Keihin CV34 to Mikuni BS34 More air restriction???

I swapped off the stock Keihin CV34 carbs that had come on my bike for a set of Mikuni BS34's that I got for cheap. The Mikuni's run perfect at higher RPMS but I have some issues. Idle is fine, but steady transition in the pilot circuit from about 2000-3000 RPM's was terrible and would cause the bike to almost die out. I've had the pilot screws from 1-2 screws out at different intervals and this did not help. I read on the forums of these carbs not running without the airbox. I have my stock keihin airbox with all pieces intact installed, so I shoved a piece of old shirt down through the middle of the air filter intake creating more of a restriction, and the bike runs soo much better. Just wondering if anyone else has experienced this need for more restriction, and if soo, any suggestions on creating the right amount of air flow for slow speeds without ruining the need for air at higher RPMS. Always something... Thanks

You need a larger (like #40) pilot jet, to get a nice smooth transition.

Also, make sure that the advancer unit rotates and returns freely. This should be lubricated periodically as per regular maintenence.

GL.

Thanks, I ordered some new pilot jets. Can't beat the advice from people like you on these forums, learning something new everyday. And thanks for mentioning the advancer I have checked and lubed it also.

I am not sure how your air box is configured, but my '82 KZ650, which has Mikuni BS32 carbs, will not run well without the top screw-on nozzle shaped piece on the air box. This part screws onto the top of the air box and it holds the filter in position. It restricts the incoming air to a diameter just over 1 1/2". Without that part in place, the bike has very little power and runs poorly if the throttle is open more than about 1/8 of the way.

DO NOT change your pilot jets. It is not the pilot jets but the needles. Depending what needles and needle jets these carbs have will affect that range. The BS34 came on many bikes from the 700-1100 and had many different jet and needle configurations. I am running these carbs on a very hot 1150 with a highly ported head and .420 lift cams and am running stock pilot jets and am still too rich. My pilots are 37.5. Give us some more information about the carburetors. Such as what they came from needle jet size jet needle size pilot size and so on. some of them had different slides and they have different reaction times. you canfind out more about my bike project here.

kzrider.com/index.php?option=com_kunena&...w&catid=11&id=169462

Describe the sounds you hear when you try to accelerate. Describe what it feels like in detail. Being too rich also gives a bogging feeling. I have the same situation at the exact same rpm range, and it is because I am too rich, not too lean

Nice Bike otakar, looks like you put a lot of work into it. I do also have backfiring on decel in this range another good indication of lean running. Like I said when I richen it up and cover the airbox opening (which does have the original filter restrictor holder in place) it runs great. I could be wrong, but TeK9iNe's advice seemed to make sense to me. I've already got the jets coming monday, so I'll stay hopeful for now. If not I'll have $12 of worthless brass to throw into my toolbox.

Tek9ine... after messing with 2 different sets of carbs on and off for months, your suggestion finally got her pulling smooth and strong all the way. I went from the stock 37.5's to 42.5 pilot jets and I bet this bike hasn't run this sweet for a long time. Thanks again!

Update and further question. Now that I have low speeds sorted out, I've been able to ride more. I've noticed a stumble at around 6500RPM only in gear under load. If you keep on it, it will try to fight through, but really does seem to lose its power there. Also have some backfiring through the exhaust in higer gears when letting off the throttle. Any advice on what this could be caused by? Thanks, Brian

On those BS34 carbs...... Never go up on pilot unless you are still lean at 3 - 3.5 full turns out with the pilot fuel screw.

At 2 turns, you're not maxing out the pilot fuel circuit flow

Otto, I've been biting my lip about your HOT 1170 motor with those oh so marvelous BS34's.

1 - you said you have to shift down 2 gears at 60MPH to have any real passing power.

2- Now you reveal you've got a rich stumble at 3K

Yet you may well be the Internet's greatest advocate for running BS34 carbs

You're bike has been finished for a year and those problems persist. Toss those pot metal boat anchors and put some real carburetors on that beast of a GPz..... You might just be surprised :woohoo:

I understand if a guy has no budget to work with or is just trying to get a stock bike running again. No problem.

On the other hand, going through the effort to build what is intended to be a hot street motor and then top it off with those things...... :dry:

Now don't get mad

Yet you may well be the Internet's greatest advocate for running BS34 carbs

Um, no, that would be WiredGeorge

But George only advocates them for near stock bikes, not big cc monsters.

I am no advocate for any carb either just trying to get my old junker running.
I'm happy with my larger pilots.. Runs fine on that circuit. I think I need to shim up the needles a bit for more gas at high rpm's does this sound like a resonable guess? Ah hell why even ask for $5 In shims I'm trying it anyways..