What causes a carbureted motorcycle to be so hard to start, whenever it is just mildly cool (< 60 deg. F.)?
Whenever it is cooler than 60 or so F., it is very difficult to start without wetting the spark plugs. Other carbureted four-cycle small engines that use the same aspirated carburetion principle (except having 1 for the whole engine instead of 1 per cylinder, for performance reasons) and the same ignition principle (a single-electrode spark plug) on snow blowers, garden tractors, air compressors, generators, etc. that are designed to be routinely used in much colder weather are so easy to start in much colder weather, some without electric starters, and with no starting fluid (ether) sprayed into the intake. Is it because the Japanese have them geared so low that they then have to run at high RPM’s and thus have to have low-temperature-range spark plugs, so that at operating speed in summer weather you don’t start to get detonation, which more-easily damages aluminum pistons vs. the steel ones in small-engine power equipment?
If small-engine power equipment were always this hard to start in just mildly cold temperatures, not many people would buy them.
1994 Honda Magna 750cc (the new style V-4 engine, not the V-45) (or, what Honda simply calls a "VF750C"). The choke is manually operated.
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By maddojo, September 3, 2010 @ 4:50 pm
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, September 3, 2010 @ 4:50 pm
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, September 3, 2010 @ 4:50 pm
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.