Idle RPM of single cylinder 4 cycle engine

[Brian Rupnow]

I have now built 8 or 9 i.c. engines. About half of them are hit and miss 4 cycle with "atmospheric" intake valves, and about half are overhead valve 4 cycle engines. The side valve engine I just finished is a single cylinder 4 cycle engine with two cam driven valves. I can get it to idle down to 700 rpm, but that is about the lower limit. I just spent half an hour doing an internet search to see what an "average" idle speed is for single cylinder 4 cycle engines with two conventional cam driven valves, but it was a case of "too much information". What about you other i.c. engine builders out there, with access to tachometers? What idle speed do you achieve with single cylinder 4 cycle, conventionally valved engines? I know that an enormous flywheel or two flywheels will make a difference, but I am talking about engines with single flywheels that fall into a more or less "normal" size range. I would be interested in hearing what low rpm consistent idle you are able to achieve.----Brian

 

[Brian Rupnow]

Here we are at "maximum slow".
I played a bit more with the needle valve setting on the new carburetor and was able to get the engine idle down to 670 rpm, but it simply won't go any slower than that.
[ame]https://www.youtube.com/watch?v=CQ0vm7rpf4U&feature=youtu.be[/ame]

 

[digiex-chris]

I don't have much advice, just got a few data points.

I can't get any of my commercial (OS .56 ci) 4 strokes that low. I'm happy if I can get it to 2000 rpm! The instruction manual for this one OS I have says 2000 rpm is about right but it maxes out at 9000 with the prop I'm using.

My single cylinder motorcycle idles at 1200 rpm.

I wonder if we can define idle as a percentage of maximum?

 

[Brian Rupnow]

Chris--I have been able to get my Fat Flywheeled Webster engine to idle at below 400 rpm.

 

[Swifty]

Brian, my Nemmet Lynx idles just under 2000rpm, I would like to get it lower, it certainly looks slower than that. I'm only using the in line electronic tachometer that's available for the ignition system that I used, maybe a hand held unit should be on the shopping list.

Paul.

 

[RichD]

I would think compression ratio, cam profiles, valve springs and carburetion(sizing) would be the main factors once you're sure the flywheel is heavy enough.
Rich