New MK2 owner in need of help

[MFerioli]

Hi guys, thanks for taking the time to help.

I have a 1991 mk2 that is really rough to ride. It does this "hunting" or bucking or missing thing when holding the throttle in one position. Makes it really annoying to ride. Sometimes it revs as if the throttle is stuck open, stays at a higher rev even when the throttle is closed. This miss or skip seems to happen to some degree at all throttle positions, I've opened it up pretty far and it still wasn't smooth the whole run. She starts up first kick and seems to idle fine, aside from all the smoke for the first few mins.

Everything is stock as far as I can tell. I've put in new carb orings, replaced both jets with stock, new float pin, and new spark plug. Cleaned the carb and have the mixture screw at 1.25 (reccommended stock setting). I had a look at the reed and it seems fine. The air filter is new as well. the floats seem to bet set at 16mm as far as I could measure. When I pulled the old plug, it was really black, rich I assume, but why. The air box and boots all seem to seal nicely and are not cracked. I also ran it with the petcock on and in the reserve setting just incase something wasn't right, but no help. The power valve thing is moving when I rev it, but I have no idea how much or when it is supposed to move.

This is my first two stroke, I know they aren't as smooth as fours, but I've read posts on here where people say their crm is nice and smooth on the road. I figured I'd give this fix a go myself before its off to the mechanic.

Thanks again for any help, it's greatly appreciated.
Mike

[pudzie83]

sounds to me like a seal is on its way out in your crank pal

[MFerioli]

Would I notice anything else if I had a crank seal going? Like oil leaking out? How does a bad seal affect engine operation?

Thanks for the reply

[knackeredMk1]

There are a number of things it could be. One of which is above. Is the bike modified in any way - exhaust etc ...........

What size were the jets that you took out .

Where about in the country are you based .

[MFerioli]

The bike is stock as far as I can tell. Stock looking black exhaust. The jets were stock, but I replaced them anyways figuring they may be worn after more than 20 years. I actually live in Japan, I'm not Japanese though. I can get stock parts locally, but aftermarket is a bit tougher.

How much affect does the mixture screw have? Will it do anything above 1/4 throttle?

[knackeredMk1]

Mixture screw won't do anything above 1/4 throttle.

Have you checked the oil pump setting .

There may be one cause for your problems but I suspect there are several issues.

[MFerioli]

The oil pump marks line up almost exactly. Maybe a 64th of an inch difference at most. The old plug was really black when I pulled it, not sure if that means it was really rich or if it was oil build up. My symptoms seem to point at lean though...maybe??

She smokes pretty darn good for the first 4 minutes or so of idling, then it calms down to almost nothing. Is that normal?

[fishtail]

sounds similar to something I was experiencing....( ie smoke on starting and fouled plugs... - was ok riding at any speed.. after warmed up )

still dont know If I solved it yet - I took the head off and it seemed like a rebore was a good idea.. waiting for some parts to land to put it back together..

but not sure if that was causing my smoke out and fouled plugs... but it's something that needed doing!

[MFerioli]

I rode it yesterday and things seem worse after going through the carb thoroughly. Same as before but now at more open throttle positions, the power just seems to plateau and it sounds like a high speed skipping of some kind.

How do these engines limit themselves? I could only get about half throttle in second gear before I seemed to be at max power and it sounded and felt terrible.

I may pull the head today and see how it looks in there.

[fishtail]

have to say I'm not the font of knowledge here.. ( i come here for help myself! )

seems like things have got worse?

those sound a bit like my failed attempts at jetting......... ie when the jets were wrong wouldnt get power,,, ( wished I'd made notes cos I cant remember which way round it was.. ! ) ( my jetting is far from standard on the main jet )

also what condition is your carb in? I was having issues with my float - as the float pivot pin mounting point had become very large on one side! so float height was inconsistant!

it could be more than one problem at once?

[fishtail]

powervalve video

http://www.youtube.com/watch?v=nY5Ur4Lm6-E

[MFerioli]

Fishtail,
Thanks for your continued help even though you admit to not being an expert, it is greatly appreciated as I go through this learning process.
I pulled the power valve cover again and I'd some revving like in that video you posted. Mine seems to match up with that just fine. I also popped one of those cables off and the valve seems to flop around easily.

The carb seems pretty nice inside, the foot doesn't have much play, seems to move just like any other float I've seen.

I pulled the new spark plug after two 15 minute rides and it is already very black and oily. From all the things I've read that seems like an oil pump issue or case oil leaking by a seal...maybe?

Also when holding the throttle in one position today, I noticed it seems to make a cracking sound a lot at higher revs. Almost like a mini back fire sound or a detonation or something. I dont know how to describe it very well, Ill try and make a video and take a picture of the plug.

[MFerioli]

Should I try needle position before ordering new main jets? how much difference can needle position make?

I assume you may need to change jets on a stock bike when it is twenty + years old and things are worn out a bit. Or is that a crazy assumption? I guess I'm not sure why else I would need anything other than stock jets if the rest of the bike is stock.

Thanks again in advance for everyone's help, I'm already learning a ton from this site.

[MFerioli]

I pulled the exhaust just now and there is definitely an oily build up at the exhaust port, not enough that it's dripping out when the exhaust is on, but it's wet.

The good news is that the cylender walls, head, and piston are extremely clean. Shiny in fact. I didn't expect that at all. The power valve moves full range freely.

Is draining the oil and running premix the best way of verifying an oil pump issue? I've read about people having issues with pumps leaking oil through them when not running. I assume that a pump worn in that way could cause excess oil during running. Or am I wrong?

[knackeredMk1]

Just to chaeck that you are using the 'correct' starting/warming up proceedure. - viewtopic.php?f=20&t=8231

Leaving the choke on for longer than is neccessary can screw things up. Also not reving through (cleaning out) when warmed up. Once you have got it oiled up it can take a while to clean if you are too gentle with the throttle.

Crank seals could definately be a problem.