Float level adjustment Q's

[Grizz]

Got a '80 with the bs34 carbs, brass floats.

originally set them to 27.3mm, went through carb tuning procedure outlined here:

http://www.factorypro.com/tech/tech_tuning_procedures/tuning_carbtune,CV,lower_rpm_engines.html

running 132.5 main jets, giving me good pull at >2500 rpms. non-adjustable needle clips.

step 3 in that link is adjusting float height. 27.3mm was too rich. when i cruise at 1500-2000 rpms, then all at once open it to full throttle, the bike sputters and pops. did this around the block, check spark plugs and they were black. thats why i figure float height needs to increase. this problem goes away when i increase my initial cruising rpm. ex. if im cruising at 2100 rpms, then i give it full throttle, i get a nice pull and i go faster.

i increased the float height to 29.3mm and it is still the same thing. at this point i moved on to mixture screw and syncing, then i set idle speed. so as of right now, this seems to be my only problem with the carbs (cleaned and checked all components).
(not very concerned about this problem because 1/2 to 3/4 throttle at any rpm works, so the bike is still very rideable.)




TOO LONG; DIDNT READ: 27.3mm = bad, 29.3mm = still bad.

should i keep increasing the float height? when would increasing become unreasonable (32mm? 35mm? etc.). also, does anyone have any tips for adjusting float height with minimum hassle?

fully removing the carbs from the bike is a pain. i appreciate any and all help guys

 

[650Skull]

Here is the holy grail of the XS650 carb Guide.

Others will tell you your float level and also direct you to this tutorial.

 

[TwoManyXS1Bs]

That FactoryPro website is quite interesting. That particular carb tuning link is very informative, aids tuning understanding, but is oriented towards low-rpm engines.

A cheap addition to your tuning toolbox is the petcock(s). There's about one minute of fuel in the float bowls. Switching the petcock(s) off during a carb evaluation, and knowing that the fuel level will drop during the test, and observing what happens when, can add insight to the carb tuning matrix...

 

[CDNTX650]

I have been dealing with a similar issue, slightly higher rpm though. I can go from a stop great, gentle or hard throttle. But if i am cruising at around 2500 or so for a bit and get on it hard it cuts out. My pilots are one up, screws set at around 2. No needle shims and 140mj. Uni pods and mikes brit mufflers. Plugs look great, bike runs and idles great.....i know its a rich problem there. Doesnt do it cold. Might try stock pilots again.

 

[TwoManyXS1Bs]

Sometimes you can experience what's called 'fuel delivery lag', where the fuel needs a little more time to reach its outlet. Sometimes solved by raising the fuel level (petcock test helps here), other times by tweaking the fuel/air (bleed) balance, or could be a sluggish slide diaphram.

Then again, could be rich (since it doesn't do it cold). Petcock test...

 

[5twins]

Do you have the rubber plugs in place over the pilots? They need to be there or the bike will run rich, especially the pilot circuit. Another often overlooked item is the o-ring that seals the float needle seat into the carb body. If it's bad, the needle seat can pass fuel around its outside, not just through the metering orifice as it should. If it's a small leak, running at higher speeds and RPMs may be able to use up that extra fuel without causing problems. At low speeds/RPMs though, the bowls could be over-filling.

 

[CDNTX650]

I have rebuilt these bs34 right from bodies up, new throttle shaft seals and all and new orings on the needle seat for the floats. Using clear tube, i can measure and it will not rise after float shuts off. Im just at the flange where the bowlsteps out, not the gasket seam. Yeah plugs are in place on the pilots. New orings and washers on pilot screws. This one is a head scratcher....maybe try leaner on the pilots.

 

[CDNTX650]

One bigger pilot shouldnt be too rich should it? I can put both screws in enough to make it not run...so i wouldnt think to rich.

 

[5twins]

My post was intended for Grizz but we can take a shot at your problem as well. A flat spot is usually a lean indicator. No, one up on the pilots shouldn't be too rich. Your jetting seems to be good, usually what works for most. You might try a main jet change, up or down one, to see what that indicates, but I'm thinking maybe what you really need is a tad richer in the idle circuit. Now, going up another size on the pilots could be too much but there's a way around that on the BS34s - the air jets. You can achieve richness levels midway between pilot jet sizes by tinkering with them.

A #135 small round main is used for the air jet in the BS34s. By reducing it a size or two, you can richen the pilot circuit a bit more. I see you have a '74. You should have the small round main sizes needed to try this (low 130s or a 127.5).

 

[weekendrider]

5twins is this an option besides shimming the needle or in addition to shimming the needle?

 

[weekendrider]

There's about one minute of fuel in the float bowls.

I never thought to time the event. I know the distance to dying seems to vary with terrain.

 

[Grizz]

did another test this morning, same thing as before except i did it while the engine was cold. the lag and sputtering is much less noticeable when cold, so this confirms my suspicion that @ 1500-2000 rpms the bike is rich.

@650skull - yup ive read that before. went by the factory pro write up bc it seemed to put the same info in a how to. after rereading the carb guide, it looks like i should try decreasing pilot jet rather than increasing float height.

@twomanyxs1bs - thats a great idea, problem is my petcock doesnt have an off position. ive thrown a clamp on the fuel line before, but i dont think thats a great idea while riding.

@5twins - i have the plugs, new o-rings on the float needle seat. in fact new float needles as well, used the rebuild kit from mikes.

so, now it looks like i should put the floats back to 27.3mm and decrease the pilot jet. problem is i wont be able to do this for a few weeks. got a long road trip in a couple days, not enough time to order any parts. i will let yall know how it goes when i get back to ny. like i said before its still rideable, thanks for all the help

 

[5twins]

Well, that air jet feeds the idle circuit, so I guess you would use it in combination with needle changes if you have an idle to midrange transition problem. I suppose you could do one or the other, or both, whatever it took to solve the issue. Running pods, I think an air jet change and experimenting with various sizes would be easier (access-wise).

I'm basing this on a problem I had years back on another bike. My jetting was close but it turned out the main was one size too big. The bike ran fine and revved up through all the gears from a standing start just great. However, if you cruised for a while in the midrange, then rolled the throttle open to accelerate, it would stumble a bit before taking off. I think the main being just a little too big was loading up the plugs during extended easy midrange cruising.

This problem could be similar but at the other end of the midrange, the lower part, and be a just slightly lean condition. The bike runs up fine through the gears under large throttle applications, but has a problem after some cruise time at a steady RPM in the lower midrange area (2500 RPM).

This is the fine, fine tuning stage of dialing your carbs in. You can usually get them close in a couple tuning sessions but the little glitches take some miles of use under varying conditions before they may show themselves. You're close so it usually doesn't take much of a change to fix things, but some of this fine, fine tuning or a little tweaking may be needed.

 

[gggGary]

Grizz all stock, airboxes, filter, exhaust? or?????

Doing the dance with an 82


Highjack (kinda)

BS34 foam straight through shorty mufflers on stock special headers (yes, too loud, but this is a fixer upper) thick green UNI foam wrapped over stock frames in stock air boxes, not oiled yet, BP7ES plugs, chased a >5K flat spot from 137.5s up to 142.5 a step at a time. They also are two steps up on the pilots. Put in a new charging rotor with a noticeably stronger pick up magnet and 5.6 ohms slip to slip instead of 4.6 and now the flat spot is gone. Now I gotta work the mains back down. With the new rotor, a full throttle pull plug chop looks like this.



I swear a wide band O2 gauge is coming to my garage soon.

 

[CDNTX650]

Thanks for the tips, i have some of those small mains around...have to swap em out. I did try a 142 main and it felt soggy, a 137.5 did the surge after letting off the gas at high rpm. So im gonna stay with the 140for now.