New topend low compression???

S

snowmanhat

New member
Joined
Feb 8, 2009
Messages
50
Location
Pocono PA
Just rebuit the top end of my SX500R '01. New pistons, rings, wristpins and bearings and the compression is lower that before.

Before disassembling I had 115 on one cylinder and 120 on the other. Now I have 110 on both cylinders. The cylinder walls looked like new so I didn't bother to hone - only 1000 miles on engine. The engine still had its original pistons so I did'nt think to bore and replace with oversized.The reason I rebuilt was to pick up more compression.

Will the compression increase as the ring seat over time? If so how many psi can I expect to pick up - hoping for 125 psi at minimum.

Thank you for your helpl.
 
oh and 1000 miles is nothing on an engine, i hate to tell you but yo uwasted money... 115 and 120 are probably stock.... that motor was just getting going at 1000 miles
 
tyler440 said:
bingo... tear it apart and put a cross hatch on it, your rings will NEVER seat without it, and thats why you have lower compression
Click to expand...


Well for one thing its a nikasil coated bore and it only had 1000 miles. Most of the time you dont even hone nikasil cyl you just wash/scrup them with a green scothbrite pad. Also any motor you rebuild will have lower compression until the rings seat. Usually around 110 psi is normal after a rebuild been there done that. DONT tear it down just break it in properly and the numbers will come up to 125-130psi. I rebuilt my cr250 this summer and she had 120 psi, within 10 hours operating time she had 180psi and I did NOTHING to that cyl, maybe spent 5 mins prepping it. Nikasil is very hard and once the crosshatches are worn down usually the cyl needs to be recoated.
 
Last edited:
The cross hatch marks from factory were obvious in both cylinders before I reassembled. As instructed on piston box I mixed 50:1 mix in addition to my regular injection. Also, within the first 15 minutes on stand at mid to low RPMs its red overheat light came on - the side rails felt warm but the coolant hose to the head was very hot. Do you think it got air bound?Will I be okay?
 
snowmanhat said:
The cross hatch marks from factory were obvious in both cylinders before I reassembled. As instructed on piston box I mixed 50:1 mix in addition to my regular injection. Also, within the first 15 minutes on stand at mid to low RPMs its red overheat light came on - the side rails felt warm but the coolant hose to the head was very hot. Do you think it got air bound?Will I be okay?
Click to expand...

You probably didnt bleed the system properly maybe? That would be my guess that there is still air in the system. I wouldnt run it to much on the stand you need a load on the motor to break it in properly.
 
snowmanhat said:
\ Also, within the first 15 minutes on stand at mid to low RPMs its red overheat light came on
Click to expand...
??????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????
 
I don't know, I had a problem before with a leaking headgasket and it overheated. A new head gasket fixed the problem. This time I am at a loss for why it is overheating. Maybe air like Bluemonster mentioned - I'll have to look at again.
 
How about..... you left it running on a stand for about 15 minutes with no snow on the exchangers to cool the engine is why it overheated, and the compression is low. You don't do that!
 
I ran mine on the stand in the beginning just after the rebuild to see how it runs.Just revving a bit to fluctuate the rpms.I made sure my water was moving and all exchangers were warm.On my 600 the water wasn't circulating,so shut it down,opened bleed hole and reved it until a steady stream came out.Then exchangers began to warm up.
My 600 had over 12,000 km's on it..cyl crosshatch were great.Scrubbed cyl good and put back together with new pistons,rings..After about 260 km's on the motor ,my compression numbers climbed from 125 to in the 150 PSI range.She runs great and is snappy then any other time.Love them 600's.
 
Any ideas on how to break it in without snow? I wouldn't want to drive 5hrs North just to have it overheat again. That would really bit.
 
snowmanhat said:
Any ideas on how to break it in without snow? I wouldn't want to drive 5hrs North just to have it overheat again. That would really bit.
Click to expand...

I would run it n get the air out. Lift the engine up and get the air out.

Then I would lube the slide and go around the yard.....keep the slides lubed
Do that till you are happy
 
whats a 2001 sxr 700 run for normal or whats spec for compression.. mine has 8800 miles and is 115-120
 
my 2001 SXR600 with 360 km's on the re-build now is posting 150 PSI across all 3 cylinders..I have 1 layer gasket installed also..but pressure built up good for cylinders with near 12,500 km's on it..good cross hatch and a good scrub and wash to them..no honing.What more can be said here..give it time..
 
You must log in or register to reply here.


Back
Top