Cure for an ankle buster?

[JimF]

Yes, timing is now 40 degrees BTDC.

8K rpm achieved on an unloaded engine but a quick ride (engine loaded) still could not get over 6K. The plug fowled with dry soot.

I haven't reviewed the Mikuni manual but I am wondering if I might have the enrichener lever in the wrong position. The lever says "choke" but it does not choke but rather introduces or removes an additional fuel circuit.

I swear, I can find equal amounts of websites that say "lever down" is enrichner off as I can that say "lever up" is enrichner off.

[graeme]

Jim, I have a Mikuni on a Commando. Lever down = plunger lifted up is choke or enricher on.
Lever up, plunger down is choke off.
Hope that helps.
graeme

[double diamond]

Assuming you have the timing set correctly, point gap, good spark, etc. call it good for the time being . If your Mikuni has a lever type “choke” then, with the lever lifted up, the enrichening circuit is closed, i.e. the shaft the lever is attached to is pushed all the way down into the carb body. That shaft has a piston on the end of it with a rubber plug on the end of the piston. This rubber plug obstructs the enrichening circuit fuel inlet in this position. When the piston is lifted by pushing the “choke” lever down, it opens a fuel passage to the float chamber as well as opening an air passage that originates at the carb bellmouth, passes over the fuel passage and continues to an opening in the carb venturi downstream of the slide. I’ve encountered these piston shafts that are too short for the carb they’re installed in and won’t close completely. To verify the piston shaft is the correct length, unscrew the enrichening assembly from the carb, remove the lever and reinstall the enrichening assembly in the carb body. With the piston shaft seated in the bottom of the bore the groove that the choke lever engages should be well clear of the rubber cap on the top of the enrichening assembly. If the groove disappears into the rubber cap, the piston shaft is too short. Chances are slim that this is your problem but it’s worth checking if all else fails. Check the float level. The float lever should be parallel to the float bowl gasket surface. Mikuni calls out a spec of 15-17mm from the gasket surface but this measurement usually works out to be setting the float lever parallel with the gasket surface. If the needle valve has a spring loaded pin, adjust the float lever so this spring loaded pin is not depressed when you determine the height. Adjust the height by bending the tab that contacts the needle valve. Make sure there’s no debris in the needle valve and that it’s preventing fuel flow when closed. Standard Mikuni VM26 jetting is 190 main, 60 pilot, 182 N8 needle jet, 5F21 needle, 2.0 slide cut and .5 air jet. The standard spec is usually pretty close for a 2-stroke, but probably rich for a 4-stroke. For what it’s worth, here are some jetting specs I have recorded or a VM30: 190 main, 25 pilot, P6 needle jet, 6DH4 needle, 3.0 slide cut, 2.0 air jet. The standard jetting for a VM30 is richer(250, 40, P5, 6F5, 2.5, 2.0 respectively) so if your VM26 has standard jetting, it too is probably rich. Only knowing your main jet size (190), you’re running the same main jet in a 26mm carb that is called out for a 30mm carb that’s been tuned to a Ducati single. Going by other symptoms you describe, it sounds like the carb is set too rich. Also, in order to determine what needs correcting, the throttle setting is important to record, since different metering circuits affect certain throttle openings. It’s challenging to correct jetting by describing symptoms but hopefully the above will set you in the right direction. Matt

[graeme]

Jim, just a thought, are you adding upper cylinder lube or lead replacement or anything else to the fuel?
I've seen additives do sooty things to plugs and make engines run poorly.
graeme

[JimF]

I had come to the conclusion that "lever up" was enrichner off, but I didn't want to sway the forum's responses.

I could put an SSI29 on the bike that I would build up from parts, but:

A) I like that this Mikuni is fitted with a Scrambler air filter

and

B) I would be starting from scratch with a new carb which lays to waste all the effort I have expended so far and puts me further behind in the hope that I could tune the SSI from scratch faster than I can refine this Mikuni.


I think I will take the Mikuni off and tear it down a bit. I'll check and record all the jet/needle numbers and I will make sure the enrichner plunger has a good seal and that it reaches the bottom of the tube.

I don't have any fuel running out of the carb so I think the float valve is OK but perhaps there is a setting for float level that is too generous.

I'll also try to use compressed air to purge the fuel passages.

If it still runs rich after that I expect I'll run the engine without the air filter attached to see if the unimpeded air flow doesn't balance the mixture.

Thanks everyone!

[ecurbruce]

Jim,
Have you tried depressing the enricher lever while you're riding with throttle open, say 3/4 throttle?
It'll tell ya if the enricher is working, and give ya an idea if you're rich or lean...
BTW did you say what size your mikuni is?

Bruce

[JimF]

I don't know what size it is. The most recent time that I had it detached I looked and could find no markings anywhere on it. I intend to measure it this next time I go through it to determine its size.

I hate to sound ignorant but Is it the bore of the slide, the intake bore or the outlet bore (I presume it's the same as the inlet) that defines the carb size?

[DewCatTea-Bob]

" I don't know what size it is.
I intend to measure it this next time I go through it to determine its size.
Is it the bore of the slide, the intake bore or the outlet bore (I presume it's the same as the inlet) that defines the carb size? "

____ The 'size' of a carb is the diameter within it's inlet's throat-passage at the point where the throttle-slide blocks-off the throat-passage, however the outlet-bore size (where it's easier to measure the actual diameter), should be the same diameter-size.

____ I'm certain I recall that somebody-else registered here was once -(a couple years back) also going-though trying to 'jet' a Mikuni-carb for their Duke... And I believe I recall agreeing that finding reliable jet-specs for MIKUNIs on Duke-engines is sketchy, but that finding them for XL250 Honda models, is far more common & much better established ! _ And I had then advised that the Mikuni-jetting recommended for the XL250 would be the most perfect starting-point to begin-from, when jet-tuning the Mikuni for a 250-Duke.
It should be relatively very easy to search the w.net for Mikuni-jetting part-size numbers recommended for XL250.
__ Also, I found a jetting-recommendation for a Mikuni on a 250-BSA (which has a bore & stroke of 67x70mm [unlike XL250 & 250-DUKEs] ), which ought help confirm proper jetting-sizes for your project. - LINK: http://www.b50.org/phpBB2/viewtopic.php ... 166#p24413


Hopeful-Cheers,
-Bob

[ecurbruce]

Jim,
Have you considered whether you trust the coil on that bike? Could it be the duty cycle is not taking it past 6000 RPM?

My (adult) son, Mike, the motocross racer in the family, was telling me about a time he was trying to re-jet a mikuni carb between race heats, (similar symptoms, cutting out at high RPM, and under load,) only to find out all was solved by replacing the CDI.
I don't know if it pertains to your situation, I'm just saying...

Bruce

[JimF]

Thanks Bruce,

I surely have a spare coil somewhere and at this point it's worth a try.

I did try running without the air cleaner to see if more air would lean it out and get me above 6K, but this proved fruitless.

One other question I have pertains to the intake tract wich is a rubber tube, approx 2" long. I've not seen this done before. And now, considering the thing never ever got over 6000 RPM for the previous owner, I can't trust anything.

intake tract.jpg

Is there any possible relationship between my rev limit and this style/length of intake tract?

Thanks again.

Jim