Question about choke

I' just curious. I have seen others mention this but never seen an explanation. I cold weather when I have to use the choke after about 45 seconds the rpm will run up to about 4000. I lighten up on the enrichment and it falls back to normal. What is happening to cause it to run up to 4000 rpm? Otherwise, the bike starts and runs really well. I must be something with these bikes, because none of my 3 Hondas do it.

 

Here you go, courtesy of Dwayne Verhey:

"Now on to the starter circuit. As you know, the starter jet (size #40, same as the pilot fuel jet) is pressed into the bottom of the float bowl. Above it is a small reservoir. A brass suction tube ("straw") extends down into that reservoir, above the starter jet.

The straw has a tiny hole on one side near the top --- just below the carb body --- which functions as an air jet since the carb bowl is vented to atmosphere through the hole and recess cast into the carb body.

In a static state (i.e. no fuel flowing through the circuit) as when the bike has sat all night, the fuel level in the starter jet reservoir is level with the fuel in the carb bowl. So, when the "choke" lever is applied and the bike started, fuel flows from the starter jet reservoir up the brass tube, past the plunger and into the engine, enriching the mixture. A tiny amount of air is also introduced into this stream by the hole in the side of the straw. This gives the cold engine that jolt of extra-rich mixture that it needs to get started. More fuel enters the starter jet reservoir from below, but the size of the starter jet limits the flow to less than the rate the straw is sucking it out, so the level drops to the bottom of the straw and the straw starts sucking even more air, leaning out the mixture.

It is at this moment that the freshly started XJ engine suddenly increases from 1,000 or 1,500 rpm up to 3,000 or 4,000 rpm all by itself, as if the throttle had been opened. And in effect, it has --- we now have twice as much fuel and nearly twice as much air entering the engine as we would have on the pilot circuit alone.

At this point the operator usually starts easing back on the "choke" lever, to reduce the rpm. The fuel eventually starts to rise in the reservoir again, but that's OK because the starter plungers are lowered and restricting the overall contribution of the starter circuit. The engine is still cold and runs quite happily on the now (slightly) rich fuel mixture.

But what happens if the operator suddenly changes his mind and goes back to full "choke"? The engine stalls, because of the sudden gulp of extra fuel while the starter jet reservoir is again drained down."

 

Thanks. That makes sense. I guess the later Hondas don’t work that way. I’m going to dig up a Hatachi diagram and follow the above through the carb.

 

My KLR650, Goldwing1500 and Seca650 all carbureted and all do the same thing. Specifically, with the enrichment circuit on...rpm's rise as the engine is warmed up.
My understanding is,
When your engine is cold there are no hot surfaces which aid in vaporization. The fuel is in the form of wet droplets that can't be ignited by the spark plug. It also condenses on the intake tract. Hence the need for a cold start mechanism. When the enrichener circuit adds fuel, only a small amount is actually vaporized and burned...enough to start the engine.
As the engine warms the heat from the combustion chamber aids in the vaporization. The rpm increase is due to the fuel being vaporized more efficiently.

My 2 cents.

 

Interesting. All my Hondas have carbs and they never race although they do run a little faster with the choke on.

 

My Goldwing 1500 (CV carb) will increase from about 1500rpm to about 2000rpm within 30 seconds. The KLR (also CV carb) from 1200 to 1500.

 

Here's a pic of a typical cold start circuit I found in a Haynes carb manual. Although not identified specifically as a Hitachi carb...looks almost identical to me.

 

I still can remember the Kawasaki's from the 80's. Either they didn't run or they did around 3-4K cold.
There were many questions and concerns about it in those days.

 

Do they have a choke that covers the intake side of carb or an enrichment circuit? I know my Rebel with a choke will run a little higher but the XJ will hit ~4k as it warms up.

I had an 82 KZ750 4cyl before my Maxim, that thing would scream if you left the choke on during warm up. That bike gave me the most trouble in tuning that I've ever had on a bike in my entire life.

 

Most of the 4 stroke kawasaki's did not live longer then appr. 20-30k km's.
But then again.. they were used hard right from the start (cold that is).
There was hardly any time for warmups; you jumped on them en GOOO lol.