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.

 

Fired up the bike just now and the problem wasn't there. Revved it a bunch of times and nothing, so I let it warm up completely. Once warmed up, gave it a little rev (to about 2k rpm) and it jumped to 4k and stayed there. I didn't have propane, so I sprayed some carb cleaner around. The idle seemed to raise just a tad when I spayed the #1 cylinder manifold coupler and the throttle seals. Then I went around and sprayed #3 and #4 and nothing happened... Then I went to #2. I sprayed the coupler a good amount and the idle sky rocketed to 4k. Just to make sure it wasn't a coincidence, I did it again, and again, and again. Looks like I'm going to have to crack off these manifold couplers. Hopefully this fixes the problem.

 

Not necessarily there are several ways outlined on this site to fix them on the bike. With the risk of snapping an intake bolt of in the head very great on these I'd try the on bike repairs first.

 

The Number - 1 Rule of Old Motorcycle Maintenance & Repairs.

"DO NOT do something that will make a Bad situation, ... WORSE!"

The Manifold Bolts are prone to seizure.
Making matters worse; they're made of SOFT Steel.
You will "Feel" like the Bolt is >> Turning <<.
Actually, the Bolt is >> Twisting <<, ... and then ... it SNAPS!
Very Bad Situation.

Remove the Carbs.
Smooch some RTV Black, ... into the Mating Crease all-around the Manifold.
Two coats.
Make the Joint ""AIR TIGHT""

Leave removal of the Manifolds for a Winter Project when you can Pull the Head and have a Machine Shop deal with fragments of Broken Fasteners seized in the Ears of the Intake Manifold Ports.

 

I got them off, but only cyl #1 and 2, 3 and 4 didn't want to budge so I left them alone. The gasket was completely dried up and fell apart right when I got the coupler off.

I used some RTV to make a new gasket, just waiting for it to dry. I'll put some more around it, to ensure that it's sealed up on all 4 couplers.

 

hello everyone. i just built my YICS tool, fixed the annoying petcock, Now with the vacuum gauges. I have a big question that i can't seem to find on the internet. How many inches of vacuum should there be when syncing them?

 

Haynes manual says:
...."No specific vacuum reading is given by the manufacturer, but it is essential that the two carburettors (1 and 2) give the same reading."
I've not had to sync the carbs myself, yet. So I'm afraid that's about all the help I can give you. Maybe someone has a shop service manual that has the numbers in it.

 

well it looks after giving it a shot today resulted in utter failure. I can't even get a damn reading on the stupid gauges. Even after throttling it higher. they just bounce all around the stinking thing. tomorrow i'm gonna try it again but i'm gonna use the B12 trick to see if there is a vacuum leak. This bike seems to be in really good condition for it's age. I've seen other ones that are FAR worse shape and giving people all sorts of problems. But this one i got lucky with. I hope that i can sync it up tomorrow. but if anyone knows a way to make the needles stop fluttering all the place, please give me advice.

kinda frustrates me though, should of bought a CB750 since i'm an actual honda guy.

 

maybe there is a vacuum leak. But ALL of the needles are doing it. Could it be the YICS port? i built the tool to the right specs and everything! i don't get what's going on here! and i've reved it to see if it would stable out but it isn't!!! AAAAAAAAARGH!

 

OH MY GOD! i think i found out why it didn't work! i never tightened the YICS tool. DUH! i didn't know that it was supposed to expand in there like that ROFL!

 

xjmike.... don't thread jack start your own.

To answer your question a vacuum gauge will bounce up and down with the engine pulse. You need a restriction in the vacuum line to smooth it out to a useable level. There are also many other home build synch gauges that work.

carbonxe... glad to hear you didn't make it any worse by breaking a bolt off.

 

I did that once before with an exhaust manifold bolt, so I knew the consequences. I only used a standard Allen key, so there was little to no torque being applied. Luckily the one I needed to come off came off, and the gasket was NOT pretty. I just picked up some RTV black, so I'm going to plaster that stuff all over the manifold couplers later, then hopefully I can re-vac sync the carbs and get this bike running properly.

 

oh noes! i'm about ready to steal this thread as my evil secret plot!

"To answer your question a vacuum gauge will bounce up and down with the engine pulse" saying it as if i'm not aware.

also this is a technical discussion forum. people should be able to talk about whatever the need help with.

 

Some people don't know that a gauge will bounce up and down. How am I to know that you were already aware of that?

It is a technical discussion. There is plenty of space for everyone to get their help with their subject in their thread without confusing people by jumping in the middle of someone elses.

 

I DO KNOW THAT. and i already fixed it. there was no reason to be rude.

 

Gentlemen, please.

Mike, it was a pretty rude thread-jack. Chill.

carbonxe, are we talking about a 750 with Hitachis? It's not in your sig, wanted to be sure. If the carbs are still off, take a minute and pop the clips off the throttle shafts on the outside of the #1 and #4 carbs, pluck out the nylon washers, and inspect the throttle shaft seals you can see. If those are toast, you can be reasonably sure the inners are too. If they're fine, it's no guarantee that the rest are but if the outers are bad you know the inners will be too.

No sense putting the bike back together and having to pull the carbs again if the seals are bad too.

 

Take the Rack off the Bike.
Hold the Rack Vertical.
Flush the four Thottle Shafts at the 6 O'Clock position with Isopropyl alcohol.
Invert.
Flush the remaining four.

Employ an assistant.
Have them "work-the-throttles" Open~Closed.
Apply 4 Drops of CaliKleen RBR Rubber Conditioner to the 6 O'Clock Throttle Shafts.
Invert.
Do the other four.

Let set for 30 Minutes.
Reapply Conditioner.

Cross fingers.
Reinstall Carbs.
Light candle.
Pray to Motorcycle Gods and fire that mother up!

http://store.caig.com/s.nl/it.A/id.1671 ... tegory=179

 

Not trying to jack this thread, so apologies if I do......I have read enough posts to know not to dunk the whole rack or soak it due to the possibility of damaging the throttle shaft seals. When cleaning my lowers and spraying lots of carb cleaner from various positioning of the rack, i wondered if the carb spray will damage those seals just from the spraying of the lower parts? Have any of you experienced that or does it seem okay for them to get hit with some spray as long as they don't get soaked with it? Just trying to be proactive instead of reactive, lol.

Thanks!

 

one thing i've learned about the carbs is that it is HARD to find parts for them.

 

Not as long as we have Chacal around it doesn't seem to be, but money on the other hand, is HARD to come by for alot of us, lol, especially if we have joint checking accounts, if you know what I mean, lol.

 

I already checked the outside seals and they're fine, but no clue about the inner ones. At this point, it only takes 5 mins to get the carbs off the bike. I'm waiting on the RTV to dry right now, then I'll fire up the bike and see if the problem went away, if not, I'll check all of the throttle seals. A little more work this way, but I'll be sure of what the problem was.

Also, I have no clue if my carbs are Hitachi or Mikuni, I never figured out what the differences were between the two.

 

Mikuni's are the ones that have:

MIKUNI ... written on them!

 

Then I guess mine are Hitachi's, unless the words are worn off, lol.

 

You can also look at the Carb "Tops"

Mikuni's are round.
Hitachi's have a little "Ramp" on them.

 

what model of hitachis generally found on XJ bikes are the HSC32's. best if you buy a rebuild kit, replace some of the rusted bolts with STAINLESS steel ones, which last almost forever.

 

No clue what solved the problem, but the RPM hang is gone. I just went out, fired it up, let it warm up and gave it a little rev, no hang...gave it more rev, no hang...gave it one hell of a rev (to like 7000rpm lol) and the RPM just plummeted to idle. Now I just need to get my license next week, adjust the rear brake pedal so the rear brakes actually work, get new tires, and bleed the front brakes, and she's finished!

 

After a good ride you may find it hanging again. If it does reach in and turn the idle speed knob dow (out) until it settles down. May take a try or two. Probably only a half turn will get it.

 

The problem did happen when I rode it about 2 weeks ago (just up and down my driveway), but that was when it was really bad. It didn't do it while I was actually moving, just when I was sitting on the bike revving it. To get the idle to drop back down, I just put it in gear and let the clutch out a little. The drag from the drivetrain was enough to get the idle to drop back down.

 

Find a spot where the Throttles work perfectly.
Move the Bars Left and Right and make sure the Cable isn't getting tugged.

Then, SECURE the Cable Sheath firmly in the Mount.
Cut a 2" Length of Duct Tape and lay it on over the Sheath and "Iron-it" on with a Blade from a Screwdriver.

Then, take a longer section of Tape and wrap it around the Frame Bar over the Tape you Ironed-on.

The Throttles ought to Open and Shut and NOT "Race" if you have to maneuver the Bike.

 

here's another scenario I got for ya...

when doing a running vac sync, (no air leaks already checked), and the motor gets warm (too hot to touch headers) my throttle increases steadily. when I use a long screwdriver to push down on the screws the idle drops back down (from 2.5k down to 1.2k rpms). it's almost as if the cable is pulling the throttle but that's not the case since there is always slack. somehow the throttle shaft is ever-so-slightly rotating as the bike gets warm. any thoughts?

btw: carbs were overhauled and all rubber was replaced including throttle shaft seals

 

Yes but you don't want to hear it.

You didn't get the carbs all dead lined-up on the rack when you reinstalled them and the linkages are not in a "straight line" anymore.