who has experience, please tell me.

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R/C Modelsport

Open the carb wide open and keep pulling on the pull start until it doesn't go up anymore. It should read somewhere around 120 un-modified to run properly. Lapped .015" it will read somewhere ~150 plus. You may need a new piston ring and a piston if it is worn.

kengentry5000

My Feedback: (2)

Anyone know where I can pick up a compression gauge like this in the US? How much should I expect to pay?

R/C Modelsport

$30-$40 for the Actron CP7828 from Pep Boys, Kragen, etc. etc. Other brands are available. You will just need to be sure it comes with a 10mm fitting. Most don't so be careful.

Amazon has it for $35.04.

rangerfredbob

My Feedback: (13)

I got mine at walmart for $20...

if the engine is not ran, IE fresh ring (you said you just got a big bore kit) the compression will be lower than after it is broken in, if it's that low, add some oil through the spark plug hole before trying to start to help the compression some. also, it depends on how you were testing compression, most say to pull the cord 5 times (a good 10 or so compression strokes)

as for just general compression, 120+ is normal and in good shape, around 90 is getting on the tired side but will run fine, 60 and lower most likely won't run at all usually means a stuck or broken ring or the piston and cylinder wall is scored...

kiyoung

Thank you
Today, I checked compression again after 1 liter gasoline run.
As you said, I removed air filter and open the carb wide open. I checked the leakage of the compression gauge with soap water.
No leakage.

I got just 75PSI.
I think compression is very good, because when I pull a starter, my car is lifted just like when an engine was new. Also, when I drove yesterday, the engine was very powerful just like when an engine was new.

So, I think the compression number I got today is somehow influenced by the length of the cable of the gauge.
Am I right? What's your opinion?

rangerfredbob

My Feedback: (13)

there's a tire valve in the hose somewhere holding in the pressure, is it at the end that has the spark plug threads? if it's not and is up near the gauge, then yes, the hose length is making an impact on your compression reading, two stroke engines have a very short period when they are actually compressing the air/fuel mixture (distance between top of exhaust port and the end of piston travel)

how many times are you pulling the cord to get your reading?

PAPE

I cheked my compresion once on my stock zen 26 and it was 90psi, had about 2 gallons

kiyoung

I pulled the cord about twenty times. And yes, my compression gauge has two check valves. One is at the beginning of the tube and the other is at the beginning of the gauge.
So, I think these two check valves (which has a spring) have some resistance. That is why I am having low compression numbers.

Also, I tested in two different ways. One test was done with choke closed and card closed. The other test was done with choke opened and card opened.
I got the same number.

Thank you very much.