Can the Idle screw vibrate out of adjustment on carbs?

So I've been reading on this forum that these XJ's like crusing around 5-6k, so for the past few days I've been trying it and I have to say it rides like a completely different bike....I love it!!!

My question is this. Is it possible to that the idle mixture screw can vibrate out of adjustment on the carbs?

The bike has been running perfectly for about 2 1/2 weeks with no problems. Today I was on my way home cruising at about 4,500 rpm and the bike starts to jerk a little bit, like it was running out of gas or randomly getting extra shots of gas. So I reached down to turn on the reserve and look at my odometer; there was no way I was running out of gas, I'd only gone 80 miles since fill up. Nonetheless, put on reserve, but nothing changed. Drove about another mile home, pulled into the driveway and, what the heck!? It was idling at 3,000 rpm. What??? So I jump off the bike and to my disapproval, I adjust it down to 1050 at the idle screw. I swore I would never touch that things again once I had it running smoothly.

Is it possible that the engine is running hotter because I'm driving at higher rpms so it's effecting compression or pilot mixture? Or could the carbs have vibrated out of adjustment? Help would be appreciated.

 

If the spring and oring is on the mixture screw it is not likley it will move. On the other hand it is possible for the idle speed screw to work its way loose, I've had this happed and installed a new spring and the problen was solved. Have you checked the throttle cable to see if it may be causing the problem. They can and will bind up if not routed correctly. If the cable is fraying it will stick and make your engine run high rpms.

 

New spring? That might work. And I will make sure to check my throttle cable. My initial thought was that the choke was stuck open a little, but it wasn't. I didn't think about the throttle cable. I'll have to check it out tomorrow after work.

 

Work on Saturday SUCKS, thats riding day.

 

Yeah the good thing is it's at a golf course and I'm out by 8:30am so I have ALLLLLLLL day to ride!

 

They shouldn't vibrate out of adjustment.

To prevent that they might ...
Put two wraps of Teflon Tape on the Threaded Barrel.

The Teflon will act as an Anti-Vibration Protection as well as AntiSeize.

 

Rick this is why you get paid the big bucks! From one New Englander to another, You're Wicked SmAHHt. LOL.

I just hope this thing starts tomorrow morning. I will be very upset if I can't ride tomorrow, after I touched that adjustment screw. It's supposed to be a beautiful day!

 

Alright, so I was right. On the way to work today I stalled out and couldn't get restarted. Good thing I was on a hill and was able to pop start it! Got it to work messed with the idle adjustment. Back to semi-normal as far as idle is concerned, but I'm still getting that sputter/jolting when I try to hold the rpm at a constant.

My gut is telling me an internal carb problem.....
Any suggestions?

 

Sure- - you just wore out a throttle shaft seal.
Check this by using the unlit propane method, or spray ether thru a tube at individual seal areas while running.

Unfortunately, you'd have to break the carbs completely down to change all 8.

 

You mean I get to take apart my carbs AGAIN!!! YAY!!! *sigh* I guess I could have eliminated that by replacing those the first time.

Could you explain the propane/ether method to me?

 

While your engine is idling, you carefully add the flammable fluid as precisely as you can, to the area of each shaft seal, then note an increase- - or even decrease in the idle RPM or sound.

You can see which carb is the lazy one using a spray bottle on the 4 headers.

I've used a garden hose, flooding the water over vacuum leaks, the plug drouns out, but DON"T do this to an XJ, due to the small combustion chamber.

 

You might discover an AIR Leak at where the Intake Manifold mates to the Cylinder Head.

WARNING:
Those Cheap Cap Screws on them Manifolds will bust-off on you in a heartbeat.
If they come loose you win. It they are seized; don't break them.

I treat a Manifold Leak there by ""Smooching"" a Bead of RTV all around the Manifold, ... (Like I was glazing a Window Pane).

There's NO sense even touching the Cap Screws this late in the season.
You break 'em, ... you're out-of-commission.

Smooch-'em up with the RTV and keep on rolling.

THEY WII -->FEEL<-- LIKE THEY ARE TURNING.
Until it suddenly goes, "Crack"!!!

 

Thanks for the suggestions. How would I look for an air leak around the manifold? I went for two rides today. The first one, it never jumped around. The second ride, it did it almost as soon as I got out of the driveway. Strange that it's inconsistant.

 

Have you tried runnng with the petcock on PRI? You could have a petcock problem or a cracked/split vacuum line.

You could also have a gooked up gas cap vent; the next time it happpens pop the cap open and see if the symptoms suddenly go away.

 

Bad Gas?

Go back to the scene of the crime.

‘The bike has been running perfectly for about 2 1/2 weeks with no problems.’
So you did no changes to the bike for 2.5 weeks right?

‘there was no way I was running out of gas, I'd only gone 80 miles since fill up.’
Oops – a change !
Had you put any gas in her during the 2.5 weeks before this last gas-up?

‘It was idling at 3,000 rpm.’
Possibly another separate issue by coincidence?

Maybe try new gas. Yes draining a full tank of (possibly perfectly good) gas is a pain in the a. Drain just a little into glass jar and see what comes out. Should be no debris, no swirls of something other than gas, no water separated to the bottom.
Change that in-line fuel filter, or install one if lacking.

 

I got bad Gas with some kind of crap in it that actually managed to get through an Inline Filter and Clog-up my Pilot Jets.

Bad Filters is a concern.

Loss of Vacuum to the Petcock is a devilish gremlin.
A KINKED or a COLLAPSED upon itself Non-Vacuum-Rated Hose will shut-off Fuel Supply.

 

Yeah, I still haven't installed an in-line fuel filter. The bike didn't go only 80 miles in 2.5 weeks. I've been driving it about 100 miles a week, back and forth to work. Always fill up at the same gas station.

The only thing that changed was the way I was driving, i.e. cruising at 5k instead of about 3k.

I have been eyeing my petcock lines and they do look a little dry. Maybe I'm having trouble there.