Topic: Will a F motor fit in an A frame?

[Mr Freeze]

Hey guys, I have a 1978 CB750A that is currently running, but just learned that it needs a head gasket. I happen to have an opportunity to buy a 1978 CB750 F donor motor for a good price.

Does anyone have any insight to whether this swap is possible?

[70CB750]

I know that 78F fits 73K frame - with alterations to the swingarm and left rear shock mount - yours should fit unless A frame is different.

[Old Scrambler]

The motor will bolt in...............but I do not know for sure if the chain size and sprockets will match..........check both pitch and width at the rear.  That's quite a change for performance................be aware...........the F2-3 heads are known to develop worn valve-guides at less than 20,000 miles.

[Mr Freeze]

Well I think if I get my hands on the donor bike that I'm going to do a head and valve job on it before I even install it. I just talked to the guy and it's a K motor.

[70CB750]

If its a K, you should be fine with the valve guides and I venture to say that sprockets will line up too.

Does A have disc brake in the rear?

[Mr Freeze]

No there isn't a rear disc brake on the A. Check this ad out. Guy said it was a K, the brake setup is pointing to an F.

http://philadelphia.craigslist.org/mcy/4428060169.html

[Old Scrambler]

Looks like an F0 or F1 motor (short intake rubbers) in an F2-3 chassis with a K-7-8 rear fender.  Try and get some frame and motor IDs.  You can make it work if that motor is good....................

[Mr Freeze]

Yeah I don't know if maybe it's a K motor with some parts from an F, like the boots and carbs? It's some kind of hybrid cause weren't the 1978 motors black? It's an odd duck but I'm really just looking for a good motor so it don't care much about the rest.

Anyways, I'm going to go see it Monday, check compression, and all that madness. I have a set of 74 K carbs that I'd fit to it. Everything else on my bike is ready to go and I'll probably stash away my A motor for something in the future.

[Grnrngr]

Don't forget to get all the clutch handle parts also..

[hondamaticdave]

I'm thinking of doing this myself. I love the style of the A. Bigger tank, gl1000 style wheels. My thoughts are that this would be awesome with a strong K motor. Maybe 836 kit. Sounds like fun to me. Very much so.

[brandEn]

If the sprockets don't line up I have a cush drive from my 1978 F3 comstar wheels that might help.

[hondamaticdave]

Sweet. Well my motor is leaking oil slightly from the head gasket, near the chain tensioner. Its not a big problem really. I also don't like the high rpm at 70. For the cost of sprockets, chain, head gasket, and all that, its not to far of a stretch to just change out to a K motor and be done with it.

[dave the welder]

don't forget you have to mount an oil tank

[hondamaticdave]

Yep this is true. I'd have to figure that out. Thanks for the info.