I recently resurected A 1979 yamaha YZ 125,runs but falls on its face,stumbles when given full throttle,it's b

Question

runs better when cold yet still has no power to burn out from a stand still fine on the power slides into a turn though,so long as you gently give it throttle,gun it and she'll stall,I have no idea of the specs so its all guesstimating float levels and neddle screws ,and the fuel mix i have heard of is 20:1 lots of smoke and oil dripping from exaust ,inside of the cylinder looks good no deposits or scratches,any suggestions folks would be greatly appreciated

Answer 1

28 yr old ?

Sounds like the place to start is with a genuine carburettor cleaning.

Runs better cold?,,cuz it's not metering enough fuel

Have to feather throttle?,,thats a plugged up emulsion tube/nozzle/Circuit.
No Transition delivery,no intermediate range fuel,you're babying it into main jet range

Float level is not real critical on those,,it's got an over-capacity to accomodate a bit Low level,,,and it WILL overflow if it's any much high at all.
IF the float arm hasn't been mangled over the years,,,the valve tab on it being set "straight across" is usually very close.

If it was set TOO low,,or parts upside down,,,it's not likely it would rev up much at all for starvation

You can try just removing main jet,then the nozzle that screws into.
(*It pushes UP thru the carb after main is removed)
See how much crap is coating that.
Realize it has a series of holes so small they're almost invisible when plugged up.

Cleaning that alone may do it,,,but it really should have a proper cleaning

Careful not to damage it,,,very sensitive part.

Do need to cut fuel blend in half of what you're using.
3~4 ozs/gallon Max,,,32~40:1,,,,depending on what oil you're using

Good Luck with it

Answer 2

Hey I used to race one of these when it was new and I was young! They were great looking bikes.

There are many places you need to look.
1. Do a compression test when cold and when the engine is warmed up. Smaller mx bikes eat rings so they may be worn out.

2. You should really have an air leak test done. This is done by removing the exhaust pipe and carb and pressurizing the motor. Leaks in the seals and gaskets will show up. The motor is old and probably the seals are shot. If the clutch side seal is bad it will draw in tranny oil when warmed up. If the left seal is bad it will run lean and burn down.

3. When this bike came out 20:1 was common because most oils sucked. You can't mix many of the new oils at that ratio without the entire top end gumming up. Use only premium fuel without ethanol and follow the ratio on your oil bottle. 32:1 - 40:1 is probably going to be in the best range for this bike.

4. Your float level may be too high. Most mx bikes in this era are probably in the 3-5mm range. The only true way to check a float level is with a gauge attached to the bowl that shows the actual level of fuel. If you have a drain on the bowl you can often make your own with a clear hose. Hook up the carb to a fuel source, open the drain and when the gas stops moving up the hose measure the level from the top of the bowl.

If you go through those steps you should be able to fix the problem and be off and running. I'd pick up a used manual on e-bay so you have all the specs. By the way, the clutches in those sucked so check that out if you end up pulling the clutch cover to replace the right side seal.

Answer 3

It sounds like the jetting is off. Also 20:1 sounds like a lot of oil. If you are into the bike you should invest in a service manual. Anyway, things like running well only when cold or hot and only in certain rev ranges is usually the jetting. If it runs better cold then it is probably rich, but you need to get the oil mix correct first because that has a direct impact on the air:fuel ratio. The higher the proportion of oil, the less fuel there is, so you would need a leaner jet.

Answer 4

20:1 is way to rich on the oil side. Run it at 32:1