Vintage Dirt Bike Q & A

I've raced this bike once, then tore it down some and put new suspension on it and plastics also a seat.

When I got the bike it was running pretty well on pump gas, but I have a hit and miss trying to find non-ethanol gas so switched to AV gas, by the end of the 4th moto when bike started running more on the AV gas it started blubbering and running very rich. When I put the bike back together, it is still doing the same thing and running very rich, will not start and run below half throttle. It had a 178 main jet in the Keihin PE01, I tried a 175 and it is running a little better at half to full throttle, but still blubbering and not sounding clean at all. Let off the throttle and it dies.

My next step will be to take the carb apart and clean all the jets and check the float height. When pulled the plug it was very oily and black. Something is amiss. Not quite sure what..

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Kristi J
1979 CR250 with 43mm forks, Race Tech springs and Gold Valve emulators, Works Performance shocks

might check the float level,it can possibly be the float valve no longer sealing or a sticking float.

as for fuel,i would suggest using 110 octane Race gas and you can get that at a speed shop that sells hop up parts for cars and such.

i had seen some engines let go catastrophically from running AV gas.valves torched off at the stems,things like that.

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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!!

I bought 5 gallons of VP110 and that is what the bike will be running now, no more AV gas. I am going through the Keihin now, hopefully I can get it working some. I am trying to possibly go to Mikuni, at least you can get parts for it, the big thing for it, is the jetting. I know a guy who put one on his 1979 CR125 and seized the bike because it was running to lean.

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Kristi J
1979 CR250 with 43mm forks, Race Tech springs and Gold Valve emulators, Works Performance shocks

Pilot 35
Air screw 1.75 out
Slide 2.5
Needle Jet 159-P8
Needle 6FJ6
Clip #2
Main 360-380
Float height 19mm

Jetting for VM 36mm Mikuni. Unless you are in Alaska or at the top of a mountain, this will get you going...

With a good carb and merely reasonable jetting, you simply don't need race gas for bikes of this age, assuming no high compression engine mods. I ran a 50/50 pump/VP C-12 blend for a while simply because I had a CR480R that did need a blend. The CR250R will be fine on 91 pump gas.

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Michael Stiles
1979 Honda CR250R Elsinore | 2006 Husqvarna SM510R | 2007 Service Honda CR500R-AF

So, let's say you ARE on the top of a mountain -- I live in Golden, Colorado. I'm trying to get a '79 CR250 up-and-running, and it's jetted for sea level. The dirt track by my home (Thunder Valley) is at about 6000'. Got any suggestions for a good starting point. Oh, and we're still talking about a Mikuni 36mm VM. And I'd prefer to use commercial gas, but if I have to, I'll go with VM110. AND, is there a good place to get a rebuild kit for this carb?

TIA!

if you are at high altitude,the air there is thinner and you will need to go leaner on the jetting.i do not have any starting points on a Mikuni as i am still running the stock Kehin carb on my 1979 CR250.

however i had been thinking of running a Kehin 38 mm Air Striker carb and a V force 3 reed cage assembly along with some port cleanup,maybe a aftermarket pipe,perhaps someday.
i know it would not be legal to race with that setup but it should provide many wide eyes and cheap thrills while riding 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!!