over revving problem

I am having a slight problem with my 82 xj 650 seca. It takes a while to warm up to the point where
the choke can be turned off. It will Idle smoothly at 1k rpms and will accelerate ok, but after about a mile
the rpms tend to stay high when I let off on the throttle to shift. I stop the bike turn down the idle adstment
to to 1k rpms again, the bike then runs fine, but when I shut the engine off and let it cool it is hard to start
without engaging the choke all the way and turning the idle up again and the cycle is repeated when I ride it.
Also the 1+2 pipes get hot right away but the 3+4 pipes heat slowly and don't reach full temp. until I ride for
a mile. I've gone through the carbs twice, cleaning everything. the slides All drop with a clunk when lifted,
no flaws in the diaphrams, the float needles and seats are new and I wet set the float level. The compression
on 4 cylinders is all within 5 lbs. +- 120 at 7500' elevation. Ideas ? Coils, pilot fuel circuit, wrong main jet size,
stuck accelerator needle ? It has a standard airbox/filter. No air leaks.

 

XJ650 and XJ750 air-cooled engines:
Minimum: 128 psi
Standard: 156 psi
Maximum: 171 psi
Max. variance between lowest and highest: 14 psi

- 7000' ASL, multiply your readings by 1.233 to get a "true" compression reading.
- 8000' ASL, multiply your readings by 1.272 to get a "true" compression reading.

so 120 =147.96 compression is good
color tune and running sync?
valves in spec?

air leak is possible for hard starts

idle should be set after riding for 10 miles to ensure bike is fully warmed up

have you tested spark plug caps?
what do the plugs look like?

 

I just had the carbs out and reinstalled them carefully. No leaking boots. I reshimed the valves last year.
The plugs all had good spark. I bench synched the carbs. I think I might check the coils and try swapping
tci's with a couple of spares I scrounged.

 

Bench synching gets you close but a colortune and running synch gets the carbs dialed in.

New throttle shaft seals?

 

Good chance your running synch is off , listen to the exhaust any popping sound at idle usually is a sign or coming off the throttle down shifting quickly . I use a 4 gauge vacuum manifold to balance mine . All 4 are a light brown color on my 750 Seca .

 

first swap the spark plugs and caps to other side and see if the problem follows the caps then do the same with the coils. if it were 1&4 or 2&3 I would suspect pickup coils or tci .
your problem is 3&4. Remember that your ignition system is a wasted spark ignition cylinder 1&4 both fire together as do 2&3.
so your problem should be with the end delivery of spark ,ie sparkplug caps corroded wire ends to caps or spark plugs. could be your wires are arcing out fire up bike then look under the tank for light show in the dark.
after eliminating spark(fire) it is time to look at carbs ,check enrichment circuit and wells in the bowls then pilot jet circuit

 

From what you have already done, my guess is that your problem is either a bad sync or pilot screw setting, or perhaps both. Recheck the sync first since that’s the easiest. If that is fine move to the pilot screws. If you have a colortune plug use it and close the pilot screw until you get a white (lean) flame, then open the pilot screw just until the white goes away. Repeat for all cylinders. Install new plugs and take an easy 15 mile ride. Remove the plugs an see what they look like. You are looking for a light tan or gray color in the insulation. If it is all white open the pilot screw about the width of a dime. Ride again, read the plugs, and adjust again if needed. Repeat until all plugs have the light gray/tan color. I did mine over a couple of weeks. You will probably find that the pilot screw setting are not the same for all carbs. My pilot screws are set between about 1 3/4 and slightly more than 2. If you do not have a colortune start with the pilot screws at 1 1/2. Just don’t do any prolonged high speed riding until you get the right color on the plugs. When I did this my mpg went up dramatically and the starting and running issues went away.

 

Thanks so much guys. I will I'll follow up on those suggestions