Do I need new carb bodies?

Hey gang.

My 750 is running pretty good, but it has a quirk: If I'm running at about 4K and briefly twist it to say, 6-7K, it sounds like one or two of the cylinders are misfiring. This is more noticeable in lower gears, where the engine can pick up revs quicker.
If I smoothly roll on the throttle, or just hammer it from idle, there is no misfiring. It sounds pretty good all the way up to redline.
Idle is perfect, sync is perfect, I've adjusted float levels (tried higher and lower than spec), mixtures are all nice bunsen-blue.

I've replaced or mic'd pretty much everything in the carbs other than the bodies...
The needles do not show any significant wear. There are no tears in the diaphragms.
One thing I did notice is that the slides have some sideways play inside the bores. I'd say between 1/128" and 1/64". They come down pretty quick, quicker than I've seen with all my other bikes. I've tried one of the slides in a spare 650 carb body, and they do not have any play at all, which tells me the slides themselves are not wore out. Is it possible that the bores in the carbs are out of spec and allowing vacuum and/or fuel past the bore walls?

I'm about ready to source another carb rack... I actually have a set of 650 bodies, but one of them has a broken pillar (I don't know where the broken piece is). I also don't have the 750 air jets, and the ones in my 750 are stripped and will not come out without destroying them...

I don't know what else to try

Ideas?

 

Thry this

http://cgi.ebay.com/ebaymotors/XJ750-XJ ... ccessories

 

Throttle shaft seals?

Dried-out (and/or split into rings) throttle shaft seals can pass the "propane test" at rest (or very little opening) but then LEAK at greater throttle openings as the shaft rotates.

You didn't mention if you had attended to them yet.

 

the slides are anodized aluminum so the bore will ware first but you would see scuffing on the inlet and engine sides of the bore and the left and right would look much better
i've seen ware on the engine side of the bore about 1/4 of the way up but it cleaned up and worked ok
might try changing the plugs and checking the float height

 

The shaft seals are about 10K miles old. Got them from Chacal some time ago. I'll do the carb cleaner/propane test this evening.
Float height is to spec, and the plugs are brand new, but I didn't think to look for scuff marks... So that's on the list too.

 

Is it wrong to consider this an acceptable quirk? I tend to think a 30 year old bike deserves a smooth right hand.

 

w00t! 4 new carb bodies, 1 new slide, and she's all better. No more off-idle rich stumble, and no more mid-throttle issues.

It comes down to idle kinda slow, but I haven't had a chance to tune it all the way yet. Still, night and day difference. I'm not ashamed to ride it now :mrgreen:

 

To restore Performance to your 30-Year-Old Carbs.

Start with an Extraordinarily Meticulous Carb Cleaning.

Then, restore the Finish to the Carb Body Diaphragm Piston Bore.
>Remove the Oxidation and years of Dull Film Buildup.
>POLISH the Carb Body Bore to a Friction Free luster.


Performance Restoring and Performance Improving Polish.

 

Well, I took the carbs off again, and found that I latched the big throttle shaft spring on the wrong notch so it did not snap back like it should. I upped the main jets to 124 and pilots to 41, since I have the 4-1 exhaust, a free-flowing air filter and an enlarged intake port on the airbox. I checked the pistons one more time, and they all come down smooth as silk. Just as it was starting to get dark, I busted out the color-tune plug and got the mixtures dead-on.

Holy Cow! I don't think she ever ran this good. I kept barking the rear tire because I wasn't used to the new-found take-off power. I can't wait to go to work tomorrow! :mrgreen: