[Louie Uribe]

Hello fellas,

I've got an issue that I simply can't figure on how to lean out the Mikuni carb on my 98 Yamaha Timberwolf 250 4X4. I recently cleaned the carb in my ultrasoic cleaner. All the passages and orifices were clear and flowed brake cleaner and air. I used a new All Balls rebuild kit wchich came with a 20 pilot jet and an 85 & 105 main jet. Stock jetting is 20 pilot and 85 main with 1.5 turns out at the air screw. The stock needle clip position is 2 from the bottom with a washer. All new o-rings, needle, etc.

The issue I'm having is that it runs extremely rich (spark plug is sooty due to the excess fuel). The choke plunger o-ring was changed out with the one in the rebuild kit but it won't fully seat inside the plunger barrel (obviously the o-ring is too big). I ended up using an o-ring that somewhat fits correctly and the plunger will seat fully in the bore. The plunger was also adjusted to where it clears the enrichment hole on the cylinder side of the carburetor. Should it be adjusted that far up when activating the choke? Does the plunger needle no seat fully inside the enrichment tube that comes up from the inside of the carb when the choke id deactivated? Could it be allowing fuel to continously enter causing the rich condition? Or, could it be that the plunger bore needs a tighter fitting o-ring or new choke assembly?

Caveat: I've moved the needle clip from the stock position to the 3rd and 2nd position from the top and still no change in hopes of leaning out the carb. Also, no amount of air screw adjustement leans out the carb unless it is fully closed (screwed in all the way). At that point, and only then is there a change in engine speed (idles faster - leaning out). You would expect the engine to die with the air screw fully seated - closed. I keep leaning toward the choke being the issue causing this extremely rich condition.

I'm at my wits end and seek assistance from those who have more experience than I. I work on mostly small engines (weeders, chainsaws, mowers, etc...) so I know about gasoline engines. However, I'm not very saavy when it comes to motorcyle / ATV engines but I do understand the concept. Any help would be greatly appreciated.

Louie

 

[Mech]

If the choke plunger goes right in then it should not let any air or fuel past. It's the hole in the seat/bottom of the hole that has to be closed off to stop the fuel. If there is an air leak from where the cable goes in it only communicates with the passage going to the aircleaner end of the carby. Also, that type of choke only works when the throttle is closed, when you open the throttle there is not enough vacuum on the choke circuit to draw the fuel up from the bowl.

Are you sure all the hoses are on right, and clear ? Float level right ? Not too much oil on the air filter ? Did you eyeball the new jets to see they were accurate ? I've seen new cheap spare jets with different numbers which all looked to be the same size before.

Is it running rich at idle only or with throttle too ? If it's rich at wider throttle settings then perhaps the emulsion tube is still dirty.

 

[Louie Uribe]

Thanks for your reply. The only hose that connects the carburetor to airbox is the vent tube which is clear of any obstructions. The hose is currently not connected but i dont think it would make much difference since it vents to atmosphere. Please correct me if I am wrong.

Float level is adjusted as per the service manaul 21mm-22mm (0.83"-0.87"). Air filter is good to go. I did eyeball the pilot and main jets and appeared to be the same as the OE jets. I will be going back to the OE jets and air screw and see if theres a difference. I'll also be placing thr needle clip back to the factory location (2nd from the bottom) along with the washer. Now, the carb is missing the baffle that sits over the main jet. I don't know if that will cause any issues I'm having. I don't think it does.

The choke plunger is adjusted to where it exposes the enrichment hole when activated and seals off the hole when it's deactivated. Is this correct or should the plunger just expose enough of the hole to allow fuel in when activated and when it's deactivated, the plunger sits deeper in the bore and the needle bottoms out?

The issue is at idle I believe. There's no engine response when the air screw is at 1.5 turns out until it is screwed in all the way and bottomed out. That's when you hear the engine respond and it begins to gain speed - leaning of the engine. However, once it transitions from the pilot jet to the main jet (while driving - under load) it runs great. Even at idle the engine will rev easily. It's just that transition from the pilot jet to the main jet where it bogs due to too much fuel. The proof is in the spark plug being extremely sooty.

I hope I've been able to accurately explain my situation. Meanwhile, I'm going to take the carb apart again, dip it in the ultrasonic cleaner, make sure the float level is accurate, and reinstall with the OE pilot and main jet.

 

[Mech]

I'm not familiar with yamahas, so I'm presuming this is a cv/diaphragm carby ? If not then read "slide" where I say butterfly..

As long as the choke plunger goes fully in and seats/seals on the seat in the bottom of the hole it should not be causing a problem. Perhaps check that rubber seat in the plunger.
The bowl baffle I agree won't be the problem.
If it's only the idle circuit that's running rich then either the float level, idle fuel jet's too big, or idle air bleed must be blocked.
Are you sure that the idle speed/throttle opening is set right ? There are two or more tiny holes near the butterfly(or slide if it's a cable on slide carb), and when the throttle is closed one hole is on each side of the butterfly. Both holes are connected and connect to the air and fuel passage of the idle mixture. With the butterfly closed the hole nearer the aircleaner allows air to be drawn into the idle fuel passage and dilutes the fuel, then as the butterfly opens and vacuum comes onto that aircleaner side hole it starts to discharge fuel as well which causes the mixture to become richer. That gives an automatically compensated mixture to prevent flat spots on acceleration. If we have the butterfly open too far when we are adjusting the idle mixture we are adjusting the mixture with fuel being discharged out of both holes instead of adjusting it to be right with the extra air being drawn in through that aircleaner side hole. It wouldn't normally be such a big difference that it would need the mixture screw to be right in onto it's seat though. Even with both holes discharging the mixture screw should still lean the mixture before it needs to be right on it's seat. If it had the wrong idle mixture screw it might cause a problem though.. A combination of wrong needle and wrong throttle setting perhaps might cause this problem.

 

[Mech]

It doesn't have a vacuum operated fuel tap does it ? A hole in the diaphgram would cause flooding..

 

[Louie Uribe]

No, it does not have a diaphragm. It is a slide operated carburetor.

I'm going to take the choke out of play and adjust the plunger so that it is completely bottomed out in the bore. I'll dribble a little fuel into the carburetor and start it. If there still no change with the air screw then:

1. there's an obstruction somewhere in the carb
2. the pilot jet is too big (I don't think it is)
3. the float level is not correct (I don't think it's that either)

Either way, I'll be relpacing the the pilot and main jets with the OE jets I still have, the air screw as well, and repositioning the needle clip back to factory placement. I'll also check the float level. I'll check the slide bore and make sure the small orifices are clear of any debris and go from there.

Crossing my fingers!

 

[Mech]

The diaphragm I meant was one in a vacuum operated fuel tap.
And yeah the choke has to bottom out. If it's lifted even slightly it lets fuel through..

 

[Mech]

Idle mixture adjustment needles came in different tapers and diameters.. That might be wrong.

 

[Mech]

Looking in the manual I see the float chamber overflow hose has a one way valve in it.. If that's on the wrong way it could cause flooding.

 

[Louie Uribe]

The diaphragm you are talking about regarding the fuel tap... the fuel tap / petcock is a gravity feeds the carburetor.

I went back and used the OE needle with the clip in the factory position.

So the float chamber overflow currently does not have a hose and the barbed fitting is venting into the atmosphere. Fuel isn't dripping from the carburetor or the overflow barb which would indicate that the float level is correct and not overflowing into the carburetor. I have ordered a new choke plunger to just to rule that out. I'm really hoping its the choke plunger assembly that's bad.

 

[Mech]

The main jet and the slide needle won't be the trouble.

If the overflow isn't dripping fuel then it's not likely to be the trouble either. Even a high float level should respond to the idle mixture screw. The float bowl does have to be vented though or the vacuum will build up in there and start sucking fuel out of all the discharge ports.

If it runs good with throttle then the problem will be in the idle circuit..

Did you check the air bleed jets near the aircleaner end of the carb for blockages ? One of those is for the idle mixture and one's for the high speed running. If the idle one was blocked then no air would be getting mixed with the idle fuel before it gets controlled by the idle mixture screw, so it would run rich.

 

[Louie Uribe]

The engine revs nicely without a hitch. It's just down low where it stumbles until it transitions into the main jet. It definitely has to be the idle circuit. The air bleed jets / orifices are clear of debris and flow brake cleaner and air. Ugh... this is getting to be annoying. Haha... I will win though! It will only beat me if I have to buy a new carburetor (aftermarket since OE Mikuni are not available).

 

[Louie Uribe]

I'll double check again in the morning. Thanks for all your help. I'll keep you in the loop on the progress.

 

[Louie Uribe]

UPDATE:

So, after removing the carburetor for the ump-teenth time... I again went through every orofice and jet, etc... Well, after removing the main jet stack and pilot jet, I sprayed brake cleaner through air bleed jet located at the bottom of the carb on the air filter side and noticed it wasn't travel into main jet bore. I ran a welding tip cleaner and it would stop. I figured that's where my clog was located.

I guess in my haste to reassemble the carburetor with the rebuild kit, I put an o-ring in the main jet stack - specifically where the needle holder is inserted into the carb body. Well, that o-ring plugged the hole which didn't allow air into the idle circuit; hence it ran fat and had no adjustnment with the air screw. Removing the needle holder and o-ring allowed brake cleaner to be sprayed into the air bleed jet without obstruction. In looking at the service manual, an o-ring does not go there.

I reassembled, air screw did respond to very minute turns and now it runs great. It idles and responds to acceleration without bogging from the idle circuit into the transition to the main jet. I'm pleased that I was able to get thisa issue squared away.

Moral of the story.... always refeer to the service manual!

 

[Mech]

There are several overlapping transition points in those carbs.
At idle the idle jet and idle air jet supply fuel/air mix to the idle mixture screw. The screw controls how much of that fuel/air mix gets mixed with the other air coming in through the discharge hole under the slide that's nearer the air cleaner, and the highly diluted fuel/air mixture then gets discharged through the other discharge hole nearer the engine. Then as the slide rises a little fuel starts to get drawn out of both those holes which reduces the amount of air being drawn in through the air cleaner side hole which makes that mixture slightly richer. That's the first of the transition points. That transition from one discharge hole to two prevents stumbling just off idle. If we adjust the mixture with the slide too high then fuel is getting discharged through both holes and we have to wind the mixture screw in too far to compensate for that, which then causes a stumble/flat spot when we d try to open the throttle. It's important when adjusting the mixture that we have the slide down as far as possible.
Somewhere in that system is where your problem lays.

The other transition points are controlled by the slide cut away, and then the emulsion tube's holes and the capacity of fuel in the drilling the emulsion tube is in. It's important to clean the tiny holes in the emulsion tubes side, and the drilling it goes in if it has crud in it reducing it's capacity for fuel. The tiny holes in the emulsion tube start off being covered with fuel at low throttle settings and that fuel is drawn into the tube along with fuel from the main jet and discharged past the slide needle, but as the fuel consumption increases with throttle opening the drilling starts to empty and more of the tiny holes are exposed allowing air to be drawn through them diluting the mixture being discharged out past the slide needle. Eventually the main jet is controlling the amount of fuel being drawn in and the tiny holes are mixing air into that fuel so that the fuel being discharged past the slide needle is diluted. That process gives a rich mixture as we open the throttle, but then leans it after a short time when the drilling for the emulsion tube is emptied. The overall effect is similar to an accelerator pump.

Hope that helps..

 

[Louie Uribe]

Thanks for your help brother. I totally appreciate your taking time out to explain things to me. Very, very much appreciated.

 

[Mech]

Ha.. All good man.. Glad you got to the bottom of it..

 

[Ulfthednar]

LOL. right when I was going to third the notion of a blocked passage.
I solemnly sear I have never done anything that dumb in my . . . damn. Im not a politician. I cant lie.

 

[Mech]

It takes practice Ulf..

 

[Louie Uribe]

I’ve slowly been learning from my mistakes. What sucks is when you get ahead of yourself and… well, I put an o-ring where it didn’t belong and brought frustration, swearing, and more frustration. I’m just happy that I was able to figure it out: not without help and suggestions.