JS Motorsports Carbs

Marc

If you go online and search for Commando roadsters you will see that they all have the petcocks located mid way between the amal carb and the intake valve cover. Its the same as in the photos I posted on the previous page showing Keihin carbs with Roadster tanks. I've looked at a dozen 750 roadsters and they are all the same with plenty of clearance for the Keihin carbs. Your tank is different than the stock Roadster tanks I'm seeing.

You will either have to make a custom short barb (silver solder a very short 90 deg barb to the threaded part of the fitting that screws into the tank) and an inline petcock or get an original tank with the petcocks located forward and out of the way same as shown below.

This is the original Roadster tank you need.

Hey Marc,

I think Jim is right. I've looked at a lot of roadster tanks, and none have the petcock located where yours is. I could be wrong, because of the picture angle, but the shape of the tank bottom edge in your picture does not look like a normal roadster tank. It should be a smooth curve down to the bottom edge, but yours looks like it has some sort of bent flange there. Could you post a picture of your bike from the side? I'm really curious what your tank looks like.

Not trying to pick on your bike, just curious about what you have. A fair number of folks have fitted Jim's carbs to roadster Commandos with no such issues, and yours is clearly different in configuration from theirs.

Ken

Another question vrt. fitting Keihin CRS carbs. Has anyone tried to pair these carbs with the black box air filter? I know the carbs are further back than the Amal Concentric and possibly has greater length. I was hoping that rubber bellows of a suitable size and shape would do the trick. I hate buying a pair just to find that space is too limited to make it work.

-Knut

I was wondering the same thing about the black box air filter. It works fine, and I'd be very reluctant to dump it just to slot in a new pair of carbs. To be honest new carbs are quite a way down my shopping list right now, but I'd like to know for future reference.

I don't think the Keihins (or any other aftermarket brand of carb) will fit a stock aircleaner. All aftermarket carbs are a bit longer.

Here she is...







It's hard to see in the picture but I'm 90% sure this imprint on the bottom of the tank says Pakistan.



I'll assume Pakistan is part of the U.K., right? :lol:

If anyone has an original tank that they are willing to sell then please let me know. I'm in the US now but I will be in Northern Ireland for the Ulster GP next month.

That's a shame. The petcocks are for sure not in the normal location. You could probably plug them, and braze in a new pair in the correct location. Might even be able to save the paint job with the use of heat blocking paste, etc. Another alternative is a new EMGO tank, made in Taiwan and pretty high quality. They are stamped from dies, not pounded out by hand, and the finish is very nice. I've bought two of them in the last few years, and am quite happy with them. Petcocks are in the right place, too.

Ken

Marc Sullivan said:

If anyone has an original tank that they are willing to sell then please let me know. I'm in the US now but I will be in Northern Ireland for the Ulster GP next month.

Click to expand...

I have a new Emgo Roadster tank if you're interested (they're good - trust me :wink: ). I'm in NW England.

Send me a PM for more info

@Marc

Take a look at this http://www.nortonraceparts.com/cafe-racer-parts.html about half way down the page BTM 15 Confined Space Fuel Tap

It may be an answer to your issues?

mdt-son said:

Another question vrt. fitting Keihin CRS carbs. Has anyone tried to pair these carbs with the black box air filter? I know the carbs are further back than the Amal Concentric and possibly has greater length. I was hoping that rubber bellows of a suitable size and shape would do the trick. I hate buying a pair just to find that space is too limited to make it work.

-Knut

Click to expand...

It's funny you should ask about this. A new Emgo tank came in the mail yesterday and it fits well over the carbs. So that took care of the petcock carb contact issue. My problem now is the difficulty installing the K&N air filter that came with the carbs kit. Please take a look at the picture below. May have to click on it to make it bigger.




As you can see in the pic, the air cleaner bumps into the arm that runs from the metal plate in front of the battery to where it is bolted to the frame. In the picture, the bolt is actually taken out of the frame in order to lift that arm up a bit but I still cannot lift the K&N filter up high enough to slip onto the carbs. I guess I could take the whole plate out and give the filter some room but the ignition key tumbler is attached to the plate and so is the battery box. I'm not much of a metal fabrication guy. I also thought about cutting that arm off but my gut says the air filter will still not fit because the back side will be in contact with the plate and the top of the filter may hit the bracket mounting the oil reservoir. Though I could be wrong.

Has anyone else had this problem? Does anyone have any suggestions for me? The velocity stacks that came with the carbs fit fine. If I added filter screens onto the stacks would the screens work well enough for regular riding? Or would I still be getting a lot of contaminants into the motor? I appreciate any thoughts on the matter.

How about just slipping a (cheap) compressible UNI sock filter right over each velocity stack? Or maybe screen filters from http://www.pjmotorsports.com/velocity-stack-filters.html


Or a super fancy splatter screen from the cupboard.

Beaneater

I don't know what kind of oil tank you have that has the strap so low. See the two photos below and you can see the strap is located above the air cleaner.




A small screen on the velo stack will be very restrictive.

I removed the air filter back plate and remounted the ignition switch on a strip of steel screwed to the battery tray but not far from the original switch. This setup also frees up space for my Boyer powerbox to live below the filter with a small plate fitted to the screw holes that are redundant from the air filter plate.

I never thought of that metal plate being part of the stock air cleaner but now I see how it is. I'll pop that off asap and see if it fits better. Thanks again for your help Jseng1 and gripper.

I removed the back of the airbox and moved the oil reservoir arm a little to the right and it now fits like a glove. Thanks again for your assistance.

So after a year and a half I finally got the carbs on my bike. They're awesome and I'm glad I bought them. Thank you all for your assistance.

I've been running Jim's carbs on my '71 Commando for about 2 years. They are not flat slides but 31mm round slide "competition only" (Ha) Keihins. I love them. I screwed around with jets and needles because the plugs were rich. When I got 'em to run tan, it was so lean the bike ran like shit. So I forgot about plug readings and set it where it ran best, and that was just as Jim shipped them. The only weak spot was the rubber manifolds that deformed easily when I tightened the clamps tight enough for the carbs not to sag (They're a snug fit and don't sag far, but I didn't want them resting against the frame). So I got some of that super Gates hose from NAPA, attached a spring retainer between the top of the carbs and frame, and problem solved.

Tangozulu said:

I've been running Jim's carbs on my '71 Commando for about 2 years. They are not flat slides but 31mm round slide "competition only" (Ha) Keihins. I love them. I screwed around with jets and needles because the plugs were rich. When I got 'em to run tan, it was so lean the bike ran like shit. So I forgot about plug readings and set it where it ran best, and that was just as Jim shipped them. The only weak spot was the rubber manifolds that deformed easily when I tightened the clamps tight enough for the carbs not to sag (They're a snug fit and don't sag far, but I didn't want them resting against the frame). So I got some of that super Gates hose from NAPA, attached a spring retainer between the top of the carbs and frame, and problem solved.

Click to expand...

They all go out with Gates rubber hose now - been doing that for awhile.

jseng1 said:

I don't think the Keihins (or any other aftermarket brand of carb) will fit a stock aircleaner. All aftermarket carbs are a bit longer.

Click to expand...

Now that Jim has sorted the cable wheel issue, providing syncronized action and a single cable pull, I have to raise the question again: Has anyone tried fitting the CR(S) carb to a Mk2A or Mk3 and retaining the black box air filter? There are pictures of CR(S) and FCR 33's mated to shallow hamcam air filters, but that's it. The effectiveness of a shallow hamcam type filter (source??) may be questionable!?

-Knut

mdt-son said:

Now that Jim has sorted the cable wheel issue, providing syncronized action and a single cable pull, I have to raise the question again: Has anyone tried fitting the CR(S) carb to a Mk2A or Mk3 and retaining the black box air filter? There are pictures of CR(S) and FCR 33's mated to shallow hamcam air filters, but that's it. The effectiveness of a shallow hamcam type filter (source??) may be questionable!?

-Knut

Click to expand...

I would like to fit these carbs also using my black plastic air filter box .