Thread: What to do about low compression
05-13-2008, 10:42 PM #1
What to do about low compression
1996 SLT 780
SBT Reman installed in June of 2006
Hours??? I'm not sure, but there's not a whole lot on this engine.
I'm sorry this is long, but I'm trying to get my ski lined out and have a good riding season.
Okay, before anyone jumps on me, no I haven't pulled the domes and inspected the cylinders. I had some issues last fall and I'm beginning to troubleshoot them and a compression check was first on the list. My pressures were:
MAG = 110 PSI
CEN = 105 PSI
PTO = 115 PSI
I know on outboards, a 10% or greater difference means it's time to tear down. So, on these cylinders it appears I'm basically at that point, especially since the OEM spec is 120 -150 PSI. These readings were taken on a cold engine, topped off battery, all plugs pulled and the throttle wide open.
So here are my questions?
1) If the cylinder walls and pistons are okay, would it be worth my time to just tear down, hone the walls, and install new rings?
2) Are the SBT pistons fair quality, or should I replace them with another brand?
3) If replacement, what brand? Polaris? Wiseco? WSM?
4) Are the SBT cylinder jobs worth trusting? Should I just punch the cylinders out to the next 0.5mm and install new pistons?
I know that SBT isn't the most popular supplier around here, and I have my own experience to question they're products. However, I was in a pinch when I bought this motor and I just wanted to get something in and be back running.
Okay, here's the next issue. The ski has always been difficult to start. Basically, I have to prime the lines and hold the throttle wide open to start the ski. The carbs were rebuilt in '06 and this starting condition existed before that, so there is something else going on. I do have new needles and seats to install to ensure I'm not getting any fuel leaking past.
Is the starting linked to my low compression? Does this sound like a spark issue? What is the best troubleshooting method for tracking down weak spark conditions?
I know I've asked a lot here, but everyone here is fantastic and I'm just looking for some direction in getting this ski running well.
05-13-2008, 10:49 PM #2
i go with polaris oem pistons an rings or wsm are prettty good too ill let other guy chime in too good luck,hard starting can be due to low comp others with help you better
Last edited by RiverRat19405; 05-13-2008 at 10:51 PM.
05-14-2008, 12:19 AM #3
SBT lowers the compression of their engines, we won't get into why.
I'd pull the domes and inspect the cylinder walls.
If they look good, I'd send the domes off and get them cut to raise the compression back up to the 120 zone and hope the rest of the engine holds together.
As for their honing abilities, they seem to have saved that for their customers.
BUT let's give them the benefit of the doubt and pretend somebody knew what they were doing.
05-14-2008, 10:10 AM #4
- Join Date
- Jul 2007
- near Toronto, Canada
Prime the lines;
Is the fuel draining out of the fuel lines when the ski is sitting?
What fuel pump do you have - single or triple outlet?
Weak spark - what makes you think the spark is weak?
Have you tested the stator coils with an ohm-meter, per the Service Manual?
What is the part number on your CDI?
You MAY be in need of the CDI+stator upgrade kit (never can remember the details regarding that update)
Edit: Apparently not
Last edited by K447; 05-14-2008 at 11:07 AM.
05-14-2008, 10:23 AM #5
05-14-2008, 01:41 PM #6
05-14-2008, 01:51 PM #7
should not the plugs be checked individually and with the other 2 plugs installed huh? and also should not the compression be checked after there has been an engine warm up huh? having the rings and cylinder walls coated with fuel and 2 stroke oil will definitely raise compression and be actually giving you a compression reading of real motor operation condition no? just something that you might want to consider. or maybe im thinking out load and im completely wrong........................
05-14-2008, 02:09 PM #8
Thanks for all the help do far.
05-14-2008, 02:13 PM #9
05-14-2008, 02:19 PM #10
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