Yamaha IT200 Questions - engine running away

Hey everybody. I don't usually post questions like this but I recently purchased a 1986 Yamaha IT200 in pretty rough shape. I have basically zero familiarity with the IT200, but it seems like it has a lot of potential to be a really nice little trail bike so far.

Unfortunately it has some issues. First off, while the bike runs, it always wants to take off as soon as you kick it to life - even with the throttle shut. I have double checked that the throttle slide is inserted correctly. The throttle cable is not sticking and the slide shuts all the way correctly. The slide seems to be shutting correctly (and has been inspected through the air cleaner).

Does anybody have any wisdom to offer on this issue? The carb has been meticulously cleaned but the main jet is definitely too rich for my current elevation in Flagstaff, AZ.

There are two thumbwheels on the side of the carb - one appears to be an airscrew adjustment and the other appears to be the idle speed adjuster screw for the slide stop. The air adjuster I have guessed to be one full turn from the seat. The idle speed/slide stop adjuster is backed out all the way. Theoretically at these settings the bike should barely run with the throttle shut, let alone start running away from me.

Any ideas are appreciated.

Thanks! :lol:

sounds like you may have an air leak somewhere. Most likely culprit would be the intake manifold. Inspect for cracks, maybe change the gasket at the reed cage. Check to be sure the choke is also working correctly.

Yes that might make sense. There is also a funky little box connected to the intake post carburetor. Not sure what it is or what it does as I've never seen anything like it before. Looks like that might be a potential leak source too... It's just called an "air chamber" on the fiches I've been looking at. Can't find any research on the design concept though. Leave it to Yamaha to put some odd intake system on an otherwise simple little bike though. They always seem to have some form of trickery or some gimmick to make more power. :p

Yes that might make sense. There is also a funky little box connected to the intake post carburetor. Not sure what it is or what it does as I've never seen anything like it before. Looks like that might be a potential leak source too... It's just called an "air chamber" on the fiches I've been looking at. Can't find any research on the design concept though. Leave it to Yamaha to put some odd intake system on an otherwise simple little bike though. They always seem to have some form of trickery or some gimmick to make more power. :p

That's the old YEIS, boost bottle. Forgot what it stands for, but it does work to add a little bottom end apparently. My IT is too old for that. You could check the hose for potential leaks there. Check also the base and head gaskets for leaks (more likely the base than head) as well. That's another common area.

That's the old YEIS, boost bottle. Forgot what it stands for, but it does work to add a little bottom end apparently. My IT is too old for that. You could check the hose for potential leaks there. Check also the base and head gaskets for leaks (more likely the base than head) as well. That's another common area.

Can chipped or leaky reeds cause this problem?

Can chipped or leaky reeds cause this problem?

doubt it would cause this particular problem, but malfunctioning reeds will cause a noticable loss in power.

OK I found the culprit today. The base gasket was an excellent suggestion, but it turns out the answer was right under my nose all along. In my years of wrenching I've never seen a bike run with such a severe crack in the cylinder. The crack appears to have started around the left side of the cylinder and snakes around behind the expansion chamber.

Tomorrow I am going to take it apart and see if there is any way I can try to braze it together myself, using an emry cloth to try to make sure the crack leaves nothing for the piston ring to catch on accidentally. I have to see if the cylinder is plated first and if I am able to do it without severe risk of warpage on the mating face for the case. I figure at the least I have very little to lose.

Worst case scenario I will find a junkyard jug and try to make something work.

Honestly though this crack is hideous. I would have never in a million years believed the bike could run with such a crack. I wonder how I missed it on my initial inspection...

OK I found the culprit today. The base gasket was an excellent suggestion, but it turns out the answer was right under my nose all along. In my years of wrenching I've never seen a bike run with such a severe crack in the cylinder. The crack appears to have started around the left side of the cylinder and snakes around behind the expansion chamber.

Tomorrow I am going to take it apart and see if there is any way I can try to braze it together myself, using an emry cloth to try to make sure the crack leaves nothing for the piston ring to catch on accidentally. I have to see if the cylinder is plated first and if I am able to do it without severe risk of warpage on the mating face for the case. I figure at the least I have very little to lose.

Worst case scenario I will find a junkyard jug and try to make something work.

Honestly though this crack is hideous. I would have never in a million years believed the bike could run with such a crack. I wonder how I missed it on my initial inspection...

:p but at least you found it. The cylinders on these old bikes are not plated. You'd be better off sending it out for a new sleeve or like you said, searching for a used jug. If your repair doesn't work, you'll be out a piston kit at least as well as a jug.

Well I say I have little to lose because even if I do get a used jug for it, I'd probably have to bore or at least hone that jug, and no sense doing that and just leaving the old piston.... Hopefully I don't mess up badly enough to saw a crankshaft but I'm really not too concerned. If I break it, I'll fix it again.

I figure I'll give a shot at stop drilling and brazing the crack myself, rubbing the cylinder wall down with a an emry cloth and possibly a hone before putting it back together. I've never tried anything like it before and it seems like it might be fun to attempt.

I suspect that the crack is in fact entirely south of where the piston ring travels although removing the expansion chamber and degreasing the entire area should reveal whether my suspicions are true or not. If true, it means the risk of drilling and brazing the jug are quite minimal, meaning that I will go for it - albeit I will probably reassemble with thicker silicone gasket material rather than an OEM cylinder base gasket.

I'll post back whatever I decide to do though. Too early to say for sure, and I am mired down in college homework so I probably won't do anything until this weekend. It really seems like the little bike has a lot of potential to be a really solid trail machine though when I'm through with her. There aren't many other glaring mechanical flaws besides the missing front brake caliper, aforementioned jug crack, and the need for a new throttle and new clutch cable. The levers and bars aren't bent, the frame is straight, while it isn't pretty, all the body work is there and intact. The chain, sprockets, wheels and tires are all in decent shape - minor bent front wheel but I don't think it's bent badly enough to affect anything. The bike goes through all its gears and the clutch works quite nicely. The lighting system even works!

I've owned several IT 200's. One of them, I was riding back to the truck when it started vibrating badly. Stopped to check it out, the cylinder had cracked from the front to the back on one side, down very low. I considered having someone weld it but just bought a new cyl/piston kit instead. It didn't go through the cyl. wall in my case. Good luck with the repair, those are really good little bikes.

ahhh the good old YEIS or Yamaha Energy Induction System i believe. if i remember correctly it is a relative to a power valve. from what i've heard it gives a shot of power to your low end when it needs it. my It-200 is a great trail bike and so far has never let me down i wish you the best of luck for you and your new to you toy.

Well I did not end up brazing the cylinder myself but rather I am attempting to located a junkyard jug and piston to throw in the bike. Temporarily, with all new parts from Japan I am shifting my efforts to my 1974 Suzuki TS 125. I seized it up back in August riding in the desert when I had an air cleaner unexpectedly go south on me. Just got in from pressing the main bearings and getting the bottom end (including the transmission) all back together successfully. The top end will be done within the week and hopefully get her back on the trail so I can focus on my IT. It is actually not so much my IT 200 as it is temporarily mine until my buddy can afford to buy it from me, as we agreed when I bought the bike that if it works out it will become his first trail bike so we can go riding together (I have a sum total of 6 other bikes anyway).

I've heard from an IT200 guru that the cylinder cracking is common, and is due to the upper motor mount that hooks the head to the frame. He very strongly suggests removal of that mounting bracket completely... fwiw.

I had an IT 200 back in '85. I installed a 240 big bore kit in it and could pull the 250s on the straightaways. During a local grand prix, I came down off a jump and the bike started running like crap and idled really strange. I had pulled the cylinder apart right where it bolts to the engine cases. I had the cylinder heli-arc'd (The cylinder is aluminum, can't "braze" aluminum) back together, reassembled it, pressure tested for leaks, and then left the upper engine stay (bolts to the head) off. Apparently when you hammer those things really hard, the frame flexes a bit. If the cylinder head stay is in place, it literally pulls the engine apart, the cylinder pulls off the cases. Removing that upper mount hid the fact the frame was moving around. I loved that scoot, could out turn the bigger bikes in the tight stuff, and at a minimum keep up, or pull them on the straights....way too much motor for the suspension...They still use that basic motor in the Blaster quad.....

Sounds reasonable to me. I was planning on trying to engineer an iso-mount system with some rubber suspension bushings from a car but perhaps just leaving it off completely is best.

The update is that I put a junkyard jug and piston in it. I still have much much more to do to the bike before it will truly make a good trail bike, but at the moment I have paused my restoration to focus on paying down school debts. By winter time though I should be well on the road to making the bike into a really great trail rider. :crazy:

The YEIS Yamaha Energy Induction System was designed to smooth out pulses in the intake tract flow at low and mid rpm to broaden and smoothen the power.

I believe those old Answer Roost Boost bottles so popular in the 80s and early 90s were closely related to this concept.

my IT-2

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Mine is nowhere near as nice as that one (you're look worn but not torn).

My IT is an '86 so it has a front disc brake. Among the major issues left to fix are:

1) New reeds - teh old ones are really worn out.

2) Shock rebuilding?

3) Intake manifold - the old one is too hard to use anymore

4) Front brake - somebody removed it. It will need a caliper and line at the minimum - the mast cylinder is questionable.

It was a $200 project. Put about 160 worth of cables and hardware into it and about 200 into the top end. I anticipate spending another 200-300 to make it a good rider with a combination of new and junkyard salvaged parts. The body work will always be pretty ugly on it though I think...

I'm just wondering how this project is going and to say that that top mount is important.

It's all that kept the jug on my bike when all the base bolts shook loose.

On 3/2/2009 at 10:16 AM, Helmet_Hair said:

That's the old YEIS, boost bottle. Forgot what it stands for, but it does work to add a little bottom end apparently. My IT is too old for that. You could check the hose for potential leaks there. Check also the base and head gaskets for leaks (more likely the base than head) as well. That's another common area.

Yamaha Energy Induction System. Like all boost bottles it stores atomized fuel, which translates to more linear power delivery. 

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