Comments on: What causes a carbureted motorcycle to be so hard to start, whenever it is just mildly cool (< 60 deg. F.)?

By: maddojo

maddojo — Sat, 04 Jul 2009 08:06:30 +0000

Cold fuel vaporizes very slowly, so when starting a cold engine, a very rich fuel/air ratio is needed. When the engine warms, so does the fuel going into it, so the choke can be released. Many newer carburated engines have automatic chokes, so it may not seem apparent. An no, it isn’t due to it being a Japanese engine. Before widespread fuel injection, autos had automatic chokes. You had to depress the gas pedal to the floor to engage it. Before that, there was the choke knob you had to pull in order to crank a cold engine.
Here is a link to a very nice explanation.
Great question.

By: molitor

molitor — Sat, 04 Jul 2009 08:06:30 +0000

The choke or enriching circuit on the carburetor(s) in question is most likely in need of service / adjustment.
Not sure what you mean by wetting the spark plugs.
Carburetor aspiration principle is a very different animal than applied carburetor tuning. Differences in application require different methods.
Ease of starting, low gearing necessitated by low torque output and the resultant high RPMs have little to do with Spark plug heat range. Which is the plugs ability to shed or retain heat while at operating temperatures. You have given this some thought but it is a crooked line of logic. Detonation is caused by the premature ignition of fuel due to a mismatch of octane requirement for a given compression ratio or other factors. Tidbit higher octane actually resists compression ignition (detonation) more than low octane, it’s actually harder to light in the combustion chamber.
Never seen a steel piston in anything from a lawn mower to a Lycoming aircraft engine.
Now be a good fellow and give us the year make and model of the bike in question and the failure of the choke / enricher can be addressed without mangling any other mechanical terms. to put it simply:
Yer choke is broke, bloke.

By: modelplus2000

modelplus2000 — Sat, 04 Jul 2009 08:06:30 +0000

Your carb, and choke need adjusted. All I have ever had are MCs W/carbs, and with quite a few, making adjustments fixed the problem. My husband has an 83 Magna VF750C, and we never had any problems with the carb.