Super Jet Is this an Airleak, or not?

[retroicon]

Hey guys,

Alright, so I have another long thread, but I think it's a little too long and scaring people off... so, here goes.

New rebuild, and the ski is running away immediately when started. Before you ask, it has all new fuel lines, new fuel petcock, new primer, all new gaskets, new crankseals front and rear.

I've tried the ski with the fuel line directly to the carbs, no primer in line, and the same thing happens.

Just did a few leakdown tests, and when starting at 11lbs, it's down to about 6.5lbs after 10 minutes... it seems to be a consistent 5lb loss each time over a period of 10 minutes.

Eventually, after 2-3 hours, the pressure on the leakdown test goes all the way down to 0lbs. And yes, I'm sure the leakdown test is being performed properly, and that my seals over the carb openings, and exhaust manifold are airtight, and not leaking.

The thing is, after searching it seems that some people think 5lbs loss over 10 minutes isn't enough to cause a runaway, and other people do... so, I'm hoping to get some kind of consensus here.

Right now I can't go out and start it. The fuel tank is out, and the exhaust pipe is off, only the manifold is on the ski right now... and, it's a bitch and a half to put that damn mod-pipe back on.

Oh, just in case it matters. I did check compression and the front cylinder is at about 210, and the rear seems to be about 200. I haven't had time to run the ski at all, so there has been no break-in to even things out yet.

-G

P.S. Yes, I know that this is technically an airleak because it's losing pressure. What I'm asking is, do you think it's severe enough to be causing the runaways?

 

[D-Roc]

i had 8 psi difference from front to back on a motor i built and after break in, it was about 5 psi difference. every motor leaks a bit. you passed the leakdown test IMO. i would try running your fuel lines without the fuel selector. just put the filters on and hook them up to your carb/s. zip tie all of the lines and bypass the primer for now. when going to start your motor check that your throttle plates are not stuck open or out of sync. do not prime your new motor by pumping fuel down the throat. hand prime the fuel system so it has fuel in the lines. ( put your hand over the carb and choke it.) 2 strokes need lots of fuel to start when cold and if you pour gas in or give it a few prime the motor fires and goes lean instantly. fatten your low speed screw/s a 1/2 turn and check your idle/cable for proper settings.

 

[WaveDemon]

5lbs in 10 min. is very bad. In a leakdown test there should be no leak. Your leak is obviously too much as seen by your run away.

I have attached the Watcon Leakdown test procedure.

 

[WaveDemon]

i had 8 psi difference from front to back on a motor i built and after break in, it was about 5 psi difference. every motor leaks a bit. you passed the leakdown test IMO. i would try running your fuel lines without the fuel selector. just put the filters on and hook them up to your carb/s. zip tie all of the lines and bypass the primer for now. when going to start your motor check that your throttle plates are not stuck open or out of sync. do not prime your new motor by pumping fuel down the throat. hand prime the fuel system so it has fuel in the lines. ( put your hand over the carb and choke it.) 2 strokes need lots of fuel to start when cold and if you pour gas in or give it a few prime the motor fires and goes lean instantly. fatten your low speed screw/s a 1/2 turn and check your idle/cable for proper settings.

I think you're talking about a compression test. not a leak down test.

 

[D-Roc]

no, i know what a leakdown test is. he was talking about the difference from front to back. hence my experience with that. i doubt that is a fast enough leak to cause a lean runaway. think about how long the it took to loose 5lbs. and how long that air would have been used up in the motor. 5lbs in 10 seconds is a bad airleak. IMO.

 

[WaveDemon]

no, i know what a leakdown test is. he was talking about the difference from front to back. hence my experience with that. i doubt that is a fast enough leak to cause a lean runaway. think about how long the it took to loose 5lbs. and how long that air would have been used up in the motor. 5lbs in 10 seconds is a bad airleak. IMO.

OK, I see what you were saying now but lets not confuse the OP.

JUST TO BE CLEAR!

Compression tests have nothing to do with air leaks or run aways.
Differences in cylinder compression have nothing to with run aways.
There is no reason to talk about compression with run aways.

His motor runs away
His motor failed the leak down test
His motor has an air leak that needs to be fixed.

Read the PDF I posted to help you locate the airleak on your motor. You don't need to check your fuel selector/carb/fuel filter/fuel lines etc. Just your motor at the gasketed and sealed locations.

 

[D-Roc]

your right, sorry for the confusion but he was worried about his compression difference.
And yeah that has nothing to do with a runaway motor. however, i have seen several motors. brand new, from LPW and XS do a runaway cold start up, and it is NOT because of an airleak. His motor did not fail the test IMO. 90% of motors on the water have an airleak, just not enough to cause a problem. lean carbs and air in the fuel lines will run a motor away. On a used motor i check for grime around the front and back seals. blown o-rings are easy and a base gasket leak will run a motor away. When using motor parts from 1995 it becomes more difficult to get a motor to seal because of the condition of parts. If my motor holds some pressure for 5 min and i spray soapy water and see no bubbles, it is a pass. IMO of course.

WaveDemon i will bet you a beer it is not an airleak. lol.

 

[kawishaul]

What fuel are you running? Could the pressure be causing dieseling?

 

[waterfreak]

10 PSI difference in compression doesn't seem right! it won't cause a runaway motor but it does raise an eyebrow!

 

[D-Roc]

when i had 8 psi difference i raised an eyebrow and asked the question. I was told on a ported motor that it wasn't anything to worry about. but i always wondered why it read different. i ran it for years and had no problem.
so to recap:
8 psi....forget about it, 10 psi ehhhhhh raise an eyebrow, 12-13 something is happening....20-30 psi oh yeah its f-ed now. haha.

 

[WaveDemon]

10 PSI difference in compression doesn't seem right! it won't cause a runaway motor but it does raise an eyebrow!

the motor is not broken in yet. Wait to raise an eyebrow until after the machining process is complete... after the motor is broken in.

 

[D-Roc]

too late, he raised an eyebrow. haha.

 

[waterfreak]

the motor is not broken in yet. Wait to raise an eyebrow until after the machining process is complete... after the motor is broken in.

maybe it was broken in while running away!! ouch!

 

[WaveDemon]

my point is one problem at a time. the motor doesn't hold pressure and his motor runs away. symptom + diagnosed problem = great starting point

 

[D-Roc]

his motor holds some pressure, more than mine ever did. if he has an airleak that will cost me a beer, my guess is the intake portion somewhere. possibly the carb to intake area. also, some underknowledgable ski wrenchers may mistake an open throttle as a lean runaway because they fire the motor up and it revs up high and then panic and runaround for a couple seconds until they kill the motor. a good runaway is for a minute of holding the stop button. lol. something that make you say, ohhhh that ain't good.

 

[waterfreak]

I think is problem is in the carbs...but that's just my opinion!

 

[easyrhino]

On a cool start will a motor run away when one carb is lean because each carb has its own restrictor and fuel pump?

 

[WaveDemon]

his motor holds some pressure, more than mine ever did. if he has an airleak that will cost me a beer, my guess is the intake portion somewhere. possibly the carb to intake area. also, some underknowledgable ski wrenchers may mistake an open throttle as a lean runaway because they fire the motor up and it revs up high and then panic and runaround for a couple seconds until they kill the motor. a good runaway is for a minute of holding the stop button. lol. something that make you say, ohhhh that ain't good.

that's a good point, I assumed when he said run away that his stop button didn't work. That may be a bad assumption.

 

[Nuklear6]

I do a lot with high performance 2 strokes (build micro sprint engines) and however I do agree that there maybe a SLIGHT air leak. I do not feel as if that preassure drop is enought to cause a lean run away. I have seen engines with much higher compression (running methanol of course) have worse of a pressure drop due to badly worn "ridges" on the hardened seal surface of an older crank shaft, that ran just fine and offen won main events.

As far as this engine goes I think some more info would be good (ie. What tyype of carbs, intake, and electroics being used). First thing most people over look with there carberators is the idle balance of the two throttle plates (if ur running duals). I use a .003 feeler gauge and set the cable side carb first with the idle screw, then use the linkage inbetween the carbs to set the slave carb to that same feeler gauge. You will need to open the idle screw back up for it to idle once this is set.

If this is a run away the stop button doesn't fix you may want to take a look at the condition of the pulse line and how well it is on both ends. Slight crack or issue in that and ur gonna have big probledms! Also make sure if u just re built your fual pump or pumps, that you don't have an issue with the installation on the two check valve discs in the center section of each pump.

 

[Nuklear6]

Double posted sorry!