Cafe Racer Forum banner

1 - 6 of 6 Posts

·
Registered
Joined
·
1,523 Posts
Discussion Starter #1
here is the scenario. 1976 cb750. only performance mod shorty 4 into 4 drag pipes.

The bike runs well at all throttle levels under 5,000 rpm. feels very very torquey.

when asked to go over 5000rpms, it starts to pop and missfire unless the throttle input is around 15%. Is there a certain jet i should look at? i don't feel that the ignition is at fault.


'72 cb750
'76 cb550
'76 cb750
 

·
Registered
Joined
·
242 Posts
Sounds like ignition to me also. How is the battery? Low primary ignition voltage (battery weak) could cause that behavior as could timing, points floating, weak coils (unlikely but possible) or bad condenser.

What is the history of how it developed? All of a sudden or increasingly noticed over time?

Mike O.
 

·
Registered
Joined
·
687 Posts
just a guess but i haven't heard anyone mention advance springs going bad.

...connoisseur of slack...
 

·
Registered
Joined
·
838 Posts
quote:
just a guess but i haven't heard anyone mention advance springs going bad.

...connoisseur of slack...
Typically when advance springs go bad they either stretch or break. This allows the timing to advance too soon and/or be advanced too far in the lower RPM range. If nothing else was wrong, worn springs would cause idle and low end performance to be crap due to erratic and over advanced timing. High RPM running would be fine where the fully advanced timing would be correct.

" I don't feel that the ignition is at fault."

Have you inspected the points? Are they gapped properly? Are the contacts burned? Does the condenser hold a charge? Have you checked the ignition timing? At idle and at full advance? Does the timing sweep smoothly and fully as the RPMs increase? Is the timing stable at steady throttle?
All ignition settings and operations need to be checked and proven functional before carburetion can be acurately addressed.


Cheers, Bret @ Glass from the Past
 

·
Registered
Joined
·
1,523 Posts
Discussion Starter #6
update:
the points were adjust correctly, but they didn't look great so i replaced them. also put 110 main jets in. now she runs great! so while i don't know which item fixed it, its fixed.

while removing the main jet of number 4 carb, i noticed a small crack in the casing around the spot where the main jet goes. yeah, it ended up breaking. So i jb'ed that peice back on and waited a couple hours. then put the new jet and holder on. we'll see how long that lasts. thanks for the answers and tips!

'72 cb750
'76 cb550
'76 cb750
 
1 - 6 of 6 Posts
Top