Best carb combo for 750 big bore kit and Shell #1 cam?

[EarthHeavy]

Hey guys getting ready to get my engine back together and am starting to think about carbs - as the title says, looking for the optimal carb setup for 750 big bore kit with a shell #1 cam - I have a pair of BS38, and the stock carbs that came on the engine (1980 heritage special) - any recommendations on carb and jet combo? Thanks for reading! Cheers, jase

 

[Signal]

I dont know if it is the optimal set up but I like it. CVK carbs off GPZ/EN 500.
The bike has a similar tune as yours. I bought the bike with VM36 carbs fitted the CVK carbs are much easier to live with.

 

[Slowshed]

Glad you posted the question. I’m researching what direction to go and the 750 kit seems to be a favorite. I’m curious about how the cam performs … mid range or top end, etc.

 

[halfmile]

With a 750BBK and Shell #1 cam you might want to think about a better than stock clutch. 750 kit is a noticeable difference. Just sayin.

 

[zrx1100]

+1 on beefing up the clutch. Also don’t forget the exhaust part. I did a 700cc kit and opening up the intakes and the exhaust was key to unlocking the potential of the motor.

 

[nighthog]

I have a 750BBK, ported head and Shell #1 cam. I opted for VM34 carbs and that all seems to be working very well - I also have a stock clutch which has been holding up fine so far (about 1000 miles on the motor, so not too many when fully run in).

 

[EarthHeavy]

With a 750BBK and Shell #1 cam you might want to think about a better than stock clutch. 750 kit is a noticeable difference. Just sayin.

Ok I didnt consider this - Do you have any recommendations for a better clutch? Is it as easy as swapping out the discs for a different set?

 

[EarthHeavy]

+1 on beefing up the clutch. Also don’t forget the exhaust part. I did a 700cc kit and opening up the intakes and the exhaust was key to unlocking the potential of the motor.

+1 on beefing up the clutch. Also don’t forget the exhaust part. I did a 700cc kit and opening up the intakes and the exhaust was key to unlocking the potential of the motor.

I bought a DIY exhasut kit from TCbros - guessing what you are talking about is going with a larger diameter exhaust tube? Which would lead me to believe that I need to open up the exhaust port on the head as well?

 

[EarthHeavy]

I dont know if it is the optimal set up but I like it. CVK carbs off GPZ/EN 500.
The bike has a similar tune as yours. I bought the bike with VM36 carbs fitted the CVK carbs are much easier to live with.

Interesting take - never thought of using kawasaki carbs - is there a boot adapter available or do simply they bolt up? What is it you like better?

 

[gggGary]

Ok I didnt consider this - Do you have any recommendations for a better clutch? Is it as easy as swapping out the discs for a different set?

https://www.xs650.com/threads/interest-in-8-plate-clutch-conversions.56486/

 

[EarthHeavy]

I have a 750BBK, ported head and Shell #1 cam. I opted for VM34 carbs and that all seems to be working very well - I also have a stock clutch which has been holding up fine so far (about 1000 miles on the motor, so not too many when fully run in).

Ok, thanks for the advice - I was leaning towards using the BS38s bc of the bigger size, did you need to rejet your 34's to make them run richer for the larger bore?

 

[EarthHeavy]

https://www.xs650.com/threads/interest-in-8-plate-clutch-conversions.56486/

Thanks gary I appreciate the link

 

[zrx1100]

I bought a DIY exhasut kit from TCbros - guessing what you are talking about is going with a larger diameter exhaust tube? Which would lead me to believe that I need to open up the exhaust port on the head as well?

Don’t go with 1.75” diameter headers. 1.5” is probably optimum. Pairing that with open core mufflers and Uni foam filters ( without the end caps) is what really opened up the motor potential.

No need to open up the exhaust port

 

[zrx1100]

Ok, thanks for the advice - I was leaning towards using the BS38s bc of the bigger size, did you need to rejet your 34's to make them run richer for the larger bore?

FYI. I am running BS38s and they seem to work well, but never tried the VMs

 

[EarthHeavy]

Don’t go with 1.75” diameter headers. 1.5” is probably optimum. Pairing that with open core mufflers and Uni foam filters ( without the end caps) is what really opened up the motor potential.

No need to open up the exhaust port

Right on, I was planning on fabbing a set of 1.5" tubes and leaving them open with no baffles

 

[zrx1100]

Right on, I was planning on fabbing a set of 1.5" tubes and leaving them open with no baffles

You still benefit from a muffler. I am using open core cone engineering mufflers. They work very well and not bad at all as far as noise

 

[zrx1100]

Here is a good video on benefits of mufflers

 

[nighthog]

VM34 jetting - there's lots of info on this site and what works best will depend on many variables. I started with the XS650 kit from Niche Cycle:

Carb kit from Niche Cycle: http://www.nichecycle.com/ncs/mikuni/carb-kits/yamaha/ncs235-yamaha-xs650-70-79-vm34-carb-kit.html
More carb details: http://www.nichecycle.com/ncs/mikuni/carb-kits/yamaha/mikuni-34mm-yamaha-xs650-carburetor-set.html

Then made a small change as it looked a bit lean for my setup. Fitted – variation from supplied:
Main Jet 180
Pilot Jet 20
Air jet removed

I have a good response all through the rev range, and no bogging when giving it a handful at any point, so I think I've lucked out and and got it pretty close to ideal.

Goes like stink, by the way

 

[Ratranger]

The best of the best are probably the topham tm34/tm36 carbs. Flatslide pumpers, down side is they are expensive. But it is a pretty piece of kit.

 

[sparky4123]

I had the VM34 on a 750 kit bike and they seemed to work OK. They had been set up by a previous owner so not sure of the jetting. Since then I have built a number of bikes, and never had any problems getting the stock BS38’s to function properly. A stock bike with open pipes needed one size up on the pilot and I left the mains alone as it ran fine. A 707 kit bike with pods and open exhaust, 1 size up on both jets and I got a nice colour on the plugs, but possibly 2 sizes up on the pilot might be better as I have a tiny hesitation off idle.