[kitjv]

I have a '93 900 Superlight (same 900 SS engine) that literally overnight developed 2 symptoms: (1) While riding, when I close the throttle at a stoplight, the engine takes as long as 10 seconds to return to idle. (2) Although possibly unrelated, the engine backfires when the throttle is chopped on deceleration. Otherwise, the bike runs fine.

Assuming a problem in the fuel system, I removed, thoroughly cleaned & rebuilt the carbs (stock Mikunis) replacing all O-rings & gaskets. The carb slides appeared to move smoothly & the diaphrams seemed in good supple condition with no tears nor cracks.

After reassembling everything, the same problems exist. I would appreciate any thoughts. Thank you.

 

[88sporty]

Mine does that to. I just got it running (after sitting in a garage for 5 years) If the choke is on, it goes to 3k ~ 3.5k rpm. With the choke off, it still will stay at ~3krpm. Sometimes it will go to 1k rpm by itself, but mostly I have to hold the brake, slip the clutch to load the engine and them the rpms drop.

I havent rebuit the carbs, but there are no leaks, no air leaks. Cleaned with about 1/3rd can of carb cleaner on the cam/springs and when I checked for air leaks I used wd40 so now said springs are well lubed if nothing else.:think:

 

[mac]

Both sound like lean conditions.

 

[YellowDuck]

I agree - lean. An air leak can definitely cause that, and they are not always easy to find.

 

[jmmmrm]

I would check for air leaks. Spray WD40 on the intake side (all around where the fitttings are) and see if the revs increase, if so, you have a leak.

As for the backfire, I was told that that can be due to an exhaust leak.

 

[kitjv]

Both sound like lean conditions.

I assume that I can check to see if it is indeed a lean condition by simply backing out (turning counterclockwise) the idle mixture screw on each carb.

But what I find puzzling is that the symptoms developed overnight without having done anything to the bike.

 

[bigbadbass]

I assume that I can check to see if it is indeed a lean condition by simply backing out (turning counterclockwise) the idle mixture screw on each carb.

But what I find puzzling is that the symptoms developed overnight without having done anything to the bike.

a split rubber manifold could produce immediate lean symptoms. i'd think you'd want to evaluate and examine your whole fuel system, from fuel filter to floats to jets to properly identify and correct problem. binding cables or linkage, too.

 

[RobAlley]

I have a '93 900 Superlight (same 900 SS engine) that literally overnight developed 2 symptoms: (1) While riding, when I close the throttle at a stoplight, the engine takes as long as 10 seconds to return to idle. (2) Although possibly unrelated, the engine backfires when the throttle is chopped on deceleration. Otherwise, the bike runs fine.

(1) Definitely sounds like a lean idle mix. Try turning your idle mix screws out half a turn at a time, letting the bike warm up fully (10-15 minutes riding for me at the moment) and seeing if the problem persists. My bike seems to want the screws to be 4.25 turns out from fully closed. If you adjust the mix much you may also have to readjust the idle speed with the throttle stop. All adjustments are possible without removing anything from the bike - remember to adjust both idle screw by the same amount.

(2) Sounds like an exhaust leak - my front cylinder exhaust mount nuts constantly work loose and allow this to happen.

I just went through both symptoms after re jetting my bike, much improved now but taking it to the dyno next week to get it all spot on.

Rob.

 

[kitjv]

a split rubber manifold could produce immediate lean symptoms. i'd think you'd want to evaluate and examine your whole fuel system, from fuel filter to floats to jets to properly identify and correct problem. binding cables or linkage, too.

I replaced the fuel filter, completely cleaned & rebuilt the carbs (as previously mentioned). I examined the rubber manifolds (but will do so again). I replaced the throttle cables. No binding nor restrictions.

However, could the symptoms be due to the carb slides not quickly returning to the closed position? Although I already examined the plastic slides & the diaphrams when the carbs were apart, I think that I will view the slides through the carb bores to see if they are hanging up. Make sense?

Thanks, everyone, for your thoughts. I will keep you posted.

 

[Joe-B]

Be sure you didn't pinch the gas tank vent hose (or one of the fuel lines). If the vent hose is partially blocked or pinched, or the one-way valve is blocked, the partial vacume in the tank can cause lean running. You probably only have one problem; a lean mixture can result in both of your symptoms. You're sure the float levels didn't get tweeked when you had the float bowls open? It's easy to do.
Joe

 

[kitjv]

Be sure you didn't pinch the gas tank vent hose (or one of the fuel lines). If the vent hose is partially blocked or pinched, or the one-way valve is blocked, the partial vacume in the tank can cause lean running. You probably only have one problem; a lean mixture can result in both of your symptoms. You're sure the float levels didn't get tweeked when you had the float bowls open? It's easy to do.
Joe

After rebuilding the carbs, I ran the bike w/o the gas tank vent hose detached. And the fuel line was not pinched. So I eliminate those possibilities.

I was careful when disassembling the float bowls. So the float levels were unaffected. Again, remember that the symptoms I am experiencing existed PRIOR to the carb rebuild.

I hope to be able to continue working on the bike in a day or two. I will pursue the ideas that you guys suggested & report back.

If anyone, has any other thoughts, I would appreciate it. Again, THANK YOU!

 

[kitjv]

Well, here is the latest. The problems that I originally cited in this thread seem to be resolved After completely rebuilding the carbs, I continued to have the problems of the engine rpms "hanging up" & not returning to idle, as well as backfiring on deceleration.

So, I backed out the idle mixture screws 4 1/2 full turns (i.e. an additional 1/2 turn from the previous setting) & then reset the idle speed screw. Voila! Problems disappeared.

Thanks to some of your suggestion, the problems were indeed due to a lean idle mixture. I still cannot figure out how these problems developed overnight. But there are just some things that I really don't have to know.

Thanks, everyone!!

 

[88sporty]

I turned my screws out to about 4 1/2 turns and it seemed to be running fine. But,I was getting a cup of coffee when I happend to glance at the bike and i see a stream of fluids leaking.

it was the overflow drain, crap stuck float. Rode it home and all I could think of was that stupid movie "Torque" when he gets a hole in the tank and the fire is following him. I was hoping that no one would throw out a cigarette.

So I am getting new emulsion tubes (its already apart....) and waiting on jets.

 

[kitjv]

Glad to hear that adjusting the idle mixture worked. Remember to check carb synch after you have everything buttoned up again.

When I rebuilt my carbs, I was surprised that no rebuild kit was available. It was a bit of a pain to be sure that I identified every O-ring & gasket.