FZ750 Super High Idle Problem

[lostinthe202]

Hiya!

I recently brought an FZ750 back from the grave,

I cleaned the carbs, went through the ignition system, rebuilt the brakes, etc etc.

After turning it over about a billion times it now runs but it won't idle down. It's idling around 2k rpms. Further, if I even twist the throttle a single degree, it will act as if I just gunned it reving up to 8 or 9k then slowly settling back down to 2k.

The throttle cable is free moving and not pinched, the idle screw seems to have no effect except to increase the idle even more.

I checked for vacuum leaks on the intake boots by squirting wd40 around each one with no change in behavior.

I can see the choke lever operating the rod back and forth but I don't know what the choke lever actually does inside the carb. When I opened the carbs up to replace the needle jets they were pretty clean, none of the jets (main or pilot) were clogged and the slides moved freely. the diaphragms appeared to be in good shape.

Does this sound like anything in particular? Stuck slide? Stuck choke? Clogged something?

Any suggestions?

Thanks!

Will-

 

[Hellgate]

Have you cleaned and lubed the throttle cables? The high rpm, and the slight twist and the throttle sticking at a high rpm sound like it could be the cables.

 

[lostinthe202]

Throttle cable moves free and easy. What's weird is that I will twist the throttle say 1/16th of a turn yet it throttles up like I had twisted it 1/4 of a turn even though I didn't

Will-

 

[Powerman]

Maybe it has Amsoil in the crankcase.

 

[RJ2112]

The carbs on that monkey are CV, right?

The long delay to drop back down in speed, happened to me on a Suzuki I owned, when I shimmed the carb needles, and put a breather plate in backwards. The CV throttle body thingy is supposed to keep the speed of the air flow through the carb constant. It gets lifted higher and higher out of the way (which exposes the tapered part of the needle) and then is supposed to 'fall' back into place. The little plate I had reversed put a much smaller apature in line to control the rate the body 'fell'.... and it took over a second to drop back down to idle after a throttle 'blip'.

Maybe that's a part of what's happened here.....

 

[lostinthe202]

Maybe it has Amsoil in the crankcase.

Not sure what this is?

The carbs on that monkey are CV, right?

The long delay to drop back down in speed, happened to me on a Suzuki I owned, when I shimmed the carb needles, and put a breather plate in backwards. The CV throttle body thingy is supposed to keep the speed of the air flow through the carb constant. It gets lifted higher and higher out of the way (which exposes the tapered part of the needle) and then is supposed to 'fall' back into place. The little plate I had reversed put a much smaller apature in line to control the rate the body 'fell'.... and it took over a second to drop back down to idle after a throttle 'blip'.

Maybe that's a part of what's happened here.....

Now this made some sense. I was thinking this over and I looked over the spare set of carbs I have to see how things go together and see if there is maybe something I didn't notice that should've been cleaned, etc. I also looked up a "how it works" on the web about CV carbs which mentions the slide needle controlling idle air/fuel mixture. I have no idea if the web link I found applies to the carb style I've got but it got me to thinking. I pulled the slides from the spare carb bank and took the needle assembly out. I noticed that the needle has this little tab that orients the needle the same way every time you reassemble. I can't remember if I noticed that the last time or not (though I didn't miss it this time so I'm doubtful I missed it last time) but it's a lead at any rate. This weekend I'm to pull the carbs and see if perhaps these are hanging open. it's the best theory I've got so far.

I'll report back in case anyone cares.

Cheers,

Will-

 

[RJ2112]

If the needles are attached to the gizmo that looks like a float and are pulled 'up'
out of the jet..... that's a CV carb.

CVs are really a single circuit setup, in my view. Instead of 2 or 3 different circuits that engage sequentially, the slide pulls the needle out of the jet.... it can have multiple tapers, so the point at which the fuel delivery changes can be changed by changing the height of the needle with respect to the slide. 'raising' the needle ensures more fuel for any airflow. A needle that also gets skinnier than the OEM would increase the amount of fuel delivered for any rate of airflow.

In my case, it wasn't the needle itself that was oriented 180° out.... it was a small metal disc that rode on top of the slide assembly. I recall there being at least one diaphragm in each carb that was critical, as well.

If one of the four carbs is buggered, it would be really difficult to tell it was significantly different.... you may want to borrow a manometer to see if the carbs are synchronized. It's possible you'd be able to tell from the spark plugs; if one is much more wet with fuel, you'd know something was different in that carb, at least.

 

[lostinthe202]

Just read this great article explaining CV carbs,

Cvcarbs

I think I must have one or more of the slide needles stuck open. The start circuit plungers are all moving freely, I can see that from outside the carb so I don't think that's the problem. The jetting hasn't been messed with since this was a properly running bike and the throttle plate is moving freely as well so I don't see what else it could be.

I don't have a manometer yet, I'm going to make one of those canning jar deals as described by clymer manuals circa 1970. I had one years ago and it worked well (with 10w oil instead of water). But again this was supposed to be a properly running bike before it sat in a field for five years and I'm the only one that messed with it and I didn't separate the carb bodies when I did the cleaning so I don't think they are too far out. Besides, if one or more of the needles are out of position, it will show on the manometer even though that isn't the problem, you know what I mean?



Thanks again!

Will-

 

[Hellgate]

Will I think you're on the right path; something is stuck or no moving smoothly.

On CV carbs the cable pulls it open and vacuum closes it. I'm guessing, as you're thinking, one of the slides isn't closing.

I assume you've already done this but remove the slides and blast it out with carb cleaner. Also look for corrosion inside of the carb body. Something is preventing it from closing.

Looks like on hellva project.

 

[lostinthe202]

Thanks Pete, yeah the carbs were pretty dern clean when I opened them up, I think the PO must've drained them before the bike sat which I will be eternally grateful for. I took the slides apart when I clean the carbs pulling the needle out. The needle has this little peg that is supposed to sit in a hole so it has the same orientation every time you put it in. I can't remember if I noticed that or not so I'll check it out this weekend when I have the time.

And yeah, I've been working on this sucker all winter,

rebuilt the brakes, new pads and stainless braided lines, rebuilt master
rebuilt clutch master and slave
Regreased the steering and swingarm
new wheel bearings
new fork seals (still gotta find some dust caps)
went through the electrical system and cleaned what needed it
new battery
new plugs
new tires
new pet****
cleaned fuel sender
cleaned gas tank
replaced various broken bits.
I have some body work which will get paint, it won't be a show bike, but it should be a good A -> B bike which is what I'm after.

I'll post some pics once I get it painted.

Cheers,

Will-

 

[lostinthe202]

Not much interest in this thread, but for the archives here's the solution to this problem,

The culprit was part of the diaphragm for the #3 carb was not seated in it's groove when I put the cap on and it pinched the diaphragm so it was not actually sealing. Stupid problem, but these are such simple carbs, it was bound to be a simple solution eh?

Will-

 

[RJ2112]

Not much interest in this thread, but for the archives here's the solution to this problem,

The culprit was part of the diaphragm for the #3 carb was not seated in it's groove when I put the cap on and it pinched the diaphragm so it was not actually sealing. Stupid problem, but these are such simple carbs, it was bound to be a simple solution eh?

Will-

The cool part is you found it, and fixed it. Not so simple with FI when it goes bad.....

 

[lostinthe202]

The cool part is you found it, and fixed it. Not so simple with FI when it goes bad.....

Yeah? I have no experience with FI bikes, hell this is my first CV carb bike! But I'd imagine it's like a car, expensive injectors, high pressure lines, expensive fuel pump, etc.

Will-

 

[Hellgate]

I'm glad to hear it was an easy, and no cost fix. Be sure to keep us in the loop with the project.

 

[lostinthe202]

Will do, if it's not raining next weekend I'll try and get it painted then snap some pics. I can't wait to get this puppy on the road. Just waiting for my chain/sprockets to arrive....

 

[runningman72]

get the carbs balanced I had the same prob If your mech inclinded buy a vacum gauge and do it your self it simple