Topic: Are 77 and 78 750 F motors really bad for gas mileage ?

[754]

 Mine is terrible,  fresh motor, run in now, not any better.
 I Been getting between 33 to 43 to a US gallon.
 With K motors I got 48 to 53.... and I am running the roundtops of those same motors. 110 mains
 What do you guys get.

[Don R]

 My 76 F got awful mileage but it had tweaked floats and drilled jets. It was way better afterward we figured that out.

[scottly]

How did you mate up the roundtops to the larger F2/3 spigots?

[754]

I oiled them up and they popped on, fairly easy..
 I should check the clamps.

[BobR]

 My F2 engine with earlier carbs (roundtops?) 115 mains seems to drink gas though I never measured my mileage. Bob

[seanbarney41]

My F3 with roundtops got low 40's generally, but I was always giving it the beans.  I might have made 50mpg on a long straight ride...so about the same as all my other cb750's

[scottly]

Frank has to give it "the beans" just to get it rolling. That still doesn't explain the difference in mileage with the F motor; should be very near the same.

[PeWe]

My K6 has always consumed around 0.6L/10km (40 MPG). Mixed speeds, city and highways. I think I have been down to 43 MPG riding really slow no twisting cruising around 70kph (44 mph). 836 with ported head and cam did not increase, except WOT.

Now with 1000cc and 32mm carbs mostly around 0.7L/10Km (34 MPG), this with some twisting. I have cruised closer to 0.6L/10km (40 MPG) when riding really slow which is very difficult.

[Bailgang]

My 77 F gets mid 30's to low 40 mpg but it's never been properly rebuilt either. The heads have new guides and valves and although the pistons have been re-ringed the pistons show their age and cyls could use at least a 1 size up overbore and pistons to match to get a good seal.

[2wheels]

Maybe the gearing, what sprockets do you have?
My F3 spins about 5000 rpm at 60 mph.

[Don R]

 I did go up a tooth on the front sprocket, the F turns more rpm at cruising speed. I'm thinking it went from 5200 to 4800 at indicated 75. That's just from memory but it seemed more relaxed on the interstate.

[754]

Just read in the PD 42 thread that an F3 uses 105 main, and a 77K uses 110.
 I run roundtops  and am probably rich with 110.. that or its my needles.
 But stil, a bit of a surprise the F3 runs a smaller jet..

[TwoTired]

The F3 had a high pressure muffler.  Likely the reason for the smaller main.

Cheers,

[flybox1]

I was always pleased with 40mpg +/-

[70CB750]

Red gets 35 to 38 mpg, but she has 657 carburetors.

[Skoti]

My 1976 F1 used to get about 30 mpg (UK) until I renewed the needle jets and needles, now getting around 50 mpg but needs warmed up on the choke a fair bit when engine cold. Normally I'm cruising around 65mph two up.

(Gearing 60mph at 3500rpm)