Twin JRC carb field report

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acadian

acadian

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Installed a new set of 30mm JRC's on Friday. Starts first kick out of the box, jetted to 220 and she pulls above idle like a dream, beautiful response, great tan on the plugs. Problem seems to be erratic idle. Now I traced a leak in the balance tube, corrected it, got both idle mixtures set to produce a nice tan burn on the plugs but she's still going up and down at idle. At settings that produce a stable idle, she runs too rich and fouls the plugs. Anyway, my overall feedback on these carbs is positive, and can't beat the performance for the cost....
 
Any pix?

Where'd ypu get them? What are you using for an air cleaner?

The report didn't sound all that glowing but maybe you'll get the jetting figured out. What were you using before the JRCs?
 
Noted your joy to get the odd ball carbs pleasing to ride.
Make sure battery is topped off and diddle with timing.
Old mech. advance slopping about gives similar up/down idle
as does electronic ign on too low volts.

Its possible a hidden smoke conduction connection is jostled
at idle but not on the fly.

Maybe a float level diddle to compensate for the Commando
canted engine few carbs normal float level can tolerate well.

Nice plug chop report.

hobot
 
The main jet was a #122 . I'm suspecting the idle was stronger with the screw near closed??
 
Just guessing but screwing in idle air screw enrichenes mix
so implies a low fuel float level baseline to me.
Also clogged and wrong size stuff too, but not likely on
a new installation.

hobot
 
The voltage at 1200 RPM is 13 so I'm ruling that out. They seem to take a bit longer than the old Amal's to reach operating temp, but as I said the difference above idle is noticeable. I stand corrected on the 220's. I'm going to recheck valve lash this evening, and probably enrichen the idle mixture a tad. The carbs don't look half bad either, and she doesn't stall when shutting the throttle from higher openings anymore which is a real treat when coming to a quick stop behind some dough head in a minivan cutting me off.

p.s. I'll pot some pics of the carbs and the beautiful wheels Rick Harret @ Highway Cycles built me
 
Glad you got such a problem to ponder on A Running Nice Commando.
A nice idle is so pleasant and so desirable in traffic.
13 V @ 1300 is very good, must have that Sparx uint working for ya.
But hey its a Commando so seems almost anything affects
everything else.
Sneaky air leaks to sneak smoke leaks can give same symptoms
as too small or lean an idle jet, so can low float level.
Let us know what news you find for full range engine sounds.

hobot
 
extend the throttle cable until the the engines racing around 2K. Pull on plug and adjust the cable adjuster on top of the carb to 1k and the same to the other cylinder. Lower each slide exactly the same amount with both plug connected. Slack in cable has to be removed to set idle! :D
 
Reporting back, had them on the bike for a few weeks now and I have to say I'm very happy. Have mostly sorted the idle issue by moving to larger airfilters and cleaning some glass deposits off the intake valve stems. At throttle openings above idle she performs beautifully, pulls like a combat, most likely the result of the higher efficiency flat slides. Took her down to Chicago and back for Mods & Rockers (1600 Km), cruised effortlessly at 75 mph, no hiccups. The insta start is a plus from all engine temp's, and having finished them off with some nice braided fuel line they don't look half bad either.
 
Twin JRC carb field report
 
Here they are completely set up for Nortons - a bolt on kit, Jetted and dialed in with cables, aircleaners and manifolds. After a year of testing they are ready for release. These are 32mm and ideally suited for Nortons.

see them at the new
http://www.JSmotorsport.com
webpage.

Twin JRC carb field report


Jim Schmidt
 
They are contracted out to PWK by Keihin. Keihin designed them. I was asked to fabricate needles for them so they would work well with Nortons. So what started out expecting to be an easy job turned into a lot of blasting up & down country roads doing plug chops at various throttle settings. I made a bunch of needles until it ran perfectly. I went through a whole year of testing. In the end I was really impressed with the results and decided to sell them as a complete, easy to install kit. I don't think you can come anywhere close to the price with any other flat side carbs.

see them in the products page at:
http://www.jsmotorsport.com

Jim
 
speirmoor said:
Twin JRC carb field report
Click to expand...

Will post some when I get home. So far the only real irritant (aside from fabbing a properly routed braided fuel supply), is the air mixture screw on the gearbox side of the bike. Both air screws are on the left of the carb, and there is only about 1/4" of clearance between them making adjustments difficult at best without removing the primary side carb. I managed to MacGyver an adjuster tool out of a piece of spring steel just tiny enough, and with a 90 degree end, to fit in between.
 
What are peoples thoughts on removing the balance tube with these flat slides? Yay or nay?
 
That' easy. Yay, if it idles stronger and Nay if it doesn't.... did you try opening up the valve clearances ? as the engine might have a Norris cam grind in it.... If it has a stock grind in it , I'd have another for fiberglass resin on the exhaust valve stems as well as the intakes. All the PWKs that I've seen are wayyyy toooo rich on on the the main. choose your clip position for a mid range that makes you feel at least 10 years younger and had a strong steady idle..... As the bike had idle and low speed issues with a very good set of Amals, I'm not convinced th carbs are the cause for the weaker idle..
Congrats on the great trip, great to see people riding Nortons instead of just owning Nortons Rick
 
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