XK140 starts on first tick over, starting carb activates and idle is 1,100. I’m in temperate climate so shortly the thermo switch breaks ground and the idle drops to around 650-675. After driving for some distance and the engine reaches temperature the idle returns to 1,100. I have checked all throttle linkage, return springs, etc. -all seems in good order. Because the idle returns to the exact starting carb rpm I am suspecting the thermo switch may be faulty - easy enough to check. Any thoughts on that would be appreciated.
But my question is regarding the starting carb tapered needle and auxiliary jet. If the function of the starting carb is to rich the mixture, why is the needle is drawn down into the jet? It appears that it would lean the mixture instead of richening it for the initial start.
Alan
A very good question. I’m just trying to set mine up and would like to know the answer to Alan’s question. The ‘needle’ on my 140’s ASC looks to me to be a step-down in diameter rather than a point, but it has a slight taper where the step occurs.
Maybe when the device is off, this taper rests against its seat, so no fuel passes through the jet. When the device is actuated, the solenoid opens the round flap valve which causes the needle to rise against the spring pressure?
Alan, I think you would be able to tell if the ASC had come on again by the reappearance of the hissing sound.
This might help:
When the choke is activated, the solenoid raises the flat plate up opening the path for the fuel to run through the starting carb. The engine vacuum draws the needle down into its position at the jet. The adjustment nut on the needle determines the position of the needle in the jet. That’s why the needle can only be adjusted when the “hisser” is operating. Once the the solenoid is shut off, the vacuum is stopped, the fuel is stopped, and the needle via the spring returns to its rest position off the jet.
I hope this helps.
Mike,
Thanks for your response. I did figure out the mechanics of the starter carb, but why is the needle drawn down leaning the mixture on startup when the function of a choke is to enrich the mixture?
Alan
When the starting carb has vacuum, the needle is drawn down by the vacuum to where it is stopped by the adjustment nut, in other words as far as it needs to be. You can adjust it up and down for summer or winter. When the starting carb is shut off, no vacuum, the needle has no effect at all, it’s in the ready position, ready to be pulled down when the vacuum is switched on.
Hi Alan,
When you are cranking the engine there is virtually no vacuum so the needle stays up and allows a maximum amount of fuel into the mixture. When the engine starts running the vacuum pulls the needle down to create the proper mixture for a running cold engine. If the cold engine stumbles the vacuum decreases, the needle goes up and enriches the mixture more until the engine gets back up to speed. If it weren’t for these effects there would be no reason to have a movable needle at all.
Actually the solenoid plate opens the path for air to flow. The fuel is always there, making your garage smell.
I can’t add much but the needle will also rise (adding extra fuel) whenever the manifold vacuum drops, that is when increasing throttle or otherwise increasing load. The ASC is doing something throughout all (cold) operating ranges.
Tho figure out if the ASC is to blame pull up the needle while „it“ happens. It should be up against the stop and nothing should change.
Mike,
Thanks for the explanation. All makes sense now.
Alan
You don’t mention what position the Starting Carb Needle is after warm-up. If it has returned to the neutral position( not drawn down) , I’d suspect you have a vacuum leak in your intake system. I had the same problem after switching Carbs