Idling problem

I just got my 82 XJ750 Maxim running today and ran into an idling problem. When I first bought the bike, the bike wouldn't idle, you needed to keep the throttle open a little bit. Now, after finally getting it running again (sat for 2 weeks without oil) the idle is all over the place. I let the bike warm up some and it was idling fine with the choke off, but a little low (800rpm). I wasn't sure which way to turn the idle adjustment screw, so I slowly turned it and figured out which way was up and which was down. I adjusted it to get it right around 1000RPM and everything was fine. I let it sit and idle for a few minutes then out of nowhere, the bike started to rev it's way to around 5000RPM. I hit the throttle a tiny bit to see if it would drop back down, but it revved up to 6000RPM and stayed there, so I killed it.

I played around with the idle again, it would idle low, then eventually die. Then I bumped it up a little and it would idle fine. After a while, it didn't rev itself up to 5000RPM and it idled just fine at 1000. However, if I gave it some gas, it would rev up, then stay there. The engine wouldn't go back down to idle.

I need to sync my carbs, so I'm hoping that is the issue here. Has anyone else run into this problem, or have any idea what the issue could be?

Any help is greatly appreciated.

 

I had the same problem, Cleaning syncing the carbs fixed the issue for me.

 

Same problem here. Tried everthing to fix it.
to do list
Clean carbs
Synch carbs
Check for air leaks with a unlit propan torch.
check to make sure all 4 Choke circuit plungers are fully seating (this turned out to be my problem)

 

That's something that alot of people overlook when cleaning their carbs -- you really have to make sure it opens and flows freely, and then closes completely.

Point being, I would clean your carbs and double check your enrichment circuitry (choke), carbonxe. Sounds like it's getting way too much fuel.

 

You need an Advanced Carb Cleaning.
Clean the Carbs PLUS Scrub and Polish the Bores.

Upon completing the Cleaning, ... Bench Sync the Carbs with a very thin Feeler.
Quarter-inch wide Strips of 3 X 5 Card will Bench Sync the Throttles darn-near closed.

With the Throttles Closed you will get the Bike to Idle on the Piloot Mixture Screws.

Once you have the Bike Idling nicely, ... you are just a couple of more steps of Fine Tuning before she's running Tip-top.

The process begins with Absolute Cleanliness of the Carbs.
Removal of the Emulsion Tubes
Removal of the Pilot Mixture Screws
Flushing all the Parts, Jets and Passages
Scrubbing and Polishing the Diaphragm Piston Bores.

And when its all done right, ...

YOU, ... are "Dialed In"!

 

clean sync the carbs but also check your shaft seals before you pull them that could be part of your erratic idle. Just give them a squirt and see what happens. Better to find out now so if they are bad you can fix them while they are apart

 

IN ORDER, no shortcuts and no skipping steps:

-check and adjust valve clearances.

-compression test (for peace of mind after valve adjust.)

-carbs CLEAN CLEAN CLEAN; pass "clunk" test; enrichment wells clear. Go through the recommended carb service sequence in the book.

-float levels checked using gas and the clear tube method.

-bench sync, idle mixture screws 2.5 turns out to start.

Then you will need to do a running vacuum sync.

Take shortcuts, or skip steps, and you'll be going back again time and again until you finally do it right. No quick and easy fix.

 

Thanks Fitz. Compression is low, but still within spec. I skipped the valve clearance check for now (cleaning the carbs seemed to be the easier 'first' step, but the valves will be adjusted). Clunk test was a breeze, passed with flying colors after I put the carbs back together. I bench sync'd the carbs using small paper clips.

Now, before vacuum syncing, I should check and adjust the valve clearance, correct? After putting the clean carbs back on the bike, it fired up, but idled VERY high, but it seemed to idle fine after re-adjusting the idle. I ran out of fuel before figuring out if the problem went away, but it seemed to idle better than before.

Also, I'm not sure if it was because I ran out of fuel, or if I adjusted the idle too low...but which way do I need to turn the idle adjustment screw to raise idle? Is it clockwise = raise, counterclockwise = lower?

 

I guess no one checks shaft seals anymore

 

Is it clockwise = raise, counterclockwise = lower?

Correct

 

I was not sure couldn't remember if it came up thru the bottom or went down thru the top the screw that is

 

2 months later, and everything finished on the list, and the idle still hangs some times. When I ride the bike, everything is fine. When I come to a stop, the revs drop back down to idle with no issue. However, if it's sitting and idling and I rev it, it sometimes gets 'stuck' at a higher RPM. Sometimes it creeps it's way back down, sometimes I have to kill the engine, but usually if I lower the idle with the big idle adjustment screw, it fixes the problem. The bike idles beautifully, rides nice, revs nice, etc...I just have this issue with revving the motor and I just can't figure out what it is.

One thing I have noticed, is that the bike acts weird if I turn the fuel off then back on. I have an on/off switch in place of the petcock because my petcock was leaking. If I turn the fuel valve to 'off,' the idle drops, even though there is a sufficient amount of fuel still in the line, then when I turn the valve 'on,' the idle raises up again. Could my float levels be off and that's what is causing the sporadic idle hang?

 

Check your throttle cable. Lube it and check for fraying.

As for the valve shout off. I have one on my bike, if I leave it in the off position I can ride about 100 yard and it dies. When I look at the line I still see fuel in it (clear line) and I have to put it on prim and open the shut off valve to be able to start (it’s a pain in the but when you’re in the middle of an intersection )

 

The cable is fine, that's not getting stuck. I do all this with the gas tank off the bike, so I can actually see that the linkage is freely moving when I use the throttle.

 

You might have an "Intermittent" Air Leak.

The O-ring on the Intake Manifold could be flattened and fried-out. The resulting "Shrinkage" lets Air sneak-in when the Vacuum gets high enough to permit Air to leak past the O-ring and re-seal when the Vacuum is relaxed.

To Seal the Manifolds without risking BREAKING A BOLT, ...

Remove the Carbs.
Using an RTV Sealant, ... "Wipe" and "Smush" RTV alont the Joint where the Manifold and Head mate.

Create a BEAD all around the Manifold's back edge.

Don't tighten or loosen the Manifold Securing Bolts until you are well aware of the risks involved attempting to remove potentially SEIZED Fasteners.

 

Thanks, I'll have to try that out on Sunday. I really didn't want to have to pull the carbs off the bike again, but it looks like I'm going to have to do it. I guess this time I can give the outside of the carbs a thorough cleaning.

 

I wasn't too keen on pulling off my carbs when I was investigating an air leak. Now I can do it in about 10 minutes !

 

I've already done it twice. It's not that it's difficult, it's just a PITA to do lol.

Actually, it's not getting them off the bike that's the problem, it's getting them back on the bike.

I hope this solves the problem. I have only 2 more weeks until I get my license, and I don't want to be without a properly running bike when I have it.

 

I pulled off the carbs and redid everything to them (clean, sync, floats, etc), put them back on the bike and the problem is still there.

Out of curiosity, I decided to pull the vacuum line off the petcock to create a vacuum leak. The bike bogged down a little bit because of the leak, but when I revved the motor, it dropped right back down to idle like it was supposed to. I'm completely stumped on this one.

 

Propane test the throttle shaft seals. I would bet they are leaking on carb 2. Mine was doing the samething and when I did the test I found the seals leaking. I have no idea why they are not included in the carb cleaning. The shaft seals are made out of rubber and they harden when not used. They are 20 plus years old after all.