New product - 34mm flatslide carbs

For those who just can't get enough - now in 34mm. Identical to JS 32mm flatslides except for the bore. Requires longer JS junction box & special cables because of increased slide travel.

34mm manifolds sleeved down to 32mm on head side also available.

Hi Jim, for which kind of engine are they designed ? mine is a 750 with your stage one cam (piston/rods, too!), maney two/one exhaust,lightened crank (with steel fllywheel) Maney jugs (no importance in that case!) and a standart RH 5 head (so 32mm port, see my post in the head posts), should i go to 32 or 34 carbies ??

marinatlas said:

Hi Jim, for which kind of engine are they designed ? mine is a 750 with your stage one cam (piston/rods, too!), maney two/one exhaust,lightened crank (with steel fllywheel) Maney jugs (no importance in that case!) and a standart RH 5 head (so 32mm port, see my post in the head posts), should i go to 32 or 34 carbies ??

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If its a a full race engine then go the with 34s
If its a street bike then I suggest 32s
The 34s will work fine on the street but it could be overkill for a 750.

How about 920 engines---hot street applications. Using you stage 2 cam kit---FulAuto head ?

303 fmj said:

How about 920 engines---hot street applications. Using you stage 2 cam kit---FulAuto head ?

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Yes definitely the 34s.

I'd suggest the 34s for all 920s and some 850s (preferably performance motors and for those who want just a little more HP). Also fine for 750s that are pumped up to race tune or for those who want all they can reasonably get. On my street 750 the 34s actually seem about the same as the 32s. Same smooth idle and linear throttle response with no flat spots. Its just that once you are at 3/4 throttle with the 34s on a near stock 750 - that last bit of throttle doesn't seem to make any difference. If I had a hotter cam then it would be a different story. Flat slide carbs work so well that it does not feel over carburated - even on a mild 750 Norton. THey just give you a little more head room.

But any high performance motor runs best with a hot cam, free breathing pipes, 34 or 36 mm carbs and maniolds that taper down to a 32mm (or 30mm minimum) head. And a little port work around the guide to raise the roof to give a more vertical air/fuel shot straight down at the valve and into the combustion chamber.

The nice thing about these 34mm carbs is that the overall body size is small and they fit without having to be splayed out to prevent them from rubbing each other. You can't this with Mikunis or Del Ortos. This makes them ideal for Nortons.

How new are these?. i bought some 32mm's in February and was advised that for racing or if I went to 920cc then they were ideal and just up the main jets? Have you run dyno tests Jim?

Hi Jim,
Dare I ask, do you have any plans in offering one of your flatslide carbs as a single?

Is there a left and right carb? I am running 32's on the 850 and they are brilliant except they are both right side and difficult to adjust

The 34mm flatslide carbs are a brand new option. I knew nothing about them a month ago. And yes they are identical to the 32s except for the bore. The right side carb has the idle speed moved so it is easy to adjust but the idle mix is still where it is on the 32mm carb with the lever so you can adjust it.

I could offer a single 34mm carb setup if someone wants it. The single kit is not on the jsmotorsport website so contact me directly at:

jim@jsmotorsport.com