Revving problem

XJ900F, been off the road for 9 years, finally got round to reviving the beast, on a tight budget.
Whilst I had the carbs off, I checked them over. Did the clunk test OK, set all the float heights, diaphragms are good, inlet rubbers all sealed.
Bike starts instantly, revs up OK but slow to rev down again. It idles alright.
If I rev to 3k it takes 2 secs to go back to idle.
There's a bit of popping going on as well.
Any ideas? Cheers

 

carbs out of sync, possible leak around butterfly valve and or seals, mine was doing the same thing until replaces a damaged butterfly valve and done a sync...

 

sounds lean
check those plugs, they may tell ya a story.

 

Just been up the road and back about a mile, bike pulls OK, by the time I got back if I blipped the throttle it would go to 3k and stay there. If I cut the engine and restarted it idles OK, but still revs up and stays.
Took the plugs out, looks lean on two maybe three plugs.
How many turns out should the screws be and which way to richen it up?

 

Counterclockwise to enrichen it, but only go a dimes width at a time, then check, dimes width, check, till you get her dialed in perfectly. Very well could need a sync job as well.

 

I changing-out the Carbs on a Bike I have here in the garage because the Carbs wouldn't return to Idle after the Throttles were opened.

The problem, as it turns out ... was Throttle Shaft Seals.
Just enough Air was getting in through the Seals to keep the Idle up.
The Bike was Colortuned.
Synced-up perfectly.

Just wouldn't return to Idle.
Very discouraging having been so close ... just to have to start over with building another rack of Carbs.

 

That means the carbs have to come out again.
If I change the throttle seals, how much of a job is it?
Can I get the seals in the UK?
Thanks for your help

 

Would the propane trick work if the throttle seals were leaking?

 

No. But it did find a massive leak on the Number-4 Manifold where a Cap Screw was broken.
I has no choice but to seal the leaking Manifold with High-temp RTV Black.
Several coats worked into the space between the Manifold and Head.
Encouraged by the immediate increase in performance ... I added two additional coaks of Seakant all around that Manifold.

The leak got cured so well that the Number-4 Vacuum balanced perfectly with 1, 2, and 3 to sync the rack without an extensive repair.

 

you might try testing the throttle shaft seals with carb cleaner... With the bike running, just spray it right on the ends of the throttle shafts (where those c-clips are) and if the idle drops briefly, that seal is bad. My carbs on the 750 drove me nuts till I replaced those seals...

 

Actually, spraying or probing with Propane will make the Idle spike.

You'll hear the rev's increase as the Carb Cleaner or Propane gets-in through the leaks and add more Fuel to the Mixture.

 

Ok, no oil in the bike at the moment, will try again tomorrow with the propane

 

Wouldn't spraying carb cleaner on the shaft seals cause them to go bad, if they weren't already? I mean, they are rubber, are they not? And Carb cleaner breaks rubber down.

 

The Carb Cleaner in the Spray Can isn't harmful to Rubber Parts and Seals.

The Carb Cleaner in the Dip Tank is the one that will cause damage to the Throttle Shaft Seals and everything rubber it touches.

Spraying Carb Cleaner on the Manifolds, Boots and Throttle Seal Hinges has been a Test for locating Air Leaks for as long as I can remember.

 

I have only ever used the spray can stuff. And every can I have ever used had a warning to not spray rubber or painted items as damage would occur. That is why I have always used starter fluid to check for leaks, instead of carb cleaner.

 

I have some starter fluid, will give that a go, love the smell too

 

Using Starting Fluid to detect an Intake Leak is an extremely hazardous and potentially very dangerous situation.
You risk being burned and starting a fire.
You risk having the Starting Fluid become ignited by a backfire and the resulting ignition of the material (ether) will flash so rapidly that it is possible for you to become severely burned, blinded or worse.

The same cautions apply to other products; such as WD-40 and other aerosol sprays used for leak detection.
But, the other products DO NOT have a Flash and Explosive quality anywhere even close to the nature of Starting Fluid.

A backfire may cause the spray ... AND ... the vapors to ignite so fast you will not see the flames that burn you.

Please read the Hazards and Warning on the label of the Starting Fluid Can before you decide to direct the spray from a can of Starting Fluid on a running engine.

 

Oops, well warned that man, I will take your advice and go back to the propane!

 

Ok, oil is now in the engine, starts instantly, idles at 1000rpm after warm up.
I have tried propane all over the carbs and manifolds but there is no change in engine revs at all.
What do I do next?

 

The Mikuni's on the 900 will let you play with the Mixtures and respond very well to Fine Tuning.

Check the Vacuum Sync
Read the Plugs
Make adjustments to the Pilot Mixtures based on how the Plugs look treating each Cylinder individually.