CARB SYNC

Hi there, i have a 1982 xj 750 maxim, i fitted a k and n air filter to it the other day, and now when i start the bike up, the revs shoot up and stay at around 4000, if i engage gear and slowly let out the clutch, the revs go down, but if i pull in the clutch again, the revs shoot up once again. Are the carbs needing sync'd or have i ruined my beloved machine? please help, p.s i have put the old air filter back in.

 

Hi, me again, on the throttle cable at the part that is beside the carbs, it is held in place by a metal lever, is this meant to be really loose? should it have a spring attached to it, to firm it up? mine is really wobbly, and doesn't feel right, any ideas xj-ers? thanks, Jim.

 

Donno about the throttle cable on your bike, but when you change to a K&N you do have to resynch the carbs, and possibly rejet. You've change the air flow of your bike, so adjusting the fuel to match is necessary.

 

The throttle cable generally comes down in the center of the rack. You sure you aren't talking about the "choke" (enrichening) cable, which is usually located on the LH side of the rack?

 

i deal with this phenomenon frequently myself. even if its just slightly higher revs if im at a stoplight or something and i dont want people to think im "showing off" i will do this. always wondered what the mechanics behind it are. something with the clutch plates and the engine or something perhaps?

 

You haven't ruined anything.
But, 4,000 rpm's IS NOT Idle.
To make 4-Grand the Plant is getting Main Jet supplied Fuel

Make the Bike IDLE with as little Throttle help as possible.

Adjust the Idle Rod OUT to close the Butterflys.
Tweak the Pilot Mixture Screws to get the Bike to Idle on the Pilot Circuit.

"Preset" the Mixture Screws to "Just-shy" of 3-turns out.

 

It's because the carbs are out of sync (or your float levels are off) and you're "pulling the motor down" until you engage the clutch and turn it loose to rev back up.

It's known as a "hanging idle" or "racing" and it's because one or more carbs is lifting it's slide when it shouldn't be and because the CV carbs respond to manifold vacuum, the other three soon join in.

It's not a "phenomenon" its a (classic) symptom. Of being out of sync.

 

Hi all, thanks for the advice, i have disconnected the throttle cable, and the choke cable, the throttle hand grip twists freely and snaps back with no stickiness. i then started the her up with the cables disconnected, and the engine still revved straight away to 5000 rpm. Any suggestions guys, i'm pulling my hair out here. Thanks in advance guys. Jim.

 

Hi again, forgot to mention that when i turn the idle screw out, it has no effect in bringing the revs down, but it does make them go up when i screw it in !!!!

 

It only takes ONE carb having its butterfly open too far when the others are closed to cause this.

You need to wet-verify the float levels and do a good accurate bench sync. You probably knocked one of the carbs way out of sync in the R&R process.

 

I'm wondering if one of the floats stuck open and ended up flooding the crankcase.

 

For the Plant to make 5,000 rpm's ... the Throttles are Open and the Engine is being Fueled by MAIN Jets.

The Linkage is keeping Throttle(s) Open.

Check:

• Cable Attachment device to Linkages stuck under back of Head.
• Intake Manifold Clamps improperly fitted fouling Linkage movement.
• Carbs not fully seated into Manifolds.
• Swollen Throttle Shaft Seals preventing Throttle(s) from closing.

 

Hi guys, i've taken off the carbs again, done a bench sync, and re-fitted them, the bike idled fine, after a bit of tweeking, but when i gave it some throttle, the revs went back up to 5000, and stayed there, the only way to bring them down, is to turn the ignition off. (The vaccuum hose that goes from the head to the tank), if i take it off, the revs come down momentarily, but if i connect the hose to the tank, the revs slowly go back up to 5000!
I removed the rubber boots from the carbs to the airbox, and started her up, there is plenty of suction in each carb. Also, how do you check the float levels?

 

http://www.xj4ever.com/setting%20fuel%20levels.pdf

 

The Membrane within the Vacuum Unit could be perforated allowing SOME Fuel to be introduced to the Engine.

I don't thing enough to make 5K rpm's, though.

To make 5-Grand, ... those Throttles are opened at least a third.

 

How does it run on PRI? What's the condition of the vacuum hose?

 

Hi guys, i removed the carbs (again), one of the sliders was sticking, the rubber diaphram was not central in the carb top, creating the sticking. I checked all of the diaphrams and they were all fine, i put the carbs back on, did the vac leak test with wd40, there does not appear to be any leaks whatsoever. I started the bike up, and she was idling nicely at around 1000rpm, i let her tick over like this for a few minutes, then i blipped the throttle a bit,. and the revs went up again to around 4500. I turned her off via the ignition key, then turned her back on, the revs went down again, then slowly, but gradually went back up again to the usual 4500. I turned the idle screw fully out, but it made no difference. I put the bike in gear and slowly let out the clutch, the revs then went down to a more normal rpm. I have checked everything that all you guys, with your experience have suggested, but the only way the revs come down, is by either turning the bike off, or putting it in gear and releasing the clutch. Do you think i should just give her a blast down the road, and see what happens? Thanks for all your help guys, you have been great. Jim.

 

A "hanging idle" is a classic symptom of being out of sync.

If your valve clearances are in spec;

and your float levels truly and accurately "wet-set"

then the next step is a running vacuum sync with YICS blocked.

Until these things have been attended to, you won't get it to return "smartly" to idle.

Honest.

 

Hi all, regarding the high revs at idle, when i first start the bike she idles fine, then after a few minutes, when the engine warms up, the revs shoot up, is this connected? i.e when the bike warms up the revs shoot up to 4500, could the ignition coils have anything to do with my problem? Thanks for listening to me!

 

hi , every time i change gear my revs shoot up and stay up, my idle screw is turned right out, theoretically, the bike shouldn't idle, but it sits at 4000. When at traffic lights, i need to let the clutch out to biting point, and hold on to the brakes, to bring the revs down, if anyone has any ideas i would be most grateful, thanks , Jim.

 

Here's an idea:

What I DON'T GET is why you keep completely ignoring the answers you've been given.

THIS IS YOUR ANSWER. Your carbs/motor are NOT IN SYNC. It is the problem and therefore the only effective answer you're going to get. Asking the same question over and over until you get an answer you "like" won't work.

To recap:

-valve clearances in spec;

-float levels accurately "wet-set"

-running vacuum sync with YICS blocked.

-then final mixture tuning.

IN ORDER.

Until ALL OF these things have been attended to, you won't get it to idle properly.

 

hi bigfitz, will do.
thanks.