Maximum revs at idle..!

[Eikari]

Its a 98 GSX Ltd. Been doing some engine overhaul, changing one piston, cleaning the carbs etc..

So after all has been put together the engine goes to maximum revs, 7140 rpm, when i hit the start button. It was running on the hose and i didnt see any leak in the exhaust. The throttle cable and the plates in the carbs move freely.

Air leak, faulty rev limiter?

 

[Strizzo]

Sounds like an air leak or misadjusted carbs.

 

[Sportster-2001-951C-Stock]

If combustion was not supported by ignition system, then engine was dieseling, this is known as runaway and should be stopped immediately to avoid engine damage from overspeeding. The best way to stop this runaway is by pulling in the choke plates completely ASAP, so always make certain you're choke is adjusted and working properly, this is your most reliable mechanism for stopping a runaway engine.

Lean dieseling can be caused by an air leak but the high performance 2 stoke engine is known to run like a bat out of Hades even if everything is tight and all adjustments are close to normal. You may simply have set the idle speed too high, or adjusted the throttle cable improperly(too tight, or binding).

So yes, check your work for air leaks, could be a crank seal or intake manifold gasket somewhere air leak or just a slight misadjustment.

This diesel runaway event cannot happen in the water while the jet pump is loaded with water.

 

[Eikari]

It shuts off when i pull the lanyard off. I have never pulled the choke. So probably an air leak somewhere.. Can a tight throttle cable make it runaway although i never touched the throttle?

 

[Sportster-2001-951C-Stock]

It shuts off when i pull the lanyard off. I have never pulled the choke. So probably an air leak somewhere.. Can a tight throttle cable make it runaway although i never touched the throttle?

Absolutely, this will happen if the throttle plates are sticking open, there is no load on the engine so off it goes!

If it stopped by removing the lanyard, it's more than likely the throttle plats are not fully returning to the idle stop screw or the idle stop screw is improperly adjusted. Could also be the butterflies are not synchronized correctly, one may be open too much.

 

[Eikari]

Ok, check the throttle cable and the carb sync. I adjust the carb linkage visually, but there´s probably a better way to sync it close to perfect.

That exhaust pipe is always in the way for any repairs. Always removing it..

 

[Sportster-2001-951C-Stock]

That exhaust pipe is always in the way for any repairs. Always removing it..

A popular argument, whether or not to remove the exhaust. I avoid doing that and am used to working in tight places using a mirror, so I can usually do this without removing the pipe.... Some how.... maybe my small hands square mirror, and flashlight working upside down suspended from the ceiling?

You may also have an air leak, it's possible, any of the above are possibilities. Carb base gaskets are often held in place while installing by heavy grease.

Full closed on both throttle plates(butterflies) with idle screw fully backed out, no slop/play in linkage adjustment. Full open produces both plates(butterflies) parallel. I guess you have the service manual instructions for butterfly synchronization.

Remember of course to recheck/adjust the oil pump lever arm alignment also, early and often. Especially after carb linkage adjustment is final. I'm an oil nut, I always squirt oil down both barrels b/c there is none from oil pump at idle.

Syncing the butterflies for later skies are easier with both carbs on bench carbs mounted to top plate but yours are somewhat different I think, with horizontal butterfly shaft, not vertical shaft like mine so I'm not familiar with the detail.

 

[Sportster-2001-951C-Stock]

Also make sure there is no air leak at rave valve pressure hose if your ski has this, it's at PTO end of crankcase just forward of rear motor mount on port side.

 

[Eikari]

Have checked all the hoses and all looks good, throttle- and oil cable move freely. I think its now the carb gaskets or perhaps the engine base gasket. Can i rule out that its not the MPEM ( faulty rev limiter)?

 

[Strizzo]

Have checked all the hoses and all looks good, throttle- and oil cable move freely. I think its now the carb gaskets or perhaps the engine base gasket. Can i rule out that its not the MPEM ( faulty rev limiter)?

not likely to be MPEM related, the engine needs air, fuel, and spark to run at high rpm, if the throttle is closed and no air leaks, it won't have the air and fuel to free rev.

did you check that there is a little slack in the throttle cable at idle, if the cable isn't adjusted right the lever can be holding it open. make sure that the linkage goes back and hits the idle screw when you release the throttle.

 

[Eikari]

the linkage goes back , carbs synced, the idle screw is the all the way out, somewhere the engine is getting more air than it should.

 

[Strizzo]

then start looking downstream of the carbs for air leaks or bad gaskets. make sure you don't have any leaks at the carb base. Sportster951c is more familiar with the 951 specifics than I am, so he would know the trouble areas for air leaks a bit better.

 

[- BiRkS_]

Have you made absolutly sure that the carbs are tight to the block? did you replace the carb base gaskets when you had the engine apart?

You say you had the cylinders off for replacing a piston... yes it is possible to have an air leak there, but more likely to have a water leak. Just use a water spray bottle and squirt the gaskets down. Listen for a high pitched whistling sound, or where the water quickly disappears. That would show you an air leak. Start the ski up and play with the choke to keep the engine rpm down. then start looking for that air leak.

Worst case senario, you have bad crank seals. The PTO can be checked, but the mag is pretty much impossible inside the ski.

 

[Eikari]

The engine was rebuild about 2 years ago, rebuild crankshaft, cylinders etc. .5mm overbore. What do you mean by squirting the gaskets? English is not my first language.. Make them wet on the outside to the mating surfaces?

 

[- BiRkS_]

Spraying the water on the gaskets. Its better than carb cleaner as it wont damage your seals. Basically you are trying to cover the air leak, and that will change the engine rpm

 

[Sportster-2001-951C-Stock]

How about sitting in the water with the jet pump full of water, how does it run? If not good as if low on power, don't keep running it.

I still think maybe you have an air leak somewhere if the throttle butterflies are fully closing and the engine wants to rev high.

Carb base gaskets, intake manifold gaskets at the reed valves(951 motor), are the first that come to mind, could also be a leaky crankshaft seal on one end of the crankshaft, even cylinder base gasket is possible(incorrect one?). Agree if cylinder base gasket is leaking, it might also leak water there from the water jacket.

You might consider pressure testing the engine to locate any air leaks.

 

[Eikari]

Took the carbs off today, carb gaskets look OK. Time for a engine removal then and do a leak test. Possibly the cylinder base gasket is leaking water and not air tight..