Carb issue

[milesmiles]

A little background. 98' MZ 660. Uses what basically is a xt600 carb.

After a several attempts I can get the bike running, can't turn On the choke it seems to kill the bike. The bike doesn't idle well, until its warm I have to give it a slight amount of throttle. It still dies sometimes

So I was adjusting the ..pilot air screw? And it didn't seem to matter. Matter of fact it came all the way out and still ran. Now I can put my finger over the whole and feel suction, but even if I keep my finger on the hole it doesn't change anything.

So there you have it! Yes I've cleaned the carbs. I'm kind I don't know what to do now

[Dragonfly]

You have a vacuum leak somewhere.

[Dragonfly]

In other words "Its getting air from somewhere else".

[milesmiles]

Ok good good good that could very very likely be it. Now time to start taking apart again

[fixxervi6]

I have a vacum thingy if you want to test your lines

[shilka99]

Is your carb a standard XT600 or is it an aftermarket twin choke carb?
If the pilot screw has no effect, the pilot jet is too small. This bike came from out of state right? Its probably tuned for a colder climate and isnt happy running in this heat (air loses density as temperature rises).
Typically, if the pilot screw has no effect it means that when it comes all the way out it still isnt flowing enough air to lean the mixture enough. A larger pilot jet will flow more air with the screw out. The right size jet should make the mixture too lean after more than 3-5 turns out thereby killing the engine.
A larger pilot jet will allow you to adjust idle speed accurately. You want to get the highest rpm possible with the pilot screw then adjust the idle speed externally.
If you want to check for a vacuum leak, get a MAP gas cylinder (the type used to solder plumbing joints) and turn it on without lighting it, then point the gas at the carb mount rubber and any gaskets or o-ring joints. If the engine revs up at any point thats where your leak is. (Make sure you do this outside. And don't light up until you're done...)

[milesmiles]

From pictures It's exactly the same as an xt600(slow and fast side). Aftermarket exhaust and the pilot screw is bigger to make up for that. Maybe I need to get back to the basics. And start all over again... with factory settings, but from what I understand the bike was running just fine how it is now. If I did go back to factory settings the jet would be smaller. (That shouldn't help it)

So lets say I go to a bigger jet....next size up or what? I've really never done this, but what you're saying makes sense to me.

Ugh...this thing is getting annoying taking apart lol

[Grinner]

Welcome to the fun world of wrenchin!

[milesmiles]

Just a little side note, I took it around my apartment a bit...ooohhh so fun

[Striple]

Just a little side note, I took it around my apartment a bit...ooohhh so fun

To the bathroom and back to the kitchen? Full throttle?




PS: Sorry, just one of those days...

[milesmiles]

I can do it

[shilka99]

Remember theres a big difference between main jet and pilot jet. The pilot jet adjusts the mixture at idle only. At idle, the aftermarket exhaust will have little to no effect on jetting. Going to a smaller pilot jet will restrict air to the mixture making it richer. Its already too rich, which is why the choke kills the engine. (the choke makes the mixture richer. Too much richness kills the motor)
You need to get more air into the mix to lean it out.
Once you find a source of pilot jets, get a few. They are small and cheap - cheaper than shipping if you only buy 1 or 2. I would get a few mains also in case you need to adjust jetting once the engine is running reliably. 2 sizes richer and 2 sizes smaller are always good to have. Thats just my recommendation.

How does it run at mid to full throttle?

[milesmiles]

Seems to run fairly good... Maybe hesitant a little. Hard for me to say as I just got it back together tonight and haven't fiddled too much with it.

[shilka99]

Here's a great demonstration of stoichiometric mixture vs too rich.
Remember, an engine wont fire if it has too much fuel.

This video shows perfectly what happens when there is too much fuel in the engine. (fast forward to 1:45 for the slow motion, but the whole video is fun)


The fireball only happens where the mixture is right. Notice all the unburned fuel droplets below the flame. This region is too rich. The fuel didn't atomize so falls away unburned.

[milesmiles]

Ugh what does it mean if i can turn the fuel mixture screw All the way in? If i start backing it out it seems to run worse. On that note....which way should i adjust the carb needle, up or down?

[shilka99]

Remember you've been having problems with excess fuel in the mix. winding the screw out - just about any screw valve - opens the orifice more allowing more fuel in. And as we already know too much fuel will kill the engine.

The best way to go with this carb is back to stock. Most stock needles arent adjustable so if yours is that means its probably part of an aftermarket jet kit which may confuse things a little. The needle could even be worn - the taper could have a shoulder in it that prevents a smooth transition in mixture throughout the rev range. that could/will throw another wrench in the works.

If this one is adjustable, moving the clip higher will lean out the mixture. Adjust it in stages and go for a ride after each adjustment and see what happens.

[LeanHard]

Im not sure what type of carb you have but Carbs will have 2 types of pilot crews. Air screw or fuel screw. If it was fuel and you turned it all the way in it wouldnt idle. If it was air it would run poorly and run lean. They have opposite effects. If the bike is turned on and it starts running high rpm without the throttle being opened you have an air leak that is "uncontrolled". This meaning its on the engine side of the carb, usually the intake boot cracked or bad seal. You can usually find the leak by spraying carb cleaner or brake cleaner around the suspected leak and if the motor changes in rpm either high or low then you know its pulling air in from that location. If the pilot screw has a spring washer and O-ring with it it is most likely a fuel screw. It is very easy to miss a fuel circut in the cleaning prosses. Keep that in mind. Unless the bike runs decent at idle dont bother messing with the main fuel needle position.

[milesmiles]

yes it is an adjustable needle. I'll start adjusting that and report back. thanks

[milesmiles]

Update: I decided i wasn't making much progress so i was going back to stock jets(as much as i could, one jet i cannot find it, so i'm running a different one that should be better). I have a 135 secondary main jet?, on it. The bike calls for a 130. There's not one single shop here that has a 130, so i bought the closest, 132.5 Popped it in and the bike started right up, but it ran best when the pilot screw was turned all the way in. I leaned out the needle all i could as well. That tells me i really need to find the 130 jet.

I'm moving in the right direction!

[shilka99]

Just to clarify - i said move the clip higher. By this I assumed it was clear that higher means further from the pointy end, closer to the flat end. Is this the direction you went?

[milesmiles]

yep, that's exactly what i did. With the jet being too big i needed to lean the needle out correct?

[milesmiles]

Update: dropped a smaller jet in it and leaned the needle out. Runs like a top, starts easy and has a good umph of power. I need to replace a few coolant hoses and work out a few electrical issues and we should be up and fully running!