Thread: SL750 Help
01-25-2010, 09:51 PM #1
- Join Date
- Jan 2010
Hey Guys I have a SL750 and I have been doing some troubleshooting to determine why my machine isn't reashing its max RPM's. With a good battery I compression tested the cylinders and got consistent readings of around 100psi for all three. (is this "ok"?) (disclaimer this is with a replacement engine.) therefore I'm not sure if I should expect between 120-150psi like factory engines. Probably not. I have spark to all three cylinders. Basically it tops out in the low 40mph range.It feels like it hits a brick wall when you go over waves. It was used all last season in this condition. I ride my other machine so for the past few years I've avoided working on it because quite honestly its very reliable, it always starts and is something from my friends to ride when they come out on the water with me. I only have a single outlet fuel pump, I have not had the carbs apart and I am in the process of looking at the cylinders to look for holes and damage. What is the significance of the CDI unit inside the electrical box, several years ago that was replaced. Sadly enough that was too long ago for me to remember if I had this performance problem directly after that was replaced or sometime down the road. Any thoughts?
I would be greatful for advice in determining the most likely cause of the problem.
Fuel issues, ignition? compression/gaskets?
01-25-2010, 11:39 PM #2
- Join Date
- Oct 2006
- Cleveland OH
Welcome to the hulk Mike.
Sounds like it can be any of those things. You need to start checking those systems out, and eliminating possiblities.
01-26-2010, 01:12 PM #3
I would start with the carbs, they seem to be the culprit more times than not, by the sound of it your motor is OK. 100psi is fine, a tad on the low side but as long as they are all the same +/- 5psi your ok. Compression testers can vary as well a different one might read 110 on all cylinders.
This is what I would do in order:
Pull carbs and inspect for corrosion, crud and fuel gum buildup. Check diaphragms for pinholes and coating condition.
Take apart fuel pump and inspect check valves, a lot of times aluminum oxide builds up on the sealing surface.
Pull your reed valves, if there is a slight chip on the corners or crack it will make hard starting and contribute to the lean condition your trying to avoid.
Inspect rear crank seal for any signs of leakage, If everything looks a-ok then your front crank seal may be the culprit.
If you really want to get down and dirty pull off the exhaust manifold and block the exhaust and intake ports, then do a pressure check.
One more thing: pull the plugs and turn the motor by hand(using the drive shaft coupler). you should be able to feel a smooth resistance with no 'catches' in the rotation. If there is a hard spot while you are rotating the crank then you most likely have a lower rod bearing going out.
Check these things and report back, we'll help you get to the bottom of this.
01-26-2010, 01:28 PM #4
- Join Date
- Jul 2007
- near Toronto, Canada
Test again with another gauge. All spark plugs out, throttle held wide open.
Tell us what the piston wash pattern on the tops of the pistons looks like (looking through the plug hole, with a small flashlight).
Are the pistons all black, or are there clean metal areas around the edges?
If there are clean areas, how large are they (roughly)?
What model year is your SL 750?
Who rebuilt the engine?
Was compression tested after it was rebuilt?
The CDI controls the ignition and timing. What is the part number on the CDI you have in there now?
You say you have spark on all three cylinders. Do the spark plugs all look about the same, in terms of color and appearance?
Are the plug tips and center insulator brownish, black, or very white?
What part number are the spark plugs?
Have a look here;
Polaris Fuji (blue) engine
01-26-2010, 03:12 PM #5
XLint, Joyrider, and K447 are all pointing you down the right path. All 2 strokes have this simple formula:
Fuel + Compression + Spark = GO
You have spark, so 1/3 of the equation is satisfied.
You have compression. A little low, but even is the key.
With 2/3 of the equation solved, that means we're talking fuel delivery. From your description, it sounds like you have a completely original fuel setup. If it were me, I would run through the entire fuel system replacing the OEM hoses (known to degrade and collapse), delete the autocock (known to cause restriction), and upgrade to a 3-outlet fuel pump (OEM single outlet known to cause MAG lean condition).
Once all the typical problems are addressed I would then take Joyrider's advice and run through the carbs being sure everything is clean, in good condition, and that pop-off is correct. Also, make sure to set high and low speed needles back to factory settings. After that's done I would put new tubing on the oil pump lines if you're still using the auto oiler.
The 'TRUE' scientific way to address this would be to fix each fuel system problem individually, test for effectiveness, then if the problem isn't remedied try the next item. However, the above mentioned fixes are all very common problems, so it would make sense just to do them and have all these potential failure points checked off.
With all the fuel system addressed you can now begin to test the ski and see how it runs. Like K447 said, you need to check piston wash after you've run for a bit with the updated fuel system. Do a search here or on the 'net to see examples of good and bad piston wash to get an idea what you're looking to see.
Last AUgust at Starved Rock, Tenjuna was fighting his SL650 because it seemed to not reach max RPM. With the help of Randy and others he was able to get the carbs pretty well tuned and discovered about 500+ RPM that he didn't have before. The moral of the story is carb tuning makes a world of difference. Factory settings for the needles should get you close. If you run the skis after doing the fixes, piston wash is fine, and you're near max RPM I don't think I'd mess with it since this is just a recreational ski. However, if the piston wash isn't quite right after the fuel fixes, then you might want to spend some time tuning to find that extra performance.
01-26-2010, 04:30 PM #6
You might try removing the spark plug boots and trim the wires back 1/4" or so. Reinstall the boots and give the ski a try.
01-26-2010, 05:22 PM #7
01-27-2010, 08:53 PM #8
- Join Date
- Jan 2010
Thanks for the help. OK first off it is a 95 Sl750. When looking through the plug hole you can see almost a dime size area of clean piston directly under the spark plug. Then there is a blackish area around it thats about 3/4" around or so, then you can see clean pistion around the outside of that. All three look about the same. should I pull the top of the cylinders off to get a better look?
A local Polaris dealer did the engine swap several years ago, no I dont know what the compresstion was after it was changed. At the time I wasn't the one maintaining the machine.
These are the CDI part numbers printed on the part just like shown below
Then odd as it may seem there is a mitsubishi logo on it as well. Are those the numbers your refering too? Is this the wrong CDI?
The spark plugs all look roughly the same the tips are blackish and the insulators are brownish with some white showing. What do you think? I am using BPR7ES plugs like it calls for.
This machine will litteraly burn a tank of gas in 2 hours, no premix. (basically when its run the rider almost always runs it wide open because of the slow performance) is this normal or am I just running way to rich and need carb adjustments?
AGAIN, THANKS FOR THE HELP
01-27-2010, 09:15 PM #9
Welcome to the hulk.
Your piston wash seams a bit rich but I dont think that is your problem. I would take a peek at the jet pump. Do you know what RPM's it is hitting at WOT? The CDI 78W is from a 780. 2 hours is about right for a 750 tank.
01-27-2010, 10:03 PM #10
- Join Date
- Oct 2006
- Cleveland OH
I'll have to do a little digging, but the 780 CDI, I think is from a SLT. Which still uses 24* of advance, just like the 94/95 750 uses.
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