Inconsistent stumbling under acceleration; fuel/air problem?

Here's my current setup: 1982 XJ650J with stock 4-into-2 exhaust. "Hiflo-filtro" brand cheap paper air filter. Stock airbox was cut up by P.O. but I welded plastic back to replicate the stock design as best I could. I am not 100% sure it is exactly as restrictive now as it was when brand new. Hitachi carbs are running 3 turns out idle mixture screw on cylinders 1, 3, and 4. Carb 2's mixture screw is stipped out at an unknown setting. Pilot jets are 41. Jet needles are completely stock with no shim. Slides are relatively clean and go "clink" but not "clunk". Main jets are 112. Brand new spark plugs. Plugs read snowman-in-a-blizzard white. No YICS modifications done. I'm 75% sure the clutch isn't slipping. I can do a burnout on pavement. Any signs of slippage most likely are sloppy rider input.

Here's my problem: when I accelerate hard at >60% throttle and run every gear out the engine stumbles as I get to the top end of the rev range. For example: Dead stop in first gear to ~40mph, no problem. ~40mph in second gear to ~60mph there is stumbling between 50 and 60. Repeat symptoms of second gear for all higher gears; stumbling as I approach redline. I have never had a working tachometer so I ride guestimating that redline is slightly above 40, 60, 80, 100, and 120.

Does anyone have insight as to what could be causing this? I'd like to get a good clean pull all throughout the rev range especially since the meat of the power is at high rpm's.

My current thought is to really polish the slides and bores and see what that gets me. As a side question, what material would be best for polishing without scratching the slides?

 

From what you said about the colur of your plugs, it sounds like your engine is starving at high revs. For some reason, your carbs don't suck enough gasoline or they suck too much air.

 

Re: Inconsistent stumbling under acceleration; fuel/air prob

My plan of action is to

A) Check with brake cleaner for air leaks for the thousandth time.
B) Polish the slides and bodies with 1000 grit sandpaper.
C) Go up to 114 main jets.

Hopefully one of those will solve my problem.

 

Sounds like a good plan, specially steps B and C, but A is easy to achieve and you'd better know for sure there is no air leak.

I'd personnally polish with 1500 grit for the final touch.

 

Re: Inconsistent stumbling under acceleration; fuel/air prob

Yeah, I might as well suck it up and go by the auto parts store for some 2000 grit. I have 1000 grit on-hand but it takes a bit too much metal off for my taste.

 

I do believe I have read on this forum that you should not try to polish the slides themselves, only the carb housing. Can that be confirmed by someone?

 

Correct. Quite often the slides have a coating on them that can be damaged by polishing; I know for sure this is the case with Mikunis. Not sure if this applies to all the Hitachis or not.

Additional things to consider:
-I see no mention of valve clearances; tight valves could be very well be contributing to the issue.
-Float levels; not only are they critical to overall operation they could also be a contributing factor here.
-That #2 mixture screw; gotta get that fixed. It can throw off the other three and could also be part of the problem.

 

Re: Inconsistent stumbling under acceleration; fuel/air prob

I knew that no matter how much I wrote down I would forget something. I did the valves about a month ago. Two of the 8 were out of spec. I checked the float levels around the same time and all were on point at 3mm. As far as the mixture screw goes I am going to need a new carb body. It's a train wreck in there.

I checked for air leaks with brake clean and didn't find any. I ran some 1000 grit on the slides and carb bodies but it didn't do too much. They were already in pretty good shape.

I also turned the working idle screws to 3.25 from 3.00 turns. The idle is still expectantly lumpy but is a little more stable.

I ordered some 116 hitachi main jets because I couldn't find a good price on 114's.