Topic: Continuing Carburetor Woes revisited

[tiptopdadddy]

This is to add to my previous thread, “Continuing Carburetor Woes”.

It’s funny to look back to a year ago and realize how much I didn’t know.  But it’s still fun to learn.

Just to reiterate the facts:
1973 Honda cb750
Big bore kit (836cc?)
4-into-1 pipes, no baffle
Kei Hin carburetors with stock idle jet, .17 main jet
Needle setting 2nd notch from blunt end
Pods (K&N?)
Slide height set using “bench synch” method with flashlight
Air/fuel mix screw all the way in
Idle screw all the way out and dis-engaged
Switched to “new” used boots soaked in Xylol/Wintergreen mixture
8 new SS hose clamps
Reset the valves per the manual

The bike is running well and pulls strong.  Previously there was some stumbling when it switched from the slow jet to the fast jet in 4th gear around 4000rpm.  Moving the needle up corrected this problem.

My current issue is getting the idle to come down.  The bike acts as if it is getting about 40% too much air idling in neutral or with the clutch pulled in while in gear.  With the mix and idle screws at their extremes I can’t get below a very steady 2000rpm, even after the bike is warm for over 30 minutes.  I checked for air leaks with starter fluid while the bike was running, none found. 

I have a few hypothesis:
Big bore motor with stock valve settings is sucking too much air.
Pods allow too much air.
Carbs need vacuum synch.

The fixes would be:
Re-adjust intake valves, but to what setting?
Add green lawn mower foam and chicken wire to inside of pods to restrict air flow.
Take to local carb shop for vacuum synch, which will be done eventually.

Am I off here and or is there something I might have missed?  Any advice is mucho appreciated.

[ekpent]

If your air screws are all the way in and you are not even using the idle screw adjustment at all on the rack if I read correctly than something sounds wrong..Never had to do anything to radical for my 836. Are these stock 73 carbs? not sure what a .17 main jet is??

[tiptopdadddy]

Yes, they are stock '73 Kei Hin carbs.  .17 is the size of the main jets I was recomended to run for the big bore/pods/4-into-1 setup.  I am using a stock idle jets.  The fuel levels seem to be fine.  It is the high idling that has me scratching my head.

Too much air, but from where?

[flybox1]

not sure what a .17 main jet is??

+1
most jet sizes are mm's  .17" is like 430 mms 

[tiptopdadddy]

Ok, lets just say that the fuel supply is no longer the issue.  Does anyone know if this is a common occurrence and would it seem to be caused by the amount of vacuum, the amount of air or the difference in the synch of the carbs?

[TwoTired]

Carbs need vacuum synch.

Of course they do.
Four Independent air/fuel inlets and a common crankshaft.  Which of the four will dominate the crankshaft RPM if they are not pulling as a team?

Pods allow too much air.

This is probably the dream of most pod advocates. And, it is based on mythology rather than physics.

Pod do two immediate things.  They shorten the inlet duct, and they change the pressure drop across the filter membrane.  The original carb settings were set for physical factors.  Change them and the carb's setting are invalidated.
Particularly at idle, the "too much air" assumption is ludicrous.  The engine consumes the same amount of air for idle speed, regardless of air inlet factors.  If the inlet can supply enough air to achieve 8000 RPM, 1000 RPM will not tax the inlet system.

The engine creates a negative pressure and that pressure is equalized from an atmospheric source.  The pressure differential is greatest at the intake valve and tapers to non-existent at the air supply point.  The jet tube exist points are located along this path and supply fuel according to the differential pressure that exists at that point.  Shortening the duct makes it closer to atmospheric pressure than the pressure at the intake valve.  If the jet tube "sees" a lower pressure differential, it transfers less fuel for any given jet size selected for the tube.
Then there is the filter membrane itself to consider.  Each membrane design will have its own characteristic pressure drop across it as it passes air. 

The stock filter has a fairly controlled resistance X and the pods have Y (which varies by manufacturer).  If you assume the stock filter has more pressure drop across it than the selected pod, then this also changes (in this case lowers) the pressure observed at the exit ports of the jet tube in the carb throat.  The closer you bring those jet exit ports toward outside atmospheric pressure the less fuel will flow through them.  ... Which is why the carbs don't continue to deliver fuel when the engine is stopped.

Sync your carbs.
Then see if you have enough adjustment range of the air screws to get the proper over rich mixture these carbs need to diminish throttle twist stumble. 

Good luck!