Mixture screws fully closed. Cylinders still firing rich. wow. 3rd gen.

...perhaps bad mixture screw o rings?

 

My money is on stuck open choke (mixture enrichment) valves. Either the choke slide rack is being held open by the cable with no endplay, or something is bent holding the slide open, or there is some debris keeping it from shutting completely.

But you might need to give more info such as is it stock or jet kitted? Pilot jet size?

Is it Running rich just at idle, or at a certain throttle position, or across the entire throttle range?

Float levels correct or too high? Debris in the Inlet valve and not shutting off, etc... Lots of possible causes depending upon the symptoms...

If you do need those orings i could mail you 4 of them for a $1.

 

I haven't messed with carbs in quite some time but are the "mixture screws" the same as the "pilot jets"? "Pilot jets" are also called "air screws" because they just allow air to be drawn into the carb manifold. If you're running rich at idle to half-throttle, then you need to back the screws out to allow more air to flow past them. If they aren't the same as "pilot jet" forget what I just wrote. :biggrin:

 

Forget for sure !!! Pilot jets are not also called air screws.

Different types of carbs use different circuits to slightly adjust idle mixture, and carbs like ours use a pointed screw to add or subtract FUEL supplied (not air) to a small hole downstream from the throttle plates, so turning the screw clockwise decreases fuel flow for a leaner mix. Some other bikes do use air screws but not ours.

Kennybobby has covered almost everything that could be messed up.

 

Thanks guys. The enrichment plungers are not stuck open. I have cleaned & cleaned and checked them many times. Made sure cable is not hanging them open. No bent rods or mechanics. They snap shut when pulled. I've had the carbs off the bike prolly 25ish times and checked the enrichment circuit everytime. perhaps the rubber on the plungers is still not sealing right. Jets & pilot are all stock. Carbs are super clean. I jumped the fuel relay and listened to the pump as it filled the bowls. The pump stops clicking so that tells me the float valves are closing. Tested that many times. Float level is at 12mm vs. the spec 9mm. Not sure why this is the case but when i set them to 9mm, the bike is super rich. 12mm has the bike running better than ever. Plugs have always dry carbon fouled. Seems to run rich thru entire throttle range. To my delight I pulled plugs today and 2&4 are almost perfect! I hope! 1&3 are dark brown(better than before) but still to rich. The bowl fuel level in 1&3 was about 2mm higher than 2&4 so I will chase that today. I am I correct in assuming the only places fuel can enter bowls other than main fuel supply is from enrichment? Plug pics are 1,3,2,4.

 

oh and the mixture screws are totally closed on all four which doesnt seem right at all!

 

I will simplify this for you.

The PILOT JET is a JET and it is INSIDE the carb float bowl.
No adjustment is possible. Fuel just goes through the hole.

The Pilot air screw is on the OUTSIDE of the carb and is
adjustable to adjust how much fuel or air can be allowed in.
On older motorcycles with non vacuum operated carbs the pilot air screw really did adjust the amount of air by lifting the slide up or down.

The more modern vacuum operated carbs have a fuel enrichment screw commonly called the "pilot air screw" but actually adjust the amount of fuel.

This is what the engine works on for the first 1/4 throttle.
Pilot jet AND fuel (enricment screw) also known as pilot air adjustment screw.

 

What is your idle speed?

Are the butterfly throttle valves closing shut when you let off the throttle or do you have the idle screw turned in keeping the butterfly open? Open the mixture screws 2 turns and back off the idle speed screw to shut the butterfly valve. Also make sure you have a little free play in the throttle cable such that it is not holding the butterfly open.

did you look at the rubber tip of the fuel inlet needle valve? Is it in good shape or deformed, clean with no debris that might keep the valve open?

 

Nice. I will adjust the butterflys now. Float valves have smidge of a wear ring, very little. They are clean...anal about that. I thought closing the mix screws would kill the cylinders. Is that normal? And float hieght of 12mm cant be normal but It seems to be leaning the bike out. I have wondered with a working float height, roughly where should the fuel level be in the bowl? That could give some sort of guideline if I am way off or not. I have been checking to see if all bowls have the same level with clear tubing, but Im not using that method for float height of course. Do ya'll agree the most recent plug pics of 2&4 are pretty much good? If so I can dial in 1&3.

 

starting with idle at 1k, I backed out the mix screws 2 turns. idle increased from 1k to about 2k. when i turned them clockwise to closed, idle dropped to about 600. The butterflys snap shut with the idle adjust screwed out to where it does not touch the throttle wheel and cables dont hold butters open. Getting the bike to idle at 1k, with mix screws closed required just under one turn of the idle adjust. At this position exhaust has gassy note still.

 

You're sure gettin closer, but somehow too much fuel is getting to 2 cylinders. Make sure idle jets look the same and have not been drilled. Some idjiot over-tightening mixture screws could crack that area, letting too much fuel pass.

Each cylinder should quit firing at anything less than about 1 turn out if idle jets are correct.

 

And if idle jets are good?(didnt look drilled to me). would new mix screw o rings help? And what do over tightened mix screws look like...

 

Bad screws= screw tips trunkated not like fine point.


Maybe you should try new O-rings, but i believe (with spring and washer) that they seal air infiltration through threads, preventing more air= leaner mix

 

Okay so somewhere between 2 turns out and closed on the mixture screws, the idle goes down from 2000 to 600, with the butterfly throttle valves all shut? If so then you are close to getting this dialed in.

When you increase the idle speed using the idle speed screw you are uncovering the transfer ports located under the edge of the butterfly valve which is letting too much fuel in. One turn of the speed screw is about 1mm or 0.040"--that's too big a gap for the throttle valve.

The '90 service manual is pretty useless--it does not give any of the usual procedures for adjusting idle speed by dialing in the mixture screws. But basically it works like this: start at 2 turns out with the throttle closed, then turn the mixture screws in 1/4 turn at a time till the idle comes down to ~1000. Then turn the speed screw in to raise the idle to 1200. Then turn the mixture screws in, one at a time, until the idle speed drops about 50 rpms, then open that screw 1/8 turn and go to the next carb. After doing all 4, then lower the idle to 1000 using the speed screw. Take it for a ride and see how it responds.

This is all assuming you have a clean air filter and the air jets are not clogged with debris, and that the throttle valves are somewhat close to being synchronous.

 

Heres a thought: You say 2 & 4 are jetted perfect, but 1 & 3 are rich. Possibly the real problem is in the 2 & 4 circuit, and all you have done is adjusted the mixture to compensate for 2/4 problems, which means 1 & 3 are fine.


You said the carbs have been off about 25X. Have you checked the carb gaskets? Mist a squirt of starter fluid at the gasket joint and see if RPM's rise. If they do, it shows an air leak, that would normally cause a lean condition. Since you overcompensated 2/4 to run perfect, but 1/3 are rich, there could be a gasket air leak on 2/4.

Also, have you tried running the motor with the air filter removed?

 

rustedroot: hmm. interesting. good point. I have new boots. made sure to lightly lube them and make sure they are seated. Dont have new gaskets. I used a propane torch and shot it right against the boots and gasket joints. no diff in idle. I will test again. The idle doesnt seem to hang up anymore, but it will rise after riding and bike is hot. Perhaps that indicates some rubber somewhere is swelling/shrinking causing vac leak. So idle differs based on bike temp. I have started the bike with air filter off. What should i look for?

 

It is normal for idle to rise after the bike is warmed up. That is why you keep the choke on until the engine is warm enough that it can idle.
I am serious.
That is why you SET the idle AFTER it is warmed up, so that when the bike IS warmed up that it will not idle too high. See what I mean? Do not set the idle when it is not fully warmed up.

Cylinders 2&3 WILL be a little leaner and warmer most of the time. They are in the middle and do not get cooled as well as the #1 and #4 cylinders.

 

Almost ALL service manuals are messed up in some way except ONE.
It was the service manual for the Renault R5. It had a photo of the part in the car and out of the car,
it and a diagram showing the parts and internal parts, it had a brief description of how the part works, it told how to test the part,
and it had this for every single part.

One of the worst.... Volkswagon manual for Idiots.
For instance... When you look up removal of speedo cable from the dash its just says -Good Luck!
Not kidding.
But removing the tray under the dash first does make it easier.

In that same manual it also does not list pilot air screw setting. Should be 1.5 turns.

The drawings are funny though.

 

Sure it do: reduced pumping drag after oil gets warm almost always makes a hot engine idle faster.

 

kennybobby re: Idle