JS Motorsports Carbs

acotrel said:

I don't jet by 'trial and error', I use a system. It is easy, after the motor starts and runs. The symptoms of wrong jetting are obvious. The difficult part is getting the most out of your bike using mikuni needles which are gr ound with thee stages of taper. It then becomes a combination of gearing, handling, and where you want the most power. I don't use Amal needles or needle jets. I use methanol fuel with 6DP6 Mikuni needles in AMal Mk2 carbs, and I made the needle jets at 0.117 inch. The Amal alcohol needle jets are always 0.120 inch, and their X and Y needles are just bloody stupid.

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I very much appreciate your input and thank you for helping out with my questions. Unfortunately, what you just said is way over my head but hopefully it helps other guys with their carbs.

Johnnymac said:

crusadersports said:

Johnnymac said:

Question about jetting ...again. and she started up on 2nd kick after sitting for 6 months. I was very happy. Carbs still run great now that I dropped the size of the main jet. Idles perfectly.


The only issue I have that still bugs me is when I slowly throttle from idle. It is rather hesitant at very low RPM's and sometimes stalls. If I keep the idle at 1000/1100, I have no trouble. But sub 1000, I have to constantly tickle the throttle when I come to a stop. I can let it drop to idle.... But tickling it up above 1000 is the issue.

From what I've read, it is the pilot jet. Is this a correct assessment? Do i need to go up in size or down? Thanks guys!

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Johnnie, as a long term user how have you found these carbs? Are they plug and play?Like to hear how well they last, how regularly they need balancing. Whats mpg are you getting? Thanks mate.

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Hey Crusader, I would say that these carbs are plug and play... in that you can easily bolt them on and connect them... but everyone's bike is different and the jetting will be trial and error. In my case, there continues to be a lot of fine tuning (I'm a perfectionist with very little time to ride...much less work... what takes me months might take you a day) and at the time, really no agreed upon PERFECT way to route the fuel lines. I spent a lot of time designing solutions for fuel line routing and fuel filters. (Which I'm very proud to say, JS
They certainly run great and I have had great support from the JS. I would certainly buy them again.[/quote

Thanks Johnnie, considering how much attention these carbs generated on this forum its good to hear that they seem to stay in tune and last so well. 'lasting well' and 'carburettors' arent always associated with Cdo... The JS solution does sound like a worthwhile upgrade. Thanks again.

jseng1 said:

Johnnymac said:

Question about jetting ...again. Got the Commando out for the first time of the year and she started up on 2nd kick after sitting for 6 months. I was very happy. Carbs still run great now that I dropped the size of the main jet. Idles perfectly.


The only issue I have that still bugs me is when I slowly throttle from idle. It is rather hesitant at very low RPM's and sometimes stalls. If I keep the idle at 1000/1100, I have no trouble. But sub 1000, I have to constantly tickle the throttle when I come to a stop. I can let it drop to idle.... But tickling it up above 1000 is the issue.

From what I've read, it is the pilot jet. Is this a correct assessment? Do i need to go up in size or down? Thanks guys!

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If you have a perfect idle then you may want to leave the idle jets alone. The needle jets are what has the most effect just off idle. Pull the chokes to see if that fixes the off idle problem - if so then go to the bigger .105" needle jets. Or go one size richer with the pilot jets (but this may not make enough difference).

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Hi Guys,

I took her out today for nice ride. But I still have the same problem coming off of idle. I upped the idle to about 1500, and that helps, but it's not a good solution. I literally have to tickle the throttle when I come to a stop in order to take off again. If I don't, -and just let it idle perfectly, it will cough and want to die. I want to say that I think it's getting to much fuel off of idle but that is just a guess. The same thing happens with the choke...I was expecting the RPMs to increase when I pulled the chokes open..... as soon as I did pull the chokes after it's warmed at up at idle to see if that helps.... it instantly stalls. I'm open to your suggestions here.

Hi Jonny,
I've just been fettling the JS carbs on mine and they are good but the pilot air bleed screws only need to be 1/8 -1/4 of a turn out ie 45-90degrees to get a good tickover and good pickup. The RH carb of mine is 1/4 turn out and the left 1/8, much more than that and the slow running is erratic and lumpy.

Thanks gripper. I will double check that.

I had to go with a #35 slow jet in the right and a #40 in the left. That and needle clip positions were also different between carbs for proper running.

There is nothing outstanding with these carbs. Other than the fact that they offer some flatside perfomance and that they actually fit, they are somewhat cheap and also inexpensive.

pvisseriii said:

I had to go with a #35 slow jet in the right and a #40 in the left. That and needle clip positions were also different between carbs for proper running.

There is nothing outstanding with these carbs. Other than the fact that they offer some flatside perfomance and that they actually fit, they are somewhat cheap and also inexpensive.

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Well, I will say this, I feel like I have spent more time adjusting these carbs than I have riding. I have 38 Pilots Jets on mine... I'm wondering if maybe the 35's would be better in both.

I checked the pilot mixture screws and they are 1/4 out. I can't spend anymore time on this. I am going to take it to be professionally tuned.

One more thing, for those of you out there with this JS Setup.... how tight is the throttle to twist? I'm a pretty young guy... but driving around the city with frequent starts and stops causes my wrist to cramp. I feel like i'm getting a work out just twisting the throttle. I have nice, gradual bends in the cables so I wouldn't think that it should be this difficult to twist.

Johnnymac said:

One more thing, for those of you out there with this JS Setup.... how tight is the throttle to twist? I'm a pretty young guy... but driving around the city with frequent starts and stops causes my wrist to cramp. I feel like i'm getting a work out just twisting the throttle. I have nice, gradual bends in the cables so I wouldn't think that it should be this difficult to twist.

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I believe the function required and updated spring to give a better closing action of the slide. There is a silver and a gold colored spring. The gold colored spring is the updated one. You just need to ride more.

The JS flatside carbs actually have weaker springs than the original Amals. So twistgrip effort should not be a problem.

The silver springs measure .031" thick at 3-1/2 length and they work fine. These are standard and I don't feel any stiffness at all.

The gold springs measure .036" thick at 3-1/2 length and are a little stronger and are more secure with the throttle return. Available on request only.

One customer did contact me about stiffness but it turned out to be a binding twistgrip problem and was fixed.

I have had my JS flat slides for well over two years, I was in the very first group of purchasers.

Some observations:

I am overall really very happy with these carbs, they idle well and they flow the mixture strongly throughout the rpm range with no flat spots. I like em.

The price that Jim offers them for is very reasonable especially given that everything from manifolds to air filters is included. They come pre jetted for either 750 or 850 usage.

From what I can tell of reading other users comments, and my own experience, is the fiddlyness that is sometimes but not always necessary in sorting out the idle settings.

By idle settings I mean both the idle or pilot jet size and also how much to open up the air mixture screws.

I believe that the faster taper of the air screws than one may be used to with other carbs leads people to overcompensate one way or the other and get easily frustrated.

No two bikes are the same and so Jim can only set them up with typical jetting, almost everyone agrees that the needle position and main jets are pretty close to spot on at delivery. Some may find they need to tweak things there, I did not.

Back to the idle settings: there is a BIG difference because of the taper between having the air screws set at say 1/8 to 1/4 turn out from fully seated, does not sound like much but it is the difference between too rich and the plugs are load up a bit and too lean where the motor can pop on deceleration. If the motor tends to run on and not idle down when coming to a stop then one or both air screws are set a tad too lean and need to be turned in to richen the idle mixture, small changes make larger than you think differences.

ya gotta be patient, make sure the cables are free and non binding, start with a brand new set of plugs so your plug color readings reflect only this new set of carbs and not the ones you took off, if one plug starts getting darker than the other then you know to turn out the air screw a tiny bit more to lean it out, it the motor pops while backing off the throttle then turn in the air screw on that side just a tiny bit as it is telling you the idle setting is too lean.

Once I got mine dialed in they have stayed that way for two years now.

Jim's recent suggestion to shut off the fuel taps a block or so from home is a good one as the ethanol in todays gas can cause the float to stick and then leak upon the next start up.

Agree, its like stay up floats,,now shall i have stay up floats,,,or WHAT?????

Carbonfibre said:

Why on earth are customers for Amal carbs being asked to pay extra for a product which seems slightly more fit for purpose intended? This seems to be a bit of joke, as it suggests that products still being sold for a little less money are not fit for purpose, and shouldnt be considered by anyone wanting to actually use them on a bike?

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I looked over this entire thread and couldn't find what I was looking for so I apologize if this was already brought up.

Here we go...

I just installed the JS Motorsport Keihin CRS carbs onto my '71 commando. They look great but unfortunately both petcocks on my factory roadster tank are in direct contact with the linkage bit between the carbs. I'm unable to get the back end of the tank anywhere near to the frame. Please take a look at the photo below. Somebody suggested that I try installing a 90 degree angled barb connector piece to each bung hole in the tank and then install in-line petcocks farther down the fuel line avoiding the carbs. This sounds like a good idea to me. Any thoughts? Anyone know where I can order a couple of these connectors with bsp threads?

Thanks



The petcock on the other side in this pic is sitting right on top of the linkage

what a pain i hope i don't have this

Looking at your picture, I would suggest that 90 degree taps of the type sold by cNw would fit and probably be the best / neatest overall solution.

I don't think Matt lists them on his web site, you'll probably have to contact him.

There may be other sources, but I've bought mine from Matt as I know they're good ones and not cheap / inferior knock offs!

Marc Sullivan said:

I just installed the JS Motorsport Keihin CRS carbs onto my '71 commando. They look great but unfortunately both petcocks on my factory roadster tank are in direct contact with the linkage bit between the carbs...

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Marc
I haven't had any problems with other customers with factory tanks. Maybe your roadster is different. See the tank below. You can see that the petcock is not located where yours is located. You can also see that he used a 90 deg petcock barb. But I don't think a 90 deg barb will solve your problem unless you fit a very short 90 deg barb directly to the tank and then fit an inline shutoff valve somewhere in the fuel line where its out of the way. This might work if you can find a 90 deg barb that fits the tank (or make one up).


Heres a view of the tank above.


Here is another photo of the JSM Keihin CRS carbs on another Norton showing that the petcocks are located further forward than yours.


Here's a view of the tank above.


Here's a photo of what I think is an aftermarket tank where the petcocks are moved forward (not backward).



Maybe someone else could confirm if they have a tank like Marc's with the petcocks moved rearward. Were there some factory tanks made with the petcocks moved rearward instead of being forward as usual? I've only seen aftermarket tanks that had the petcocks moved. It doesn't make sense that the factory would move the petcocks.

Thanks for getting back to me guys. I'm giving some serious thought to these here http://thebonnevilleshop.com/triump...ngles-90-1-4-bsp-petcock-fitting-pn-tbs-0920/ and then using an inline shut off valve farther downstream. Jseng you may be right. They may still be in contact with the carbs. Unfortunately I'm away from home until Saturday so I'm unable to take a second look at my bike, including the distance from the petcock bore hole to the carbs once the petcocks are removed. I'll wait until then before I make an order for them. If that doesn't work out then I'll probably pick up a modified tank from Legendary Motorcycle http://legendary-motorcycles.com/index.html. His stuff looks great.

Marc Sullivan said:

Thanks for getting back to me guys. I'm giving some serious thought to these here http://thebonnevilleshop.com/triump...ngles-90-1-4-bsp-petcock-fitting-pn-tbs-0920/ and then using an inline shut off valve farther downstream. Jseng you may be right. They may still be in contact with the carbs. Unfortunately I'm away from home until Saturday so I'm unable to take a second look at my bike, including the distance from the petcock bore hole to the carbs once the petcocks are removed. I'll wait until then before I make an order for them. If that doesn't work out then I'll probably pick up a modified tank from Legendary Motorcycle http://legendary-motorcycles.com/index.html. His stuff looks great.

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those fittings you show are female fitting not male,they wont screw into your tank they screw onto the fuel tap,you could get a 1/4 bsp male hose tail with an inline tap, maybe solder a short bit of copper tube in the fitting to give yourself a "main" tap and leave the over side plain for a "reserve"

baz said:

Marc Sullivan said:

Thanks for getting back to me guys. I'm giving some serious thought to these here http://thebonnevilleshop.com/triump...ngles-90-1-4-bsp-petcock-fitting-pn-tbs-0920/ and then using an inline shut off valve farther downstream. Jseng you may be right. They may still be in contact with the carbs. Unfortunately I'm away from home until Saturday so I'm unable to take a second look at my bike, including the distance from the petcock bore hole to the carbs once the petcocks are removed. I'll wait until then before I make an order for them. If that doesn't work out then I'll probably pick up a modified tank from Legendary Motorcycle http://legendary-motorcycles.com/index.html. His stuff looks great.

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those fittings you show are female fitting not male,they wont screw into your tank they screw onto the fuel tap,you could get a 1/4 bsp male hose tail with an inline tap, maybe solder a short bit of copper tube in the fitting to give yourself a "main" tap and leave the over side plain for a "reserve"

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Oh jeez thanks for pointing that out. I missed an obvious one.

Different petcock location makes sense