Carb adjusting not working...

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madjap

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Have tried the 'dead cylinder' method now with zero changes. I'm gonna give you guys some info and see if there might be a consensus pointing to my issues.... Doing the DC method and I noticed that I have a 'slowly surging or drifting' idle. It does it no matter what adjustment I make to the air mix screw. I'm TRYING to keep my motor idles down as low as I can during this test and I can screw my air mix screw all the way to stop and it isn't really changing my idle. Doesn't seem to matter much WHERE I set the air mix screw, I get the surging idle, 'runs up for a while, starts slowing, then picks up again'. I tried spraying WD40 around my intake manifold checking for vac leaks, but it's soooooo hard to tell any change! Certainly no immediate or obvious change. Oh and I checked my float level using the see thru tube and all is well there. When I have both spark plugs hooked up, I have excellent throttle response and no issues with high RPM's, it's the idle that is KILLING me here...! Just a up and down 'hunting' idle. Thing starts good, raps out good. Pulls good.
It's KIRRING ME!!! Thanks, madjap

EDIT: 72 motor, good compression, BS34's, very recently and completely cleaned, new fuel filter and lines, long straight pipes WITH baffles, foam air filter (not cone).
 
I have never used propane..........................its just a tip passed on by others on the site. It doesn't really matter whether it goes up or down, its the change in rpm that should be obvious. However I suspect the rpm will increase.
 
Air leak causing no response to mixture screw changes? I've never seen that. If the idle mix was lean due to an air leak the test cylinder would die well before the mix screw closed. Not knowing the history of your bike and carburetors, I'd suggest that you check to be sure that the pilot air jet is unobstructed and that pilot jets are correct series. Baffles crammed into straight pipes result in a very restrictive exhaust system. You may need to lean the pilot jets and/or pilot air jets a step. Make sure that the pilot jet chambers have rubber plugs in place and that O rings and washers are in place on mix screws. Once you have the motor responding to mix screw settings, check timing with a strobe.
 
Not unusual to have to clean a pilot jet orifice or choke orifice in the float bowl more than once.
How far open is the throttle stop screw?
bs34 throttle stop.JPG

It should look real close to this, this no matter what pipes, air filters, or jetting, you have. This screw setting results in the plate having just that one idle fuel hole uncovered. If the screw here is turned in much further, the other idle screw holes get uncovered and the mix screw no longer will offer much effect.
There can be many reasons your plates are not at the proper position. a clogged orifice, a weak cylinder, an air leak, improper spark or timing carb snych to name a few. 2many recently did a nice illustrated explanation of the throttle plate vs idle circuit relationship.
 
Unable to find the sweet spot when adjusting the mixture screw:
1) May be that the butterflys are open too much........................reduce rpm as low as possible, i.e. close to a stall
2) The pilot jet may be too large......................reduce the pilot jet size by one step.
 
So I went down and tried the dead cylinder method on period piece. I came up with the same results as madjap, doing dead cylinder tests,no noticeable difference with the idle screws unless they were basically seated, put em 3 turns out and called it good.
But my idle is steady, doesn't drift other than the usual raising a bit as the engine warms up
 
I've been following Madjap's progress and I think it's just dirty idle passage ways no fuel getting through the idle passage ways and the screw won't adjust anything.... it's drawing the fuel at idle from the mains .... so Gary's idle screw settings should prove that !
clean the choke passageways ( bowl too) and the idle passageways screws out and compressed air and make darn sure you get wd-40 out those 3 little holes on each carb.... that is where your engine gets it's fuel at idle ! or is supposed to !!!!!!!
yes that means another session of the carbs on the bench and all apart but ONE more time won't kill ya ! <Grin> ( I didn't think so either when I had to do it the 4th time myself! ).... but I'ed almost guarantee you its still dirty carbs ! these carbs have the hardest to clean holes in them than I've ever seen on motorcycle carbs ! ..... carb cleaner is our friend ! try not to get it in your eyes while spraying down the idle screw hole ! .....like I DID ! .... BUT IT DID COME THROUGH THE HOLES ! ha !
.....
Bob..........
 
2M is right, the E.P.A. strangled BS34s don't respond to mix screw adjustments like the earlier carbs do. You may not find that half turn or so "sweet spot" where the idle speed is fastest and smoothest, and falls off if you go past it in either direction. As Gary noted, sometimes nothing happens until you pretty much close the screws completely. For that reason, I don't use the dead cylinder method or even try to do one carb at a time with the BS34s. I use a little bit different technique. I set both carbs together at the same time, relying on little throttle blips to check my setting.

I start with both screws set the same at about 3 turns out. I then start turning both in about 1/4 turn at a time, keeping them the same, checking each adjustment with little throttle blips. I'm watching for popping and/or hanging RPMs. This indicates I'm getting too lean. That can start happening by 2 to 2.5 turns out. When it does, I start working the screws back out until the lean symptoms go away. That's usually somewhere in the 3 to 3.5 turns out range. Make sure you keep the screws set the same because having them mismatched can give these lean symptoms too.

As 2M also mentioned, the proper way to set these is with an exhaust gas analyzer, but who has one of those?
 
when I used the dead cylinder test on my bike , a 83'xs650 Heritage Special. i had no problems at all just hearing the increase of the RPM from a slow idle, ... granted my carbs were very close to being synced at that time already... so that may or may not make a difference.
....it's been my experience through the years that when an Idle mixture screw doesn't change the running it's almost always due to dirt and crap in the carburetor.,... but that doesn't mean it's chipped in stone and infallible !..... these BS34CV carbs are very picky and need everything Correct before they operate Right ! ..... you can take that to the bank ! there is no such thing as being just close on the float adjustment, or having a small vacuum leak or even One plugged passageway in the carb... it ALL has to be RIGHT !
.......
Yes you do not have to use the Dead cylinder method, there are other ways , usually used by more experienced mechanics that can hear the subtle change in the settings, Many guys use the feel the exhaust method when at idle you adjust the idle air screws to the same puff of exhaust the pressure is what your feeling for and they must be the same....
I've even heard of guys syncing the carbs this way by using a very high idle setting and then adjusting the sync screw for equil pressure out both sides...... don't be confused,..... there are 2 synchronizations going on here, one at idle to get the burn just right at idle and equal on both sides, and then for the throttle once it's off idle and turning a decent RPM the throttles must move in unison to give power and balance to the engine.
.....
my 2 coppers !
Bob...........
 

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