Topic: Should main jet size match the head or the carb?

[Stoli]

I'm putting the carbs from a K6 on the head from a K4. If my research is correct, the K4s had stock main jet size #120 while the K6 had #105. If I wanted a starting point for tuning this configuration, should I start with #120s or #105s?

[MiGhost]

You will always be safer starting out on the rich side.
You risk heat damage if you start out on the lean side.

[PeWe]

Check here about the different models with different jettings and other differencies like airbox, cam, ignition timing (when full adv kick in)
http://forums.sohc4.net/index.php?topic=7401.msg67248#msg67248

[lrutt]

Yup, what else do you have on the intake or exhaust side? if all stock, I'd go 120. That's what I ended up with on my K6 but it has a KN in the stock air box.

[lucky]

The carb does not care what engine it is on.

Since you are moving them to a different engine that is in the same family of engines
It will make very little difference.
As the years went by the EPA just wanted a leaner and leaner mixture.
The carbs got so stingy with gas that then the 77-78's had to have an accelerator pump and nozzles.

These bikes are only ridden a very short period in their lives for reasons of weather or pleasure . That little bit of extra gas used  may add up  but I do not like the solutions being thrown on to the design at the last minute.


I have put 1970 CB750 carbs (the good ones) on a 1978 CB750 and it ran so much better it was unbelievable, even with the larger valves.

[ekpent]

Technically the K4 had a #110 main jet up to serial number 2304500. K6 did have a #105.

[Stoli]

The more I thought about this, after I posted it, the more I realized it was a stupid question. From what I can tell, the carbs and airbox were the same from '74 to '76 so the possible differences were in the rest of the stuff (head, exhaust, and Honda's tuning). I'll be putting velocity stacks on it and mildly baffled drag pipes so I'll obviously need to make some changes. I just wanted a starting point. Nice to know where the strike zone is before you step up to the plate and start making adjustments to your swing. I'm thinking I might start with 42s and 120s and moving the needle clip one down from center to raise the needles a bit. Total shot in the dark. Neither the carbs nor the engine came off a running bike.

Thanks for the feedback. All good answers.