Super Jet Is this an Airleak, or not?

[D-Roc]

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

is the ski out of water and no load on the pump? When my buddy uses to fire up his fancy pants motor with the stock airboxes he would dump fuel down them because he had no primer. the ski instantly fires up and idles quite high until it gets its own fuel that is metered by the jets for the proper idle. so yeah it "could" in the right situtaion. i seen cold motors run away and hot motors on the trailer almost out of fuel run away. both ran fine in the water and had no airleaks.

 

[easyrhino]

when it runs away, should you open the throttle?

 

[waterfreak]

when it runs away, should you open the throttle?

yes!...and hit the stop button!

 

[D-Roc]

One problem is we tend to loose our minds for a second when it starts screaming all tight sounding. haha it is funny to see when its not your ski and wonder how the heck can the motor rev that high without the throttle plates open. the hot motor on the trailer was the worst i seen because it was out of fuel and opening the throttle and stop button did not slow it down. that was a long one at it musta sucked every last dribble of fuel from the entire system. I had a bad flywheel that took out my front bearing and seal. I realized the airleak when i jumped the ski out of the water and the motor revved up, same with a blown base gasket. sometimes finding it in the water is better than in the garage or trailer.

 

[AtomicPunk]

that was better (or worse) than a cash for clunkers video.

 

[retroicon]

So some people think it's an airleak worth doing a teardown on, and other people are saying just look at the carbs?

The carbs are freshly re-built, with Mikuni parts including new jets, springs, diaphrams, and gaskets... and I re-opened them up a 2nd time, then again a 3rd time just to be sure everything was in there, in the right locations, and that nothing was restricted anywhere in either of the carbs. I also checked the sync, and both butterflies are closed.

Let's focus on one thing at a time... I'm losing about 5lbs over 10 minutes. Can you give me a yes, or no... do you think this is an airleak that could cause a lean condition runaway on starting up cold? These are all cold starts, on the trailer, under no load.

And just to be clear... yes, pressing the stop button DOES stop the motor immediately.

-G

 

[easyrhino]

Why pressure check? Fuel/Air blowing out of case just makes a mess. Vacuum check makes more since that would lean out the mix. When building an outboard lower unit, we had specs on vacuum and pressure.

 

[D-Roc]

sounds like bad tuning and not an airleak. did you handprime the fuel system? did you check your idle, did you rebuild your carbs right so that they are working proper and not leaned out.
what happens if you rev it up with the throttle when it is "running away"
are the fuel lines all hooked up proper?

 

[tom21]

New rebuild, and the ski is running away immediately when started.
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?

so you know its an airleak and still want to run a new rebuild. are you just crazy loaded with money or what? regardless of runaway or not why would you not address the airleak? you are also overlooking that you sealed everything properly and that your gauge is not what is leaking. plug the hose on the gauge and and put it at 11 and see if it looses pressure. if its good then proceed to pressurize the motor and spray it with soapy water to find the leak. fix it and move forward.

as mentioned you should crank the engine with the plug wires off and grounded while choking the carbs til they have fuel. this will prevent the runaway from no fuel in carbs. If you are sure of your jetting and ability to rebuild you should be good. any airleaks in the lines should be double checked before you start also.

start from the beginning, dont be lazy. it will only come back to haunt you later and usually at the worst possible time.

 

[waterfreak]

and what about the jetting? are you sure that the carbs are jetted properly for your application? have you tried opening up the adjustment screws ?

 

[waterfreak]

and another thing...make sure the tip of your adjustments screws are not broken and possibly stuck inside the carb...I've seen this before and it kicked my azz once!...just once!

 

[Big Kahuna]

Have you checked to make sure both butterflies on the carbs are closed with there is no throttle being pulled?? I ran across this years ago on a buddies ski. I pulled those damn carbs about 3 times. Could not figure out what was going on. What I found was the rear carb was out of sync and this would act like a runaway. Check Your Butterfly's

 

[retroicon]

sounds like bad tuning and not an airleak. did you handprime the fuel system? did you check your idle, did you rebuild your carbs right so that they are working proper and not leaned out.
what happens if you rev it up with the throttle when it is "running away"
are the fuel lines all hooked up proper?

100% sure the fuel lines are hooked up properly, and 100% sure the carbs are built properly, and that the pop-off is even across both carbs...

Also 100% sure the lines were primed, using both a primer, and on another occassion, by blowing through the check valve on the tank and waiting until fuel ran back in through the return.

Carbs have low jets in that were recommended by 3 separate builders (reputable builders), and they're opened 2 full turns more than they should be based on recommended jetting.

 

[retroicon]

Have you checked to make sure both butterflies on the carbs are closed with there is no throttle being pulled?? I ran across this years ago on a buddies ski. I pulled those damn carbs about 3 times. Could not figure out what was going on. What I found was the rear carb was out of sync and this would act like a runaway. Check Your Butterfly's

I'm starting up by manually opening the throttle a little bit with my hand. I took the throttle cable off entirely. As for making sure they'r 100% in sync, is there some recommended way of doing this other than just a visual look at the plates? Based on everything I'm seeing, they look to be in perfect sync, but I can't figure out how you'd ever measure it to be sure...

 

[retroicon]

and another thing...make sure the tip of your adjustments screws are not broken and possibly stuck inside the carb...I've seen this before and it kicked my azz once!...just once!

Brand new adjustment screws, and they're perfect on both carbs... both the high, and the low. There's also nothing stopping up any of the internal passages in the carbs... they're 100% clear. I'm sure of it.

 

[retroicon]

so you know its an airleak and still want to run a new rebuild. are you just crazy loaded with money or what? regardless of runaway or not why would you not address the airleak? you are also overlooking that you sealed everything properly and that your gauge is not what is leaking. plug the hose on the gauge and and put it at 11 and see if it looses pressure. if its good then proceed to pressurize the motor and spray it with soapy water to find the leak. fix it and move forward.

as mentioned you should crank the engine with the plug wires off and grounded while choking the carbs til they have fuel. this will prevent the runaway from no fuel in carbs. If you are sure of your jetting and ability to rebuild you should be good. any airleaks in the lines should be double checked before you start also.

start from the beginning, dont be lazy. it will only come back to haunt you later and usually at the worst possible time.

100% sure the blockoff plates are NOT where the pressure is leaking. I already did the soapy water test and found a small leak on one of the blockoffs and fixed that before getting the 5lb loss mentioned in the original post.... but I haven't found the leak itself yet either. I'm guessing it's the front crankseal since that's the only seal not exposed right now, it's still behind the flywheel cover.

 

[retroicon]

and what about the jetting? are you sure that the carbs are jetted properly for your application? have you tried opening up the adjustment screws ?

It's a 701, bored out to 83mm, freeride ported by LPW, ADA head, Mod Pipe, with the stock dual 38's and stock reed cages and reeds. Jetting is as follows:

In there right now:
145 main jet (brand new OEM mikuni) 1 turn out was recommended - it's at 2 out now.
75 pilot jet (brand new OEM mikuni) - 1.5 turn out was recommended, it's at 3 out right now.
needle and seat 1.5 (brand new OEM mikuni)
spring - 115 stock (brand new OEM mikuni)

I tried replacing the springs too, just in case they were off a little bit, and also tested the pop-off pressure... both carbs were within a pound on pop-off testing.

-G

 

[Watty]

Th air leak is a small problem that should be addressed, but I don't think it's causing the run away. Too much fuel, or too much air seem to be the case here. How does the fuel pump on the carb look? Is there fuel flowing out of the return into the tank at a good rate?

I had a similar problem on the weekend with a ski. It would only run when choking the carbs, and would run away rather than idle. It turned out to be the low speed screws were set way too lean. It's probably not the case here though as it looks like you have plenty of fuel in the low speed. Keep plugging away, you'll find it.

 

[D-Roc]

can you run the ski in the water so it at least doesn't run away? you can use propane or ether sprayed around the motor while running to find if it is the airleak that is pulling the revs.

 

[Big Kahuna]

I'm starting up by manually opening the throttle a little bit with my hand. I took the throttle cable off entirely. As for making sure they'r 100% in sync, is there some recommended way of doing this other than just a visual look at the plates? Based on everything I'm seeing, they look to be in perfect sync, but I can't figure out how you'd ever measure it to be sure...

Loosen the coupler and force the butterflies closed, then tighten the coupler back up. If these are OEM Duals, then there is a set screw that you adjust that will close both butterflies. Do this with the cable disconnected.