Differences between C72/77 and CB72/77

arthur305 · Post by **arthur305** » Sun May 13, 2012 10:48 pm

Hi All, I have just bought a CB77 with a C72 based engine. When buying I knew that the engine was a 250cc, but I had wrongly assumed a CB72!!. Turns out that it is a C72.
The seller is a motorcycle mechanic, and I am hoping that he knew what he was doing and that all will be OK - but I do not know.
So can anyone tell me what the differences between a C72 and CB72 are, and what I have to look out for?

Thanks in advance, Arthur

G-Man · Post by **G-Man** » Mon May 14, 2012 2:03 am

Arthur

Welcome to the Forum.

The C72/77 is a touring machine with a 360 degree crank, single carburettor and pressed-steel frame and forks.

The CB is, as you know, a tube frame sports version of the above, with twin carbs and 180 degree firing interval.

The engines are structurally the same but vary in the crankshaft (180 vs 360 pins) and the cylinder head (1 inlet vs 2) and the camshaft ( to go with the crank type).

It may be that you have a set of C72 crankcases with all CB other parts. To check, pull off the points cover at the RHS of the cylinder head. Does it have one set of points (C72) or 2 (CB72)? Do you have one or two carbs? I'm guessing that you have a CB72 top half on a C72 bottom half.

Sens some pictures and we'll be able to sort you out. If the engine runs the mechanic probably knew what he was doing as the parts are pretty much interchangeable.

G

arthur305 · Post by **arthur305** » Sat May 19, 2012 1:04 am

Hi Gman - many thanks for your reply (and help).
I have now had a chance to look at my latest purchase in a bit more detail - so here goes.
The bottom half is definitely C72, and has a 360 degree crankshaft (checked by testing that both pistons go up together and down together). There are no points in the head, so I was not able to verify that way.

The head has twin carbs. The points cam has 2 lobes (at 180degrees).

This suggests a CB72/77 top half on a C72/77 bottom half.

This takes me into unknown territory!. Will this configuration actually work?.
Are there any differences in the C72 and CB72 barrels (eg oil ways or other)?
Will I need to change the points cam to fire both cylinders at once?
Should I be on the lookout for C77 head and barrels, or should I lookout for a CB77 bottomend?

The frame is a CB77 (but the rear swinging arm has been bent - perhaps in a crash), hopefully I can get it straightenend - else I will need another one of them!.

Thanks, Arthur

jensen · Post by **jensen** » Sat May 19, 2012 2:01 am

Hi,

NO points ? Are you sure ?

Does the left cam has the tacho gear on it ?

I would try to rotate the crank by hand and see / feel if the valves are touching the pistons first.

It could be a C72 lower end, and a CB 72/77 head with C72/77 cams in it.

any pictures ?

Jensen

arthur305 · Post by **arthur305** » Sat May 19, 2012 7:08 pm

Hi Jensen,
The bike is not complete and was being built up as a classic race bike, scrambler or cafe racer depending upon the then current whim of the previous owner.

There are no points and no back plate and the head currently has just a "plain" cover over the rev counter side of the camshaft.

What photos would be appropriate to help identifying/clearing up the situation?.
Are there any markings/numbers on the cams which would help identify them?.
Sorry to sound so "out of it", but the learning curve must start somewhere.
Thanks, Arthur

arthur305 · Post by **arthur305** » Sun May 20, 2012 2:07 am

Hi all,
I have had a bit moe of a play with the engine (out of the frame now).

When turning the engine over there does not appear to be any contact - but at the low speeds I can manage, that may be false.

When the cylinders are at TDC (Top Dead Centre) (the Cam has 0 and F showing and 12o'clock) the LH rockers can rattle when lifted & dropped by hand (both valves well and truly closed), the RH rockers are still under pressure Inl just moving into "Open" and Exh just moving to "Close".

So next I checked the part numbers for C77 and CB77 camshafts and point shaft.
C77has RH camshaft 14111-259-020 (superceded by 14111-259-030), LH xamshaft 14121-259-020, Points shaft 30310-259-030. The CB77 has RH canshaft 14111-268-030, LH camshaft 14121-268-050 (superceded by 14121-268-060), the Points Shaft is 30310-268-010. Sounds like they have used the same parts?. I still have a slight problem with the Points Shaft - my logic suggests the C77 should have a single lobe, and the CB77 a double lobe?.
Any feedback/correction/help?.
(I have a few photos, but am not really sure what to take a picture of!).
Any/all help appreciated. Arthur

arthur305 · Post by **arthur305** » Mon May 21, 2012 5:38 pm

Hi all, Looks like my task of mating the CB77 top end onto a C72 bottom end would be quite difficult.
I understand from other contacts that the C72/77 barrels are slightly different than the CB72/77 in that the oilways/galleries are in different positions - anyone verify this?

I guess I will have to start looking for a CB77 bottom end.
Arthur