Honda-SOHC
SOHC.co.uk Forums => CB750 => Topic started by: sohc bob on August 22, 2020, 03:05:05 PM
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Hi all,
After recently aquiring a 1977 cb750f2 im over the moon with it, yes it has a few things i want to do to it but over all it starts on the button and idles once warm, however when riding it seems to take ages to gain decent speed both rev counter and speedo seem to climb slowly, im beginning to think it may be over geared, the bike rolls freely and brakes dont bind. Rear sprocket is 43 teeth not sure about front still need to remove the cover, speed wise in 5th gear its doing 60 at 4k, 75 at 5k and 90 at 6k is this right any advice will be greatly appreciated, thanks in advance
Bob
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That looks about Ok.
The motor needs revving.
Sent from my SM-A750FN using Tapatalk
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With the throttle wide open it just seems so slow to rev
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If you want spirited riding. Keep the revs above 6k, and don't change till above 8k.
If it still feels sluggish. Check timing and mixture
Sent from my SM-A750FN using Tapatalk
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Points gap in spec and timing spot on bike idles evenly with no issues through rev range so im guessing carburation is pretty good or am i expecting too much im not after masses of speed but it feels muted ii has a motad 4/1 plugs are new as are caps and air filter
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The bike is nearly 50yrs old.
Will it run to 8k in 4th?
Sent from my SM-A750FN using Tapatalk
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it will hit 8K in fourth if you wait long enough, ive checked the sprockets and it has 15/43 does anyone know if this is correct
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Check the parts book it will tell you
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checked mine...43T rear sprocket is what I have...so I reckon that is right. Same with the 15T gearbox sprocket.
Judging also by your speeds in gears...if you are pulling 90mph at 6000rpm in top, that sounds right to me. You are not going to see much over 110 unless you are flat on the tank. Maybe you could hit the dizzy heights of 120mph if you try hard.
You would need to hold the revs in each gear, though, or be prepared to wait. These bikes don;t have anything like the power of a modern 750
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Thanks for the replys maybe I am expecting too much from it I had a 77 f2 4 years ago and don't recall it being like this plus maybe I've been spoilt by modern bikes although I am suprised that I can run at 2500 rpm in top and open the throttle and she picks up with no hesitation seems to be quite torquey low down
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Speeds in the gears are about the same as mine so would suggest the gearing is correct. Mine picks up very quickly but you need to drop gears and rev the thing. If you try to accelerate in 5th gear it is very sluggish. You didn't mention what gears and revs you were using.
On these old bikes it can often be too much mechanical sympathy causing the issue, give it some beans, the old girls love it.
On the carb issue, poor pick up could be a stuck accelerator pump or ruptured diaphragm.
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Back in the 70's took an 18 month old 28000 mile F2 to the IOM TT, wouldnt go over 95, smoked like a pig and dropped to 150 mpp of oil.
First one i came accross with the worn out exhaust valve guides.
The F2 cam was different and may have been changed in your bikes life and if its not had exhaust guides it will need them-----they ALL did and it was not done as a recall or under warantee unless the problem occured in the first 12 months
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There's a fix for the top end problems from CycleX in the States involving hardened guides, supposed to be a permanent fix. I hope it is as that's the fix I used on my F2. If anybody needs it this is the link http://www.cyclexchange.net/Honda%20cb750%20(%2077-78%20)%20%20F%202%20Section%20Page.htm (http://www.cyclexchange.net/Honda%20cb750%20(%2077-78%20)%20%20F%202%20Section%20Page.htm)
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The replacement parts from Honda were extremely hard
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The CycleX Kibblewhite parts are also longer to better support the valve and a more modern material. They reckon to have had an engine to 60k without any issues at all from the exhaust valves.
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Hi the bike burns no oil and does not smoke i have canned the knackers of it only a couple of times at full throttle in all gears and yes in the lower gears it accelerates better as you would expect and i only did this to try its not my intention to thrash it but as ive said all along i just thought it would rev quicker than it does i may try a 14 tooth frnt sprocket anyone tried this opinions please
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Too small for the chain, if you must alter the gearing alter the rear sprocket but not sure its worth it, admittedley Honda only spent 100's of 1000 developing this model but they normally got it right
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The 630 chain wasn't the ultimate solution though ;D
The 14 tooth sprocket was used by Honda on some models, so it can't have been too bad. And I have seen a 15/41 drive quoted too.