low compression after rebuild

[mandylink]

So in the fall my compression was 62 PSI when it calls for 99PSI decided to do a top end rebuild. Cleaned the hole top end this winter taking my time and assembled with new piston and rings and all gaskets. Just did a light 500km break in and then compression tested today and the reading was at 82PSI. So a 20 PSI increase but still not 99.

So I guess the next issues will be getting the new Valves?

[MeCH_MaN]

Adjust valves first

[MeCH_MaN]

A quick note...
Did head gasket on my 01 Jeep Cherokee. I was lapping in the valves, when I removed them they weren't sealing because of corrosion on the faces. The faces had pitted. Something I've seen in the past, may apply if it's high mileage.

[mandylink]

the bike only has 8500km on it. I will recheck the valves tomorrow but I am sure I set them up right during the rebuild.

[brianhare]

That 99psi is with the decomp still working and perfectly adjusted valves,no worn parts and fully seated rings.....

 

loosen the right exhaust rocker 80 thou and the decomp will be out of the readings,,giving you full cylinder pressure...

 

I would never rely on a com p test with the decomp opening an exhaust valve at cranking rpm........also you have to have the throttle wide open to do the test properly..

 

See what you get with the decomp inactive....and let me know..

 

B

[mandylink]

1 hour ago, brianhare said:

That 99psi is with the decomp still working and perfectly adjusted valves,no worn parts and fully seated rings.....

 

loosen the right exhaust rocker 80 thou and the decomp will be out of the readings,,giving you full cylinder pressure...

 

I would never rely on a com p test with the decomp opening an exhaust valve at cranking rpm........also you have to have the throttle wide open to do the test properly..

 

See what you get with the decomp inactive....and let me know..

 

B

I will try that tomorrow

[mandylink]

I did have the throttle wide open when I did the test

[mandylink]

On ‎09‎/‎05‎/‎2017 at 7:29 PM, mandylink said:

I will try that tomorrow

OK so the results are 120 PSI that is low from what I read

[JoeRC51]

Was the cylinder honed , measured for diameter and roundness or did you just slap a new parts in it ?

[mandylink]

yes it was honed and checked at the dealer

 

[JoeRC51]

Do a leakdown on it. Piston at TDC on the compression stroke. Sprayed a dish soap/water mix into the intake and exhaust ports and look for bubbles and/or hissing.

[braindead0]

If you have a CV full throttle doens't cut it, you need to hold the slide fully open or pull the carb.. just in case..

[mandylink]

22 minutes ago, braindead0 said:

If you have a CV full throttle doens't cut it, you need to hold the slide fully open or pull the carb.. just in case..

OK so I took your advice and held the slide all the way up with my finger in the carb the number dropped to from 120 with just throttle to 90 PSI with slide all the way up. must be valves so I Guess my next step is to figure out how to do the leak down test to pin point where the loss is

[braindead0]

Still have the decomp 'out of the picture'?

Are you sure your gauge is working properly?  Maybe check on another vehicle?

Personally never done a leakdown test, so I can't help you there.  Good luck!

[mandylink]

yes that is with the decomp out of the test. Starter is lagging a little to turn the motor over so it is deactivated. I have two new testers and both are giving me the exact same reading

[mandylink]

OK so leak down test complete. Showing a 43% loss of compression. loud hissing from exhaust with a little hissing from carb Valve job needed.

[brianhare]

i have never rebuilt an engine without a complete check of the head......a good 3 angle valve job is a must...

 

B

[mandylink]

yes I should have this is my first 4 stroke rebuild. I will have to take the head to someone to have it done may be over my head to do and don't have the right tools for this job.

[beezer]

Experience is the best teacher.  Not a big deal to take the head off and have it redone.

Make sure who ever you pick has done it before.

[mandylink]

how the hell do you get the head cover off with engine still in the bike there is no clearance to do it