1990 Suzuki 40 skip at WOT

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blackshear

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I recently purchased a 1990 Suzuki 40/25 jet drive that has seen very little use over the years. It is a 2 stroke with oil injection system. It cranks up quickly and idles perfect but at WOT it has a skip. I can back off the throttle slightly and the skip goes away. Is it the rev-limiter kicking in or something else? I am running premix gas because I didn’t trust the oil injection system but it appears to working. I guess the extra rich gas oil mixture could be causing the skip but it doesn’t make sense that it would only skip a WOT. Any ideas what my issue might be?
 
I had the same thing happen with a 25 hp. I had a tach so I knew I wasn’t on the rev limiter. Turns out the fuel line connectors on the motor and tank were old and not making a good seal. The fuel pump couldn’t keep up with the air leak.


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Could be a few things. Start with the easy stuff.

Look up inside the pump and see how badly the impeller is chipped/worn. Check the clearance between the impeller and then housing, it should be 1/16" or thereabouts. If it's really loose, you may have to re-shim. If shimming is not possible or the impeller is trashed, it puts less load on the engine, allows the engine to run over speed and then the ignition cut limiter turns off the spark momentarily--which could be what you're experiencing. Pretty common on jet drives. If it's really worn you may have to replace the impeller and the sleeve. Welcome to jet drives. The day the motor is bought new is the day it performs the best, from there on out, performance goes downhill until the pump is rebuilt and/or the engine is rebuilt. Jets put a different load on the engine; which is exactly why Yamaha uses a different ECU for the EFI jet 4 strokes than the prop EFI 4 strokes. Others may use similar tactics, I just don't know about them.

Another possibility is a fuel starvation issue. Does the primer bulb suck down flat when at full throttle/full load? If it does, the pickup, line, or bulb could have a restriction (kink?) OR the vent on the tank isn't venting properly. If not, could be restriction between the bulb and carburetor, or at the carburetors themselves.

Could also be ignition related. Sometimes guys will remove/clean carbs and upon reinstalling them, don't bother to check carb sync and ignition timing linkages--both can cause issues, with sync issue causing a rougher idle (most apparent with a prop drive in neutral). Might not notice it much on a jet because it's constantly under a load.

Could also be an issue with the oil tank float, OR overheat warning. I believe both are tied to the CDI to cut ignition if the tank is "low" or the engine's running hotter than the CDI thinks it should be.
 
Thanks for all the responses, you guys are the best. After reading all the excellent information I have to believe it's most likely the rev limiter. It only seems to have the skip when I am by myself and only at WOT. Idle is good and mid-range is good. The skip is quick and short unlike fuel starvation issues that I have experienced in the past. I will give it a try again this weekend with a fishing buddy and see if it goes away.

I originally thought that my problem might have something to do with me running 50:1 premix along having oil in the tank mounted on the engine. As it turns out the oil injection system is using oil from the tank but I'm not sure if it is mixing properly. I will probably go up to 100:1 on the next tank and see how it runs. I just don't have confidence in the oil injection system on a motor this old. Is there any way to verify that the oil injection is working properly?
 

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