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Carbs AGAIN!!!

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Shawzee

Shawzee

Joined
Nov 1, 2020
Messages
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Location
Christiana, TN
Hi all!,
OK, I cannot get these carbs to sync worth a s--t. I ordered and installed a 6Sigma jet kit, a 48 pilot and a 140 main. I have pod filters and I had 12" shortiy mufflers on it. I have since put megaphone mufflers on. It still ran terrible so I removed the battles and it seemed to improve some. #3 plug still was soot while 1&2 were fine but got horrible gas mileage ( 33mpg). I have been trying to sync the carbs to #2 cylinder they run 2-3" off of vacuum. Number three plug runs black soot while 1 & 2 are grey to brown ash color. I have cleaned the carbs adjusted the floats to the proper height, 25.7 mm, I raised the needle jet to the #4 position and this bike will not run right. Am I looking in the right area??? I'm ready to make this things planter!!! Also my compression is 150# for all three cylinders.
 
Someone will be along soon who knows a whole lot more about this than me, but a vacuum leak or three can cause the symptoms you describe. So can stuff like leaking pilot needles.
 
Thanks, I've checked for vacuum leaks and have none. My needle jets look fine. I may have to mess around with different size main jets. I WILL GET THIS RIGHT!!
 
i said pilot needles.That tiny o ring can get hard and flat and leak fuel no matter what the pilot setting. But I don't even know what carbs or what kind of bike this is.
 
Not sure what engine /bike you have , but here is some geriatric (oops - I mean generic) multi carb suggestions

Have you;
Changed all o-rings, seals, etc
Verified all passages clear
Verified floats are not leaking or heavy - swap them about in the carbs or check levels wet
Checked for throttle shaft radial play
Verified compression is OK
Verified full voltage to all coil(s)
Swapped coils to see if soot changes cylinders (may not be able to do this)

Just some ideas

me pondering - aren't all carb issues geriatric???
 
Thanks for your replies. My bike is 79 xs750 special with Mikuni 34vs carbs. I'll follow your suggestions. Thanks again!!!
 
From my car carb days; set mixture, then sync.
If the idle mixture is not set right, it will throw off the synch settings if you are trying to do it by vacuum readings.

With one plug sooting up, I suspect the mixture is not right there.
 
dukey33 said:
If the idle mixture is not set right, it will throw off the synch settings if you are trying to do it by vacuum readings.
Click to expand...

Likewise if there is even the most tiny vacuum leak, you can't sync by vacuum. And sometimes you can't get rid of the tiniest of vacuum leaks, at least not easily, like throttle shaft, or boot/head interface, petcock diaphragm leaks, etc. And we usually convince ourselves there are no vacuum leaks without really knowing where all to check. I like to just replace every single o-ring and go over every vacuum fitting and the diaphragms with a fine tooth comb.

dukey33 said:
With one plug sooting up, I suspect the mixture is not right there.
Click to expand...

That's why I suggested checking the pilot needles, on the Mikuni carbs I have here (more modern...) they have a needle with a spring, a washer, and a tiny o-ring. Most folks ignore this when they rebuild carbs, but that o-ring is almost always toast, so once you adjust the pilot mixture you break the seal and it'll leak. Then it runs rich no matter where you put the pilot mixture setting. Soot on one plug would have me changing these o-rings promptly.
 
I have checked the jet needles, I know about that "o" ring you mentioned. I have the bike running, set the idle mixture and sync the carbs and they are right on point. The bike runs well at idle, then will start to break up at 50-55 mph. It won't go over 65 mph. I have a 48 pilot and a 142 main in the carbs and The needle jet is in The #4 position. The plugs are burning nicely now. I truly think my problem is in the jetting. I'll 'll check out all areas for vacuum leaks. Thanks for all your input.
 
you keep saying "jet needle", which I not the same thing as a "pilot needle".

But your carbs might not even have pilot needles.
 
When I referred to the needle jet I'm referring to the needle on the bottom of the cylindrical slide attached to the diaphram that goes into the emulsion tube.
 
Turn the carb over. The "screw" you turn to adjust the pilot mixture, that's the pilot needle. At least on the carbs I am most familiar with, dunno your carbs.
 
I looked at the diagram of my carbs and there is nothing listed as a pilot needle. We have to be talking about two different types of carbs. Anyway, I put a 152 main jet in and that seemed to have solved the problem. The bike starts right up and is running well. Thanks for your help.
 
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