Yet another Jetting question

[rlkarren]

I replaced the stock 24mm Mikuni with a modern 28mm Mikuni carb on my (94) Wombat and I'm still working on dialing in the jetting.

The carb came with a 50/190 (Pilot/Main) and I knew that that would be too rich for my application so I pulled out my jets and began the process. I verified that the 50/190 was too rich and during the process brought it down to 35/140. (I think it may still be a little lean in the Main jet though.)

Anyway, after some reading, I determined that I was probably lean on the pilot even still, because the bike would not immediately return to idle speed if I revved. I replaced the pilot with a 45 and saw marked improvement, however, I can set the idle speed and air adjustment so that the idle is nice and smooth and has that "fat" and even feel to it, but as soon as I set it down on the kickstand, the idle immediately drops and the bike dies. Further, after riding around a little at 1/2 - full throttle, the bike still hangs on a slightly higher idle when I come to a stop, but then drops to desired idle speed after about 5 seconds.

I am close enough on jetting that I could live with what I have, but I don't believe it is optimal yet.

I think I am dealing with two, possibly three problems or combination of two or more. 1) air screw adjustment still needs attention, 2) Float level may be off, and possible 3) Main Jet is too lean. (Plug color is slightly lighter than the dark chocolate brown/black than I am accustomed to).

So i am wondering, what is typical behavior if the float level is too high vs too low? And how does the float level effect jetting?

Thanks,

Roger

[hodakamax]

Roger, I made the same carb change on my 94A. Mine has a reed on it. I to worked down to about a 140 main from a 200. I'd have to check on the pilot jet though. Comparing notes with others with the same change they were running much larger mains than we are. Ed (Bullfrog) mentioned that the stock 94 air filter has a pinch point which is the rubber snorkel on the outer cover. It sets too close to the filter and causes a restriction. I haven't removed it and tried new jetting yet. It could be a factor if you are running the stock 94 air cleaner. Maybe Ed will have something to add to this discussion.

Max

[Bullfrog]

Dagnabbit! I did post a response . . . and it disappeared into the ethers. Can't take the time to re-type it right now. Will get back to it late tonite.

Ed

[hodakamax]

Hmm, Ed, I stayed up way past my bedtime for your report. I was just trying to fill in.

Max

PS--I too have launched some long ones into the ethers, aggravating huh? (Worse yet I'm a slow typer.)

[Dale]

Sorry Roger, but I do not have much in the way of wisdom to provide for your jetting situation, but I do have a suggestion for posting here on the forum to avoid the frustration of lost text. Prior to hitting the Submit button, I always select all of my text by pressing [ctrl]+A and then copy that text by [ctrl]+C. Then when the inevitable happens and the Submit button fails, you can paste the text right back with the keystroke [ctrl]+V.

In addition, should you accidently erase or lose text, you can always try the keystroke [ctrl]+Z to undo the last action.

Of course, there are other ways to perform these actions, but I find these keystrokes are the easiest and most efficient to use. Yes, it is a little bit of a pain, but a lot less frustrating than realizing that your thoughts just vaporized right before your eyes!
Dale

[JayLael]

The slow to return to idle thing just screams air leak. Are you sure the main seals are ok? I recomend really giving this a close look.
Float level can cause stalling at idle if it is too high. One sure fire way to know if it is too high is as follows. Get the machine running at idle and get the front end way below the back end, say on a steep ditch bank or something improvised. The idea is to simulate a steep dirt bank drop off scenario. If the machine becomes flooded out, there you go. Simply lower the float level until the machine will stay running when it's nose is steeply down hill. I ended up adding little washers under the float seat until the pissing stopped, this on a brand new mikuni 26 mm from Sudco. The float level can be too low, wherein eventually at wide open throttle the bike starves for fuel, then as you close the throttle some, it picks up and runs good until the float bowl runs dry again. Trials bikes benefit more from lowered float levels than say street or mx bikes, where fuel starvation can become an issue.

[Bullfrog]

I too am a bit worried about a small air leak. If you have an air leak, it MUST be identified and corrected.

I would expect your new carb to be properly set for float level. Generally, the float arms should be parallel to the float bowl gasket surface. (Carb upside down and float arm/tang resting on needle valve "post".

I would highly recommend that you complete tuning on the pilot jet BEFORE doing main jet tuning. The pilot circuit delivers fuel at ALL throttle settings - so if you select a "proper" main jet and then go back and change the pilot jet . . . it WILL change the mixture at full throttle (and you'll likely have to re-tune the main jet).

Generally, the pilot jet is very close when the pilot air screw "dials in" somewhere between 1 and 2 turns "out". Procedure calls for warming up the engine and setting the idle speed with the idle speed screw. Then adjust the pilot air screw "in" and "out" slowly to find the smoothest running setting. Since the smoothest running setting is providing the "right" mixture, the idle speed may have increased. You can re-adjust the idle speed, then slowly re-adjust the pilot air screw for smoothest running again. IF the pilot air screw setting ends up between 1 and 2 turns out, you probably have the correct pilot jet. If your pilot air screw setting ends up at less than 1 turn out, your pilot jet is too small and you need to try the next step richer size pilot jet. If your pilot air screw setting ends up more than 2 turns out, your pilot jet is too large and you need to try the next step leaner size pilot jet.

Ed
PS: Sorry I didn't get this entry tapped in last night - business stuff kept me going till wayeeeee too late to attempt writing this message without rushing it.

[rlkarren]

Thank you very much for taking the time to respond. I like Jay's idea for checking float level. With all the mountain trails to ride around here, that test might come in handy.

The weather turned and we had a some illnesses so I didn't get to work on it until yesterday. After reading your posts I was convinced that there had to be a leak somewhere. I haven't found it yet, but I'll keep looking.

In my defense.. I'm aware that tuning the Main while simultaneously tuning the Pilot is an exercise in futility, but I thought I had it. Obviously I didn't or don't have the correct Pilot. so, we're back to square one: Tuning the pilot, leaving all other jets alone until I can get the Pilot optimized.

I was especially looking for this tidbit of info:

IF the pilot air screw setting ends up between 1 and 2 turns out, you probably have the correct pilot jet. If your pilot air screw setting ends up at less than 1 turn out, your pilot jet is too small and you need to try the next step richer size pilot jet. If your pilot air screw setting ends up more than 2 turns out, your pilot jet is too large and you need to try the next step leaner size pilot jet.

I've seen it many times before but couldn't find it again when I needed it. Thanks again, Ed

I'll post again when I get it right.

Roger

[Bullfrog]

IF, you can't seem to get the pilot jet "dialed in" . . . THEN you have a bit more evidence that a leak exists.

Ed

[Bill2001]

Timely thread. I just bought a refurbished Wombat with new, new, new everything and it has the potential for being a tabula rasa on jetting, etc. A whole 'nuther Universe to putter with.

--Bill