Topic: A 750F and the contradictory mileage calculations

[super pasty white guy]

Well, while on a ride yesterday, I went onto reserve at 115 miles (normal-  I fill up roughly once a week so I hit reserve often enough...) but the most convenient gas station was ~20 miles away.

This got me to thinking about how many miles are left when I hit reserve so I looked up the cycle world review article which says 1.3 gallons out of a total of 4.8.  That leaves me 3.5 gallons before reserve, which means that at my usual 40mpg I should get 140 miles before switching.

So, what's going on here?  I can't find the capacities in the repair manual.  I figure that it could be that cycle world got it wrong, or that the PO replaced the tap with one from another model with a different standpipe height, or that my tank is a different capacity.  Anyone with the same model (a 76) want to weigh in with their usual mpg and miles to reserve figs?

And just for giggles, here's a picture from the trip.

spwg

[mlinder]

How hard did you ride it last week?
More in town riding than freeway?
I've gotten as little as 15-20mpg on my 750, but spent 250 miles near, at, or over, redline (or going like hell to get up there).
I've also gotten 40+ on it...

It just depends on how you are riding.

[super pasty white guy]

I was nearly entirely highway and unless it was really out of tune, I've never been below 35mpg.  I checked my mpg at fillup and got 39.  I've never been up around 140 to reserve like the cycle world article would suggest should be my usual mileage onto reserve.

spwg

[mlinder]

.....#$%*ty gas?

[jhasewhite]

My 76 750f acts the same way, as little as 20mpg racing - or up to 40-45 on an easy cruise. I usually hit reserve @ about 110 miles, but do most of my riding in the city.

[TwoTired]

Assuming you have a stock tank...

Not all of the tank capacity is usable because of the tunnel in the tank for the frame spine.  The fuel exit is at the bottom of one side only. To get the fuel from the other side you need to slosh it over via braking action and fuel inertia changes.  A steady cruise on the freeway doesn't slosh much fuel over the tanks tunnel hump.


Cheers,

[fuzzybutt]

does your fuel have a 10% ethanol blend? i know that kills the mileage for me in the car

[KB02]

I got about the same mileage on my F1 and hit reserve at about 120 miles or so.

What Twotired say, though, does make a lot of sence. Theres a lot of gas that is just never used.

[TomC]

Hi super pasty white guy
     About a month ago I went on to reserve less that a mile from the gas station on my CB750F1. I put in 3.3 Gallons. As I find the CB750F1 very tiring to push the most I have put in is about 3.8 gallons.
     What is the most that anyone has put in there stock CB750F 0-3 gas tank?
          TomC in Ohio

[jtb]

I gas up at 180 on my F2.  When it had 115 mains, I always had to switch arount 140, but with its current 110's it gets to 190 or so before I have to switch.

[gregk]

I get 120 miles on the main tank on my 76 750F. 

Greg

[Porscheguy912]

Well I found out the hard way that my reserve is clogged!

On my first real long cruise on my 78 750F i sputtered out at 117 miles while approaching the Ton, switched to reserve and got nothing.

I was quite in shock because my old 74 550K got much farther than that on a smaller tank before hitting reserve.
I did rough math and figured i was getting around 25mpg on the the 750F. Far different than 50+ on the 550K.

I'm running 120 mains, 150 psi dry compression with +/- 3 psi,  recently rebuilt/sonic cleaned/synced carbs, pods and a 4-1 wrapped header with a cherry bomb exhaust on my current 1978 CB750F.

I am getting a pinging on the left bank under load. but i dont believe that is causing bad MPGs.

Funny story; A guy stopped to help me that said he had a 78 750K when he was younger.
He had straps and so we looped through the forks around the frame by the exhaust and attached the other end to his trailer hitch. He pulled me off the expressway to a gas station. It worked out pretty sweet!

[TomC]

Hi Porscheguy912
    Did you fill up when you got to the gas station? If so how much did you put in?
    I ran out of gas and switched to reserve and nothing happened. That is how I know about pushing it. The problem was not that reserve was plugged. The problem was that the rubber gasket between the petcock and the standpipe and strainer that goes into the tank was not sealing between on and reserve so that all the gas had been used.
     A trick for getting better gas millage. Use an unmodified CB400F1 gas tank. I had been running one for some time and about 10 mpg better mileage.
          TomC in Ohio

[Porscheguy912]

good point tom,
Now that you mention it i'm pretty sure my tank was almost empty. That means i'm getting even worse gas mileage
I dont have a rubber gasket (it ripped when i was inspecting the petcock) and there was no strainer on it either. i'm running a large in-line filter. Would that mean its dumping gas from both tubes though? i thought that rubber o-ring just helped the whole thing from leaking?

I plan on treating the inside of that tank this winter and getting a new strainer filter element.

[super pasty white guy]

Thanks all.  Good to know I'm on par.  Guess the only way I'll know the tank range is to 1)empty the thing out and measure full to reserve and reserve to empty and 2)  Take TT's info into account and just run it till it's empty, with ample sloshing on the way.

spwg

[TomC]

Hi super pasty white guy
     Last night I looked in the CB750F supplement of the CB750 Shop Manual. It showed tank 4.3 gal., reserve 1.3 gal.. I am looking forward to you numbers. I believe the 4.3 gal for the tank. So expect a reserve of 1 gal.
     It looks to that Honda went back to a petcock with the stand pipe as a fixed part of the petcock in 1977.
          TomC in Ohio

[david 750f]

Get 150-160 miles before reserve with a mix of city and highway driving. I never drive the bike gently, always cruising around 75-80mph on the highway. My bike mileage always increases after a carb sync and tune-up.