75 Z1B 900 Hesitation

Have 75 z1b (first day of production) with Mikuni vm28sc 1473 carbs which appear to be 73-74 early according to wiredgeorge as they have single large bowl drain and are ribbed style. I think they may have been factory installed not replaced later in life! Since being stored for years and just recently fueled up and run the engine starts easily with choke yet within half a minute can shut of enrichers and will idle at 1500-1600 with idle screw all the way out. While sitting still at idle it will rev easily to any rpm yet when you try and drive it has huge hesitation when trying to accelerate. Hesitation under load starts at 2500 thru 5000 rpm then runs great from 5000 to redline!
It has Dyna S ignition, strobe timed to "F", tried two different sets of coils, shim clearance good, synced carbs with carbtune, float levels adjusted, 112.5 main and 20 pilot. B8ES plugs dry dark fouled (carbon) when checked and replaced.
The engine will run without hesitation if you manually engauge choke slightly when getting near 2-3000 but is scary while trying to drive! Carbs are pristine inside, . No smoke out exhaust and have driven with this symptom for half an hour and still hesitates....so it is at operating temp. As I indicated earlier, the idle screw for all four carbs is out all the way and still idles at 15-1600 rpm...will go no lower! 12,000 total miles on machine since new.....any ideas welcomed as spring is just around the corner!!!

Set the Dyna at full advance then lower the idle. Use a guitar E string or strand from a wire brush to be sure the pilots are clear.
Steve

Check your mechanical advance

144inc wrote: . . . idle screw for all four carbs is out all the way and still idles at 15-1600 rpm...will go no lower! . . . .

Does pilot air mixture screw adjustment on any carb produce a change in idle rpm?

Generally, when turning a pilot air mixture screw fails to affect the rpm, it indicates that there's likely something wrong inside the carb. Could be a persistently obstructed pilot circuit in a carb that has been cleaned and appears to be clean and thought to be clean.

The obstruction might be in the orifice through which the fuel mixture exits from the pilot jet into the carb bore.

Here's a diagram that shows the air and fuel flows. together with a possible pilot circuit cleaning method.



Good Fortune!

144inc wrote: . . . idle screw for all four carbs is out all the way and still idles at 15-1600 rpm...will go no lower! . . . .

In the bench syncing procedure, the idle thumb screw should be mid-thread when equalizing the initial slide heights, which is to assure that the slides may be further lowered as may be necessary during the later running-sync procedure.

Good Fortune!

Yes, when pilot air mix screw is less than 1 1/2 turns out...like 3/4 out from gently closed the idle is reduced but I have always had other Z1's 73-75 at 1 to 1 1/2 turns out as indicated in manuals. The idle thumb adjustment brass screw that reduces/increases overall idle is not even touching or able to lower rpm only increase if screwed in.
If I screw in pilot air mix to 3/4 turns out I thought this would reduce air in mix and cause even a richer mix. My plugs appear dark, sooty already at 1 1/2 out.
In your second note it looks like I need to learn about "initial slide heights"
Thankx for the words of wisdom!!! I may not always understand BUT will always listen and learn!!

The 1 to 1 1/2 turns out on the pilot screw is an starting point. Each carb pilot screw is adjusted to the highest RPM. If you can close or go all the way open you have an internal carburetor problem. Most likely a pilot circuit problem.

Follow the instructions Patton's illustration.

On my Z1, a half a turn on the pilot air screw will slightly change the color of the corresponding spark plug.

Also make sure the spark plug wires and caps are good. I've cut 1/4 inch off the end of a couple of wires to get rid of corrosion and have had caps go bad. The caps screw on and off the wires.

Are your throttle cables adjusted too tight as to not let the throttle close all the way?