Bad compression on re-build

[cmg]

So I just finished rebuilding my 800 SXR after a busted rod. New trued and welded crank, new (used crankcase), new V Force 3 reeds, new (used 1 season) cylinder and pistons, brand new wrist pins, bearings, c-clips and piston rings.

In addition I am running a TMB non girdled head with limited domes, metal core .060 base gasket.

I rode yesterday for break in and it was very boggy on the bottom end, almost like it was trying to clear out the waterbox or was getting way to much fuel. Watching it run on the trailer in the water it appeared that some water was leaking out from under the head. I suspect this was also getting in the cylinder causing some of the issue also.

Regardless, I brought it back home and ran a compression test. I was getting 100 PSI in both cylinders. So I swapped to the factory head in case the TBM was not sealing (O-rings) and ran the tests again. This time I got 100 PSI in the front cylinder and 110 in the rear. I put a touch of oil in each cylinder and got a 15-20 PSI rise in both cylinders.

Pulled the head and everything looks like new, no scoring, no damage etc...

What may I have done wrong? Is it right to assume the rings are not sealing? What should my next step be?

 

[Motorheads5]

Did you check ring gap ?

 

[cmeripper26]

I would check the deck of the head you are using. If there is even the slightest distortion to it, there wont be a good seal against the top of the cylinders. also maybe check and re-torque all bolts.

 

[cmg]

Head deck looked good. I just took a straight edge and laid it across ensuring there were not gaps using several differant angles.

What is the best way of checking ring gap? God help me if I put the rings on wrong :dead1:

 

[cmg]

Also, checked and retorqued all head bolts.

 

[750SX]

How to check ring gap:

Disassemble the top end. Put one ring in the cylinder and push them down a little bit with the piston to make sure they are square. Then check the "gap" between the two edges with a feeler gauge.
Repeat for the other rings

 

[Kennay]

base gasket sounds tall to me... that would lower compression and make your squish larger.


I don't know much about SXR's, so it may be perfectly normal. Might want to look into that though.

 

[750SX]

base gasket sounds tall to me... that would lower compression and make your squish larger.


I don't know much about SXR's, so it may be perfectly normal. Might want to look into that though.

I think you are right. That is the gasket that is supposed to be used with drop domes.

 

[Waternut]

Base gasket is definitely way too thick. I don't have an SXR but the 750's are very similar and mine has the same gasket material as a carb gasket. I don't think it should be a metal core gasket either. The only things you need metal core for are exhaust gaskets.

Hopefully that's the only problem but just in case it's not... How many cc's are those domes in the girdled head?

Adding oil in a 2 stroke motor is not good practice when trying to determine compression because it significantly raised the compression even in a perfectly good motor. I know there are other reasons but I can't remember what they are.

 

[cmeripper26]

You could try the OEM base gasket to eliminate that as the problem. Also i would re-torque the cylinder base nuts. That base gasket does sound a little thick for the SXR's.

 

[Mark44]

Check your squish if you change heads!

mark44

 

[Big Kahuna]

I think you are right. That is the gasket that is supposed to be used with drop domes.

Bingo. The tall base gasket raises your port timing for racing applications (or general use), gives more top end. Down side is you have to run TBM's drop domes to get your compression back. I would go back with an OEM Gasket.

 

[cmg]

Thank you everyone for the advice and insight. It looks like the problem was a combination of the base gasket and domes. I mistankenly did not have the TBM limited domes "drop down", but instead had the standard TBM domes. This in conjunction with the thicker TBM gasket caused the problem.

Solution:
1) Run the standard base gasket with current TBM domes, or
2) Get the limited domes (always need a good reason to buy new parts) and run the thicker base gasket.

FYI, you all were right on this in a few different ways and mentioned several very good things to check for trouble shooting. The thicker gasket raises RPM through port timing (when combined with the proper domes). The metal core prevents blowouts of the gasket. TBM set me straight and was very helpful. The best news is that the top end is OK and I did not damage anything. Now I just got to get it back together and celebrate my first top end build with a good ride.