Topic: F3 vs K2 carbs: Which is better for an early engine?

[Patrick]

Frankenbike is starting to take shape and I'm starting to think about carbs. The bike is an F3 750, but the engine is a K2 750. I have a full complete set of F3 carbs and the appropriate manifolds and I also have enough parts of early-style carbs that I can easily put together a set (or a few sets really) of early carbs. I even have a spare set of K0 carbs and a decent cable for them. I have the manifolds from the K2 engine. I have both F3 and K2 stock air boxes. The exhaust will be a 4-1 of unknown brand (except I know it is not a Mac). The exhaust is so ugly that a rat bike builder would reject it as over the top decrepit looking, but solid and actually quite quiet. I've had it on a couple other 750s.

Which set of carbs would go best with this engine? I believe the F3 carbs were different from the earlier at least partially because of emissions issues in 1978, what with the accelerator pump and everything. Are they actually better carbs or just different? Early 750 carbs were already pretty high performance.

Frankenbike will be a daily rider. I figure I may as well get it running as well as possible. F3 parts are expensive. It may be a while before I can afford to rebuild the frozen F3 engine.

I will ask later about where all this blasted vent lines go. Must be six or seven of them. I do plan to use the F3 emissions system. I worked for the Texas environmental protection agency for seven years and I am well aware of the poisons we pump into the air every day.

Patrick

[TwoTired]

The F3 carbs were tuned for the breathing characteristics of the F3 engine along with the stock exhaust and air box filter, 28mm venturis.
The earlier carbs were tuned for the breathing characteristics of the K2 engine along with the stock exhaust and air box filter, 28mm venturis.

In either case, you have some carb tuning to do with the new exhaust and different breathing characteristics between the two engines.

I rather prefer the newer PD style carbs as you can set the idle lean and rely on the accelerator pump to get a nice/brisk throttle response.  The downside is that the idle screws have limited range (probably not a problem in this application).
And, the throttle valve needles don't have the extra adjustment grooves to fine tune the midrange throttle setting mixtures. (though 77 models do have those grooves).  Assuming you need the richen the midrange (unknown requirement), you can shim the F3 needles for some adjustment.

The early carbs rely on a very rich setting at idle to get acceleration throttle response.  Still can't snap the throttle wide open from idle without wheezing the motor (unless you REALLY set the idle and midrange slobbery rich).  However, the needles are multi grooved for midrange adjustment, and other internal parts are easier to get.  And you don't have to worry about making the accelerator pump work properly as you will with the F3 carbs. (it's not that difficult, really.)

I see the earlier carbs as "sloppy" carbs, less finicky in their adjustment settings.  The F3 carbs are more precise and give better operation after "dial in".  Kinda depends on where your priorities lie.  Just making it go, or making it run sweet.

Cheers,

[Patrick]

Actually those two ends are not  necessarily mutually exclusive, Lloyd. I consider them progressive. First I want to make it run; then I want to make it run sweetly. I want it to be like my K5 750, which has provided more than four years of trouble-free performance and has logged about 20,000 miles since I rebuilt it. It is so reliable that I let my son take it to college as his primary means of transportation.

That was a mistake. I miss riding a 750. This one will be a mongrel, but it will be unique.

I'll have to take another look at those F3 carbs.

Thanks for your input.