Rich mid range and top end.

[Mekanix]

Messing around with a 96 xp that is running very rich in mid to top end.

Carbs look good inside. Had to change the springs. Now sitting at 36 pop off. Stock jets. Stock everything.
Runs good when you pin it but then loads up and eventually clears up but takes a long time.
Choke made much worse so I knew it was too much fuel.
Noticed high speed was open so closed those and set low speed to 1 turn.

Still very rich.

Removed the flame arrestor and tried again and that cleared it up almost completely.

Ok so cleaned flame arrestor. No real change though, still back to being rich again.

Tried turning off the gas slightly
and seeing if that helped, well that did quite a bit actually. Clean crisp response and then bogg as the carbs run out of fuel.

This all shows that something in the carbs is wrong. I'm not sure what too look at yet. Both carbs are affected.


What would happen if the return fuel orifice was too clogged up?

 

[JeremyD615]

It’s a bit out in left field, but have you checked the fuel tank vent? I guess a clogged return line could cause a pressure build up, but I would think you would have to have a clogged vent line as well, or it would be releasing the pressure...

Since the carbs are cleaned up, popoff is set, and it’s affecting both cylinders, I’m thinking it’s got to be a pressurization somewhere...

 

[JJinSC]

Are you sure the jets are stock?

 

[Mekanix]

I'm sure they are stock numbers buuuut I'll replace them anyways. Po could have ran a drill bit through them for all I know.

Oh and I also drove with gas cap loose as well to eliminate tank pressure.

Has anyone experimented and blocked off the return line just to see what effect that has? I may do that on another 787 just to see.

 

[JeremyD615]

Has anyone experimented and blocked off the return line just to see what effect that has? I may do that on another 787 just to see.

I haven’t, but I’m curious, so please let us know how that experiment goes...

 

[JJinSC]

It would appear to me that blocking the return would only present certain results for a short period of time until the vacuum was created on the tank side. At which point the system would then start to lose its supply of fuel and not run properly or present opposite results.

 

[JeremyD615]

Normally, a clogged return line in a carburetor application (not just on two strokes, but in general), will cause the carburetor to see the maximum pressure that the fuel pump can create. In these Rotax motors, that pressure is not very high, so I’m not real sure how it would act...

 

[mikidymac]

Blocking off the restrictor will make it more rich as the fuel pressure will increase. So if you are already rich increasing the restrictor will make it worse.

What springs did you install, 80 gram?

 

[Mekanix]

Yup 80's. It had something in it. I dont know what but popped at 15 psi...
I'm wondering if maximum pressure from the pump is the issue. In that case I'll look at the return orifices. If not maybe I'll just install some fa's lol

Everything from the tuning manual says main jet. Since that is the throttle range its messing up in.

But if I restrict fuel flow to the carb by the fuel selector everything is fine once it starts to starve a but So I'm not sure what to think.

 

[mandriva]

I try it! I put a main jet 60 on Y return fuel! The motor run more rich ! Each 5 @8 seconds my EGT dropped around 50F 1080f to 1030f

 

[mandriva]

Yup 80's. It had something in it. I dont know what but popped at 15 psi...
I'm wondering if maximum pressure from the pump is the issue. In that case I'll look at the return orifices. If not maybe I'll just install some fa's lol

Everything from the tuning manual says main jet. Since that is the throttle range its messing up in.

But if I restrict fuel flow to the carb by the fuel selector everything is fine once it starts to starve a but So I'm not sure what to think.

Are you verified the test leak needle and seat?

 

[Mekanix]

Are you verified the test leak needle and seat?

Part of the pop off test I left them there to see if they would leak down. They didn't leak after a few minutes.

 

[etemplet]

What effect would too much clearance on the rotary valve have on top end performance?

 

[Mekanix]

I think none.
I wonder how to measure the return orifice size and flow to see what normal and what clogged slightly are like.

 

[etemplet]

I'm just wondering if it is leaking much when the other cylinder pulls in fuel you might get extra in the opposite cylinder. Piston is on the way up, not sure if there is enough vacuum or conditions correct to draw in fuel. If not through the carbs that's another path.

I have two GTXs and I've been contemplating installing a smaller main jet on one just to see how it responds. I am also gonna check the rotary valve clearance.

 

[JeremyD615]

I can’t think of a good repeatable way to measure return line flow at the throttle ranges you’re having problems at. I guess you could try adding a fuel pressure gauge and cross checking your readings against another 787... I’d offer to put a gauge on mine, but my 96 is out of commission until I get a set of cases that don’t have a crack in them...

 

[Mekanix]

Going to run a longer line between the carbs and add a gauge there.
Also going to see if I can maybe clean out the orifice with something stronger than carb cleaner.

 

[etemplet]

Yes that is a very small hole in there. I bought an ultrasonic cleaner that arrived yesterday. I have to solicit some advice on the best concoction mixture to clean the carbs. Hopefully this will be less of a mess for me, less toxic chemicals, and so a more thorough job on the carbs.

 

[JeremyD615]

I’ve heard good things about the Simple Green Extreme, as it’s an aviation product and is aluminum safe. I’ve never tried it in an ultrasonic cleaner, but I think that’s the route I would go...

 

[POPPS]

I'm just wondering if it is leaking much when the other cylinder pulls in fuel you might get extra in the opposite cylinder. Piston is on the way up, not sure if there is enough vacuum or conditions correct to draw in fuel. If not through the carbs that's another path.

I have two GTXs and I've been contemplating installing a smaller main jet on one just to see how it responds. I am also gonna check the rotary valve clearance.

When your rotary valve clearance is too wide, it will affect how fast your ski will start, it will have minimal affect at wot.

 

[etemplet]

I’ve heard good things about the Simple Green Extreme, as it’s an aviation product and is aluminum safe. I’ve never tried it in an ultrasonic cleaner, but I think that’s the route I would go...

I want something that will dissolve the crud in there that we can't get to. I"m hoping the Ultrasonic will give me a better carb rebuild. I wanna go fast on the first try.... LOL