anyone know about old dirt bikes?

[danny boy]

howdy all,
i recently acquired a 1978 yamaha 250it dirt bike. it ran for a while (though a little underpowered) and it ran very rich. at a point it simply stopped working; i rode it around the neighborhood and the next time i tried to start it it simply wouldn't start.
i fiddled with the carb and replaced the idle adjustment screw which had been lost. i also worked with the floats a little and the best i could get was the bike started (after a lot of hard kicks) but wouldn't idle. now it won't start at all.
i've never worked on bikes before, so i'm learning and i'm hoping someone can point me in the right direction.
as a final point, the bike was free and i just want a toy to play with, so i'm not realy interested in buying expensive/high coolness factor parts unless someone can convince me otherwise.

thanks for any help,
dan f.

 

[Sephrion]

what kind of ignition is it? if its a "point style" (for lack of a better term) then clean your points and see if that helps... other than that dump your gas clean the carb, check for spark etc... (and im more of a harley guy so im hoping this helps some)

 

[XTrmXJ]

Sounds like you need to do a top end rebuild (ie ring and piston) Rebuild the carb (new jets, and float).. flush the tank out

 

[njedgexj]

im not sure if thats a 2 stroke bike? if it is check out the plug if is was runnin rich its probably fouled...do a compression test on the motor....that could help tell if u need a new top end....im not sure of what the specs should be for cylinder pressure, since 2 stroke motors are different than 4 stroke motors when checking compression......id say around 120 or so psi should be ok....i got a 85 cr125...its around 85psi....make sure the gas is properly premixed..did you take apart the carb and clean good?

 

[danny boy]

thanks for the replies!
i checked compression and got about 140 (it is a 2 stroke).

i think i'm going to start getting all the changeable parts for the carb and rebuild it as best i can.

also, i will check/replace my fuel. hopefully that will cure the problem, though i'd feel pretty dumb if that was the problem all along.

thanks a lot, i'll report as updates arrive!

dan f.

 

[RichP]

Change the plug, those yd's are all 2 cycles. When you said 'old' I was figuring you were talking greeves, bultaco's, hodakas or Jawa's which were what I grew up with. All those bikes we carryied plug wrench and small files for cleaning the plug out or a match book striker in a pinch. Hell, they would carbon up full out on a scramble track and the greeves was hell on trials, had to keep blipping it above 8K just to keep the plug on that 250 from fouling while walking trees.

 

[Pflumingo]

Used to have an 80 IT175, the little brother of yours....

Definately flush out the old gas, Replace the plug (Gapped correctly, not out of the box) and make sure the return spring on the idle jet pin is on the right side of the actuator arm... I have seen several that were put on the wrong side, and they would run rich for a while, then just stop all together, The little spring is very important, and easy to get on the wrong side of the arm... When it stays that way too long, the jet gets junked up, and needs to be cleaned out real well...

That bike is an enduro classic, and as I recall, was pretty tough to beat in it's day when under the right rider.

If you decide you don't want it, just send it my way... I would be happy to drive it around for a while.

 

[Moto]

I am not too familiar with that specific bike, but if it has reed valves, that can cause the lack of power and running too rich. If that bike had reeds they would be mounted attatched inside the reed block which is on the intake side of the cylinder and usually has the carb intake boot going to it. You can check them, they look like little plastic of rubber flaps, if they are good they should have very little or no gap at all between them and the reed block.

 

[danny boy]

rich, i've tried numerous plugs, but perhaps not the right one.

flamingo, i think you're on track with that spring idea. i'm pretty unfamiliar with the carb (mikuni), could you elaborate on that a bit more?

moto, it does have reed valves, but they seem to be in pretty good shape.

thanks again,
danf.

 

[Ed A. Stevens]

The 78 IT250 should have a CDI ignition (no points).

Place a plug into the holder and ground the threads on the barrel while kicking the engine over to see (visual) spark. You do not need to kick hard, just cycle the piston through the kick starter travel.

If you have spark, the problem is fuel (you already mentioned you have compression.)

The condition of the gaskets for the carb and reed valve can also lean out the air/fuel misture. Tighten the screws on the reed valve, and the carb (better yet rebuild).

The 400's had a compression release on the head, actuated by the clutch lever, to make it easier to kick over. If the compression release sticks open it will not start easily. The 250 may be the same (CRS kicking in).

Most likely the idle jet is plugged. The Mikuni is simple to take off and apart. Pull it off and turn it over, loosen the main jet (the brass plug on the side of the float bowl), and pull the float bowl. The float hinge pin will slide out and allow you to remove the float. The three brass pins looking things in the carb body are the needle and seat, and the idle jet and choke passage. The jets can be removed with a small screwdriver. The idle jet has small holes in the cage that plug easily. The main jet is inside the plug you loosened on the float bowl.

Buy a service book as a picture is worth the cost.

The IT250 engine is nearly bulletproof, with a cast iron cylinder liner and a wide ratio trans.

With a little port work the engines can pick up both power and rpm, and not lose bottom end torque with the right reed valve tension. The wilder ported 76-78 YZ barrel will bolt up, as will many of the early monoshock YZ MX race parts. Desert racers would mix the IT, DT, and YZ parts for a competitive bike that could keep up with the KTM's and Maico's.

The early design monoshock is not as sophisticated as later suspensions but it works until it gets too hot (where it will pogo and bounce the rear a lot).

The late 70's DT's and IT's make great trail bikes. I owned and rode a highly modified street legal 78 DT400E for years. It was the last year for a street legal 400 2-stroke, and at under 300 pounds it was the lightest street legal 400 until the late XR400 4-stroke dual sport.

Have fun.

 

[dmillion]

"Old dirt bikes." That's pretty funny! My first dirt bike was a 1965, 350cc Ducati!

 

[RichP]

"Old dirt bikes." That's pretty funny! My first dirt bike was a 1965, 350cc Ducati!

My first dirt bike was a bright red 61 Cushman scooter with a 2 speed suicide stick shift on the tank. Mini copy of a HD superglide with tiny scooter tires.... wish I still had it, it was a hoot...

 

[danny boy]

thanks again guys for the responses!
after my post i realized that 78 isn't really that old, but then again i'm only two decades myself.

ed stevens, thanks for that description. tomorrow will be the day of reckoning. i'll take everything apart, buy a manual, read your guys' responses again and see what happens.

once again, thanks to everybody for their responses, and keep 'em coming!

dan f.

 

[Junior]

squish fuel and fire man

as long as she has those 3 things she'll run, maybe not well but she'll run.

before you go throwing big coin like that around, change the plug flush out the fuel and fill it with premium (you should only be running premium on a motor with that kinda squish, which by the way is higher than stock, somebody has been playing around in that motor) take the bowl off the carb, and give her hell with the carb clean, make sure all the jets flow well and everything slides freely, bolt her back together and give her a kick, should be fine.

I worked in a small engine shop for awhile and now race yamaha sleds semi-professionally, that's a basic tune-up and will solve problems 90% of the time, cheap, easy, effective.

 

[Spudboy]

My first dirt bike was a bright red 61 Cushman scooter with a 2 speed suicide stick shift on the tank. Mini copy of a HD superglide with tiny scooter tires.... wish I still had it, it was a hoot...

I had one of those - it was white though. It was a blast to ride but hard on the knees with that kickstarter under the gas tank. About half my high school would stand around to see if I could actually get it started every afternoon. I too wish I still had it.