BS-34 Carb questons

[gentlemanjim]

You may recall I rebuilt my carbs. Installed K&N's and Mikes XS 2:1 Exhaust. Bike ran great in neutral free reving easy starting. Out on the road it wouldn't get past 5,000 rpm and ran terrible sputtering and popping. It seemed like a lean condition as I put in old dirty plugs and after a 2 mile ride they were brilliant white and clean.

I pulled the carbs apart hoping to find some stupid error when I assembled them. All seemed fine.

Main Jet is 137.5, Pilot jet is 47.5 and the jet tha goes into the inlet of the carb on one side. Don't know the name is stock 5EI stamped on the end. Needles are stock also, I felt the oversize main would add enough richness to the mix.

Also there is an orifice on each carb that appears to be blind. see photo below. Is this correct? While recleaning some hard plastic like material came out of them, but appears to have a brass fixed plug.

It doesn't seem likke the carbs are my problem. So what's my problem.

 

[xjwmx]

When they drill the passages out at the factory they drill straight lines and if they wanted a curve they intersect two of those at some angle, then plug the unwanted openings with brass plugs. If you follow what you've found on down you'll see what intersects it. I don't remember right off. There are several versions of BS-34 that are real similar to each other but different. Incidentally, a good way to see if your diaphragm is sealing is to blow in the oblong hole on the air box side, and the piston should go up and hold.

 

[xsjohn]

Someone on these boards has talked this subject to death and has worke for 12 years to design jet needles for the BS34......well he is still around.............

 

[nj1639]

John is the man for the bs34's, and numerous other custom improvements.
Glad he's still around.

 

[gentlemanjim]

John, glad to see you are back. How's the vision issue? I got your PM and will give you a call. As you cansee my carbsare spotless and the jets I used were your recommendations. I'm wondering if the cause of my poor running is somewhere else. I adjusted the valves maybe thats the source to recheck. It's an '83 with TCI ignition so I think not there.

 

[twinsarehot]

hey guys, I hate to thread jack, but how do I remove the part circled in the picture, the jet holder? I sourced a carb body to replace mine, but this part is missing so Ill have to swap, I don't want to break anything as these old things are fragile. I hear I should use a wooden dowel? Should I push from the top or bottom? Advice?

thanks in advance!

 

[Lunatic]

The Needle Jet is removed from the bottom after removing the main jet. It should just slide out.

 

[grizld1]

Those are the needle jets (AKA atomizers, AKA emulsion tubes). They're installed from the top down and are retained at the bottom by the main jets. The easiest way to free them if they're stuck is to remove the carbs, remove the float bowls, loosen the mains a couple of turns, and tap the mains a bit, using a soft drift (screwdriver handle, piece of wooden dowel, etc.) until you get a little movement. Then remove the mains and push the NJ's up until you can pull them out from the top.

If there's any corrosion in the NJ's, replace them. The ID is critical, and material lost in removing corrosion will change the size enough to cause trouble. In Mikuni NJ's a difference of two number steps (the usual sizing interval) means a difference in ID of .00039" (.01 mm).

The only Mikuni NJ's that are installed and removed from the bottom are the thread-in type found in the radial smoothbore pumpers (RS, HS, HSR, TM36/68, and related designs).

 

[Lunatic]

Crap, that's what I get for answering a question when I don't have a set of carbs in front of me... as grizld1 said they slide out of the top not the bottom.

I've had mine apart often enough I should have remembered that.

 

[twinsarehot]

Thanks guys!

Lunatic - I forgot how old I was the other day, happens lol.

 

[twinsarehot]

Hey guys, another situation that I need help with...this will take a little more thinking?

My 650 started to run roughly after some neighborhood riding, basically barely staying in idol without pulling on the throttle. I immediately shut it down and pulled the carbs to clean thinking it was dirty carbs.

At this point I've replaced most of the carbs, 46 pilot jets, 135 mains, floats adjusted, johns needle kit (center clip setting). I've got pod filters and a 2-1 exhaust with a reverse cone. I had to replace the body of the left carb as the PO royally fucked up the fuel mixture screw.

Before I slapped the carbs back on, I adjusted the cam chain and the tappets. I know that the plugs are both getting a strong spark. I also changed the sprockets to 17/32. That's basically everything I've done...

So now when I start the engine, it starts first kick always but it's rough running still, barely running by itself. Strange thing is, the left carburetor seems to be backfiring the fuel mixture out the carb intake. I hear this is caused by a lean mixture, but I'm not sure that's the case. Basically sounds like the left side isn't exactly firing correctly. Carbs also aren't 100% balanced. Any help?

 

[XSLeo]

Gentleman jim, The air jet from the bell of the carbs probably reads 135 not 5EI.

 

[XSLeo]

You might try moving the needle up one notch by moving the clip down one notch. Another thing is that a hole or crack in the slide diaphram can keep the slide from opening completely.
At about 5000 rpm you should be getting into the wot area. So if the slides won' lift past this amount the carbs won't get past the midrange area. So no rpms above 5000.

 

[xsjohn]

Wouldn't move the needle clips ....the only time that has been necessary in 500 sets was one person that used a specialty cam.....that set had to be lowered one notch.........If it's not dirty carbs then it' easy to set the larger pilots to rich..........best way I have found to set them is to warm the engine to operating temp (say after 10 miles or so) the run along at low throttle (30-40 mph) and check for soot on the plug rung........just a "very" faint trace is good...setting each side individually....heavy trace is too rich and none may be lean.........here is a pic that may help..........and the wiring harness needs to be cleaned well and especially the large muti plugs that work the ignition......I have had to clean mine twice in 78 thousand with problems resembling yours.......don't forget the 2 wires at the coil..........hope this helps....

Edit.........forgot to mention.........with the larger pilots you need to do the final set in warm weather like 80 degrees and above since it will be much richer then.......and this setting should be fine for the winter ................

xsjohn

 

[twinsarehot]

Hey John!

Well the thing is, the bike really can't be ridden at the moment. By setting the larger pilots too rich, are you saying that I should try switching back to the smaller main jets? I'm hoping that it's not carb problems, because with the settings I have it ran beautifully, plugs were fine.

I'll check over the wiring, I'm hoping that will be the source of the problem. Would it be a timing problem?

 

[xsjohn]

The larger pilots are better .......for sure........xsjohn

 

[Sundie]

out of curiousity, how did the plugs look when this was happening?

 

[twinsarehot]

Not entirely too bad. I checked those first when the problem started, looked to be on the rich side, I cleaned those up before I pulled the carbs. I then cleaned the carbs and lightened the mixture with the needles (my first attempt to fix).

I then adjusted the cam chain and the tappets, before slapping the carburetor back on. Couldn't tell if it helped so I pulled them again to replace the carburetor body and some old jets; I also switched the clip back like John was saying...and this is where I left off before I posted up.

Could it be I totally messed up on the cam/tappets adjustment? I would be surprised if I did. Could it be the idle mixture screw not being dialed in? I just replaced those and haven't played around with them much, they are 3 turns out on both sides.

 

[Sundie]

way too rich, IMHO. start at 1.5 turns.

Wait, you *leaned* them to 3 turns out? Whoa.

edit - by the by, i've always found the good starting point to be one size up on the mains, stock pilot, 1.5 turns out on the idle screws for open pipes/pods. And if you're not caning it to death (which some of us do) that will probably make you happy for the rest of your riding career.

 

[twinsarehot]

no no, I haven't had it running since I replaced the mixture screws, I had just preset it to three turns out, which is what the 650garage carb guide said. I'll start over again starting at 1.5 turns. I thought 3 turns sounded a little strange.