XJ mixture screws... how many turns?

[BigRed]

To start: my engine and carbs are from an '82 XJ.

I replaced my mixture screws a couple nights ago, and I turned them all the way in, then out 1.5 turns. Now I think I've got it a little too lean... here's why: The bike feels like it's running out of gas a lot of the time, and it's not blowing any black smoke. It needs to be REAL hot before it will idle without me holding the throttle open a little. Performance feels pretty good over 4-5k RPM, but below that, it's pretty boggy and stuttery. I had to open up the idle screw to get it to idle smoothly, and now, once it's hot, it might idle smoothly around 1400-1500, then suddenly drop down to a rough idle at 1100. If I open the throttle and close it quickly, the bike dies. I checked the plugs, immediately after a ride (no idling) and they looked real clean. The ground electrode looked only slightly whitish in colour, the porcelain was totally clean, and the ring was quite black.

That was a bit of a rant intended to let carb gurus either confirm that it sounds lean, or tell me that my problem might be somthing else. I'm planning to do a carb synch tonight, but I'd like to have an idea where to set my mixture screws to hold a steady idle before I synch.

Thanks in advance,
Brian

[randy]

You're going to have to get the synch close in order to fine tune the idle mixture, it's a process that works in tandem. As you get the synch close you adjust the mixture, then synch finer and adjust mixture again.

Just get it close enough to stay running and start synching. It'll never idle or run right without a good synch.

[CatatonicBug]

My mixture screws are out over 4 turns. Don't be afraid to keep turning until the popping goes away.

[WMarshy]

A good sync and tune is in need by the sounds of it. Have you done any mods to the exhaust or air box? You may need to increase you pilot jet size if you are more than 3 turn out on the idle screws... Are your needles in the center position or have they been raised at all? How did it run before you started adjusting the mix? What size jets do you have? These are all common questions we ask when someone is having your problem and usually everything you need to know regarding these questions are answered in the maintenance section...or the numerous other carb threads.

[XJ Rob]

When I did my carbs I started out at 2 1/2 turns out. It seemed to work good for me.

[BigRed]

Maybe it was rich after all...

Why spend money for shop retards to work on my bike when I can do just as bad a job myself? I spent a little time cobbling together a YICS tool only to find that the rubber fuel line I was using wouldn't fit into the YICS gallery. It was about 40C in my garage, so by that time I was probably down about 10 pints of sweat, and in my delirious state decided that vinyl tube would be good enough for short term use. That's not actually the case... vinly melted... spent half an hour trying to get the tool out of the bike and then another hour or two cleaning out vinyl chunks. Managed to get a half-arsed synch done (partly before the vinly melted, then finished up without having the YICS blocked), and I also found that I had my mixture screws out about 2.5 turns each (thought it was less). I tried 1.25 turns, and it ran much better, but had some rather explosive popping on decel, so I'm going to try richening them slightly, and then wait for my real YICS tool to arrive from Larry (ordered this morning) before I try to mess around any more.

Thanks for tuning in

[XJOK2PLAY]

Hey, BigRed...
One thing I could recommend to ease the pain of synching XJ carbs: Get a hold of member Yahman & get one of his YICS eliminator tools. That's a big help.
Next, 2 1/2 turns out is usually the best place to start... but not always where you end up. Depends on jet sizes, or what kit you may have in there, condition of wear on the carb components... etc.

Then you need a good synch guage setup. Make sure they have the little adjustable valve in each vac line to settle out the bouncing guage needle problem.

Lastly, check out the threads here and many discussions about the best synch methods. I recommend adjusting the carbs to avhieve your highest amount of vaccuum possible, synch them as closely as you can.

I bit the bullet and bought a Dynojet kit for mine. It made a huge difference for me. My mileage has jumed several MPG since, and idle speed reliability is consistant now.

XJ carbs don't have adjustable needles/needle jet setups... DJ kit gives you that. Those jets taper w/ age, and the needles wear down... increasing diameter and altering shape. That's why the mpg's drop, and irratic idle speed, stalling, popping back on decel, as well as a multitude of other drivability problems occur from.

I made the mistake twice of synching good, but w/ low vaccuum readings. Gave me fits until I tuned by ear better first, got the vac readings up, then synched. Solved a lot of issues.

There's a lot of thread discuusions on here. Check 'em out. It'll really help.

Also... I'm assuming this engine / carb setup's not in an XJ anymore? Transplant? Just wondered if you also swapped to the 10M TCI box & the little vac switch that XJ's use that sits under the gas tank? That controls the electronic timing... High vac readings make a HUGE difference on how well that switch functions and controls ignition timing... that's why I ask...
That switch generates a voltage based off how much/ how little vac is present. That voltage is sent to the TCI, which in turn controls the coils.
It was a big issue for me as to why I couldn't get mine to stop soaring when I'd return to idle....

I also had big problems w/ popping on decel... not anymore!

HTH's ya.
Regards,
Bob

[BigRed]

Yeah; I ordered one of Yahman's YICS eliminators yesterday, so I don't think I'll try to mess with the carbs too much until it arrives... hopefully it doesn't get held up at the border for too long!

Interesting that you say that XJ carb needles aren't adjustable because mine are. They've got five rings and a circlip that hold them into the diaphragm piston. I wonder if I've got a dynojet kit already? The timing/advance system seems to be from an XS; I've got a 2H7 TCI, with the vacuum & mechanical advance under the timing cover. I did a lot of troubleshooting in this area when I was first trying to get the bike running, and everything seems to be swapped correctly. As for the carbs, I thought they were from an XJ, but maybe they're not... or maybe I've got an aftermarket kit in them.

I messed around with my mixture screws a bit last night, and then did some more reading here this morning. Looks like I've got my vacuum set too low... it was pretty stuttery on launch and acceleration, and the idle is a little funny. It might stall when I come to a stop, or it might idle up around 1500. I was messing with it at every stop light and in the parking lot trying to get it right, but I don't think I'll be able to until I raise the vacuum. I think it was half-arse-synched (no YICS eliminator) around 9"Hg. Cyl#1&2 were starting around 12&14"Hg, but I brought them down to match cyl#3. I gather that I need to increase vacuum on #3 using the idle adjustment and the #3 mixture screw, but I'm not going to try any of that until I've got the YICS eliminator, and can do it properly.

As usual, thanks for all the help!

[Yahman]

YICS Eliminator to the rescue!

Hi BigRed,

Your Eliminator shipped yesterday and should be there in about 10 to 12 days if everything goes as usual. Keep it installed and you can forget about the YICS system.

Thanks,
Larry

[BigRed]

Thanks, Larry!

Now that I think I know what I need to do, I'm really eager to get into it and get this thing running like it should. I miss my low range torque...

Here's a question: is it REALLY bad to synch without the YICS eliminator installed, or is it just slightly less accurate? It'd be nice to just get it running better than it is for now, and then do the fine tuning once the eliminator comes in. My only concern is that I could mess it up bad enough that I can't ride it.

[Yahman]

I would do a good bench synch on them, it will get you fairly close and then you can fine tune it when the Eliminator gets there. To set all four of the throttle plates to the same opening, just use a wire tie for plastic bags with the paper stripped off. There are several threads on here that go into it with more detail. Do a search for "bench synch" and you should find them.

Larry