tecumseh AH600 on Toro Snowthrower

[johnl_]

I don't understand what is happening here. It's a fixed speed set-up running at 4300 rpm, tecumseh diaphram carburetor, with new aftermarket diaphram and gasket. F3 on carb body, assembled in this order, carb body, diaphram, gasket, cover plate, primer bulb connected by hose to plate, no leaks. While running I can see it pulling fuel from the high speed nozzle, but the throttle is near the idle position, 1/8 to 1/4 open. I can completely close the H screw and the fuel flow from the main nozzle will stop, but it will keep running around 4300 rpm. If I close the L screw it will die. It seems to be able to run on the idle circuit at full rpm, and throttle plate is barely open. I pressure/vacuum tested the block and it held with no leaks. I sprayed wd40 on the throttle shaft with it running and zero rpm change, though it feels loose/worn. I put a load on it and it responded normally, has normal power.

I have another carb that I can swap.

[RoyM]

Then why worry about it?

[Skywatcher]

Hi John

This sounds perfectly normal, the engine does not need much throttle to maintain governed RPM with no load. Think of it like this, if your car has a tachometer, how much gas do you have to give it to hold the engine at 3,000 RPM when the vehicle is sitting still in park? Not much at all. This is what the governor system is doing with your engine at no load governed RPM. Now, to keep your car going at a steady 60 MPH on the flat takes more gas and it requires even more gas to keep the car going at 60 MPH up a hill. You have to adjust the throttle input with your foot to keep the car going at the same speed in varying conditions. If your car has cruise control, the cruise control adjusts the throttle input of the engine to keep your ground speed constant Think of the governor system on your snow blower as a cruise control for the engine, it keeps the engine going at the same speed no matter how much or how little load is put on the engine. Hope this makes sense to you,

Sky

[bgsengine]

I don't understand what is happening here. It's a fixed speed set-up running at 4300 rpm, tecumseh diaphram carburetor, with new aftermarket diaphram and gasket. F3 on carb body, assembled in this order, carb body, diaphram, gasket, cover plate, primer bulb connected by hose to plate, no leaks. While running I can see it pulling fuel from the high speed nozzle, but the throttle is near the idle position, 1/8 to 1/4 open. I can completely close the H screw and the fuel flow from the main nozzle will stop, but it will keep running around 4300 rpm. If I close the L screw it will die. It seems to be able to run on the idle circuit at full rpm, and throttle plate is barely open. I pressure/vacuum tested the block and it held with no leaks. I sprayed wd40 on the throttle shaft with it running and zero rpm change, though it feels loose/worn. I put a load on it and it responded normally, has normal power.

I have another carb that I can swap.

it's normal operation, if I understand what you are asking. When under no load, at idle RPM or governed No Load RPM, it will always be running off the idle circuit.. It helps if you think of it as Top No Load RPM means "High Idle"

As you said -the throttle plate is barely open (1/8 or so open) which means there is very little air flow actually over the venturi - you may get a fine mist, but as visible as it may be, it's very little - most of the fuel is being fed through the three idle circuit passages - If you remove the welch plug on the side over the idle mixing chamber, you probably notice 3 or 4 tiny holes going into the carburetor throttle opening - They are quite precisely placed too as you may notice if you look closely - when throttle plate is completely (or almost) closed, there's just one , maybe part of a second of those holes exposed to vacuum from intake side, which feeds the little bit of fuel required for idle operation.. as the throttle plate opens slightly, it reveals more of the second hole.. when a load is applied, the third/fourth holes come into play as they are "transition" ports that will feed extra fuel that is needed as the throttle plate opens, but before the main jet's fuel is able to effectively come into play.. (as air flow increases over the main jet and draws substantial fuel from it)

So, without a load, whatever your governed RPM is running at you're running almost entirely on fuel fed through the L (Low Speed, commonly called idle) circuit whether it is at 1600 RPM (dead idle where the idle stop screw will set that minimum) or 4300 RPM (Top No Load or High Idle) Once a load is applied, your H side and the main metering nozzle will kick in.