ATA-250D with Mikuni 30mm - what jets you running?

[Weldangrind]

The 30mm Mikuni that I'm familiar with has the pilot mixture screw just under the transition from the carb to the intake tube. When the screw is in that position, it controls fuel.

On a typical two-stroke, the screw is at the air filter end of the carb, and it controls air.

[Rule308]

The flexible R&D fuel screw didn't seem to want to screw in though the thread pitch and diameter looked identical and seemed to line up with the stock screw side by side, you could feel the number of threads were the same, I think, but when screwing it in it caught and went in about 1/2 turn then stopped, giving me impression it was going to cross thread, so didn't try to push it. It said it was for the Keihin FCR carbs but looks like it will be correct for the Mikuni VM26 PZ30.

Used the extended pilot screw version instead, and had a ton of spitting through carb and sputtering, eventually walked the pilot jet size to largest I had which is the 30 pilot (slow jet) and I'm ordering 35, 40 , 45, 50. Seems like the larger pilot jets finally got it running fairly well.

Does running lean sound louder, noisey, and rich sound lower and more quiet?. The reason is, the ATV has been getting quieter as I go up in pilot jet size. Think I still have to go bigger. Plug looks lean and too clean. White and clean, way to clean. Using new plugs every few jet changes.

What theoretical pilot size should I be running at ~80 degrees, 200 above sea level, K&N with UNI sleeve, add on Walker Sound FX muffler after the stock system connected with flex pipe. This is the 230cc Rhino 250 ATA-250D.

Remember this is the odd ball VM26 PZ30 30mm Mikuni carb.

[Rule308]

Originally Posted by Weldangrind

The 30mm Mikuni that I'm familiar with has the pilot mixture screw just under the transition from the carb to the intake tube. When the screw is in that position, it controls fuel.

On a typical two-stroke, the screw is at the air filter end of the carb, and it controls air.

Is the VM26 PZ30 meant for two stroke engines or is it specific to chinese ATVs mainly now with maybe a history on an older model name brand bike?

I thought I read somewhere this carb was used on old Honda bikes. Not sure if 2-stroke or 4-stroke version.

Since you know this version is a fuel screw I have it unscrewed probably too much to get my fuel flow I need, so I'm ordering some larger pilot jets.

[Rule308]

This VM26 PZ30 uses Mikuni VM28/486 pilot jets, N100.604 Main Jets, and I can't seem to find any identification for this Needle Jet.

Any idea who sells the other sizes of this?

[Weldangrind]

The only source I can guess is www.jetsrus.com

[Rule308]

Originally Posted by Weldangrind

The only source I can guess is www.jetsrus.com

They said they couldn't get this one. It has a W on the side and thats all. They have been a big help though and I've got a fourth or fifth order into them for more pilots and mains.

I've been looking for the origin of the VM26 PZ30 Mikuni and the Yamaha TTR125L 2006-2007 uses the same Pilot and Main jets types and the Needle Jet looks to be the same type (maybe another jet setting) but it has the hex top, o-ring groove and ported section just like the PZ30's does. I can't seem to find a source of information or how to buy this jet online except maybe go to Yamaha store and ask them to order the various needle jets in a certain range. Other than the jets all being the same type, overall the carb looks very similar but has a spigot mount on motor side rather than flange.

http://www.amazon.com/Carburetor-Reb...06+TTR125+carb

They use a gasket rather than O-ring. But look, the fuel screw, Main jet, Pilot jet, needle jet, etc all look the same type/family of parts.

[Rule308]

Originally Posted by Weldangrind

The 30mm Mikuni that I'm familiar with has the pilot mixture screw just under the transition from the carb to the intake tube. When the screw is in that position, it controls fuel.

On a typical two-stroke, the screw is at the air filter end of the carb, and it controls air.

I'm still sorting out my pilot size, after making 4-5 orders (still not satisfied with size) I decided to order every pilot jet (and Main Jet) in even 10s up to the max size Jets-R-Us sells so I don't have to keep placing orders except to fine tune. It appears that this fuel or air screw in question is actually an air screw. The typical Mikuni (not that this VM26 PZ30 is really a Mikuni) that has the pilot screw in the rear near the manifold makes the pilot screw an air screw. It meters air to mix with fuel in the pilot jet. I've had my "air" screw unscrewed to allow more air in and it did make it lean and start to cough, and when I screwed it in, it went rich and loaded up. Even the UFO instructions refer to this being an air screw on the VM type round slide carbs. I believe Mikuni makes carbs with both air and fuel screws on different models. From my understanding is if the screw is in the back of the carb near intake manifold (cylinder head side) then it meters air and if its located on the air filter side it meters fuel. I know the Keihen carbs have fuel screws because that is what I purchased that actually fit the Mikuni but as a fuel screw. Weird hybrid Chinese copy. I'll go back out shortly and try the screw in near full in and check plug and also at near full open and check plug and see if I can absolutely confirm this. I've been getting a lot of conflicting feedback from the carb, jets, plug reading, running conditions.

[Rule308]

Just experimented on another Mikuni of exact same model (My Spare) and screwed and unscrewed the air screw in and out and blew into the air pilot in the venturi tube edge, and you could tell (though hard to tell and ever so slightly different) by listening to the air escape the tiny port behind the needle jet (about 3/8" behind it) and the intersection of the rear of the slide and the bottom curve of carb tunnel, with finger covering and uncovering the tiny port you could hear the difference in air flow. So I'm pretty sure this is an air screw.

[Adjuster]

Another way to tell. Fully remove the screw. If it has a needle sharp point it is a fuel screw. If it has a dull rounded point it is an air screw.


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[Rule308]

Well typically going rich (adjusting pilot screw - doesn't matter if its an air or fuel screw) lowers the idle speed and going lean increases the idle speed and that's what I'm experiencing with this screw giving me feedback its an air screw, it is actually a Fuel Screw for a Keihen (bought that because I needed to replace the existing sharp pointed screw but its functioning as an air screw on this "clone" Mikuni. Its got the sharp tip for as a fuel screw and its for the Keihen, but this VM26 PZ30 Mikuni clone had the same stock sharp tip air screw not the dull oval'ed tip. I don't think this clone copy follows what typical.


The needle jet, not the needle, but the sleeve around it that holds the main jet, is another mystery part not found anywhere else s far. The Yamaha TTR125 has a similar shaped needle jet, but it doesn't appear to have the same amount of holes. I believe I'll have to cross drill (has series of cross drilled holes) this part to increase the fuel.

[Weldangrind]

I'd need to see the carb; I'm more familiar with the Mikuni that has a pilot mixture screw under the carb, near the intake tube. In that design, it controls fuel.

In general terms, two stroke carbs have a pilot mixture screw near the air cleaner end, and it controls air.