Carbs Again: Pulling My Hair Out

Okay. This has been going on for months, so I'll try to keep it short without leaving any important detail out. I bought a Colortune and went at it. The first 3 carbs went great - got them purring - but when I got to the fourth I could see the spark, but there was no ignition. No changes at idle or at higher RPM's - just the spark speeding up, but no ignition. Cranking the mixture screw all the way up and then all the way down did absolutely nothing. Pulled the carbs, gave the jets and bowls a thorough cleaning and tried agian. Same thing. Pulled them again, cleaned them again, disappointed again. Knowing I had spark I checked the compression and it is great. I have drained the bowl, opened the drain plug and put the tank on "Prime" to see if it was getting supplied, which it is.

So, yesterday I engaged the choke with the Colortune in and low-and-behold there was ignition. So I assume it's fuel related, though because it is happening at both the idle and when reving up, I don't suspect the jets (which I swear I have cleaned and cleaned). The only thing I haven't check is the float bowl fuel level, which I am going to do tonight after work.

My question is - when you engage the choke, does it change where the fuel is being drawn from? If my bowl fuel levels are too low because of the float, would engaging the choke somehow bypass that?

Any help would be greatly appreciated - the beautiful Fall riding season is fast dwindling and I don't want to miss too much!

James

 

Pilot jets draw from the very low in the bowl - check if enrichment port is clogged it is very tiny.


Check this out http://www.xjbikes.com/Forums/viewtopic/t=8918.html


edited to correct my terminology

 

Yeah, I cleaned that out pretty thoroughly too and can see light from a flashlight through it. Plus, the cylinder only fires when the choke is engaged, so it would seem that passage is clean. But reading that post may have cleared some things up. If I follow it correctly - if the float levels are off, and fuel isn't reaching the jets, then engaging the choke would draw from the bottom of the bowl where there is fuel. I think. I hope. Damn I hate carbs!

Thanks!

 

could be but would have to be very low. Hopefully that's all it is.

 

The choke circuit pulls fuel from the small brass tube that goes to the bottom of the float bowl. This is the jet in the above post. So yes, it pulls from a lower point and yes, it is a fuel related problem. I would suspect that one of the pilot mixture passages is clogged or that the float level in that carb is too low. Pilot mixture passages would include air as well as fuel. Remember there is no fuel pump so it is air flowing through the main venturi that pulls the fuel up and into the intake. So if the air passage is clogged, no fuel. IF the fuel level is too low then the air can't reach the fuel.

 

Update: I checked the float level last night and it was disappointingly fine. I managed to get the carbs off of the bike before it was too dark (I work outside and have no garage) but nothing else.

If one of the Pilot mixture passages is clogged would that explain the lack of ignition at both idle and reving? Also, would it explain the fact that I am getting ignition with the choke on?

I've taken these carbs apart many, many times - but have never gone beyond removing the jets (air and fuel) needle assembly, floats etc. How difficult is it to get into the passages within the body itself?

Thanks again!
James

 

That can do it. I used carb cleaner and compressed air to clear the body passages. If it's really clogged an appropriately sized piece of wire helps.

What works really well is to breakdown the carbs and soak them in heated lemon juice or pinesol for a couple of hours. Of course that is a lot more work but it works fantastic. You want to be sure to remove all the brass and rubber bits when you do that.

 

Sorry to be such a pain - where is the access to these air passages?

 

Maybe this will help some...

 

maybe look at the o-ring on the pilot mixture screw, it might have got jacked up putting the screw in
easy to do

 

Thanks! Actually I've been staring at that image in my Haynes manual for 2 days now. The thing that sorta throws me is that it looks like the main air jet and pilot air jet run separately from each other - yet this problem is affecting both idle and higher rpm's. Oh well, maybe I'll stumble upon something once I get it apart (again!).

Thanks!
James

 

Remember all the jets are added so to speak so if your pilot jet is clogged you would be missing that gas at all RPMs

 

Ahhhh.... that changes everything - in my mind at least! I didn't realize that! I always thought that the pilot jets somehow - magically I guess - became moot after certain RPMs. I will approach this with a new outlook tonight!

 

Yea it is not intuitive. But that is why the mixture screws effect the mix at all rpms. Besides the main only the enrichment circuit (choke) has a valve and I'm convinced it does not close completely because it the port is clogged they can be a bear to start...

 

SOUNDS LIKE YOU HAVE A PIECE OF TANK RUST CLOGING SOMETHING UP...