low compression

[scooter guy]

i have a 2017 kymco 50 KG10SA scooter and did a complete rebuild with naraku parts from crank to top end and i have low compression 60psi. i am a auto mechanic by trade, i rechecked valve clearance thats good, did a cylinder leak down test thats good, removed exhaust to see if compression would get any better it went up to 65psi. i installed a KN100.47 big bore kit, and a KN100.13 cylinder head all naraku parts. any help would be appreciated why im getting low compression.
thanks Chris

[Ruffus]

@Chris, first of all welcome here!
There are a few things which would come in my mind....
-does it fire up and runs?
-IF yes, compression tester faulty
-IF NO, ....
-100ccm cylinder all in all too high
-cylinder foot gasket too thick
-cylinder head different/too much of caving
-piston rings in-line and not 90° set
Interesting case, pls give a feedback how it goes.

[scooter guy]

ruffus
thank for getting back to me to answer some questions
it does fire up and it does run checked compression with 2 gauges and both had 60psi. not sure on gaskets but they came with the kits.cylinder head and cylinder are compatable with each other, pisten ring are spaced out 90deg. 100ccm not sure what your telling me there?
thanks chris

[Iahawk]

my first thoughts were leaking valve or rings with too large a gap or not clocked properly...or head gasket? I assume you've checked all of these...and your leak down test was good..any chance of incompatibility between your aftermarket parts?

[Iahawk]

scooter guy, you were able to buy a 100cc big bore kit for your 50cc scooter without having to bore out the case?  I wasn't aware that you could add a 100cc kit to a 50cc with no other modifications to the case?

I was watching a Rolling Wrench Denver youtube vid recently where he went through in detail how big you could go with a 125/150cc GY6 motor without having to bore the case.

There are 170cc or 180cc kits that you can add but if you go any larger (and you have a GY6 'A' case, with 54mm cylinder head bolt spacing) you'd need to bore the case.

Just thinking out loud if you may have an issue due to the larger cylinder bore?

[scooter guy]

Iahawk
i didnt add a 100cc i installed a 72cc kit. all test are fine, and the parts according to naraku are compatible.

[Ruffus]

Chris, ah... ok. Thought this KN 100 is a 100cc cylinder.
-during compression test gave you full throttle?
My thinking is still on gaskets/demaged/wrong, or cyl head.
Just a gut feeling.
If you open (motor running) your dipstick a little, is there any fuming-out?
Would you check valve-clearance again?

[scooter guy]

Ruffus
the KN100.47 is a 72cc cylinder. i haven't tried opening the dip stick while running yet ill have to try that before taring it down. I've done valve chk several times. yes full throttle while cranking for compression test. my gut feeling is cylinder head problem but not sure.
 

[Ruffus]

Chris, did this cyl head come fully assambled or as a kit?

-maybe valve stem gaskets blow or simply wrong,...
-during mounting, how well fitted the cylinder sleeve into the block?
-cylinder mounting studs allow blow by sometimes

But I would take off the head and compare/check with old one first.