Vintage Dirt Bike Q & A

Im having problems it is bogging out top end.

Is this a "new" problem, or one with no history as far as you are concerned.
Also, what is the throttle position when this occurs? 1/4?, 1/2?, 3/4?, WFO????
Need mo info to help.
OD

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74 AMPI CZ125
73 CZ400
73 CZ250 Basket case.
72 SherpaT 350
75 Montesa King Scorpion
72 Rickman 125 Enduro
70 Honda CL70
72 Honda Z50
00 DRZ400E Baja'd

I bought the bike 10-04 . it has new piston, reeds, reeds insulator,rebuilt carb, It bogs out 1/2 to wfo. check for air leaks found none. I am thinking of new carb might solve this?

Try running it with the choke on.

If it runs worse, the needle is having problems like not being seated by the spring. Could even be something like the main jet vibrated loose/off.

If it runs better, the main jet is plugged.

I have tryed new jets I have tryed going down and up it did not change .

Have you done a leakdown test to see if the crank seals are good?

What is the compression psi?

Have you checked for good connections at the coil, frame, engine, cdi and a clean flywheel?

Have you disconnected the kill button?

Have you pulled the choke out of the carb to see if the rubber on the plunger is still good and pliable?

All of those items should be done before you bother trying to jet it. An air leak or bad electrical will ruin your attempts to jet it.

Usually, the symptoms you describe can be attributed to two areas. Worn piston rings/excessive cylinder to piston clearance, or leaking crankshaft seals which would add another set of symptoms. Namely, increase smoke in the exhaust accompanied by a decrease in trans oil level and increased fouled spark plugs would indicate a leaking clutch side cank seal. Variable idle speed and slow rev-down along with unexplained dampness in the lower magneto area would indicate a leaking dry - side seal. Also easily checked by holding the mag flywheel in your hand and trying to detect movement in any direction but circular. Any detectable 'clic' of movement indicates bad crank bearings too.
OD

_________________
74 AMPI CZ125
73 CZ400
73 CZ250 Basket case.
72 SherpaT 350
75 Montesa King Scorpion
72 Rickman 125 Enduro
70 Honda CL70
72 Honda Z50
00 DRZ400E Baja'd

True those will make for a bad top end but the bottom will be even worse. He says the bottom is still strong. I've worked on several engines with those problems and most of them had melted from apparently running WOT to get forward locomotion with an absent bottom end.

put new carb runs like champ