Author Topic: Under-geared F1?  (Read 1077 times)

Offline StealthDJ

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Under-geared F1?
« on: June 27, 2022, 04:30:19 PM »
Hi all,

After a number of years since my last one (an F2), I’ve come into ownership of another SOHC 750, this one a 1976 F1 that had been dry stored for 20 years, but with a largely unknown history of modifications.

I’ve sorted the obvious issues - rotten exhaust, worn out clutch, seized brakes, plus a few others and last weekend did 240 miles over two days, mostly motorway work.

The thing that struck me, especially from what I remember of my F2, is that it feels very under-geared.  The Delkevic exhaust isn’t exactly quiet, but 70mph (around 5k rpm) feels like I’m thrashing it, whereas my memory of the F2 is that it’d pass 100mph with relative ease.  I’ve checked the drivetrain though and it’s running 18/48, which appears to be standard.  Should I be taking a couple teeth off the rear sprocket, or is this normal?

Also, from the section of the trip where my wife was following me, it seems what shows as just over 60mph on the speedo is probably closer to 70.  Wondering if I might have the wrong speedo/drive, or if it’s more likely that the internals are gummed and a refurb is required…

Offline Bryanj

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Re: Under-geared F1?
« Reply #1 on: June 27, 2022, 05:21:07 PM »
There is only one speedo drive for 19 inch wheels so its likely gummed, as long as rear wheel is 18 inch stick with standard gearing or it wont pull away at all well

Offline Trigger

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Re: Under-geared F1?
« Reply #2 on: June 27, 2022, 06:10:49 PM »
Unless you have a 16 inch rear wheel at, 4.000 rpm in top gear your clock should show 60mph  ;)

Offline Spitfire

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Re: Under-geared F1?
« Reply #3 on: June 27, 2022, 06:49:19 PM »
My 76 F1 speedo is 7 mph fast at all speeds, when I first got it back on the road I followed my wife who was in our car through Liverpool and was amazed at the speed she was going past speed cameras, so we checked the bike speedo at 30, 40, 60 and 70 and found it was consistantly fast.
I've had the bike since new so it is fairly bog standard apart from cosmetics.

Cheers

Dennis
1976 CB750F

1977 CB750F2 In bits

1964 BSA A65R In bits

Offline Skoti

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Re: Under-geared F1?
« Reply #4 on: June 27, 2022, 09:14:41 PM »
Previous owner fitted a smaller rear sprocket to my 1976 F1, it pulls 60mph @ 3500rpm.
It's nice and relaxed and pulls top gear fine when two up touring loaded with camping gear etc.

So yes I can recommend a smaller rear sprocket.
Hope this helps.

(Photo taken on route to Honda4Fest earlier this month)


   
Motorcycling is Life, anything B4 or after is just waiting...


1976 Honda CB750F1

Offline StealthDJ

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Re: Under-geared F1?
« Reply #5 on: June 27, 2022, 10:44:14 PM »
Previous owner fitted a smaller rear sprocket to my 1976 F1, it pulls 60mph @ 3500rpm.
It's nice and relaxed and pulls top gear fine when two up touring loaded with camping gear etc.

So yes I can recommend a smaller rear sprocket.
Hope this helps.

(Photo taken on route to Honda4Fest earlier this month)


   

Good to know, thanks - any idea what size sprockets / number of chain links are on it?

Offline Skoti

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Re: Under-geared F1?
« Reply #6 on: June 28, 2022, 07:51:23 AM »
I think the chain has 98 rollers, but will double check today and
I'll also count the rear sprocket teeth.
Motorcycling is Life, anything B4 or after is just waiting...


1976 Honda CB750F1

Offline Skoti

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Re: Under-geared F1?
« Reply #7 on: June 28, 2022, 10:17:47 AM »
Hi,

Just checked my F1 gearing for you.

Rear sprocket - 43t

Rear chain - 98 rollers (49 links)

Also I'm sure the front sprocket is still the standard size.


Hope this helps

Skoti
Motorcycling is Life, anything B4 or after is just waiting...


1976 Honda CB750F1

Offline StealthDJ

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Re: Under-geared F1?
« Reply #8 on: July 01, 2022, 09:44:13 AM »
Hi,

Just checked my F1 gearing for you.

Rear sprocket - 43t

Rear chain - 98 rollers (49 links)

Also I'm sure the front sprocket is still the standard size.


Hope this helps

Skoti

Thanks for checking - that's definitely quite a jump from the standard 48 rear sprocket and more than I'd have considered going otherwise.  I'll have to get the calculator out and figure out the ratios...  :)

Offline Skoti

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Re: Under-geared F1?
« Reply #9 on: July 16, 2022, 12:03:54 PM »
Hi,

Just been out on my F1 to a classic bike and cafe racer meeting for a look around.


And I've a confession to make because my earlier statement about gearing was bollocks.

 "Previous owner fitted a smaller rear sprocket to my 1976 F1, it pulls 60mph @ 3500rpm".

Today I doubled checked and it actually pulls 55mph @ 3500rpm, so sorry if I misled anyone...
Motorcycling is Life, anything B4 or after is just waiting...


1976 Honda CB750F1

 

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