Which part of the carb affects which part of the rev range?

[The Other PaulG]

Firstly, apologies because I know this information has been recorded here before, I just can't find it.

Please could someone remind me which aspect of the carb affects which bit of the bike's performance... for instance the idling is controlled by X and 1/2 to full throttle is affected by Y etc?

This will really help me to track down a rough running issue I get at certain openings.

Thanks in advance!

Paul G (the other)

 

[Wrigsby1]

Something roughly like this bit found on the net...


Pilot or idle jet system. The idle jet controls the idle and on up to quarter throttle, give or take a bit. On some carbs, like Mikuni there is an air jet too. In conjunction with the idle jet there is an idle jet air screw. This screw leans or richens the fuel mixture for a smooth idle and on up to one quarter throttle. From the idle jet, there are little passages cast into the carb that lead to holes just in front of the throttle valve or plate. There can be just one hole or there can be several, depending on the carb design. They effect the mixture as long as the vacuum, in the venturi, is over them. As the throttle opens further, the vacuum moves to the needle jet and jet needle.
The Throttle Valve. The big slide that opens and closes your throttle has a bevel angle cut in one side of the big round (can be flat, too) slide, toward the air cleaner. This angle comes in several sizes and helps control the fuel mixture from idle to about 35% open throttle.
Needle Jet. This jet doesn't really even look like a jet, but it is ! It controls the fuel mixture from 15% to 60% open throttle. It sets in the center of the carb, right over the main jet.
Jet Needle. This is the needle that rides in the throttle slide and goes into the needle jet. This needle controls the fuel mixture from 20% to 80% open throttle. It can come in many different sized tapers. Sometimes, one needle can have several tapers on it. The top end of the needle has grooves cut in it, usually five, and you can move the little clip on the end up or down to lean (down) or richen (up) the mixture. Most late model bikes have needles with only one groove cut in them. This is so you can't richen the mixture, thereby keeping the EPA happy.
Main Jet. This jet controls the fuel mixture from 60% to 100% open throttle


There is Bing specific info but I can't find it... the main thing is the Main jet is not fully operational until you are using the last third of the throttle travel and that's obviously allowing for the CV action of the carb and revs...

The main wear item for running/fuel on a Bing as it would be for most carbs is the needle jet and the needle...

 

[Padmei]

If you can get hold of the Bing book then have a read. It is really good reading & pretty well explained.
I think you can buy them online or someone may have one floating around.

 

[Beemerboff]

The only problem with all that is that , on a Bing, there is no connection between the throttle valve and the twist grip - it bounces around independently, depending on the vacuum, and spring pressure, above it.

 

[Voyager]

The butterfly spindles/spindle bearings can wear and let air in - which upsets everything (and rattles).

Just something else to look out for.

A bit of grease will seal it while you are experimenting (but isn't a fix ).

I must have a go at mine too - I have a flat spot on pick-up that needs sorting .

Bob.

 

[The Other PaulG]

Thanks guys, this is all really helpful.

It's an odd mix of symptoms that I'm trying to make sense of:

Starting - starts without choke, even on a cold day (so running rich at tickover revs?)

Tickover - fine and smooth

1/8th to 1/3rd throttle opening - a bit rough, popping and banging a bit, feels a bit imbalanced

1/3rd to full throttle - fine - smooth and powerful

Backing off throttle - once the engine is hot, it takes a few seconds to settle back to tickover. Once back at tickover revs, very happy once again.


The carbs have been stripped cleaned and all the o-rings replaced. Spraying WD40 around the carbs and air hoses doesn't highlight any air leaks. The butterflies appear to close quickly (ie not retarded by sticky cables) although I will check for leaks around their bearings, as suggested above.

As far as diagnosis goes, I'm running short of ideas

Paul

 

[Rob Farmer]

Sounds like the problems your pilot circuit. I know its ticking over ok but the first part of the throttle (around 40mph in top gear) is down to the pilot mixture.

Mark the current mixture screw settings and then turn each one until the revs peak, set the tickover and go for a spin, once its at the right temperature check the mixtures again by getting the revs to peak. Then set the tickover again.

It sounds like your way too rich on the idle circuit.

If that doesn't work for you put it back to the settings you had and think of something else to try.

 

[Beemerboff]

Check the float levels - too high could cause the rich mixture. Snowbums site has the insttrucctions for seting them by measuring the fuel depth in the bowl, and that works well for me.

There are also two air bleed passages which run parallel to the main intake - blow carb cleaner down them.
The larger one on the right should exit freely from round the slide needle, the smaller one on the left from one of the pin pricks below the butterfly.

Another thing than doesn't get much air time is checking the springs above the slides.
Bings springs are low quality and dont last long, and one usually sags before the other, which doesn't affect the balance at tickover, but makes a big difference once the slides are moving.

The latter, and stiffer, springs have a different OE part number, and work well, usually with the needle one clip lower, but you still need to check that the new ones have the same tension - just put the spring with the slide on top an the bench and measure how much the spring sags.

 

[Wrigsby1]

Sounds like the problems your pilot circuit. I know its ticking over ok but the first part of the throttle (around 40mph in top gear) is down to the pilot mixture.

Mark the current mixture screw settings and then turn each one until the revs peak, set the tickover and go for a spin, once its at the right temperature check the mixtures again by getting the revs to peak. Then set the tickover again.

It sounds like your way too rich on the idle circuit.

If that doesn't work for you put it back to the settings you had and think of something else to try.

When I read this the 1st time it gave me the lightbulb moment and helped to cure my RT's hanging idle... It makes obvious sense as does the wear that is often ignored on the needle/needle jets... Thanks Rob...

Also the OP didn't say whether he had replaced the diaphragms and checked for smooth operation of the slides...?

 

[The Other PaulG]

Update -

Thanks for the ideas, I spent a couple of hours on then bike this weekend.

Although I'd cleaned the carbs pretty carefully but without improvement, spraying carb cleaner into the air intake whilst the bike was running did seem to improve matters a bit.

I adjusted the pilot mixture screws - I was able to turn them by about 1 1/2 turns in either direction before they made any noticeable difference . This felt wrong, I'd expected them to be far more sensitive so I returned them to where they were. Any thoughts on that?

Yes, I have indeed changed the diaphragms and the slides seem to move smoothly and fall under their own weight.

Paul

 

[litelfun]

may be your exaust is drawing air if its patching or back fireing !!! if thats what hapens when you let of the throtel off

 

[The Other PaulG]

Thanks for that response.

The bike is now running pretty well - it's smooth on the uptake, it barely pops & bangs and just feels better balanced throughout. I will definitely look at the exhaust, though, to see whether I can iron out the last of the pops. It's very possible that there were a few minor contributing factors to the rough running, and that's why they take some identifying.

Although I didn't find much difference altering the mixture screws (I still don't understand that) I wonder whether the act of adjusting them in and out helped to clear some blockage??? Who knows.

The bike still has its age-old issue in being slow to return to tickover when hot - I know that volumes have been written about that so I'll just continue trying all the fixes that have worked for other people until I nail it.

P

 

[Kenny]

As you may know, pops and bangs indicate a lean mixture or there are leaks in the exhaust system. Put your hand over the end ( with a cloth - feckin' hot them exhausts ) and see if any gases are emerging elsewhere. Backing out the mixture screws will richen the mixture

 

[bumpmuncher]

just to add to the confusion a hanging idle can also be caused by sticky bob weights in the bean can