Surge

[Kool_Biker]

Not sure how to accurately describe this, but here it is for all ICOA gurus:

My CBX Z is seemingly in top notch, pulling smooth and hard. A pleasure to ride.
Carburettors are in sync with engine taking most any throttle opening from very low RPM, without any snatch or other complaints, all the way.

However, when I accelerate hard and then shut (but not snap) the throttle, the engine will surge forward for a tiny fraction of a second, before the expected 4 stroke braking effect takes over, as normal.
This is not new, the bike's doing it for ever, it is not dangerous and will not be felt if the throttle is shut gradually. But it is odd.

Any ideas? Am I missing something here? Is this 'normal'?
Carbs, air filter, and bike overall are all dead standard.

Regards

[piit]

Hi,
I have been in same situation when I was searching the best jetting combo for my carbs.
This was one symptoms of many when I was too far in rich side with jettings. However, caused some "nasty" situation when wanted to do hard motor brake before the curve

But in your case you have all standard. Just came to my mind the air-cut valve, have you try to eliminate it or replace it...at least would be very easy to try/check.

-piit-

[Kool_Biker]

Hi Piit
My air cut off valve is plugged, as per (as I remember) Mike Nixon's advice somewhere ..
No nasty situations for me, just this 'surge' as I begin to close the throttle from accelerating hard, lasting a tiny fraction of second.
Cheers

[Mello]

Hi Aris,
I also have this on my 79. Very slight surge. In the kit I bought from Randakk, it had a new diaphragm for the air cut off so I installed this. Does this contribute to the surge?

Will keep an eye on this thread to see of there are answers.
Cheers,
Mike

[daves79x]

How high is your idle set? Anything above 1000 rpm and the bike will do that.

Dave

[Kool_Biker]

Probably a little over 1000. Will check, adjust and report back.
Out of curiosity Dave, why would this be so?
Thanks!

[daves79x]

The idle circuits will be passing a bit of mixture through, if the bleed holes in the throat are too exposed, and cause the over-run. Back the idle down to about 800 rpm and see what happens. These carbs are very sensitive to that.

Dave

[Kool_Biker]

Yeap, definitely, I reduced the tick over speed from a little over 1000 to around 750 rpm and the 'surge' is completely gone.
My respects to Dave. Where would we be guys, without his unfathomable know-how and experience.

[NobleHops]

Yeap, definitely, I reduced the tick over speed from a little over 1000 to around 750 rpm and the 'surge' is completely gone.
My respects to Dave. Where would we be guys, without his unfathomable know-how and experience.

I'd have a bike in boxes and bloodier knuckles.