Hi there,

I just added a leaner main and pilot to my 04 beta rev 250. I haven't started teh bike yet, but I wanted to reset the idle and air screw to their default positions. The manual states nothing about how many turns out from fully closed are the default turns for either of these. To be honest, I don't even know *which* of the two screws is which! Is the big fat one in the center of the carb the Air screw? (and the one towards the rear of the bike the idle) or vice versa?

Can anyone help me to determine what are the default settings so I at least have a solid baseline setting for the carb before I start her up with the new jets?

many thanks,

robert

Why are you changing the jets in your bike when you don't know what you are doing????

The Manual does state the air screw should be between one and a half to two and a quarter turns out, depending on the elevation.

I was given these jets from our US Importer, as I'm told that the bikes come jetted a little fat from the factory while the bikes are being broken in, and need to be leaned out after a breakin period. Clearly the throttle response is acting like the jets are too rich.

The screw towards the rear will be the air screw. The larger one should be the idle.

It is interesting that they come rich, it would seem that most (not necessarily Beta) come lean

feetup - when you're looking at the right side of the bike the air screw is the one toward the back of the bike (closest to the air filter). Screw it in until it seats lightly and then back it out 1 - 1/2 to 1 - 3/4 turns.

I'm curious what you're changing from / to on your jet sizes. I too have a 250 Rev and have settled on 145 main, 35 pilot and 1 - 3/4 turns. I'm riding at elevations around 600 feet above sea level nominally.

Download the Mikuni tuning manual for the 'VM' series carbs at this link scroll down to 'Other Motorcycle Applications' then click on 'VM "Round Slide" Tuning Manual'.

What is your location and what changes have you made (and more importantly, why are you making changes?)

Edited March 14, 200422 yr by clarkp

It is interesting that they come rich, it would seem that most (not necessarily Beta) come lean 

JTT - I haven't figured it out yet, but it appears that the importer (and most Beta dealers) suggests that owners fit leaner jets to their Betas - or it seems to be the prevalent thinking among clubmen. My guess is that stock Betas probably don't feel as 'crisp' as, say, a Gas Gas or Scorpa - so the dealer / importer suggests leaning them out so they will feel 'snappier'. Gas Gas machines are everywhere, so if you can't beat 'em, join 'em - make the Beta feel like a Gas Gas and sell more bikes... that's my theory and I'm sticking to it...

I'm sure Ron Commo knows what he's talking about...just seems odd.

Is that blind faith or personal experience? There are those who think Ron's interests are purely self-serving. In my experience Ron has been helpful and accomodating.

And as you say, it just seems odd... Ron usually recommends a 27.5 pilot where I have moved my bike the other way - I've tested up to a #40 pilot and have backed down to the #35 - maybe a little fat, but I have happy mains.

Thank you all so much for your helpful replies. I figured out that the air screw was in the rear. Sorry for my otherwise naive questions re: air screw; it's probably been 3 years or more since I jetted a carb. Before the beta, I owned an 01 montesa which required no jetting mods. Before that I owned mx/enduro bikes and those all required some fiddlin here and there.

OK, so I made the changes today. The factory jets on my beta 250 were a 150 main, and a 35 pilot. The Jets Ron Commo hooked me up with are a 145 main, and a 27.5 pilot. The difference? WOW. significant. Throttle response is immediate. You get an instantaneous and smooth reaction, even from the most minute crack of the throttle, even in rappid succesion. Can you say "za za zing!" ...

I've always found in the past that the 1st 1/8th of the twist of the wrist was always the most difficult to achieve a perfect response; in trials no doubt that little area is very significant when riding sections. The jetting changes I made today make the beta respond with knife like precision. Zaps became much easier where timing and throttle are critical. If there is one downfall, is that these jetting changes and super fast response keep you on your toes! I noticed that as I became fatigued, I had to pay more attention to what I was doing. That snappy/precision response can bite you if you're not fully committed to each and every move. You don't want to get sloppy. Before with the fatty's in there, there was a little heasitation in the throttle right off idle, which almost acted as a slow throttle. I can't use the slow throttle because I lack the dexterity in my wrist. Besides with that black throttle, I always found that I was 'late' with my timing on obstacles because I couldn't get used to turning my wrist that far to get to the meat of the power. I definitely like the fast (aluminum) tube better.

I should mention that this setup for me is at almost sea level, but Ron says this should work pretty well all the way up to around 3K feet.

Many thanks again to all, hope to see you out there. Keep your feet up! (or at least keep the bars higher than the rest of the bike...)

-robert

Almost all bikes come rich from the factory as far as the main jet goes. Depending on altitude and who you are talking to it can be quite a difference of opinion (preferance) on the pilot jetting. At 6000 ft I run 130 main 27.5 pilot. Same jetting Zero, Techno, All the Rev 3`s.

Is that blind faith or personal experience? 

I have no personal experience with Ron, but he seems well respected in the NE and surely has considerable experience.

I personally NEVER take jetting specs on blind faith. I've always found what works for one person in one circumstance, often is nowhere near perfect for another.

Joe, you know you could probably make a tidy little sum selling those '...save the two stroke' t-shirts. Where do I send my money for one?