Overhauled Carbs VM28SC - Z1A issues

I am cross-posting this from the Z1 forum - it may be less specific an issue and I'm hoping to get some responses here, too:

I installed a set of overhauled VM28SC (correct for my Z1A) carbs and I am seeing some strange behavior I don't understand. Maybe someone here can help. I'm getting tired of pouring $ and time into the fueling - this was supposed to fix it for good.

A little background:
I bought and fixed up the bike a couple of years back. It now has stock (reproduction) 4-4 pipes, stock air box and filter (all from Z1Ent) new plugs, new points and condensers. I checked voltage to coils all good. Timing light check shows ignition dead on.

I live at 6000 feet elevation and often ride between 8000-11000 feet in the summer. The winters mostly confine me to the 5000-7000 feet altitude range.

With the original stock carbs the bike came with I never got it to run quite right. Some hesitation off idle, surging and such made me fatten the jetting up - I was last running 25 idle jets and 117.5 mains. Needle on second clip. The surging went away but it ran rich and it still hesitated off idle. At high altitude it would run badly - way too rich. Leaner jetting would result in surging under 3500 rpm or so.
This always bugged me as with a stock setup one would expect needing a leaner setup than stock (17.5 and 112.5). The bike would not run at all with the stock jetting and even worse leaned out more than that.

So I got a set of overhauled carbs - setup like stock with stock jetting - all new seals, o-rings jets, etc. Put them on yesterday. The bike started fine but after warming up would only idle with the throttle stop screw turned in very far. After the bike was warmed up completely I attached the vacuum synchronizer tool and saw each carb only pulling about 2 psi of vacuum. The bike revved up OK but vacuum stayed low. As I was standing around pondering this the idle rpm started increasing - I thought oh, she's finally warmed up and kept adjusting the idle back down by turning the stop screw out (to a more reasonable position slightly out from the middle of the range instead of almost all the way in). Vacuum went higher and higher to settle at 8 psi or so and all looked good - until I discovered the reason - my little aux fuel tank had run empty and the bike quickly thereafter died from fuel starvation.

So the bike idles correctly only just before it dies of fuel starvation - with the fuel levels low in the bowls maybe? I checked the fuel level settings with the clear hose on the new carbs and they are good (all at the lower edge of the float bowl seat except #4 is a tad high but not to a point where it would be a problem).

Why would the carbs not have any vacuum at idle until the fuel level drops? What's happening?

Any thoughts would be appreciated.

Any chance you're Frank? I was just about to suggest that you join if you weren't a member already.

Is your air injection system working? Are ur new carbs synced right? What's ur compression like? Valve Clarence good??? I was having problems with my 80 1k a4 .... Stock it uses 107.5 with a15 pilots right now I'm using the a15 pilot with 127.5 mains and she runs like a top another thing to check is all your wiring make sure everything is connected helps to pick up electrical component cleaner.... Clean all connections make sure all are crimped nice and tight and most of all make sure the main ground I clean and tight.... I'm sure u have done most of these things thought I'd mention it to bring it on te table

mtbspeedfreak: That is affirmative

Kz1k420 wrote: Is your air injection system working? Are ur new carbs synced right? What's ur compression like? Valve Clarence good??? I was having problems with my 80 1k a4 .... Stock it uses 107.5 with a15 pilots right now I'm using the a15 pilot with 127.5 mains and she runs like a top another thing to check is all your wiring make sure everything is connected helps to pick up electrical component cleaner.... Clean all connections make sure all are crimped nice and tight and most of all make sure the main ground I clean and tight.... I'm sure u have done most of these things thought I'd mention it to bring it on te table

Thanks for your suggestions.
No air injection system on the Z1 that I would know of.
valves are adjusted correctly - compression good.
ignition is A-OK.

Just thought of this, what are your intake boots like? Any chance they'd be cracked or the plates are warped?

mtbspeedfreak wrote: Just thought of this, what are your intake boots like? Any chance they'd be cracked or the plates are warped?

Plates?

The intake boots (if you mean what's often referred to as "the carb holders", i.e between the carbs and the cylinders) are new-ish and the bike passed a WD40 leak test with the old carbs (and they pulled about 7-8 psi vacuum as they should) but I can take the intake boots off and inspect them.

Typically, smaller jets are used to compensate for higher altitude where the air is thinner.

While cruising along at 3500 rpm, the fuel mixture is still being influenced by the carb pilot circuits.

Here's one method to assure the pilot circuit is clean.




Adapt same procedure for carbs with pilot mixture screw.




If not already done, would also verify function of the timing advancer by watching for it to rapidly move back and forth when viewed under a timing light, as rpm is varied by blipping the throttle.

Possibly, the advancer is hanging or sticking, or for whatever reason might be failing to move freely.
Would clean and lubricate if needed.

Good Fortune!

Patton wrote: Typically, smaller jets are used to compensate for higher altitude where the air is thinner.

While cruising along at 3500 rpm, the fuel mixture is still being influenced by the carb pilot circuits.

Here's one method to assure the pilot circuit is clean.

Adapt same procedure for carbs with pilot mixture screw.


If not already done, would also verify function of the timing advancer by watching for it to rapidly move back and forth when viewed under a timing light, as rpm is varied by blipping the throttle.

Possibly, the advancer is hanging or sticking, or for whatever reason might be failing to move freely.
Would clean and lubricate if needed.

Good Fortune!

Thanks. Pilot circuits are clean - that is not the problem.
The advance and ignition work fine (as good as they do on these bikes). Verified with timing light.

For reference from Z1E's carb guide --

28mm (VM28SC) Z1 & early Z1-A
Main Jet 112.5
Air Jet 1.0
Needle Jet P-8
Jet Needle 5J9-3 - last number is the clip position where
1 is at the top of the needle (leanest) and 5 is
at the bottom (richest)
Throttle valve (slide) 2.5mm cutaway
Pilot Jet 20.0
Air Screw setting 1 1/2 turns out


28mm (VM28SC) Late Z1-A & all Z1-B
Main Jet 112.5
Air Jet 1.0
Needle Jet O-8
Jet Needle 5J9-2 (last number is the clip position)
Throttle valve (slide) 1.5mm cutaway
Pilot Jet 17.5
Air Screw setting 1 1/4 turns out


Good Fortune!

Are all 4 choke plungers fully closed -- as they are supposed to be -- when the choke lever is OFF?

Good Fortune!

Patton wrote: Are all 4 choke plungers fully closed -- as they are supposed to be -- when the choke lever is OFF?

Good Fortune!

Yeppers. all chokes work as they should. ~