Spark Plug Heat Range and Choking

[dennis]

I've noticed that a number of lawnmowers have "automatic" chokes. I am familiar with how many work (bimetal coil near exhaust working with gov air vane). The question I have is if an engine is prone to over-choking (takes awhile to warm up), would the heat range of a spark plug have anything to do with the speed of the warm up? In other words, is a cold plug or hot plug better for starting cold engine, or would that even have any effect?

[KE4AVB]

If I remember correctly the plug temp is primary for self cleaning action though one that too high of heat range will cause pre-ignition. The exhaust temp is primary affected hot the fuel burns and this greatly affected by the air/ratio; otherwords, an engine that is too rich will burn cooler vs one that is too lean will run hotter. I have one several year that run so hot that it would self the fuel and you couldn't kill the engine even when you pulled the plug wire. I had kill it by simply overloading the engine.

I would be thinking along the lines that the choke vane is too restrictive if the engine when hot runs fine. But first I would make sure the choke linkage is adjust to specs and double check the muffler in the area to the bi-metal sensor as there might be carbon clog.

Now I haven't work any of these auto choke engine yet; just older V8s where I had the sensitivity of the thermal choke system. From what I gather it similar system except you adjust the linkage instead of the position of the bi-metal spring. Even on these V8s if the intake crossover heat chamber is plugged with carbon the choke would not work properly.

But let see what others might be thinking that has worked on these auto-choke systems on small engines.

[Luffydog]

Worked on a few but really haven't had probs with the choke on a Honda yet. Had to replace a few carbs on them tho. Few that I have seen are very slow to open. Once you pull the start handle it starts to close and then you pull rope and opens up fully after start up or suppose to from what I have seen.

[bluemower]

usually, the auto choke engines are very easy to start when cold. On Briggs, this is due to a high output ignition system and a smaller venture in the carburetor. When the engine starts, air from the fly wheel pushes against an air vane and opens the choke. A problem occurs when the engine is started and operated for about one minute - then shutoff. When an attempt is made to restart the engine 30 seconds later, the engine will not start due to flooding. In this situation, the Thermostat has not warmed up and makes no effort to partially open the choke. The choke is still spring loaded close - but the engine no longer needs full choke.

Briggs issued a service bulletin that instructs to eliminate any space between the thermostat arm and the air vane that opens the choke. This adjustment works ok at sea level. At higher altitudes, the Briggs adjustment may not solve the problem. At 7,200 feet, I install the high altitude jet and instruct owners to keep the cold engine running for at least 4 minutes.

On Hondas with the "clock" mechanism choke, this system has a problem sticking. On high altitude engines, allow the choke to open about 50 % - before pulling the rope. Later Honda auto choke systems continue to have problem just like the briggs.