2024-03-20 Hard Starting After Sitting Fix

From Scotch at kz1300.com

My ‘80A can also be difficult to start from time to time, when it's sat for many days. We think everything is set-up perfectly and yet, "it's not perfect". If it was - they would start quickly every time after sitting for any length of time. This is one of the most common problems with 1300’s that until recently I couldn't understand, despite having some experience with these carbs.

We know the fuel-bowls dry-out quickly. We also know that all the fuel galleries that deliver fuel/mixing-air are all above the fuel-level and will be void also. This includes the choke circuit.

Vacuum is required to "pull" the fuel up and through the delivery galleries and to the venturi.
So with a well charged battery providing adequate cranking rotation speed, “Why are we not getting fuel to the engine”?
Specifically - Why isn't the choke circuit delivering "starting fuel"?

Assuming the carbs have been thoroughly flushed (Cleaning Tool), the idle mix-screws are set, float-level is correct (Brass Floats anyone?), choke plungers, fast-idle cam synchronized correctly, fast-idle (cam-roller) is sitting correctly, ignition and valve tolerances within spec. and carbs are sync'd ......then what the hell is the problem?

There’s only one conclusion I can rationalize: Insufficient vacuum to lift the fuel - unless you're jump-starting and spinning the engine like crazy! …….and even then it can take considerable time to get some flame!
So if it's a lack of vacuum; where is the vacuum break? (Note: I’m not saying vacuum leak. There’s a difference!)
There are the idle-air and high-speed mixing-air inlets. (Front of carb.) Then there are the "enrichment" valves which supply additional metered air to the fuel galleries. (Unless closed by high vacuum upon deceleration). These are all metered internally but non-the-less are a vacuum-break.

Then there’s our choke circuit! When the plungers are lifted they are supposed to allow a greater amount of fuel (relative to the idle circuit) to pass into the large delivery port on the engine side of the throttle-plates. It's a logical assumption that this should be a High Vacuum area! If there is no choke fuel being drawn up efficiently then where’s the vacuum that should be providing the fuel lift? And let’s keep in mind that most 1300’s have lots of miles, have known ring-sealing issues and for many other reasons will not be “factory-tight”
If you look into the bottom of the choke plunger-well, you'll see three large ports. Two (facing the engine side) that deliver choke-fuel to the intake manifold. The other (facing the air-filter side) is the same diameter and height above the choke plunger seat. This port sources air from the (atmospheric) underside of the slide-diaphragm. With the throttle plates closed at full-choke- the need for some additional mixing air seems logical. I'm calling this port the “Choke Mixing-Air” port.

However: When the plungers are lifted they expose these 3 ports the same amount, simultaneously. And these ports are huge! This is a major vacuum-break and it defies logic – to me.

To see if my theory had any validity, I restricted the 3 “Choke Mixing-Air” ports. The photos are self-explanatory. Silicone and brass tube. This is a 75% reduction in size so it makes sense that the vacuum applied to the choke circuit from the Intake stroke should be significantly increased by the reduction in the size of the “Choke Mixing-Air” port.

Before re-installing the carbs I spun the engine over about 10-12 seconds to clear any residual fuel/air remaining in the cylinders. The bowls and carbs were already dry but compressed air made certain that there was no fuel/ vapor remaining in any of the delivery runners, and then the carbs were reinstalled with the fuel supply left OFF.

The bike then sat completely fuel-dry for almost 3 full days.

I would normally expect to have to do some serious cranking to get it to start.
Turned on the fuel – listened to the bowls fill and when the “gurgling” stopped I applied FULL choke and went for the start. The engine started immediately – In less than two revolutions at which point I reduced the choke and the engine settled into a smooth 1200 RPM warm-up idle No “carb. Adjustments” were required FUCK, YA !

I’ll keep y’all posted !

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