Honda-SOHC
SOHC.co.uk Forums => CB750 => Topic started by: Grewth on August 05, 2025, 07:26:56 PM
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I really don't know what possessed me, but I bought this K6 a few days ago.
It's provenance is proving to be somewhat interesting.
Most of the bike seems to be K6, but the tank, carbs, swing arm, and instruments would appear to be off a K2.
The exhaust stubs have been removed, possibly to accommodate the aftermarket 4 into 1 exhaust, but now I've noticed that the engine number starts CB750E 251****
I seem to remember reading that a 250+ prefix means an F1 engine.
Can someone who knows confirm this please ?
I've looked in John Wyatt's book but it seems a little ambiguous about engine numbers [attachimg=1]
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A 1976 K6 engine starts with 254****.
It looks like a USA import, so it's probably a K5 engine . Has it got a VIN tag on the top down tube? If its a UK model, forget the above as the numbers and model range are different for UK models. In UK we had K2 from 72- early 1976, no K3, K4 or K5.
Edit... That tank is painted as K6 but the tap is on the wrong side for a K6.
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Thanks for the reply, but that has only added to my confusion.
The bike is a (so far unregistered in the UK) US import.
According to John Wyatt the LH fuel tap came in with the K5 in the US.
So I could well have a K3 or K4 maybe ?
That's if the tank is the original.
Other tell tale feature is the chain oiler on the output shaft.
Not a K6 feature IIRC.
The headlamp mounts are the K6 type, but we're also fitted on some of the earlier American spec bikes too.
Apart from all the above.
If 254 is a K6 engine number prefix, then my CB750F1 is fitted with a K6 engine.
Bugger !
Well at least it runs without rattling.
I wonder if the DVLA would give me the original engine number for the F1 ?
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Don't forget the F1 was 1976 and so was the K6. Honda didn't define a model by engines, they just kept fitting the same engine until they ran out of those engines.
The tank could have been changed and John Wyatts book is known to contain errors.
Being a USA model, that's 50 years old, it could have had both the engine and the frame changed over the years, plus the clocks , swing arm etc. It's probably a mystery you'll never be able to solve. What are your plans with it?
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Just planning to get it running, register it and then ride it.
Assuming that the engine still has some life left in it.
I'm not overly concerned about it being a Heinz 57 Varieties.
At least it's still a 750 Four in basically it's original form.
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I have a 73 k3 has the fuel tap on the left.
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I have a 73 k3 has the fuel tap on the left.
My information about the K5 being the first model with the LH fuel cock was gleaned from John Wyatt's book, and as stated above, it does contain errors.
I'm known for being notoriously tight fisted at times but so far I've managed to collect genuine Honda parts catalogues for everything SOHC except the K3,K4,and K5.
No doubt I'll find them all in good time.
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Good luck with it! That’s going to have an interesting “grunt” from that exhaust when you get it running.
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Looks fantastic, good luck. Glad you are going to ride it like that. My 500 is an unrestored rider too.
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BTW, there are no baffles whatsoever in that pipe !
I bought the bike with no front brake caliper.
Fortunately I managed to source one from the breakers I bought the bike from ( it has actually occurred to me that they've sold me back the same caliper that was originally on the bike)
Anyway I got a seized caliper for £46 delivered.
Three hours of prodding, poking, and roasting on the gas hob had the thing completely dismantled.
I dropped lucky this time, the caliper piston is absolutely mint, no rust pitting, and the bleed nipple unscrewed without thread damage.
So now I've ordered new seals, pads, hoses, m-cyl kit etc.
The upper hose is out of stock, but the rest still stung me for £174 !
Top hose is going to be another £38 on top
On the positive side I scrounged a pair of just about legal tyres yesterday.
The bald front tyre on the bike is a Japanese Dunlop
I'm just wondering if it could be the original ?
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I would cut the end off the pipe and put a universal silencer on the end. I've got one that is straight through but with baffle material around centre. Surprisingly quiet below 5000rpm.
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I would cut the end off the pipe and put a universal silencer on the end. I've got one that is straight through but with baffle material around centre. Surprisingly quiet below 5000rpm.
I'm already on that one.
Undid the weird little clamp thingy and pulled the rear section off yesterday, when I removed the rear wheel.
Note the high standard of engineering fixings used to mount the system, and the cunningly melted pillion footrest rubber.
I'm undecided whether to saw off the fat part, and mount a Chinese glass pack silencer, or to buy some perforated tube and make up my own glass pack.
Car exhaust tubing etc is as cheap as chips compared with anything intended for motorcycle use.
Of course it's more work for me, but I like to have something a little different to everyone else [attachimg=1]
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I once made a two into one system for my CX500, using universal flexible exhaust pipe, a Y piece from a Talbot Horizon car and a rusty BSA Goldstar silencer. I took the bike for MOT at Honda Equipe. Very cheap solution and despite strange looks from the tester, it passed without issue.
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I've lost all the photos of my £200 CB750F1 that I threw together around 1990.
I cut up three really rough aftermarket 4 into 1 exhausts, then built a hybrid system from the bits using a STICK welder.
Wheels and forks were from a CB900F, I had to weld two sprockets together to get the chain aligned right.
Seat was an aftermarket 2/4 item intended for a GS1000, but pressed into service with a fabricated steel tailpiece.
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Looks like my F1 when I bought it.
I followed the "only fix it if it's broken" mantra and all turned out OK in the end.
Please keep us posted on progress.
Good luck
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You know you'll get carried away, I had a similar idea to Skoti when I started mine, soon found myself zinc nickel plating everything in sight....and why not!
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You know you'll get carried away, I had a similar idea to Skoti when I started mine, soon found myself zinc nickel plating everything in sight....and why not!
I dunno.
Hopefully I'm older and wiser now.
I love buying up other people's abandoned projects, they're the only real bargains that I ever get.
How many times have I seen someone take a project totally to pieces, with big ideas about a concours restoration.
And never got any further, finally selling it off at an almost dead loss.
This one's got to be a rolling restoration