Vintage Dirt Bike Q & A

Finally got the bike together, long process. The bike was accually a barn find, it had 3600 miles on it and had been really abused. No lights, blinkers, horn, seat was rotted, tires were rotted, and you know the drill. But it had a fresh bore with a Wiseco piston bored 75 over. The bore may have been done yrs ago but it was clean, slick cylinder and no carbon at all on piston. It must have been really bad to bore 75 over with 3600 miles. Want to add a down swept pipe but thought I'd see if it would run before I drop the bucks! Dumped a pound or two of acorns and corn out of the exhaust before installing. Anyway, it started on the second kick, ran like a top and fouled a plug after about 5 mins, put the cap on the second plug, started first kick ran great for 5 mins fouled another plug. CRAP! my luck. Fouled 3 plugs, tranny has not lost any oil, crank bearing keeper plate has been removed so Im hoping new bearings and seals were added with the bore (who knows). Fitted it with a k&n air filter, put a fresh plug in today, removed the k&n and it started with the first crank reved out good, held my hand behind the carb and I am getting some blowback, not alot but I would think you shouldnt get any. Pipe seems to have plenty of pressure out the end. One thing I have not checked is the timing. Anybody got any ideas?

blowback = piston port engine, some is typical.

Timing will usually be noted as: hard to start, no bottom end, won't rev out or kickback, not usually fouling. Im not up to date on the MT but I assume it had points like the CR?
I would run that sucker down the road and clear it out, just letting it idle away won't clear up a loaded up bottom end. You didnt nention if it was smoking out of the ordinary.
What ratio are you mixing at?
Is the float set to spec?
Did you adjust the air screw? (warm up the engine, set high idle to approx 2000 rpm, bottom the air screw then and in or out till you get the highest smoothest idle, note the turns from bottom) In = rich/out = lean.
Nominal is 1.75 turns out, IF you are more than that out you need a leaner pilot, less than that you need a richer pilot.
When adjusting the screw go .25 turn at a time and wait 10 seconds or so for the engine to take the adjustment.
IF no idle change is noted when turning the air screw the pilot cirtuit is clogged and needs cleaning, notice the pilot circuit which includes the jet and passage ways. IF the pilot circuit is clogged it will pull from the main circuit which will be too rich for that range of idle, especially if there is wear between the needle and needle jet, which there usually is on a vintage carb.

This should get you going, if you still foul plugs then I would suspect the trans side main bearing seal may be the peoblem. You will never really see a BIG drop in trans oil since it uses so little but it is enough to load up the botton end. Old seals don't seal, replace them whenever possible.

Good luck.

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1994 AHRMA National Champion Sportsman 125 Intermediate.
1996 AHRMA National Champion Classic 125 Expert.

i will have to drag out my MT250 and do a timing check and document it for you guys.

a plugged exhaust,leaking primary side crankshaft seal,float level set too high or too much oil in the premix or very rich mixture can cause what you are talking about.

does it start easily?

does it smoke very heavily and or does the smoke really stink or is it a normal two stroke smell?

does it have a very healthy spark at the plug?

does fuel dribble out of the overflow on the carb?

if on pre mix what ratio are you running at?

does your engine/transmission oil decrease the level when the engine is running and does the idle speed increase when the dip stick is removed with it running?

air cleaner clean or stopped up?

did you clean the inside of the pipe? i do this with a torch myself when they get coked up.

_________________
(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!!

ratio is 32:1 premix, no abnormal smoke, no gas out of overflow, dont think pipe is plugged, timing seems correct, it starts easy. thx for the input, kinda at a loss!

You aren't at a loss, did you try my suggestions?????


would run that sucker down the road and clear it out, just letting it idle away won't clear up a loaded up bottom end

Did you adjust the air screw? (warm up the engine, set high idle to approx 2000 rpm, bottom the air screw then and in or out till you get the highest smoothest idle, note the turns from bottom) In = rich/out = lean.
Nominal is 1.75 turns out, IF you are more than that out you need a leaner pilot, less than that you need a richer pilot.
When adjusting the screw go .25 turn at a time and wait 10 seconds or so for the engine to take the adjustment.
IF no idle change is noted when turning the air screw the pilot cirtuit is clogged and needs cleaning, notice the pilot circuit which includes the jet and passage ways. IF the pilot circuit is clogged it will pull from the main circuit which will be too rich for that range of idle, especially if there is wear between the needle and needle jet, which there usually is on a vintage carb.

_________________
1994 AHRMA National Champion Sportsman 125 Intermediate.
1996 AHRMA National Champion Classic 125 Expert.