Another Carburetor issue...

[lostinlunacy]

Ok, so the whole thing in a nutshell. I got a '95 Katana 750 about seven months ago. Toward the end of November it started leaking gas. The advice I was given was replace the bowl gaskets and needles. I did that just today, after finally being home and the holidays being over and what not. So, the bike was tore apart for a couple months. Replacing the needles and gaskets fixed the leak just fine. I put the carbs back on and got a new issue though.

It starts up and runs fine, idles fine, rev's fine, but here is the dilemma. The idle is tuned to 1500 rpms, and when you pull on the throttle, it will rev fine, to 5, 6, or whatever rpm you pull to, but when the rpms go down again, it won't descend lower than 3000 or so. So, with the tank raised and slightly to the side, I pushed on the flange to close the venturis. They will close manually and stay at the desired idle setting, but it won't return to idle on it's own. The choke is closed. The throttle cable is not sticking. It just won't descend below 3000 rpms on its own. Any advice on why it is sticking?

The gaskets and needles were the only things I adjust, or changed at all since everything else seemed to be fully operational.
Also, I did read the Carbs 101 article and a dozen or so other carb threads on here, but nothing touched exactly on this.
In advance, your time is appreciated.
Thank you.

[steves]

Check for a vacuum leak using either a spritz of carb & choke cleaner, or an unlit propane torch.

[lostinlunacy]

Ok, I did leave out this detail - it is also back firing. That is totally new.
Now, in regard to a vacuum leak (despite my fear of appearing the novice I am), where would it be leaking from? I'm assuming the leak would be before the engine, right? That would leave the rubber mounts for the 4 carbs to the engine. They are tight, snug and proper. Then there is the carbs themselves - new gaskets on bottom, nothing done up top, so I'm assuming that I'm not looking to the bottom, right? Then there is the intake. Before I got started on this little endeavour, there hadn't even been an air filter on the bike and the air box wasn't even on it's rubber mounts to the carb. So, I did get the box fitted, and suited with a stock filter at first. Then I tried without the filter after blocking all air flow and not noticing any change (RPM's should have spiked, right?). Then in the last attempt, I completely took off the box, still the same. So, I believe by process of elimination tha the vacuum leak wouldn't be from the intake, the bowl, diaphragm cap, or the rubber engine mount (insert proper term here).
So, the only other thing that comes to mind is the choke plungers? That's a shot in the dark. But, of course, those weren't tampered with. Nothing was but the bowls, and this never happened before.
I'm hoping I don't appear like an idiot, this is just new to me and I'm trying to go about it in the most logical way I can. Returning to the vacuum leak, I guess I'm unsure of two things - where it would be leaking from and how would i tell? The last response suggested a combo of choke+carb cleaner, (lol) but what would I do with it?
Again, thank you,
Adam

[steves]

The leak could be anywhere in the air path, or any fitting on the carbs themselves with a bad or mis-fitted seal.

[Touch]

carb+choke cleaner or the unlit propane torch...the idea is you spray sections of the intake path and if the engine revs, then you leak is where you sprayed - it is a fast way to localize the hole.

I know you have checked the obvious - throttle cable recently lubed and free to move; throttle grip lubed and free; cable routed correctly (no kinks); carbs slamming shut when cable not attached and operated by hand. The high idle really sounds like something in the control path sticking. If everything from the grip to the carb is free, then I would guess the return spring in #2 carb isn't wound tight enough or more likely one of your butterflies is gummed/impeded..

[lostinlunacy]

I googled the vacuum leak idea and Touch phrased it best so far. I will give Steve's idea a shot here in a few hours, when I get to where the bike is.
And yes, the throttle cable is lubed, and free moving, throttle response is as it was before and should be. As far as the butterflies (venturi?) being being gummed or jammed, before ever putting them back on the bike I paid close attention to their amount of play when I thumbed the throttle. They all appear to moving in sync with each other, both down and up - at least that can be detected with the naked eye.
Ok, so the return spring on carb number 2. Is that where it meets 1 or three? My semi-educated guess would be the center, connecting 2 to 3, because of the relationship between 1 & 2, and 3 & 4 are as though they work as two independant pairs. And yes, it does slam shut when not attached to the bike, and with it assembled and not running, it will slam shut to where the idle level is set.
What would be the proper term for the springs and pin that connect the flanges for the 4 carbs?
So far the course of action is to test for a vacuum leak and see about tightening the return spring on carb 2, once I know for certain which that is.
Thanks again for the pointers, this is definitely a learning experience for me.
Cheers,
Adam

Oh, any thoughts on the back firing?

[Touch]

If your carbs are shutting completely when off the bike then it isn't the butterflies or return spring. You have eliminated everything I would have checked first (slow drop to idle being caused by slow closing of the throttle/butterflies). Did your diaphragm/slide assemblies move freely? The slow return to idle could also be caused by the slide closing slowly - keeping the needle out of the emulsifier and allowing more gas than you are asking for.

After carefully looking at the parts diagram, the springs that keep the butterfly/idle adjustment tensioned on all four carbs are called "springs" and the return springs are on both carbs 2 and 3. If you are confident the butterflys are moving freely and shutting completely, then you have a problem outside the idle adjustment arena and should probably focus elsewhere.

Best of luck and be sure to post the resolution.

[Andy8982]

Did you plug the drain line on the bottom of the airbox and if you're running a temporary tank you need to plug the Vaccuum line on Carb 4 too.

After that you've just got to look for it.

(now i've only worked on pre and post 98's so i'm not that familiar with the 750 carbs)

But having re-read that. If you push on the flanges you can get bike back to idle speed?
Sounds like the cable isn't adjusted right.

Try snapping the throttle open before you start it. Then start and see what it idles to. If that repeats the problem you might have to adjust your cable tension and/or idle screw.

It's my understanding that an air leak would mainly make it struggle when revved (may be wrong) but on the basis yours does all that it might just be a cable 'play' thing.

[lostinlunacy]

running the regular tank, which is a bit cramped but I"m dealing with it. All the hoses are in place, vents too.
I would think it is a cable issue accept that it does the same thing without the cable even being attached.
This just dawned on me. I think it may be a vacuum leak because completely blocking the air filter didn't do anything to the rpm level at all. Ok, this will be my last post until later today after I have had a chance to go back and tinker with it.
Thanks again.

[tone]

From memory this model has just the one throttle cable, i'm sure someone will correct me if i'm wrong lol

what can happen if the carbs are sync'ed slightly too far up the ajustment range is that the flow of air can hold the butterflies open slightly raising the tickover, this is the reason suzuki went to a 2 cable pull (to open) & pull (to shut) system Eg a positive way of closing the butterflies, over here the common term for this is throttle hang up

while the other points mentioned above are good ones & should be checked first this is also something worth considering, the sneaky way to solve the above if all else fails is to fit a small spring onto the carb end of the throttle mechinism which will positively close the butterflies

cheers tone

[md86]

Or maybe the linkage is getting caught on the intake manifold clamps ....... If you ain't careful about how you orient them , that happens .