Super Jet UPDATED 3/17: SuperJet Running Away/High Revs On Start... Need Help!

[Idok]

I dont see how it could rev like that if the throttle plates are closed. I think you should pressure test your engine.

+1

 

[FlightPlanDan]

I see you quoted the thread where I had this issue. For my ski, it turned out to be exactly what the folks on here said it was. A temporary lean condition from a "not so filled" fuel system.
Have you put water to it and let it run for a bit? The only post on that was water free, so far.
I too have no choke, but a primer.
I prime to start, but after starting it revs up (not a run-away) but revs higher than normal.

Now I just continue to work the primer til it comes down to normal.
I too fire it without water at least every other day. It does the same thing and I do the same thing.

I'm not saying your issue is not different, just that I searched high and low for another reason because it had not done this b4.

Try working the primer after the start as well as b4.

Dan


EDIT: on the tank check valve: Air should be able to get into the tank, not out.
I just blow on it to make sure I'm putting it on right.

 

[retroicon]

Yeah, I know I could be looking at an air leak, but with all new gaskets, new crank seals, and me having just dropped it in the hull, I'm trying to check everything else before I have to pull the motor back apart, make a pressure test system, and disassemble the whole thing...

It was also a fresh bore, and all mating surfaces were clean before I put it back together... honestly, I don't think it's an airleak just because I always take my time when putting motors back together, always use all new OEM gaskets, and have never had one before... but, I also know it's possible.

 

[FlightPlanDan]

I dont see how it could rev like that if the throttle plates are closed. I think you should pressure test your engine.

Throttle plates are always closed until you pull the trigger... mine will rev up without pulling the trigger. Maybe you mean choke plates? (which he does not have. Nor do I)

I'd like to hear about what it does if he continues to prime it after it fires up. He posted that the lines had air in them = lean condition til the sysytem is full of fuel. Right?

 

[retroicon]

Throttle plates are always closed until you pull the trigger... mine will rev up without pulling the trigger. Maybe you mean choke plates? (which he does not have. Nor do I)

I'd like to hear about what it does if he continues to prime it after it fires up. He posted that the lines had air in them = lean condition til the sysytem is full of fuel. Right?

Yes, that's right... lines look to be full of air. I'm a little nervous about priming the hell out of it though because, if there is an airleak, that seems like the worst thing you could do. I mean, adding more fuel to an engine that's on the verge of running away can't be a good idea.

I think the next step is to richen the low jets to about 3 turns, which should be overkill, and see what happens. If it's lean, that should correct the lean condition I'd think... barring too much air in the lines.

I just blew air through the check valve to fill the lines, and i'm going to be trying to start it up soon and see what happens. I'll post back soon...

 

[Matt_E]

I'm a little nervous about priming the hell out of it though because, if there is an airleak, that seems like the worst thing you could do. I mean, adding more fuel to an engine that's on the verge of running away can't be a good idea.

It's one of the best things you can do. Adding fuel will cause a rich condition, and RPM's will come down.

 

[SUPERTUNE]

O.K., Two things come to my mind that I would address.

1st,
If you have a primer installed, are you running a stock fuel petcock? If so where do you have the supply 'T' tapped in what line?
My suggestion is to put the supply 'T' anywhere in the reserve line between the tank pickup and the petcock.
Always before starting ski, turn petcock to the the 'ON' position, prime the carbs with the primer sucking its suppy from the reserve line and NOT using the fuel in the 'ON' line as not to disturb the fuel needed for the carbs to not have enough fuel loss to cause a lean condition.

2nd,
Pull out a high speed adjuster and a low speed adjuster and compare the thread pitch, are they the same or is the low adjuster a fine thread pitch?
If the low adjuster is a fine thread pitch, then your adjustment of 1 1/4 turn out is way to lean.
Set it to at least at 2 1/4 turns out.

On another thought...if you have the choke plates removed, I would also suggest you use 95 gram springs and not the stock 115 gram springs. This will make the main jet you now have most likely too big by 5-10 points or so.

Chuck

 

[Waternut]

Since this is a common misconception... Fresh gaskets does not mean sealed. In my experience, fresh gaskets usually means leak guaranteed. I've assembled various engines of mine 10+ times. Some were complete and others were just the cylinder, intake, or exhaust manifold. Doesn't matter how diligent I am in cleaning up the mating surface or if I use copper spray on the gasket. So far only 2-3 have leak checked good the first time.

 

[FlightPlanDan]

Since this is a common misconception... Fresh gaskets does not mean sealed. In my experience, fresh gaskets usually means leak guaranteed. I've assembled various engines of mine 10+ times. Some were complete and others were just the cylinder, intake, or exhaust manifold. Doesn't matter how diligent I am in cleaning up the mating surface or if I use copper spray on the gasket. So far only 2-3 have leak checked good the first time.

That's pretty depressing! (not looking forward to a tear down/reassemble...even worse now!)

 

[retroicon]

O.K., Two things come to my mind that I would address.

1st,
If you have a primer installed, are you running a stock fuel petcock? If so where do you have the supply 'T' tapped in what line?
My suggestion is to put the supply 'T' anywhere in the reserve line between the tank pickup and the petcock.
Always before starting ski, turn petcock to the the 'ON' position, prime the carbs with the primer sucking its suppy from the reserve line and NOT using the fuel in the 'ON' line as not to disturb the fuel needed for the carbs to not have enough fuel loss to cause a lean condition.

2nd,
Pull out a high speed adjuster and a low speed adjuster and compare the thread pitch, are they the same or is the low adjuster a fine thread pitch?
If the low adjuster is a fine thread pitch, then your adjustment of 1 1/4 turn out is way to lean.
Set it to at least at 2 1/4 turns out.

On another thought...if you have the choke plates removed, I would also suggest you use 95 gram springs and not the stock 115 gram springs. This will make the main jet you now have most likely too big by 5-10 points or so.

Chuck

Primer is on the reserve line, and fuel is coming from the on one... checked that too unfortunately.

As for 2, I didn't know there are two types of threads on the low, and high speed adjusters ... or I guess I should say, I didn't know that was possible. I'll pull those later tonight and check.

That said, I went out to 2 1/2 turns just to try to fatten up the low, and it made no difference... still needed to put my hand over the carb to get it to start, still needed to prime it, and once it got running it started to run away on me.

Finally, I was thinking 95g springs too, I actually got some when I got the rebuild kits, and I'll give that a try if/when I start pulling things apart again.

 

[retroicon]

And... no dice. Tried to start it up with the lows out to 2 1/2 and it still runs away on me. Still needed to put my hand over one carb to manually choke it to get it to start at all, and once it does, it runs away to about 4-5k rpm now... sounds a little higher than before, which is making me think airleak even more.

Really not looking forward to taking this thing apart on a fresh rebuild... any last suggestions before I try to find the time to pull, disassemble, rebuild, and give it another go?

 

[SUPERTUNE]

You need to double check the carbs again or have someone else help you with a second set of eyes always helps, even me!

 

[Waternut]

Don't disassemble until you pressure test it. I'm got a write up on building a test kit and testing that is floating around on dozens of threads but I lost the bookmark for quick easy access.

 

[FlightPlanDan]

What about the "quickie" air leak test? (when you spray carb cleaner or something around the gasket areas while it's running and listen for rpm change?)

I've never done it. Just heard about it. Sounds like it could give good info.

 

[retroicon]

You need to double check the carbs again or have someone else help you with a second set of eyes always helps, even me!

I already did this once, but it's worth another shot since it's pretty fast, and easy to do... I checked all the jets, and pathways to make sure they were clear, cleaned the screen/filters and everything else is new. New needle/seat, new diaphrams, new jets high and low, new springs, all Mikuni stuff too... not an aftermarket offbrand.

That said, I'll do it one more time before pulling the engine apart, and while I'm in there I'll replace the springs with the 95g springs too...

 

[retroicon]

Don't disassemble until you pressure test it. I'm got a write up on building a test kit and testing that is floating around on dozens of threads but I lost the bookmark for quick easy access.

Well, you have to pull the pipe, and pull the carbs right? That's about half way to being disassembled as it is...

On another note, where is the OEM yamaha kit to do this? Everyone says to build your own pressure testing rig, but if I'm testing it, I'd rather know I did it right, and that my readings aren't because my rig is failing on me.

 

[Waternut]

Never heard of an OEM kit. However, if you notice a pressure drop, step one is to spray the test kit, connections, and around your block off points with soapy water regardless of who made it because that takes a few seconds and spraying the rest of the engine takes a careful eye and a lot of spraying.

 

[Cliff]

do you have access to another set of known good running carbs? like the ones in a riding buddies boat. would be worth a shot to bolt them on your ski and see what happens. how about the throttle cable? is it out of adjustment? i bet its goona be something simple. goodluck

 

[waterfreak]

double check the clear diaphram behind the pulse cover for any pin size hole that could be cause from some burrs or imperfections in the mikuni casting. I've seen this problem a few times

 

[retroicon]

do you have access to another set of known good running carbs? like the ones in a riding buddies boat. would be worth a shot to bolt them on your ski and see what happens. how about the throttle cable? is it out of adjustment? i bet its goona be something simple. goodluck

Nope, unfortunately I don't... I have a single 48 that came with another(spare) motor, but I don't know if it's really working, or not. I'd also have to change the manifold from my current dual, to the single, on that one to check. Throttle cable was disconnected when I was testing, so it's not that.