Engine races at idle when carbs are synced...??

ok, so heres my predicament, i am going to do a thorough carb clean/rebuild this summer, and will be doing the valve shims then aswell. However i was preparing to take my xj750 on a ride to long island from MA (about 500 miles round trip) and i figured i would atleast give the carbs a sync in their dirty and untuned condition (as i readily have access to the tools, so it wont cost me to just do it again after ive cleaned it) , so as to maximize fuel economy. Anyways, the closer i got the vacuums among cylinders the higher the engine would run, even with the idle screw all the way out and the pilot screws seemed to make little to no diffrence to the speed, just the sound of the idle. also the vacuum when synced was roughly 22 ccHg this reading is WAY to high correct? Anyways in order to even drive it, i had to intentionally untune the carbs so that i could get the idle to where it ought to be.

So my question to you guys is... what the hell is going on???

i suspect dirty carbs and incorrect valve clearances are to blame... but was wondering if someone could shed some light on this situation.

Maybe one of you had a similar situation in the past? would love to get some pointers on this one.

Thanks!
Brian

 

Mine raced up to 4,000 rpm then I hit the kill switch when I was trying to sync my carbs.
I found the shaft seals on the butterfly valves were shot, replaced them and when thru a sync without the racing problem!
A quick check would be to take the "E-clips" and plastic washers off the outside carbs shaft and have a peek at the u-rings underneath.

 

Nick's calls are usually correct Brian (he has a sixth sense). I'd pull the carb bank and have a peek.

 

dead on! it hit 4000 and stayed there! butterfly shaft seals you say?
how hard are those to replace? and where would i go about looking for new seals?

well i guess i'll go have a look atmy XJCD

 

Hi, I'm new to the XJ world so be gentle =D.

I'm having a similar problem. I just acquired a 1982 XJ550 it runs pretty well, but the at idle the engine races up just to 4k just as described. The bike wouldn't rev up when I got it. My brother and I determined that the air jets were clogged going into the float bowl. We cleaned those out, but now this is the problem we are having. He thinks that the carbs just need to be synced, but I’m not so sure. Any help would be appreciated.

Thanks,
Kevin

 

sounds to me like they are synced and you should check for a vacuum leak.

does anyone have any throttle (butterfly) shaft seals they would sell to me?

also what about the o-rings for the fuel tubes?

 

I got my shaft seals from another XJBike member and the fuel tube o-rings from Yamaha at $1.10 cdn each. My invoice shows a part number of 932100718000 for these o-rings.

I replaced these items, reinstalled the carbs and can now play with the throttle screws without the engine racing as it did before the repair!

The repair wasn't that difficult, just take your time, one carb at a time. I gently cleaned out the old crud, used one of those green scour pads to buff the shaft and the seat in the carb body. I felt the "new" shaft seals were a bit small on the OD, so I used some gasket maker silicone to seat the seals into the carb body and made sure I cleaned up any of the excess. I used some o-ring silicone lube to make things slip easily on the shaft. Reassembly went smoothly, used a piece of 1/4" glass as a flat surface to make sure carbs were in line, don't know how important this step is, I think the two rails will pull them in line anyway. Reinstalled, added fuel, and she fired right up!

Pictures on my gallery show the condition of my shaft orings and it's a no brainer that they were causing a problem!