Topic: How to chose right main jet size

[Peter_from_Denmark]

Hi  there - I'm just a newbie to this forum, so .....

I'm renovating a CB 750 - it's a somewhat mixed affair : the Frame is from a K2 (# 2096324) but the engine is from a K6, I guess (CB750E -  2550338) but I think the carbs are from a K2 also (the adjustment screw for idle speed is on the left, and no stamps "086 A" on the carb body)

So shall i go for Main jet # 105 - which is the correct one for the K6 ? or should I opt for, let say a #110. As I have to run-in new pistons, liners and so on, I would perfer to stay on "the fat side" .
I've read somewhere, that the K6 was somewhat "strangled", so maybe the #105 is too small ?

Thanks in advance


Peter 


Peder

[StephenB]

Peter, it is not too much work to change the jets, at least on my carbs it wasn't: remove airfilter bottom and you got reasonable excess to float bowls.
 I had to rejet because I went back to stock exhaust and did not know whether my carbs were stock K1s or not. So I bought #105, #110 and #120 jets from a local supplier and went through a trial and error exercise. Ended up with #120s for a K1 with HM341 exhaust, if I remember correctly!

Start with K6 stock size jets and see what's happening. Clean and adjust the carbs to stock (needle position, air mixture screw), warm it up and you will see at 5000rpm and up whether you get enough gas through the #105s or not. You'll feel it: if she goes through all 8000rpm without hesitation you got it right.

Just my 2c.

Stephen in Ontario

www.stephenbottcher.net

[kaysystems]

Peter, it is not too much work to change the jets, at least on my carbs it wasn't: remove airfilter bottom and you got reasonable excess to float bowls.
 I had to rejet because I went back to stock exhaust and did not know whether my carbs were stock K1s or not. So I bought #105, #110 and #120 jets from a local supplier and went through a trial and error exercise. Ended up with #120s for a K1 with HM341 exhaust, if I remember correctly!

Start with K6 stock size jets and see what's happening. Clean and adjust the carbs to stock (needle position, air mixture screw), warm it up and you will see at 5000rpm and up whether you get enough gas through the #105s or not. You'll feel it: if she goes through all 8000rpm without hesitation you got it right.

Just my 2c.

Stephen in Ontario

www.stephenbottcher.net

Will the main jets make a big difference to normal running? I thought they only came in at very wide throttle opennings.

David'

[kghost]

Correct David.

Partial throttle is controlled by needle clip position.

[Bob Wessner]

Peter,

You might find this interesting, I did, though I can't say it has much improved my relationship with my carbs. 

http://204.71.0.3/motoprof/moto/mcycle/carb101/carb101.html

[kaysystems]

Thanks

Looks like I have some investigating to do. I have a suspicion that the needle jets are worn. Everything else that I can think of has been checked. I'll check the valve clearances again.

David

[Peter_from_Denmark]

Peter,

You might find this interesting, I did, though I can't say it has much improved my relationship with my carbs. 

http://204.71.0.3/motoprof/moto/mcycle/carb101/carb101.html

Hey Bob - I seem to have forgotten my "childhood learning " The main jet is only in use from 75% - 100% throttle.
What a great link you posted there, thanx a lot !!

Peter