Decaying idle

[swiftwing]

I have a '79 Special that I rebuilt a few years ago. I love the bike except for one problem that drives me to want to get ride of it at times. It doesn't want to hold a steady idle.

I have balanced the carbs. I have set the floats. I have checked the vacuum shutoff diaphram. The carbs have been taken apart and cleaned. When I rebuilt the motor a couple of jets were not stock so I replaced them. The intake boots are lightly cracked on th eoutside but no sign of air leakage.

When I come to a stop the bike will idle at 1200 rpm or so. Then as I sit there the rpms will slowly drop until it starts running rough at about 600 rpm. Then ifI don't crank the throttle it will die.

Another symptom is that the initial idle rpm does change with the amount of fuel in the tank. A full tank gives maximum rpm. A low tank gives a lower initial rpm.

My first guess was that the level of fuel in the bowls changes as it sits idling causing a mixture change. However, I cannot find anything that would contribute to this.

Above 3000 rpm the motor really performs. Below that it is a little sluggish.

Any suggestions? Anyone else experience this?

[TopCatGr58]

Hey there Swiftwing,

You say you cleaned the carbs, but if you're having trouble with maintaining an idle, then you may not have gotten the pilot circuit as clean and tuned as it needs to be??

However, you also stated that it idles better with a full tank. You also say you checked the vacuum valve....the Octy for the Special. One way to double check it's possibility of affecting the fuel flow is to bypass it by putting the petcocks on PRIME and see if the symptoms appear. IF they don't, and it is able to idle okay, then the Octy is suspect....or the vacuum hoses that go to it may not be developing enough vacuum....or even leaking preventing the valve from opening all the way.

You also say you replaced some of the jets, but which ones, the MAIN JETS or the Pilot jets. If someone had been into the carbs and replaced jets, they may have used some non-mikuni jets or the wrong style mikuni ones that folks had gotten with the infamous K&L carb kits!? So...you may also need to replace the Pilot jets.

Have you done any other mods, like aftermarket pipes or Individual air filter pods??

More info on what you have...jet sizes, and such will also assist us in further diagnostics!
T.C.

[swiftwing]

I will try the PRIME position to see its effect.

I know I replaced the pilot jets with Mikuni 42.5 jets.

[mro]

pilot jets

Would think fuel would flow better at idle than at higher RPM's if there's a problem with octopus...

You mentioned synking the carbs.
Have you adjusted the pilot screws? or are they just turned out same number of turns???


mro

[blue giant]

I would think that if the diaphram in the octy was gummed up, the higher vacuum with the higher RPM's would over come the gummed or sticking octy giving the symptoms described.

Just my 2 cents.

[swiftwing]

The bike is pretty much stock as best that I can tell.

When I synched the carbs, I did adjust the idle mixtures within. I would think that it is not the initial mixture (ie jets or screw positions) since it idles fine for the first 15-20 seconds and then starts to decay to a stall.

I did tested the octy by removing the forward fuel lines from the fuel valves and removing the vacuum tube from the #2 intake. By sucking ever so slightly on the tube fuel began to pour our the fuel lines. I'd say the diaphram is good. Wouldn't you agree?

[TopCatGr58]

Okay, cracked intake boots....no sign of leaks?? How did you test them? Also folks have found problems with where they attach to the engine/head.

How's the charging system? Do you have any extra electrical devices attached, like running lights? At idle the charging system barely keeps it at 12 volts, and folks have had their bike die due to loss of power at idle with heavy drains on the system, when it gets below 10.5 volts, the TCI won't work right, and can kill the bike!? JAT
T.C.

[mro]

since it idles fine for the first 15-20 seconds and then starts to decay to a stall

So is that from a cold start or after engines warmed up?
Do you start it with the "enricher circut" or not?

Generally expect RPM's to increase with a vacuum leak.
Start it up and let idle till it dies, immediately pull a plug to see if it's wet.
Would also check if firing on all four. Can use your hand to see if there getting "hot" the same or if one or more be cooler than others. DON"T get your hand stuck to a header....can make your hand smell like burnt pork.
(15/20 seconds may not be long enough for header to get hot enough to use the "spray water on header method" to see if a cylinder is not firing properly)


mro

[swiftwing]

Still Looking for guidance

I have completely disassembled all the carbs, checked for correct jetting, cleaned everything, set the floats and reassembled them.

I balanced the carbs and get a very even idle at 1000 rpm. However, when I hit the throttle, the idle does not return to 1000 rpm. It will sit at 2500 to 3000 rpm and slowly work its way down to 1000 rpm again. If it is in gear, I can let the clutch out slightly to put a load on the engine and the idle will drop right down and stay there without touching the throttle. There is plenty of slack in the throttle cable. Without the engine running I can hear the idle stops all hitting evenly.

When I have a full tank of gas, the problem seems to be worse than when I have 1/4 tank.

Anyone else experience this? How do I get an instantaneous drop from run rpm to idle when the throttle is backed off? Thsi cannot be a normal condition is it?

[egsols]

Have you checked, thoroughly, for any vaccuum leaks?

You can use an un lit propane torch to "sniff" around the intakes and other parts of the carbs to see if it effects the idle. If the idle suddenly goes up, you have a leak.

Another problem is the butterfly seals. When you cleaned the carbs did you immerse them completely in cleaner with the seals still in place? Cleaner can have an effect on the shaft seals and you can get a leak there.

[trbig]

So you say you synchronized the carbs... With what? Everything you describe points to them NOT being synched. I'm not doubting you did it, but mine always used to do the same thing when I used a single vacuum gauge to do this. After I had properly adjusted my valves, I borrowed a good set of mercury carb sticks (Thank you Hobbyman), and got them dialed in. My bike would finally purrrr. Even John stated that that was the best my bike had ever sounded... I had to agree.

When you can sit there and see what all 4 cylinders are doing at the same time (When you adjust one it moves the others) you can truly get them synched.

Tod

[swiftwing]

Carbs were synchronized with 4 simultaneous mercury tubes and all hoses of identical length.

[mitch]

Its an XJ

You need a special tool to sync the carbs, you have to shut down a port on the head. Just checking to see if you are aware of this , sounds like a carb sync prob to me .
Doug

[mitch]

Link to tool

http://www.xs11.com/forum/showthread...=&threadid=436

[mitch]

My Bad

How i got XJ out of your post ill never know. Ill shut up now
Doug