Why Does Low Cranking Rpm Cause No Start?

[George7941]

On a fuel injected gas engine? My Honda 300 motorcycle is not starting up now after it was last run in November. I will be replacing the ten year old battery.

My question is more general. Low cranking rpm leads to compression loss past the rings but does that prevent the spark plug from igniting the mixture? The spark is there, the gas/air mixture is there albeit not compressed as much, why will the spark not ignite the mixture?

 

[4wheeldog]

On a fuel injected gas engine? My Honda 300 motorcycle is not starting up now after it was last run in November. I will be replacing the ten year old battery.

My question is more general. Low cranking rpm leads to compression loss past the rings but does that prevent the spark plug from igniting the mixture? The spark is there, the gas/air mixture is there albeit not compressed as much, why will the spark not ignite the mixture?

I think the issue is the spark is weak because battery voltage is dragged down when cranking with a low battery.

 

[NOTEVIL81]

Compression warms the air fuel mixture adding the spark plug to ignite.

 

[PandaBear]

Slow crank does not take in the same amount of air as fast crank, so the compression is different. Slow crank also does not release the exhaust at the same time as fast crank, so the remaining exhaust staying in the next intake is also different. All these things make them run very different than what the designer were designing for.

 

[JRay’07]

Not always. My Mom’s 6.0 LQ9 V8 started with a ridiculously low cranking RPM once. It sounded like the engine stopped at TDC on a cylinder and then lit off.

 

[JimPghPA]

Weak spark with low voltage battery and as others have said the compressed air is not as hot with slow crank ( maybe more time for that air to transfer heat to piston, cylinder, and head). Fuel might not vaporize as well with less abrupt movement of air. If the sparkplug gets wet with fuel ( that did not vaporize ) then it becomes much harder for the weaker electricity to jump the gap, instead of traveling on the wet insulator and never even making an actual spark.

 

[George7941]

Weak spark with low voltage battery and as others have said the compressed air is not as hot with slow crank ( maybe more time for that air to transfer heat to piston, cylinder, and head). Fuel might not vaporize as well with less abrupt movement of air. If the sparkplug gets wet with fuel ( that did not vaporize ) then it becomes much harder for the weaker electricity to jump the gap, instead of traveling on the wet insulator and never even making an actual spark.

Thanks, Jim. You have nicely stated all the reasons why the mixture might not light. Thanks to the other respondents too.

 

[George7941]

Got the bike started up today. Boosted it and sprayed starting fluid into the air intake (using a 3/4 PVC bend to guide the fluid in since the intake is recessed) and it fired up after about fifteen seconds of cranking.

 

[KrisZ]

Is this motorcycle fuel injected or carbureted? 15 seconds of cranking with starter fluid and battery boost seems excessive if it's fuel injected. Your gas likely got stale and is a contributing factor.

If it's cabrureted, slow cranking will generate weaker venturi effect and thus take longer to draw in the fuel from the carb.

Edit:
I see it's fuel injected and it's the first line of your post, I feel like a moron

 

[George7941]

I have not added any gas to the tank and the bike is running well and back to its normal two second crank for starting.

I suspect I flooded the engine and got the plugs wet the first time around. Clear Flood mode kicks in if the throttle is opened all the way and I tried it and did not help.

This is the first time in ten years that this engine has been difficult to start after the winter layover.

Got the engine running not a moment too soon - it is 15C/60F and sunny outside today and I am out on my bike.

 

[alcyon]

If its fuel injected low rpm might cause the crank position to not generate enough power to the ecu. Since it does not know engine position It wont ignite spark.

 

[George7941]

You are right. I assumed that the crank sensor on the Honda would be a Hall Effect sensor but the wiring diagram shows it to be a two wire sensor. At low rpm it mght not generate much voltage.

 

[alcyon]

You are right. I assumed that the crank sensor on the Honda would be a Hall Effect sensor but the wiring diagram shows it to be a two wire sensor. At low rpm it mght not generate much voltage.

It might not be the sensor itself, maybe just bad grounding for that voltage to the ecu. That was the case in my car.

Post in thread 'Car wont start cold and stalling at hot idle intermittent problem'

Aug 29, 2021

Problem may be solved. After sanding the ground point shown

The problem hasn't return in two weeks straight. I am planning to change the negative terminal anyway because its heavily oxidized, but yeah for the last 10 days straight no hard start or stalling. I think this ground point is grounding the ECU sensors and or distributor cam signal. So when the voltage sent back to the ecu is weak (low rpm) , the ecu thinks the engine is not turning so it turns off the spark.

• alcyon

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