Single carb conversion thread...

Hi, guys...

I have discussed this topic a bit in the context of long-term carb woes with BS38s, but I thought I'd start a dedicated thread on this topic for ease of discussion and continuity, both for me and others who might be interested.

My manifold is beautiful. It came from the shop of joe-wiseguy.com. The carb is a Mikuni VM 36mm. So far, I have the manifold mounted, and the carb and fuel line mocked up. Main and pilot jets are on the way, as is a two-into-one throttle cable of which I'll use one side.

I wanted to post a few pics because I have a few space issues and need suggestions and ideas.

The first issue is that my left side cover will have to be either VERY heavily modified, eliminated, or replaced; it will not fit unless I remove basically the front half of it. I am looking for suggestions as to what to do there. One option would be to fab flat left and right "covers" out of aluminum or stainless, but I really don't have those skills.



Secondly, you'll note that because of the placement of the carb throat, my relay(?) and whatnot from the battery is in the way of any prospective air cleaner. I was wanting to run a velocity stack or K&N cone, but the connection for the relay is right in the way. I could use a flat bracket to move the relay back about an inch, but that's not much clearance. Or, I could try to get some fuel hose (like what mounts the carb to the manifold) in an elbow, but that would alter the angle of the carb to the manifold. Rather not do that. Or, I could use some sort of filter that basically is just a breathable "cap" for the carb intake. Ideas?



If you look closely, you'll see that I'm using the customary 1/4" clear fuel line off each petcock that runs into a 5/16" brass T that will be zipped to the backbone. That flows through an in-line filter to the carb's fuel inlet. Should be fine.



So, once my jets and cable get here I'll set that up and try to get the bike running and dialed in. The cosmetic bits I have mentioned are nagging me, though.

Suggestions welcome!

TC

Hi Teecat,

Yeah it looks a bit tight - is there enough room under the sidepanel (or half sidepanel) and enough play in the wiring to flip the bracket? Doesn't look as if you need much clearance to mount a pod type filter (though it might get in the way of yer leg!)

Also doesn't look like much clearance for the bend of the throttle cable?

T.C. I would alter the side cover to fit. You can always add plastic or rubber trim to make it look factory. First I would make a fiberglass copy of my side cover and do the alterations on the copy first. Good luck The other T.C.

I hate to be that guy but they also sell one that is supposed to fit a standard without modifying the side covers and you won't have to worry about your leg hitting the carb.

Guys, thanks for your interest!

While you were writing, as luck would have it, I found a little chromed/plated metal bracket about 1.5" long with a hole in one end and a threaded post on the other! JUST what the doctor ordered! So, I mounted the new "bracket" to the existing one on the battery box with the original screw, and then mounted the relay assembly to the threaded post with a proper nut. And as you see in the pics, I may well have bought myself just enough room for a short stack, small cone, or other filter with an angled mounting flange. I turned the key and nothing exploded... I was just wanting to ask you all if it was safe to do that, but it's still metal to metal and I have the grounds/terminals in the same place, so it seems like the perfect solution for that one source of interference. Seriously considering your advice on the side cover guys... thanks!

As to the throttle cable, I have seen them going in at 90 degrees, so I'm not too worried about that, really. And it's not as sharp a bend as it looks when it comes down from under the tank.

TC

Has anyone made an updraft manifold using a Zenith Carb? It looks like there is enough room. Tony C

Oh, and guys, the carb itself is actually just behind my left knee in any position. Not really an issue there.

Filter/stack recommendations welcome. A stack seems more likely to fit as it will be closer to pipe-shaped with no protuberance.

are you running a stock battery box? the starter solenoid doesnt belong on the left side? must have moved it there? fyi on the clear fuel line. most fail unless you got the good stuff. gets brittle and flat out cracks.

joe, the mounting location of the solenoid appears to be stock. It's an early bike and the battery poles are reversed on them... positive on the left.

I only keep fuel line for about a year. Your recommendation may become important if I actually get to ride the bike with any regularity if this carb conversion solves my problems.

Oh, question:

Should I leave the air screw in the factory position when I try to start the bike, hopefully sometime this week after I get the jets and cable?

TC

So, since I can't yet try to start the bike because I the cable and jets are not here yet, I thought I'd try to go ahead and get the air cleaner issue sussed. I had made a tad more room yesterday by moving my solenoid back about two inches. So the options basically were a velocity stack (which I was leery of because of lack of meaningful filtering), a K&N cone, or a foam barrel filter. Any of those would have been aesthetically acceptable to me as long as filtering was suitable, but I was still worried about clearance issues.

So I had a thought before I went to sleep last night. I figured I'd take an automotive idea and try to apply it to the bike, as I have seen a lot of you more skilled guys/builders/modders/fabricators do... kind of a street rod application, but on a bike, you know?

So, I like this company called Spectre. They do modular automotive air induction bits; I used them for my cooling plenums. I had some bits left over that I never returned, so I used a 60- or 65-degree bend to get me out away from the solenoid. It's chromed ABS, so I put it on the bandsaw and shortened it as needed from both ends. Then I took an elastic plastic coupler and test fit it to the carb throat. It's perfect:


The only thing I don't like at this point is that the carb mouth (see it in there?) might cause some turbulence, but I don't think it will matter for a street application that rarely, if ever, sees WOT. Hope not, anyway. Maybe there's a way to smooth that out with an insert of some kind. Meh... later on. The air holes/ports for the carb are completely unobstructed, so that's the main thing.

Anyway, next, this morning at Pep Boys (infinitely more useful than my local bike shop, I'm sorry to say ), I had picked up a little mini filter designed for a street rod multi-carb application; it happens to be a Spectre piece as well. $12.00. I love this stuff. It was really light, but sturdy, and attractive.


It's designed to mount to an automotive carb throat with one little screw, but the neck snugly fits inside the elbow with a little file work. I'll drill a little hole in the rim of the elbow and use that screw to secure it. You can see here how unobtrusive it is, but I think it looks pretty trick, and there should be plenty of breathing/filtering allowance.


And it's not in my way, and weighs very little. I will make a support bracket for it, though, out of polishable aluminum stock I have left from the hardware store.


Looking down from the left:


From the left flank:

And from rear. slightly left, so you can see the "footprint"... not much of one, really.


I still have to mod the left side cover. That will be scary. But my main concern now that I have the conversion all mocked up will be getting the carb jetted and the bike running...

TC

Looks as if you're nearly there. I will be interested in how she runs with the single carb compared with the stockers. You might be starting a trend, with all the threads about carb issues (touch wood mine have been fine so far, after the initial obligatory rebuilds of course). Hope the jetting isn't too much of a PITA.

ady, thanks for your interest. Credit for this project (or trend if there is one) goes to joe-wiseguy on this forum, who is hand making several intakes for this application and has them dialed in. He has given me a pilot and main jet baseline that he reports is widely successful. My engine is stock so I don't see any obvious complications to my getting the bike running and dialed in. Hope not, anyway.

TC

TeeCat said:

joe, the mounting location of the solenoid appears to be stock. It's an early bike and the battery poles are reversed on them... positive on the left.

I only keep fuel line for about a year. Your recommendation may become important if I actually get to ride the bike with any regularity if this carb conversion solves my problems.

Oh, question:

Should I leave the air screw in the factory position when I try to start the bike, hopefully sometime this week after I get the jets and cable?

TC

Click to expand...

start with the fuel-air mix screw out about one and one quarter turn..... adjust from there. good starting point.

Thank you, sir. Will keep you posted!

TC

Only thing I would try to do is add a shield to go over the element facing upward to block out rain.


Have you ran it yet???

Your a funny fellow TC, 2 years of planning when it came to fixing your flashers, and you do all this in a week!

I think you said earlier you are planning on using a 1 into 2 cable. Sounds dodgey to me. Maybe a cable for a XT (or other single Yamaha/or any dirt bike) to Mikuni would fit? If that doesn't work, maybe change your throttle at the bars to accept a more generic cable????

P.S. I like the fact that you've dived in head first on this one!

Ghetto... that's a good idea, though I never let my bikes see rain if I can help it.

Yama...

Your a funny fellow TC, 2 years of planning when it came to fixing your flashers, and you do all this in a week!

Click to expand...

Okay, that made coffee come out of my nose. But yeah... I'm a bit of a mystery to a lot of folks. I don't even really understand myself.

As to the cable, I'm using one that will have one attachment at the throttle, and then I'll eliminate one cable on the other end, so it will be like a single cable. I'm doing it mainly due to the guarantee of a correct nipple fit at each end.

Hope to have the cable and jets early this week. If I can get the jets in and the cable connected without committing any boogery, I'll try to fire her up. *cries*

TC

looks good. nice job.

I don't rain ride either but sometimes you just get caught!

TeeCat said:

Okay, that made coffee come out of my nose. But yeah... I'm a bit of a mystery to a lot of folks. I don't even really understand myself.

TC

Click to expand...

You & me both brother