Vintage Dirt Bike Q & A

Hi, can anyone explain what difference to performance & power delivery fitting a 38mm mikuni on a 79 cr250 makes as opposed to a 36mm and which is best. Is fitting a flatslide mikuni or roundslide better on this model & why ?
Any insite into the misty world of carbs would be very much appreciated.

Well I have seen one on a cr250m from 74 so it can most likley be used on a 79 as well. I dont know but would imagine the 36 would be a little better on the bottom end as its got a smaller venturi than the 38 so its gonna have a little better vacum effect at low engine rpms. The cr250m I saw was a race bike and so the guy probably wanted a lot of top end and went with the 38.

Woosh is absolutely right. A few other points to consider involve the intended use
of the bike. If you plan to race it in sanctioned vintage events, you don't have to
worry about flat vs round slide because I've been told that anything other than the
stock shape (round) is not allowed. By the way, flat or oval shaped slides are good,
D shaped are better and flat with bat wings are best (Keihin air stryker, or Mikuni
TMX). If you just want to ride and have fun and are looking for increased reliability
and better tuneability, any modern carb would do that for you.

The stock carb on the '79 was a 36 and the bike had a very good (for the time) mid
and up powerband. A 38 will improve the up portion of the powerband at the expense
of everything else. Unless you race expert or pro, or spend a lot of time at full throttle
in the desert, I don't think you will like the new power placement.

Manufacturers didn't start running 38s on the 250's until they introduced better reeds
(mid 80s) and perfected the power valves (late 80s). These devices solved the power
placement dilema by finally allowing the engine designers the flexability to produce
more than enough power (for mere mortals) from bottom to top.

dogger

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1973 CR250M
1979 CR250R
1993 CR250R
2008 CR500R
2009 CR250R

Hey, I wanted to revive this thread with some similar questions.

I have a 74 MT250 that is getting a CR250M top end transplant. I also will be adding a DG radial head and have a PFR expansion chamber exhaust tuned for a CR. I'm going to use it for ripping around town and so will not likely be winding the engine out for extended periods of time, nor will I be racing so don't need to worry about the whole period-correct issue. I am pretty much sold on the Mikuni TMX carburetors that come in 34mm, 35mm, 36mm, and 38mm.

What size would be best for my applications?

Thanks,
Alex

if you plan to ride it on the street,i would go for the 34 mm unit.it would be a lot easier to live with.the 73-74 CR250 came stock with a 34 mm Kehin on it.

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(2) 1968 DT1 1970 RT1M,1970 DT1C 1971 RT1B Enduro 1974 MT250 1974 DT250A (2) 1975 DT400B 1974 DT100,1978 DT175
1975 CR250M1 1979 CR250R 1979 Kawasaki KZ400H1 Jesus is Lord!!