Bs 34s on an '81 H high Idle

NorazDad

NorazDad
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I have been having a heck of a time with these carbs and the running of my bike. I had a crazy post a year go about how the vent in my gas cap sent me down a rabbit hole that took months to resolve. Recently I have had a similar problem caused by me due to the collapse of the fuel line T mod I installed years ago. Essentially, the bike was running on one cylinder due to lack of fuel on the right side. Found it and fixed it.
In the meantime, I have cleaned and cleaned and futzed with these carbs for weeks. Once I found the problem and fixed it, I returned my jetting to what it was prior to the craziness, remounted the carbs for the umpteenth time and the bike fired up and acted perfectly normal...except the idle is at 2K and would not change with the air mix screw.
I am at 132.5 mains, needle at 1/2mm shim, pilot 42.5 and 135 air jet. The question is whether going up in size on the pilot will help to take the idle down, or should I look to some other screw up I might have done when I split the carbs to insert the new T?
I also recently installed Hugh's new CDI system, which I probably didn't need to do, but I was chasing rabbits.
Any advice other than to reinstall the original ignition would be most appreciated.
 
First off, you don't have an air mix screw, it's just a mix screw. It doesn't control the amount of air going into the idle circuit but rather the amount of air/fuel mix coming from the pilot jet. It isn't the usual adjustment for trying to lower (or set) idle speed. There's another screw for that, an actual idle speed adjustment screw .....

ZpMeHnq.jpg


If backing that out doesn't lower the idle speed enough then the next thing you'll need to check is the sync. Maybe you threw it off when you separated the carbs. Or maybe you didn't mesh the linkage back together correctly .....

nwZjyog.jpg
 
another thing that you might check is the throttle spring tension which will probably be weaker than when new.
Try pressing firmly down on the throttle shaft bracket with your thumb and see if the idle goes any lower .
The throttle springs are at least tension when at idle. I improved my idle by adding an extra spring to the throttle shafts which gave a more positive throttle as well.
Check also that the butterflys are centered in the venturi and there isn't too much float in either throttle shaft.

I put my mouth around the carb and blow against each of the the butterflys in turn to 'bench sync' before fitting which is surprisingly accurate
 
Ok, great advice and thanks for being kind in your responses. However, if I put a larger 45 pilot jet in, might that help to lower the idle by increasing the air flow? I am going to have to check all of that linkage again, so while I have the carbs out, I might just try the larger pilot. I don't think it could hurt, but the bike always idled well prior to my self induced curses.
 
another thing that you might check is the throttle spring tension which will probably be weaker than when new.
Try pressing firmly down on the throttle shaft bracket with your thumb and see if the idle goes any lower .
The throttle springs are at least tension when at idle. I improved my idle by adding an extra spring to the throttle shafts which gave a more positive throttle as well.
Check also that the butterflys are centered in the venturi and there isn't too much float in either throttle shaft. Will do!

I put my mouth around the carb and blow against each of the the butterflys in turn to 'bench sync' before fitting which is surprisingly accurate
That's how I found the blockage in the T, by blowing into the fuel line and flipping the floats up and down. Obviously, one wasn't getting any fuel and therefore the float needle had been rendered useless.
 
The pilot jet doesn't control air flow, it controls fuel flow. But, you have stock jet sizes now and if you've done any mods like pipes and pods, you should increase the jet sizes. Do the mains too, at least up one to a 135, but you might need two up to a 137.5. A high, hanging idle can also be a sign of running lean, so maybe the larger pilot will help.
 
If you had an air leak you would expect your idle to drift upwards and also hang after blipping the throttle so it sounds like its not an air leak issue that is causing your high idle

what rpm have you set your idle speed to ? I found with my BS38s that if its set a little too high the increased engine vacuum can start pulling fuel from the needle jet which will make it impossible to set the idle mixture correctly. Try setting you idle speed a little lower then reset your mixture screws.
Adjust the screws 1/4 turn at a time and give the engine around 10+ seconds to adjust to the new setting before evaluating and readjusting.

5twins co-wrote the excellent carb guide which you'll find in the tech section so you're in good hands ;)
 
The pilot jet doesn't control air flow, it controls fuel flow. But, you have stock jet sizes now and if you've done any mods like pipes and pods, you should increase the jet sizes. Do the mains too, at least up one to a 135, but you might need two up to a 137.5. A high, hanging idle can also be a sign of running lean, so maybe the larger pilot will help.
So, I just pulled the carbs and took a look at the butterflies on both sides. I realized that the idle screw controls them...well the one on the left and the synch screw controls the other. Anyway, the butterflies seem to be very symmetrical and close completely when adjust the idle screw. I even blew into them and found them to be tight when closed.
The big question is, how could the bike still idle at 2k with them closed? I checked my plugs after I ran the bike with a high idle and the plugs looked white and kind of ashy = lean? So, I think I am going to put the larger pilots in and see what happens.
 
The big question is, how could the bike still idle at 2k with them closed? .

because the fuel and air mixture is introduced after the butterflys ... Look at the roof of the venturis in front of the butterflys.. those holes are where your idle and choke fuel and air is delivered
have a read of the carb guide it will explain how they carbs work and how to clean , jet, sync and tune them.
 
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Did you double check throttle shaft seals? Could be sucking air if bad.
 
Did you double check throttle shaft seals? Could be sucking air if bad.
All is great.
The 45 pilots seem to really do the trick and the throttle spring is a little weak so it wasn't closing the butterflies thoroughly. I have read and studied and learned and experimented to the point that me and these BS34 are now in synch. Will do some further fine tuning, but I really want to thank you and this forum for helping me keep this bike running like it should.
There is so much to learn, but when one learns it with the help of generous guides, the thrill is ever so much greater.
Thanks again,
Rob

By the way I intend to post my experience with Hugh's CDI. I ran into some interesting mods along the way, but overall I am very happy with it. I probably have three working ignition systems, including the original.

Did I say "thanks"?
 
Here some pics of running BS34 Carbs and a pic of the Carb from the manual.......
1 d5 Adjustment fuel level XS650SJ Service manual.jpg
As can be seen, the butterfly doesn't fully seal off all the bypass holes
left..............................................right
Tet Copyright crop IMG_8159 copy.jpg Text Copyright crop IMG_8160.jpg

if you shine a light from the back it helps to see if the plates are sitting nice in the venture........There as cases where the carb throttle plate has been dragged to the side and worn into the carb body. This can be caused by not aligning the throttle plate in the center properly. they do look a bit out of center but that is also due to the pic not being exactly square on............gives an idea
copyright , crop IMG_8165.jpgcopy right , crop IMG_8164.jpg
 
I have grown attached to the carb guide...my mind is filled with thoughts of rapid acceleration.
Hey Peanut, do you have a picture of your enhanced throttle spring?
 
I have grown attached to the carb guide...my mind is filled with thoughts of rapid acceleration.
Hey Peanut, do you have a picture of your enhanced throttle spring?

It was just a 1/2" spring I had in the workshop which I attached near the sync bracket in the centre I'll try and get a picture today but I'm right in the middle of laying a new floor in 32 degree heat atm:(
I found that at idle the original spring had very low tension on my carbs and the butterflys were binding slightly on the venturi walls preventing them from fully returning to the stop setting ,epecially once the bike had reached operating temperature and the brass had expanded slightly.
i discovered this by accident when I pressed down on the throttle stop lever and the high idle immediately dropped to normal. I stripped the carbs for the umteenth time and carefully cleaned everything making sure that I centered the butterflys . I also discovered that there was too much lateral play (axial) on both throttle shafts so I rebuilt those to tolerance . There is a video about this on my channel.
 
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