How to measure maximum RPM on 2 stroke?

[FergusonTO35]

So, is there an official way to measure maximum WOT RPM on a two stroke engine? I have always found that the longer you hold it on WOT the more RPM increases, presumably due to the mixture in the crankcase leaning out. For example, when I snap the throttle on my Echo CS-501P the RPM jumps to 12.5. But, as I hold it that climbs into the 13,000's.

 

[Nutball]

I wonder if it's an echo thing, because my 2511t did that. It could be leaning out as excess fuel in the crank case dries up, but it could just be a cold cylinder getting hotter and burning fuel faster.

I measure rpm by listening to a saw tooth wave sound generator at the same time. That should get by any problems with an electric tach that could get a messy reading from a rev limiter.

 

[r7000]

it would be a simple matter of getting an [inductive] type rpm measurement device or tachometer, that wraps on the spark plug lead... see "rpm tester" on amazon. The more $$$ you spend, the better the functionality; my cheapo one the LCD display updates like once a second at best so to really capture maximum rpm is not really great but I can at least tell the relative range whether it's 7000, 9000, 11000, etc. and at least have some relatively accurate idea which is good enough for me for what I was willing to spend.
And there's also noncontact optical tachs that would measure the actual rotation or something.
As for "offical way"... a tachometer and the more expensive the better it's likely going to be, just make sure it supports the ignition oe # of cylinders and 2-stroke vs 4-stroke, or be prepared to either multiply or divide by 2.

 

[dangerousatom]

I have few of these. Cheap, small and work well

Just search 2 stroke tachometer and they pop up allover

Amazon.com

 

[EFSM]

But, as I hold it that climbs into the 13,000's.

I wonder if it's an echo thing, because my 2511t did that. It could be leaning out as excess fuel in the crank case dries up, but it could just be a cold cylinder getting hotter and burning fuel faster.

Any saw will do this until fully warmed up. Echo's tend to do it more than some due to their spark timing and lack of strato-scavenging. The 501's don't have a limiter.

 

[dangerousatom]

The RPM in the cut are what you need to tune the carb for. I shoot for +11.5k on my personal saws. 10k on anything I do for a friend or random person asking for a port job unless I know they know to not stand there and throttle-jockey the saw off load. I do see my saws hit the 13.5k range when not under load, but I dont whined them out for no reason.