CB72 or CB77 crank ?

cknight · Post by **cknight** » Mon Jun 16, 2014 2:11 pm

If the keyway in the crankshaft that locates the rotor is no longer indexed properly with the crank throws, all markings will be off. The points and camshaft will still have their standard relationship with each other, so if the cam timing is off, the ignition timing is too. That's why you need to find where TDC really is, and regroup from there. I would hope that cranking compression would be at least 135-140 pounds for reasonable performance. Chase

Tim Miller · Post by **Tim Miller** » Mon Jun 16, 2014 2:32 pm

Cliquet,

Your correct there are no marks. I made a special fixture for pressing together to hold/ locate one tooth and right-hand side rod @ exactly 12 o'clock to centerline or TDC. That normally keeps the tooth @ +-.005" from perfect centerline. I'm not sure if cam timing could be off 25deg. or not if it was off half a tooth.

I check cam timing by lobe centerline, a stock cb/cl cam normally is 106 deg. ATDC for Intake and 104 deg. BTDC for Ext. with new tight cam chain.

Normal compression with seated rings should be 125 to 150 psi.

Tim

cliquet wrote:thanks for your answers
Tim you're right but no"punch mark" on the cam gear (crank side) so the maximum error is half tooth on the cam gear ;do you think that can explain the missing 25°compression on the rotor marks?.

Chase, the ignition timing is ok (checked with stroboscop on the rotor marks)and screws fixing the points plate are in the range ;as you tell me, perhaps a wrong mark "T" on the rotor.... I am going to check it.

Do you know the normal compression (in PSI) for CB72 ?I remember (but not sure ) that it is 10,5Kgs/cm²?
Cheers
Cliquet from Paris

cknight · Post by **cknight** » Mon Jun 16, 2014 5:41 pm

Once you've verified TDC, if the 25 degrees retarded is still a good figure, with 32 teeth on the cam sprocket, 25 degrees is just over two teeth. Advancing the cam that amount should get you close without having to disassemble the crank. Regards, Chase

G-Man · Post by **G-Man** » Tue Jun 17, 2014 12:16 am

The right hand crank is built in one piece with the shaft that holds the rotor so the relationship of the rotor marks with TDC will always be correct as the keyway and crankpin positions are fixed by this part.

Therefore the cam timing can only be a maximum ½ tooth away from correct if the centre section is aligned incorrectly. ~ 5.6 degrees. Highly unlikely to be ½ tooth so it will be closer than that in reality....

G

cliquet · Post by **cliquet** » Tue Jun 17, 2014 10:16 am

thank you Gman, Cknight and Tim for these precious and precise answers!
Tim, unfortunately I didn't know your "special fixture" !thus my central cam gear (crank side) is pressed in bad position 1/2 tooth . I can understand now why it was so difficult to try to fix the cam chain to the upper cam gear (32 teeth) in normal position (punch mark at 12h) ;so I prefered put the upper cam gear ( 32 teeth) slightly in advance position (about 5°..)
A last question before coming back to my workshop:
I red in constructor's standards CB72/77 that :
valve timing in. closing angle :after lower dead point 30°. with this remark:" check at 1.1mm (in case of lift)" .
could you give me the meaning of this remark .
merci beaucoup
friendly greetings from Paris
Cliquet

Tim Miller · Post by **Tim Miller** » Tue Jun 17, 2014 1:53 pm

I suppose you could try one tooth. I would purchase Art. 113 adjustable cam gear from www.cappellinimoto.it But, beware the last one's I got the marks are all off by 90 degee's for no good reason. Or you could machine / slot your stock cam gear.

Tim

cliquet wrote:thank you Gman, Cknight and Tim for these precious and precise answers!
Tim, unfortunately I didn't know your "special fixture" !thus my central cam gear (crank side) is pressed in bad position 1/2 tooth . I can understand now why it was so difficult to try to fix the cam chain to the upper cam gear (32 teeth) in normal position (punch mark at 12h) ;so I prefered put the upper cam gear ( 32 teeth) slightly in advance position (about 5°..)
A last question before coming back to my workshop:
I red in constructor's standards CB72/77 that :
valve timing in. closing angle :after lower dead point 30°. with this remark:" check at 1.1mm (in case of lift)" .
could you give me the meaning of this remark .
merci beaucoup
friendly greetings from Paris
Cliquet

cliquet · Post by **cliquet** » Wed Jun 18, 2014 3:02 am

thanks Tim
capellino's gear is a good idea. First ,I have to check again all points and try 1 tooth more.
any idea concerning my last question " check at 1.1mm in case of lift"? .thanks again.
I'll come back next month on the forum to give you last news....(my bike is in my workshop south of France near the seaside)