very fast rhythmic surging of motor

[boyorastroboy]

Hi there,
I've got a 76 CB750 F1 Super Sport.  It's been running like a dream since I restored it about 6 years ago, but recently I've noticed a rhythmic sort of surging of the motor.  I don't notice it so much in the feel of the bike as in the sound, although the bike's performance has degraded somewhat.  It's like a regular pulsing sound about once per second as though someone were really rapidly opening and closing the throttle.  It sounds a lot like when you turn the starter motor on a car that's not firing.  I thought it might be one carb not running correctly, so I cleaned them all out and no difference.  I tried new plugs, no difference.  I've also looked into the timing.  I have a Dyna S ignition and it seems to be set correctly.  Any other thoughts?  Could a bad coil be causing this? Thanks!  

[Re-run]

Most likely I would say the carbs might be dirty. if it has been a while since you cleaned them, now might be the time. As long as you dont pull the slides out, you should not have to sync. Just drain the bowls and take them off. Remove the jets and clean them up.

See if that takes care of it.

[boyorastroboy]

Thanks for the response.  I did just clean the carbs pretty thoroughly and it didn't address the surging symptom.  I did get a decent amount of rusty dust out of the main jets and the bike seems to be running better.  (Also replaced my visu-filter with a paper element to (hopefully) prevent this from continuing.) I did carefully pull the slides out, but didn't touch the adjuster nuts at the top.  Will I need to re-sync?

[Re-run]

Hard to say. Usually if the slides come out, a sync is needed.
Did you pull out the idle jets? Those are usually the ones the clog up. You need to make sure every little hole on them is clean.

Another thing that sometimes happens is the rubbers between the carb and engine get hard and can cause air leaks.
Try spraying wd-40 around those boots and see if the idle changes.

[vern401]

Does it surge at idle or running down the road or both?

[fpcb750er]

Agree with checking the boots for vacuum leaks, just happened to my 72 750 and after I retightened all the boots and drop the float bowl to pull and clean the slow jets.

you might also want to check your timing as well, that can slip over the years, did on my old vintage cars.

[boyorastroboy]

Thanks for the suggestions, guys.  It seems to not surge at idle at all.  I notice it only when the rpms are somewhat high (have a busted tach).  I decided to dig into the timing and have found that the 1-4 cylinder was dead on, but the 2-3 were firing several degrees early.  This surprised me because I've had a Dyna ignition in the bike which has worked flawlessly for 5+ years.  What further surprised me is that I can't seem to configure the Dyna in a way that I have enough "play" to adjust the 2-3 back into time.  I called Dyna and they said that was highly unusual, but possible.  They gave me some advice on how I could re-adjust the plate by measuring off the TDC mark.  Haven't had a chance yet to implement, but will try tomorrow.  

[Re-run]

Assuming this is a dyna S, in the manuals section, there is a pdf on the instructions on set up.

[bigdaddysean]

i have the same problem on my 78 model, i cant seem to find the problem ,i have the cycle x pro ark ing. systembut i have no adjustment, its in or out it seems. please post or e-mail what you find.thanks. [e-mail]

[vern401]

1 and 4 dead? do the run off the same coil?
if so try replacing the coil.