Carburetor Problems

I hope someone can help with the carb problems on my XJ750. The engine starts on the choke and accelerates fine, nice crisp throttle response. But with the choke off it won't idle, it wants to die and when I adjust the idle knob up it will idle OK (a bit high at 1500 rpm) for a while but after opening the throttle will want to idle very high (2000-300 rpm).

Sometimes after opening and closing the throttle it will return very slowly to idle (10-15 sec).

I pulled the carbs and gave them a clean as per Ricks "Old School" post but chickened out from touching the Pilot Screws. Jets, float needles, nozzles and passages all got a clean, diaphragms are good, float levels checked and appear OK. I refitted the carbs but the fault remains the same so I've pulled them again.

This time I have recleaned everything, long soak of all the passages. removed the pilot screws (#1 - 3 & 1/3 turns in. #2 - 4 & 1/3 turns in. #3 - 3 turns in. #4 - 4 & 2/3 turns in) and cleaned that and the polished the diaphragm tubes and carb bores. I've done the "clunk test" and all are good now.

I've read Gamuru's how to on "Cleaning the Enrichment Circuit Well" so I've done that as well along with the brass tube that goes into it.

Can anyone make any other suggestions? BTW the carb boots look OK but if problem persists I'll do the WD40 routine.

Should I set the set all the pilot screws back to original settings (some seem a long way out) or set them all to 2 & 1/2?

Any suggestions appreciated.

Shane

 

Also check the diaphrams on your plungers. I was told that if those aren't working, they can also cause that problem. I'm having the same type of problem. Revs up to 2-3k and then slowly dies down and quits.

 

not sure if this is the best way to do it but it works. start the bike and without using choke run it to about 8k rpm's. push the kill switch without letting your hand off the throttle. once it finishes winding down pull the plugs and read them. make adjustments according to each individual cylinder... too lean will be white or just sparkling clean too rich will be sooty or smell like gas... your looking for a a nice tan or brown on the plugs... make small adjustments to each mixture screw in the corresponding carb/cylinder. repeat the process about a million times. then you'll have your correct mixture... long, time consuming and makes you want to pull your hair out... in other words, BUY A COLORTUNE AND DO IT IN SECONDS.

 

Shane you describe the same symptoms I had. Resist the urge to increase the idle with that master knob, since it causes that hangup at 2-3000RPM as you have noticed. After you sync your carbs and get the pilot screws adjusted, it should require very little master idle knob to get the correct idle RPM and then it will drop like a rock from 3,000RPM back down to idle.

I would start with turning in your pilot screws (leaning) 1/2 turn each from their original settings, and see where that gets you... I've tried doing it blindly and gave up though, my Colortune is now in the mail...

07spacker I'm confused by your advice to rev it up and chop it to read the plugs. I thought that was to read the main and needle jet mixture, not the pilot screw setting? I'm an admitted newbie so sorry if I'm missing something.

 

Thanks Tyler, I suspected that the idle knob adjustment was bringing another circuit into play. I'm in the process of reassembly so after reading as much as I can find on the subject I've decided I might try about 2 & 3/4 to 3 turns out on each. I can always go back to original settings if necessary.

I'd like to get it idling so I can sync, after I know I've got this id;ing issue sorted I'll move onto the tuning. Thanks for your help.

 

Don't forget to bench synch those bad boys before you hang them back on the bike.

 

Thanks Gamuru, I followed your "how to" and found it very helpful.

I've read about checking the float height, I checked the float level on the bench before I refitted the carbs to the engine last time. All seemed OK.

How do you check the float height? What do you measure from, to?