air leak(i think)

[mjnp06]

hi

i rebuilt the top end of my blaster motor about a year ago and parked it in the garage. last week i rolled it out and started tinkering with it again, cleaned the carb, did a leakdown test and found a couple air leaks around the base gasket and around the reed cage. i fixed those with new gaskets and some liquid gasket from the yamaha shop, ran the test again and everything looked good. my plan was to heat cycle the motor to break it in. it fired right up, idled nice and revved smoothly, then i noticed the 3 screws holding the cover on thumb throttle were loose so i tightened them. at the same time the motor revved to about 3/4 throttle and stayed there. i turned the key off, pulled the kill switch and it didnt quit revving, so i pulled the spark plug wire and that didnt work so i pulled the gas line off and choked it to finally shut it off. i will run another leakdown test to see if something went wrong there. anybody have any ideas how a motor runs without and spark??

thanks
mike

 

[mud runner]

well i had a similar problem with mine i was riding it and it started revving really high so i turnrd the switch off and it wouldent die so i had to stop then dump the clutch in 4 th gear but any ways what is your jetting and mods what octane gas what plug and have you leak tested after this happend

 

[Wheels]

I've never seen it happen but I think a leaned out 2 stroke motor can run hot enough that it will keep running with no spark. I'm not sure how fiddling with your throttle caused the motor to rev but I would look in to that first, then make sure your jetting is in the ball park, carb clean and leak test, then get it running well and test the carb boots for leaks.

 

[89blaster89]

yep that shouts LEAN! the motor gets so hot that it will run without the spark firing

 

[Blaaster]

Ah yes I remember it well!!!!!

A overly lean mixture will cause extreme detonation and the combustuon temperature will soar and make the electrode of the spark plug and any trace of carbon glow white hot.

The spark plug is now not needed now as there is fire in the belly and the motor will diesel!

 

[mjnp06]

well, i did another leak down test and found a very small leak on the top of the reed cage, i fixed that, retested it again and all looked good. so i put it all back together and refired it. all was well until it started getting up to operating temperature then it started racing again. i shut it down right away and it stopped like it is suposed to. im thinking the problem might be in the carb. before i started it the first time i did take the bowl off, remove the needle valve set and the main jet, soake everything in yamaha carb cleaner and re-assembled everything. mud runner, everything is stock, regular 87 octane gas, and a new ngk b9es.

 

[mjnp06]

i did clean up the ports when i re-assembled the moter

 

[mud runner]

what is your jetting

 

[Blaaster]

well, i did another leak down test and found a very small leak on the top of the reed cage, i fixed that, retested it again and all looked good. so i put it all back together and refired it. all was well until it started getting up to operating temperature then it started racing again. i shut it down right away and it stopped like it is suposed to. im thinking the problem might be in the carb. before i started it the first time i did take the bowl off, remove the needle valve set and the main jet, soake everything in yamaha carb cleaner and re-assembled everything. mud runner, everything is stock, regular 87 octane gas, and a new ngk b9es.

Wrong heat range plug, B8ES is what you should use.

Is the exhaust leaking at the cylinder?

If you crack the drain screw do you have a constant dribble of gas flowing from the drain hose?

If it slows down you have a supply problem, petcock or needle and seat.

 

[BGATZ]

Pretty sure that high revv you cant shut off is called diesieling. Has happened to me before. We tried the switch and key nothing happened. Just killed the gas and waited for it to die.

 

[Blaaster]

The spark plug is now not needed now as there is fire in the belly and the motor will diesel!

Pretty sure that high revv you cant shut off is called diesieling. Has happened to me before. We tried the switch and key nothing happened. Just killed the gas and waited for it to die.

The next time it suffers runaway kick it in gear and dump the clutch, it does a motor no good screaming its butt off with a lean mixture and limited oil.

 

[bummer981]

have you checked the rubber boot at the reed cage for cracks. It may be leaking there once it is running.

 

[mjnp06]

well i got the air leak problem taken care of now it wont stay running. if i spray a little gas in the carb it wont fire, bit if i spray some in the spark plug hole it fires right up. it wont stay running but it fires up. it sounds to me to be in the carb somewhere, but i thought it would fire by spraying gas in the carb. any ideas anyone?

 

[Braaaptor]

Reeds!! Id say check for a broken or chipped petal. The engine often won't fire, even if squirted through the carburetor, if a reed is damaged. Id pull the manifold off, take a look, and see what you've got. The reed petals must seat flat and firmly against the cage or you'll have problems.

 

[mjnp06]

i kinda thought it might be something between the carb and the jug since it wouldnt fire when i sprayed in the carb. i will pull the manifold off and see what it looks likes.

thanks

 

[Blaaster]

well i got the air leak problem taken care of now it wont stay running. if i spray a little gas in the carb it wont fire, bit if i spray some in the spark plug hole it fires right up. it wont stay running but it fires up. it sounds to me to be in the carb somewhere, but i thought it would fire by spraying gas in the carb. any ideas anyone?

Please do not make a habit of spraying fuel into the spark plug hole, it is very bad practice.

 

[mjnp06]

i normally wouldnt do that but i wanted to to see if it would fire to make sure the spark plug was firing under compression.