OK, I have a 2004 Sportster with the 951 DI engine. With all the reading on here, the biggest thing I get is to always check your compression to get the state of your engine. From what I read, anything under 135 psi, is not good. So out of curiosity I decided to check my compression. I ran the engine on the trailer for 30 seconds, pulled the plugs, grounded them & went to WOT. I get 110 psi on both cylinders . This should mean a failing motor. But my boat runs really good. Starts right up, idles great, will easily spin up to 7000 rpm, do 60 mph with 3 on board and pull a tube with 2 kids on it with no problems. I usually cruise around 5500 and 35-37 mph. So what gives? Why does my boat run so good with such low compression? Can the computer compensate that much for that low of compression? I did 2 readings to make sure of the compression. Would appreciate any comments or thoughts.
You need to hold the throttle in WOT position when measuring compression.
Last Fall before winterizing my 951 carb motor blew 120psi both cylinders after sitting stone cold for more than a week, I figure a good bit of the oil ran down into the crankcase.
So you might be blowing a bit more than 110psi and your gauge may be miscalibrated or incorrectly configured, some of the Harbor Freight gauge hoses were assembled incorrectly and yield a lower number. I'd question the gauge first, but still 110psi may not be too bad depending on the conditions involved, especially if the boat has been out of service there would most likely be some corrosion on the iron cylinder walls hindering ring seal. This is why it's so important to fog your engine after using the boat if it's going to be sitting for more than a few days, to avoid cylinder wall corrosion and loss of compression due to micro-pitting.
If you have some light cylinder wall corrosion the compression may return to 120psi+ after you've run the motor a bit, so go easy on it for the time being. FYI, excessive clearance causes piston temps to increase and eventually heat transfer out of the piston is disrupted, the piston then overheats and melts and leaving puddles of aluminum smeared on the cylinder walls then you can imagine what happens from there....
The earlier 951 motors were known to blow 150psi and later years that changed to about 130 or so, many of the later models blow 120 stone cold and are still running fine.
If you are able to confirm 110psi absolutely, then it's a bit low and you may be approaching the point of rebuild, I would pay close attention and avoid running it hard and stop running it if it does seize b/c when the 951 seizes it's known to throw rods and destroy the cases, which of course costs more than if your tore it down for rebuild in the first place. All 2-stroke motors will eventually wear out and seize, rebuilding them sometime after a couple hundred hours is normal expected maintenance.
Measure twice and cut once!
Check compression early and often on this motor, especially when you detect a drop. I hope to recheck my compression again soon too, trying to keep an eye on it.
