Need carb help

[AlfromNH]

So I got my bike running. The left side barely runs, however. What's odd is what happens when I put my finger over the brass inlet port shown in the picture below. I believe it's the idle circuit inlet, am I correct?

Anyway, when I block that port on the right carb, the bike dies almost instantly. When I block it on the left carb, in 2-3 seconds that side starts running, and the idle speeds up accordingly.

I'm trying to make sense of this by looking at the Bing manual and what I can find online, but I'm still stumped. I've cleaned the carbs twice, and blown out all the passages I can find. New floats, float needles, needles & needle jets, idle jets, mixture screws, diaphragms and all associated rubber parts.

[Mike V]

Al,

I'm no expert on these Bings but I'm going to throw out a guess or two.  There's obviously more than one possibility.  My guess is there's a blockage in one or both of the small ports just behind the butterfly. If you look very closely you will find them, but you have to look real close.  Strip a single strand of copper wire from some old insulated electric wire you have lying around and carefully run the wire through these two holes the best you can.  I would also remove the idle air screw and blow some carb cleaner through it while you're there. You should see fluid escape from these holes while forcing fluid through the idle air screw port.  No air enters through the venturi during the idle circuit while the butterfly is closed (in theory). It enters one of the small ports you're describing and pulls fuel up through the orifice past the idle air jet (as I understand it).  Make sure your idle air screw is set at factory settings when you're done and you make sure you haven't pinched or cut the o-ring.  When re-installing the idle air screw use a very, very slight smear of silicone grease and slowly turn the screw to full stop, then back out the specified turns - slowly. Just some guesses to throw out to you.

Here's a really nice little video that may help the understanding of how these CV's work.  I think it's a great illustration.  Pay special attention to the video at about 0:50 minute during the idle circuit illustration. Some Google searches will find you plenty of study material.  Probably more than you want to deal with. Good luck, I'm feeling an epiphany is coming your way.

 http://www.youtube.com/watch?v=wyspAHrMbb8

-Mike V.

[Barry]

As Mike V says you have to make sure the whole idle circuit is clear but to explaining your symptoms this would be my take on it.

The brass jet is the idle air inlet. When you look at the idle jet itself there are a number or peripheral holes which is where the air enters and mixes with the fuel passing up through the jet. This rich emulsion of air and fuel is then diluted by the air coming under the butterfly before entering the engine  If you block the idle air inlet the idle mixture seen by the engine will go richer.

Your symptoms would then suggest your left carb idle mixture is too weak as reducing the overall air supply makes it run. Your right carb is less obvious, it could already be so rich that additional richness causes it to stall or it could be the throttle stop on that side is not open enough and the reduction in overall air supply is enough to stall it.

It's interesting that to make for an efficient carb both the idle circuit and the main jet circuit on the carb have an emulsion air inlet to promote better atomisation by pre-mixing air with the fuel before it gets to the venturi. In no case is raw fuel on it's own dumped into the venturi to atomise best it can. So if either of those emulsion air supplies are blocked (and that's not so uncommon thanks to gunk ingested from the engine breather) the carb will run rich.

[AlfromNH]

I thought the right side dieing when blocking the port was correct operation. Hmmm...

On both carbs, I have used copper wire to clear both of the tiny holes by the butterfly. The hole closest to the center, I can only get about 1/4" down, the other hole I can get into the mix screw orifice. Same on both carbs.

I dismantled both carbs yesterday, all the o rings are undamaged.
I didn't try silicone grease on the screw, I'll try that.

[montmil]

Starting on the enrichener circuit can be compromised if the small jet in the bottom of the float bowl is clogged with fuel sediment. Fuel must pass thru this 'jet' to feed the enricheners. Clogs not uncommon with a carb that has been parked / ignored / abandoned for a lengthy time. This little fuel reservoir feeds the enrichener -which is actually a small carburetor rather than what many people call a choke. Small, single strand copper wire will clean it out



Often, a photo will clarify accompanying words.

[AlfromNH]

The enrichener circuit seems to be working properly, the bike starts when cold, then I can gradually back off the ec. The problems I describe are happening after the engine is warm.

[Barry]

I thought the right side dieing when blocking the port was correct operation. Hmmm...

Yes I guess you could say that. The idle should at least slow as it's losing some of the air. Not all of the air though as there is still some coming past the butterfly.  The air going through that air jet is only the pre-mix air.

On both carbs, I have used copper wire to clear both of the tiny holes by the butterfly. The hole closest to the center, I can only get about 1/4" down, the other hole I can get into the mix screw orifice. Same on both carbs.
 

That's normal. the two tiny holes behind the butterfly are the transition or bypass holes. They are connected to a little chamber (#5 on the diagram) so a cleaning wire will only go so far.  Worth cleaning them but they are not active at idle and only flow when the butterfly is opened a little.

[AlfromNH]

Thinking aloud here to try to get a better understanding...

By blocking the idle inlet, I'm disabling the idle circuit entirely; no air in, no way for vacuum to draw air/fuel thru the mix screw,etc. sounds logical and correct?

Accepting that, then with no idle circuit, the only other place for fuel to come from would be the main jet.

Am I correct to this point?

[flybot]

Good suggestions. I will add, after doing a carb rebuild you definitely need to adjust the valves or you will never get a good carb tune. I found this out the hard way. When I tried setting up my carbs the first time (without valve adjustment) they were almost completely off the idle circuit just to get it to idle. After warm up it would idle way too fast.

Also, do you have the correct starting needle valve positions from Bing? I cant remember what it is off hand, but I can get it, or someone here will know it.

[Barry]

Thinking aloud here to try to get a better understanding...

By blocking the idle inlet, I'm disabling the idle circuit entirely; no air in, no way for vacuum to draw air/fuel thru the mix screw,etc. sounds logical and correct?

Accepting that, then with no idle circuit, the only other place for fuel to come from would be the main jet.


Am I correct to this point?

....afraid not.


Think about what you have do to make an engine idle at the correct speed.  The idle stop screws have to be turned in by something like 3/4 of a turn. That means the butterfly is open slightly not much but it is open, so a little air must be passing through the main venturi and the engine will not idle without it.  The little brass jet at the inlet is only the pre-mix air which forms an emulsion in the idle circuit and helps to atomise the idle mix when it meets up with the air coming through the venturi.

Like I said above in no case does neat fuel enter the venturi, it is always pre-mixed with air to give the best chance of good atomisation. The main jet circuit works exactly the same way in principle in that the main air supply comes via the venturi and butterfly but there is also a path for pre-mix air which is routed to the area around the emulsion tube that sits on top of the needle jet.  The diagram I posted above illustrates the main circuit pre-mix air supply clearly. The idle circuit air supply is less obvious being the dot and dash line just above.

If you want the definitive description of how a Bing CV carb works read the attached document. On page 1 the first paragraph of the right hand column deals with the idle circuit and explains the two sources of air at idle.

[AlfromNH]

I'm getting a better understanding of how these carbs work. I'm still wondering what is happening when I plug the idle air inlet.

In your diagram above, the dotted line must represent the idle air inlet, which goes to the top of the idle jet. So fuel and air are drawn by vacuum to the small holes by the butterfly.

When I plug the inlet, is it then creating more vacuum at the idle jet, and  and therefore drawing more fuel thru the idle jet?

[Barry]

In your diagram above, the dotted line must represent the idle air inlet, which goes to the top of the idle jet.

Yes

So fuel and air are drawn by vacuum to the small holes by the butterfly.

Not the two small transition holes they don't flow at idle because there is insufficient vacuum above them. Look carefully at the position of the transition holes they are behind the butterfly at idle and they only flow when the butterfly opens slightly and is positioned directly above them which increases the vacuum.  The purpose of the transition holes is to smooth the transition from idle onto the needle jet. At idle the mixture only flows through the idle outlet which is the one controlled by the idle mixture screw.

When I plug the inlet, is it then creating more vacuum at the idle jet, and therefore drawing more fuel thru the idle jet?

When you plug the idle air inlet you are eliminating the pre-mix air so only neat fuel is drawn through the idle outlet. It may draw more fuel through but even if it didn't the overall idle mixture would still turn rich.

[AlfromNH]

Thanks for the detailed response, Barry, I truly appreciate it.

I'm obviously going to have to take the carb apart again and clean/inspect it. I'm just hoping to have a clearer idea of what to look for.

[AlfromNH]

well, I found the problem, or at least A problem.

One of the two tiny holes directly under the butterfly is plugged, I didn't even realize there were 2 holes there until I looked real close with a magnifier, then looked at the other carb to compare. The blockage is the same color as the aluminum carb body.

The blockage is very hard, almost like mineral deposits from hard water. I don't want to pick at is and risk enlarging the hole, so I'm soaking it over night in Pine-Sol. Hopefully that will soften it enough that I'll be able to clear it with a copper wire.

[arvo92]

I remember when I took my carbs apart 2 months ago for the first time. All looked rather fine and the carbs are in good condition but I had the vent opening in the float bowl totally plugged (bike has been standing for a long time). It took me good couple of days to soak/poke it clean. The bike still ran without it.

Now a lot of newbie talk which may be painful to somebodys ears:

To be honest, I only discovered today that in order to adjust the idle you need to tune not only the idle screw but there is another screw adjusting the butterfly and you need to open that one a bit as well. Listen to your engine and work on these two screws simultaneously.

My butterflies were totally closed and my idle dropped to 800-900 rpm when I closed the choke. No matter how much I tuned the idle mixture screws the bike was having hard time idling. The bike was struggeling to operate on these low rpms and would die after a while or when revving it up to a full throttle and then sharply releasing  the trottle back to zero. Then it would just choke and die in a second. Practically I had hard time on intersections as the bike would die and I had to keep rpms up while being in neutral.

Once you have the clog removed, follow this video. https://www.youtube.com/watch?v=a_HyX1qLPbQ
Chris Harris has made a pretty good instructional video about working on the idle. The butterfly screw might just be in a different place but that is it.