Another carb question...I swear I searched first

I just tore apart my Hitachi carbs and to my pleasant surprise, they were, for the most part, very clean. However, in the number 2 carb there was a relatively thick and seemingly fresh layer of varnish. I am at a loss as to what could cause this to happen to only one of the 4 carbs while the others were as clean as a whistle.

Any ideas?

 

Honestly I'd love to know that as well, I also own an 83 XJ750 maxim, the first time I did a carb cleaning on it after buying it, the ports and what not on all of the carbs needed cleaning from the pink build up in the float bowls as expected, but were all relatively shiny, the same #2 carb on mine was grossly covered in black grime for some unknown reason. I cleaned it thoroughly and got it looking shiny. And just yesterday (or 9k miles later) I did a routine carb cleaning and didn't notice any buildup on either of the carbs at all. But still i would love to know why it was like that to begin with, or how it's possible to prevent it in the future.

I was thinking it had to have something to do with the vacuum tube that goes to that cylinder/carb since that's the only difference between them to my knowledge. (at least on mine)

 

If it's the carb with a petcock vacuum line on it, one possibility is a leaky petcock such that it's putting fuel into the vacuum line.

Also, take a look at the design of your airbox. Is the breather input located directly below the #2 carb? What condition is your air filter in? A dirty air filter will make the carbs pull in more of what's coming in through the breather tube.

 

It also seems like there are 2 different issues going on here. I have also gotten the situation where one carb has way more varnish than another. I believe that's because it's the one that receives fuel first from the fuel line. If it gums up, then the other ones don't really get as much gas after that.

Where was the black grime on the carb? Black seems like a different situation to me, more like oil or dirt. I think the vacuum line thing is plausible because if your petcock isn't right, then either air or fuel could be coming through it, and either way it would cause problems.

 

Thanks for the suggestion, I will definitely have a look at that. Though, when I had pulled the tank to do the carbs the pet**** did not leak at all.

For further clarification, the varnish I am talking about was all in the bottom end (float bowl, fuel jets, etc), the top end on all of them was spotless with 4 solid clunks.

 

Must be a different situation then, on mine the floats/jets etc. were the same as the others, it was the throttle valve and the piston valve that were covored in grime. I don't believe the vacuum tube on mine even works, the fuel will flow freely into the carbs without the engine running in both ON and PRIME. I also just checked the vacuum tube, it's completely dry all the way up to the petcock, so it doesn't leak back into the carbs, that's for sure.

Previous owner had it sitting in a garage for 6 months before I purchased it though, that may have something to do with it, since I've purchased it, I haven't gone more than 3 days at a time without riding it and as I said, last time I checked the carbs after 9k miles of riding, all the carbs were in great shape, no buildup what-so-ever.

 

How about under vacuum?

 

Not sure what you mean, can you clarify?

 

Varnish accumulation means the Fuel is not leaving the Carb and Evaporating in the Fuel Bowl:
There is an abundance of reasons why this might happen:

Weak Manifold Vacuum
Clogged Jets and Passages
Air Leaks contributing to diminished vacuum

The Primary Issue to be investigated is:
>Manifold Vacuum

Why is the Fuel being allowed to Evaporate in the No. 2 Fuel Bowl?

Put a Vacuum Gauge on the offending Carbs Manifold and see if there is Vacuum?

Report findings.

 

My brand new gauge broke during the attempt to get the vacuum reading so will have to try again tomorrow, however, it seemed as though I was getting at least some vacuum at each one. Do I have to use the YICS tool?

 

YICS Tool ... Yes!

Vacuum Restriction Devices ... Yes! (Needed. Or the vacuum Gauge fluctuates so rapidly and violently, ... it just might self-destruct).

Fabricated "Restriction Devices"