Looking for a air leak

[azsxr]

I’m having a runaway issue on my fresh rebuild. I pressure tested it before I installed it
and it passed. I set the idle at 3000and it will stay the same but when I give it some gas it rev’s up
and takes a few sec’s to go back. When I rev it a bit more the idle will only go back to 3500
Rev it again and it runs away.
I know it a leak but I’ve check it wd 40 and couldn’t find it I even tried some starting fluid and nothing. I sprayed around the rear crank seal,carb base gaskets , cylinders to case ,manifold and cases.I pulled the carbs and manifold
everything was good but it does the same thing.
I was trying not to pull the engine but what else could it be?

 

[AKnarrowback]

Did you run it in the water yet?

 

[azsxr]

Yeah and it did the same thing more or less with out the runaway

 

[Sportster-2001-951C-Stock]

I've seen them run out of fuel on the trailer and do that too. Off-kilter carburetor mounting gaskets are common, it's easy to have one slip out of place.

If your fuel system isn't fueling correctly (too lean for whatever reason, such as an fuel line air leak of carburetor issue) they will run away. Take it to the ramp and see if it hesitates and falls on it's face there, instead of running out like it should. I bet it does, so 1st, at least low pressure (no more than 10psi test of the fuel lines from the fuel pump all the way back to the tank nipple to make sure there are no air leaks.

 

[azsxr]

When I had it on the ramp it was falling on it face.I left it tied down to the trailer and I could get it to idle but as soon as I tried to give it gas it would die. I could got it to rev after some light throttle work but he idle would go up and down 500-600.

 

[AKnarrowback]

My point is if you did a leak down test and it showed good then do not jump to a conclusion and start taking the motor apart again.

Put a load on it and see how the motor reacts. I don't know the history of the machine but I would look into carb related causes first.

Is the idle too high, how is the low end screw set, is the o-ring on the rv cover, are the intake gaskets all in place and tight, double check the carb butterfly and make sure it is closing against the idle screw, did you rebuild the carb along with the motor, is the throttle cable routed properly and not being pulled on.

If all new seals were used in the rebuild, the case halves are sealed, the leak down test showed good then double check other areas.

 

[AKnarrowback]

BTW - What motor, machine is this?

 

[azsxr]

Sorry 787
I rebuilt the carbs as well . I just posted that I had it in the water.

 

[azsxr]

I have prok flame arresters carbs jetted 75 lows 145 highs. Lows 1 turn out highs mag is 1/2 turn open pto side 3/4 turn open.
Pop off is 22.

 

[Sportster-2001-951C-Stock]

After confirming no fuel line inward air leaks, open up the low speed idle mixture screws out 4 or more turns and see if the lean runaway dieseling stops. Ordinarily this should make idle mixture plenty rich, if your carbs idle circuits are working.

And don't be so sure your carbs are up to snuff, this trips up a lot of people to the point they buy new carbs and shizam, the lean hesitation is gone. And no coincidence, so is the reason their motor was roasted in the 1st place.

 

[azsxr]

Thanks for the help. I’ll try opening up the lows this afternoon.

 

[Sportster-2001-951C-Stock]

Sure. Just to rehash, the expectation is lean runaway is eliminated by the additional fuel, same concept as applying choke tips the scale rich to stop lean runaway.

 

[azsxr]

I checked the fuel system and it was good.
I opened the lows up to 4 turns out
no more runaway but it’s slow to return to idle.

 

[Sportster-2001-951C-Stock]

Are you absolutely certain your low speed circuit was totally clean? In this drawing, it's the circuit inside the yellow circle. To have any hope of cleaning it you must remove both the low speed mixture screw and low speed brass jet to flush the passage., the three bypass holes are precision drilled and easy to be plugged due to their small diameter, make sure they're whistle clean then clean them again, stick a wire down through them.

 

[mikidymac]

4 turns out is too much more than 3 and the screws aren’t really doing much and you need to go up in jet size at that point.
The screws actually do quite a bit so don’t go more than about 1/8 turn at a time.

I also haven’t had any luck getting a good stream of cleaner through the bypass holes with the low speed screw out. If you try it with the screws out all the cleaner goes out the open adjuster hole instead of being directed out the bypass holes you are trying to clean.

 

[azsxr]

You know I had the screws out and it never “shot out” of the smallest holes but I put quite a bit of carb cleaner through them.

 

[mikidymac]

Like I said...

 

[Sportster-2001-951C-Stock]

I'm not sure what the question is, perhaps there isn't one.

 

[azsxr]

Thanks again Ill pull the carbs and try cleaning them again.

 

[Sportster-2001-951C-Stock]

Yes, give that passage another thorough cleaning, working over an old white bed sheet so you can detect schmutsa, with carb cleaner and 80psi compressed air blasting through there, you might see some corroded goo come flying out.

That's one disadvantage of aluminum carbs as opposed to zinc alloy, is internal corrosion.

 

[mikidymac]

I haven't been really happy with the newer carb cleaners and have switched to brake parts cleaner. I am not sure if it is just a California "protect us from ourselves" thing but the carb cleaner just isn't as strong as it used to be.

 

[mikidymac]

I'm not sure what the question is, perhaps there isn't one.

It wasn't really a question just an observation. If you remove the low speed screw and try to clean the little bypass holes almost all the cleaner goes out the LS screw hole instead of being forced through the small holes you are trying to clean.
I like to first spray cleaner through with the screw out to get big debris then put the LS screw back in a turn or two to force cleaner and air through the little holes.

 

[Sportster-2001-951C-Stock]

I agree on the carb cleaner in the can, all it is these days is acetone. Brake cleaner is a little faster at cutting through the crud but still just a solvent and does little to dissolve aluminum corrosion.

One of these days I'll get around to testing weak acids to dissolve the corrosion like vinegar, lemon juice or sodium hydroxide on a set of stubborn Mikunis but hesitate to suggest something I haven't tried myself. Aluminum wheel cleaner maybe?

 

[IDoSeaDoo]

I had a case leak once: it was between the cases and where the mag cover meet. It was leaking out of that little triangle. Had to pressure test the case to find it. sealed it with a bunch of silicone. I've also had an issue like this when my fuel hose clamp was loose. It was tight, but tightened in such a way as to bunch up the hose somehow and allow air. You could try plugging a fuel hose right to the carbs and dipping the other end in a tank of clean fuel. This will bypass your entire fuel plumbing system and eliminate it as a potential prolem

 

[drjack]

Looking back at the diagram that Sportster has submitted, where exactly is that check valve that's circled in blue located? And what's it's purpose, or happens if it fails?