1983 Yamaha IT490 Reving Out Of Control *Help*

[Zephyr3789]

I inherited this Yamaha from my Father in law and I believe he is trying to kill me! lol jk... Anyways, recently after kicking the bike about 20 times and getting it started it revs out of control until it finally dies. It has happened to me everytime after kicking the bike since.

I have since striped the bike down, but I haven't found anything concrete. I disassembled the carb, checked the needles, floats, throttle cable, carb slide, and head gasket, crankcase gasket, airbox to carb boot, carb to intake boot.

What I have noticed that concerned me was the left side crank case gasket was bad, a cracked fuel line that completely came apart when I removed the gas tank and the bike itself was very low on fuel. I figured these are all things that could cause a vacum leak, but I'm unsure if this would cause the bike to rev out of control???

Can someone please confirm that this is most likely my problem or should I dig deeper, ie checking crank seals??

Also, could a bad reed valve, bad intake gasket, or cracked air induction box cause this??

Any help is greatly appreciated!

Rich

 

[mathd]

This is a 2 stroke model?
Pardon my ignorence about old models.

If the left crank seal/gasket is leaking, chanches are it is sucking air in and causing your out of control rev(this will probably end seizing the engine, no lube/inadequate fuel-air ratio).

Usually a air leak into the crank/top end of the engine will cause this kind of trouble. Cracked carb boot etc.
If the fuel line was that bad, make sure to clean the carburetor and the jets. If a jet is clogged or restricted by dirt this is really not good.

You dont want any air leak to the carb and the topend/crank case (can do a leakdown test). This will cause high rev and seize the engine on a two-stroke. Dont start the engine again before you get repair done to it.

Bad reed will make almost impossible to start.
Crank seal are part of the crank/case of the engine, they must not leak.

 

[Zephyr3789]

This is a 2 stroke model?
Pardon my ignorence about old models.

If the left crank seal/gasket is leaking, chanches are it is sucking air in and causing your out of control rev(this will probably end seizing the engine, no lube/inadequate fuel-air ratio).

Usually a air leak into the crank/top end of the engine will cause this kind of trouble. Cracked carb boot etc.
If the fuel line was that bad, make sure to clean the carburetor and the jets. If a jet is clogged or restricted by dirt this is really not good.

You dont want any air leak to the carb and the topend/crank case (can do a leakdown test). This will cause high rev and seize the engine on a two-stroke. Dont start the engine again before you get repair done to it.

Bad reed will make almost impossible to start.
Crank seal are part of the crank/case of the engine, they must not leak.

First off, thank you for the reply I really appreciate it.

It is a 2-stroke.

I started it maybe two more times after the initial incident, before I started disassembly. Thank god it didn't seize on me. I don't have the money for a rebuild.

I guess I should have had the leakdown test before I broke the bike down. Now I have to replace the seals I ruined if I want to take it in for a diagnosis. Or just replace all seals and the carb boot.

Thank you for pointing me in the right direction.

Rich

 

[julien_d]

there is probably very little fuel in the bowl due to a stuck float valve. When you finally get it cranked it's running on fumes so it rev's sky high. When the fuel runs completely out it dies.

Lean the bike over and see if fuel runs out the overflow tube on the carb. If not the float setting is too low or the valve is sticking.

 

[Eric Kropp]

I would probably replace both seals and since you are that far in I would pop the top end and check the piston for any scoring and that pretty much forces you to replace the head and base gaskets, which can also suck air, 'specially the base gasket. I would check the carb mount for cracks(very likely). If the carb mount has cracks I would buy a new one if they are available and maybe a second one if you plan on keeping the bike. I would check e-bay for one to. They are very hard to come by for some older bikes like yours and mine(IT465). I have three for mine.


Eric<><

 

[beck2222]

Sorry I don't have any more to add as the above posters are much more experienced wrenchers than me, but the same happened to me after tuning up my bike, i hadn't gotten the carb-to-motor boot on all the way and it was sucking air. Great advice up there, i'm sure they'll be able to guide you in the right direction.

 

[Zephyr3789]

I'm much more confident now. Thank you guys for all the help! I have a strong feeling now that it is the crank seals. I'm going to replace all the seals (and crank bearings while I'm in there), including the head gasket and double check the carb boot. I found a few used ones on ebay for about $30, so I'll probably buy one as a spare if mine checks out ok.

I'll let you guys no how it goes when I get the parts and finish the job!!

 

[2strokerfun]

I found a few used ones on ebay for about $30, so I'll probably buy one as a spare if mine checks out ok.

That is a very good idea. It is sometimes difficult to find good condition boots for these older bikes, so I tend to buy a couple of extra off the 'bay whenever I find them at a decent price. Might not ever need them, but they are ready to go if I do. Good luck !!