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74 mx250 carb run ability issues

16 Oct 2020 19:41
Vintagecycles23
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74 mx250 carb run ability issues #1
Hello. I have a great frame off build. The bike was like a slug always with the stock carb so Grabbed a 35 pwk airstyker carb and put in on the bike. It’s really has not changed anything. It’s is peppy and runs a bit better but still pretty dead at wide open, really hard time getting into the power band. Barley even gets into the power range where the motor is ringing. Brand new motor fresh reeds, fresh exhaust, fresh modern electrical system. Still fun to ride just has no guts as a racer at the moment. Maybe I’m asking to much comparing it to my 74 cr250m ?

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16 Oct 2020 19:47
Vintagecycles23
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74 mx250 carb run ability issues #2
Again I know jetting specs would help, but I have yet to order some jets and open carb, I plan to step the main jet up a size or two. Just a curiosity if the sluggishness I’m chasing is being diagnosed correct. I ordered carb new from sudco, they jetted to best of what they guessed it would need.

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16 Oct 2020 19:51
MarkT
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74 mx250 carb run ability issues #3
Out of curiosity, what ignition system are you using?

1963 YG1-T, 1965 MG1-T, Allstate 250, 1970 CT1b, 1971 R5, 1973 AT3MX, 1974 TS400L, 1975 RD350, 1976 DT175C, 1976 Husqvarna 250CR, 1981 DT175G, 1988 DT50, 1990 "Super" DT50, 1991 RT180, 2017 XT250

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17 Oct 2020 14:44
RT325
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74 mx250 carb run ability issues #4
I had a New CR250M & also a New MX250, we sold both brands at work. They're equally good as each other on power even if they do deliver the power in a different way--Both get of their arse!!. So-- you have a new pipe but are you using an old tail muffler with an overly restrictive spark arrestor & just strangling it. Other than that, as MartT says above, what ignition do you have & is it working right, where's the timing firing btdc, things like that. Is a video a possibility at some stage. Carbs don't wear out much except for the slide needle jet & needle. Nothing else moves. If needle jet is egg shaped worn & needle worn too then it'll be rich & blubbery about half throttle or slightly less.

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18 Oct 2020 07:35
turbodan
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74 mx250 carb run ability issues #5
Carb swapping always requires major rejetting. Sometimes you can't even get the bike to start with the best guess jets installed.

At least it runs, you're doing pretty good there. You might as well order jets in both directions, leaner and richer.

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18 Oct 2020 07:49
mdscott
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74 mx250 carb run ability issues #6
Hello, did you ride the bike before you rebuilt it. Like MarkT says, ignition and those are prone to have troubles. I take it you have checked the basics, timing, air filter, muffler plugged. The exhaust looks cool, did you make it, what brand.

Thanks Mark
Roseville, Ca.

'76 Bultaco 143 Frontera
'06 GL1800
'01 KDX220
'68 YG5T 80 Trailmaster
74 DT360
75 Norton
73 CB500K
63 C200
87 PW50
64 McClane Edger

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18 Oct 2020 23:25
Vintagecycles23
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74 mx250 carb run ability issues #7
Hello everyone. I purchased this originally as a beater vintage racer, i since did the restoration and took it all the way. the exhaust is the circle y or circle f if I am not mistaken out of AZ who builds them, the ignition came with the bike, it is some after market ignition system there is a logo for it on the back fender I can check tomorrow but it is a modern update ignition system, the motor is brand new intake and all is fresh, new reeds new bottom and top end. I just find the bike to be slow and never in the power band. Almost feels like a slippery clutch but the motor never wines up enough to even mimic the power zone. Also, kicking it it has compression sure, but my cr250m bout broke my foot when I kicked it in vans, this mx250 I can kick it in vans all day, does not seem to have the same crazy compression or any kick back.

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18 Oct 2020 23:27
Vintagecycles23
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74 mx250 carb run ability issues #8
Bike was in good condition when i got it its just always seemed so slow and sluggish, I ran a hillclimb race on it and was almost a joke racing agaisnt the other guys, this year i did same race on my cr250m and i ran times equal to some of the 450 riders ! i want this mx to wake up ! i really want my little nephew to start racing with me on it.

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19 Oct 2020 01:53
RT325
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74 mx250 carb run ability issues #9
You say no kickback, not that it really should with cdi retarding for starting on the MX. The CR originally had points so timing is fixed & would kick back if you didn't kick it good. I think you said you have an aftermarket cdi system on the MX so would be good to check the timing with a strobe light to see if its close to correct & not way retarded. Not sure what it should strobe to but if it's not got the ability to alter then i suppose about 2.9 btdc like say the 70x64mm same bore & stroke as the earlier DT2MX with a small rotor points mag similar to the CR250M Elsinore. Should be every bit as good as the CR250M.

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