Cranking RPM of CBR300

[George7941]

I hooked up a scope to check on the compression, health of the starter and the new battery and was surprised to find that the cold engine, at 5C ambient temperature, was cranking over at 420rpm, Wasn't expecting it to be this high, especially since the reason for checking was that I thought the starter sounded lazy. Blue trace is the battery voltage and the yellow trace is the starter current, with a peak of 107 amps. The current includes starter relay current and the ignition coil current. Throttle wide open, in clear flood mode.
The cranking sound did not indicate high rpms, that maybe because it is a single cylinder engine and there is only one compression hump every two revs, unlike my truck which has six compression events every two revs.
Googling "CBR300 cranking rpm" produced no results.

 

[Bonz]

You weren't expecting it to be that high, what was your expectation? Did it sound lazy compared to previous starts, that weren't at 5° c? Seems like it was somewhat cold outside... The colder the slower it's going to crank for a number of reasons. No mention of oil weight, I gather it was the typical Honda 10w30? I can understand no Google info about cranking speed for a Honda CBR300, lol.

 

[George7941]

10W30. Not Honda but Mobil Delvac Super 10W30. Used in our trucks at work, so an obvious choice of oil for me.

My Sierra will crank over at 150 rpm under the same conditions. Since the Honda starter sounded lazy to me, I was expecting a cranking speed of around 120 rpm. The 420 rpm came as a big surprise. Single cylinder vs six cylinder is, I think, what made the starter sound lazy to me.

 

[Bonz]

Glad you clarified your suspicions / concerns.

 

[JWC86]

Interesting experiment for sure. I wouldn’t have expected it to be that high myself either.

Pretty sure this is the same motor as the CRF300L/Rally and the older 250cc versions of both the CBR and CRF so might be able to find some data searching for “those” bikes.

 

[CourierDriver]

You weren't expecting it to be that high, what was your expectation? Did it sound lazy compared to previous starts, that weren't at 5° c? Seems like it was somewhat cold outside... The colder the slower it's going to crank for a number of reasons. No mention of oil weight, I gather it was the typical Honda 10w30? I can understand no Google info about cranking speed for a Honda CBR300, lol.

be happy it starts,,,,,my Harley starts instantly I push the start button, no hesitation at all,,,

 

[George7941]

I was considering that I might be making an error interpreting the scope traces and maybe missing a factor of 2 and the engine was being cranked over at 210 rpm.
I got my phone out and ran the timer so that I could get an exact feel for a second. Cranked the engine over in Clear Flood Mode and listened to the rhythm of the compression stroke and the following exhaust discharge. I counted the best I could how many compression events I could hear in one second and it turned out to be three to four a second. So the engine was being spun over at 420 rpm.

The CBR300 engine idles at 1350 rpm and maybe it just will not fire at low cranking speeds like 200 rpm, which is only 1/7th of its idle speed.

Impressive job by the physically small starter cranking this 10.7 compression ratio engine this fast.

 

[Ride_Red]

Interesting experiment. Thank for posting the results.

 

[George7941]

All this was prompted by the difficulty in starting the bike up this spring. I posted about it here

G

Post in thread 'Why Does Low Cranking Rpm Cause No Start?'

Mar 22, 2025

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.

• George7941

Wanted to make sure that the starter was not on its way out.

 

[Bonz]

Good work. With that said, I've never known a starter motor to fail on a motorcycle, that I've read here or numerous other forums.

 

[BusyLittleShop]

Good work. With that said, I've never known a starter motor to fail on a motorcycle, that I've read here or numerous other forums.

In my 50 years of riding and racing I've never known a starter motor to fail on a motorcycle... usually it's the battery failing to hold a charge next it's wiring connections building resistance to the point of melting and possible fire...

 

[Bonz]

Older bikes can develop that resistance, with my bikes even though they're older, living in Colorado we don't deal with the same humidity and subsequent corrosion over the long haul as more humid locations. My experience at high elevation and dry climate says when there's a problem, it's the battery.