High rpm stutter suddenly appeared.

[cadillac]

First of all I would like to say Hi, and that I am very happy to have found a site with so many experts on exactly this very beautiful bike.

I am the owner of a Honda CB750 F from '83, it has the DOHC engine with as far as I can tell standard Keihin carbs and standard airbox. The exhaust is an aftermarket 4-1.
I have been riding this machine for 4 years now and never have had any major problems with it.

Now however I have been encountering some difficulties, after I had it to a garage to have the carbs synced, only one was off by no more than about 1 psi by the way, but afterwards I experienced a stutter in the engine at about 4500-5000 rpm range, I backed of the throttle a bit and the engine rpm increased very slowly.

I now have been trying absolute everything I can think of, I have cleaned the carbs, controlled the ignition timing also checked that the advancer can move freely, I have repaired two of the rubber hoses connecting the airbox and carbs.
I am now at my final try, adjusting the pilot screws on the carbs, which by the was is almost impossible with the carbs in place without the proper tools. But then it dawned on me, after reading in multiple places that these adjustment are only for the idle mixture.
Why am I concerned about mixture, well when I drive and the engine starts to stutter, an old mechanic told me to try pulling the choke, if it got worse or better it was the mixture, and wouldn't you know it when I pull the choke it gets worse. The only diagnose I can come to is that the engine runs rich, though no black smoke appears.

Does anyone have some experience with a problem like mine or do I have to cash in for the garage to fix the problem for me, I am a guy that loves to know how my stuff works and especially why it does not.

Again Hi, and thanks for any help you guys can provide.

[seestheday]

I would look at both electrical (spark) and gas delivery.

Did the garage do any other work other than syncing the carbs?  Did they take the carbs off?  Did they pull the spark plugs?

These bikes have known issues with weak sparks.  That is why so many people upgrade to Accel or Dyna coils.  It might even be as simple as fouled plugs though.

The stutter doesn't just happen when the bike is cold does it?  It's fully warmed up when it does this?

Mine will stutter around 4-5K RPM's when it's cold.

[cadillac]

Thank you for the speedy reply.

The garage only synced the carbs. Later I pulled the plugs to check the mixture, and at that time they looked fine. However now, after repairing the hoses between the airbox and carbs, the plugs show signs of a rich mixture. I changed the cables between the coils and plugs, because I had trouble starting, also the plugs are almost new.
I assume that the engine is at working temp. after about 15 min at about 75-80 kph, and still there is this stutter or as I presume drowning of the engine.
What can cause an engine to suddenly run rich at high rpm, surely this is pretty much fixed due to the jets, and those have not been replaced, maybe since they where put on about 30 years ago.

[seestheday]

If your spark isn't igniting the fuel on every cycle then you will run rich.