Rpm Limiter Low

G

GeoffB

Member
Hello everyone,
I’m have two karts with late orange seal engines having a problem where the limiter seems to be coming in at 5850/ 5900 on a Mychron 5.
Both have latest flywheels, I’ve changed plug, coil and Carby. Next step is sending away for dyno testing.

Has anyone had similar problems?
 
How are you checking the limiter rpm?

If you are holding the throttle open with no load, you will not get the high number because the refresh rate of the gauge is not quick enough to catch max rpm before the limiter causes a detonation/ preignition type event which slows rpm quickly. Like in 2-4 revolutions.

The best way to check the numbers is with a controlled load easing to the limiter.
Even then, you may not see the 6050 rpm, as this is the max rpm allowed. As this is an electronic device, and electronics are affected by temperature, your numbers may vary.

Some experimenting with coil temperature may bare this out.
 
MO Predator Racer said:
Wait could the OP have the mychron set on the 2 stroke setting instead of 4 stroke?
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The mychron counts sparks .
If it was on 4 stroke it will only read half rpm .
Briggs and clone fire each revolution . With one spark on the exhaust stroke .
One on the ignition stroke .
Called wasted spark .
 
The most likely scenario is your mychron is just reading inaccurately. For whatever reason the m4 is much more consistent from one unit to the next. I’ve seen the m5 from 58-6500. Put an M4 on and it goes to 6030-6050.

The engine is not the issue if you can hear it is actually on the rev limiter.
 
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95 shaw said:
max rpm before the limiter causes a detonation/ preignition
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I don't think Briggs or anyone would purposefully engineer a ignition system that would cause detonation or pre-ignition for any reason including rev control. That would not be a desirable effect.
Wiki: Pre-ignition in a spark-ignition engine is a technically different phenomenon from engine knocking, and describes the event wherein the air/fuel mixture in the cylinder ignites before the spark plug fires.
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How do you set up an ignition system to ignite the A/F before the spark plug fires?
Detonation : Detonation is identified as any spontaneous combustion occurring after the burn process has already begun, and is independent of the initial flame front. This uncontrolled event can originate from anywhere within the chamber and is usually caused by high cylinder temperatures and or pressure
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Same here. How do you program an ignition system to cause detonation after the spark event? If it could, I'd get a fuel with higher octane (passes meter) and get around it. Detonation is from other factors independent of the spark event.
If I had to use a ignition system that was programmed to do this

detonation piston.jpg


to control a rev point, I would find another class.
Again, I know what you mean, just not the right choice of words.
JMHO
 
I don't think you really understand.

So you're turning 6000 rpm, give or take.
You fill the cylinder with the correct mixture for best power, super heat it during the compression stroke, do not fire the spark plug, then open the exhaust valve where there was 1000+ degree exhaust 1/50th of a second before, and expect that mixture not to explode. This creates pressure backwards against the crankshaft.
That fits the definition of a detonation type event.

It's not the ignition creating the event. At least directly.

The big car ignition rev limiters work similarly.

You don't get that bang for nothing.
 
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95 shaw said:
You fill the cylinder with the correct mixture for best power, super heat it during the compression stroke, do not fire the spark plug, then open the exhaust valve where there was 1000+ degree exhaust 1/50th of a second before, and expect that mixture not to explode. This creates pressure backwards against the crankshaft.
Click to expand...
How do you "super heat" an A/F mixture during compression (8.5:1)? You mean like a diesel engine (~20:1)?
Where did the 1000° exhaust come from? I though that cam design and timing was suppose to work with exhaust pipe design to eliminate residual exhaust and promote low / negative pressure to start the intake cycle with a stronger signal. The any significant exhaust should be long gone while the engine goes through the intake cycle, compression cycle, power producing ignition cycle before it gets to the next exhaust valve opening cycle.
How do you create pressure against the crankshaft with the a valve open? Isn't that, in a since what the "ez-spin" compression relief does - relieve the pressure against the crank so it turns over easier? Do you use a stock screw in muffler or some type of exhaust pipe and low restriction muffler.
.
Ignition induced rev limiting is from random programmed misfire. It may make farty sounds out the pipe as it misfires, but it isn't pre-ignition or cylinder detonation. In "big cars"(?) the louder farty bang comes the un-burnt charge left in the pipe when the next actually lit off residual hits it on the next power producing cycle.
 
Head back over to Wikipedia and study a little.

Auto ignition temp of gasoline.

Exhaust temperatures (egt)

Also gases under pressure, expansion rates. Maybe even piston position in rotation and crank angles during exhaust stroke.
Dynamic compression ratio.

Just because the spark missed on 1 rotation, doesn't mean physics stops as well.

Exhaust gas speeds at temperatures and rpm is good to know as well.

Lo206 exhaust is a fixed length and size, so fewer variables there. The muffler is a factor as well.

I believe it is common to put an egt thermocouple in a spare muffler.
Perhaps some that have that capability can tell us egt temps at the end of the exhaust system.

Fyi 530 to 550 is the auto ignition temp of gasoline.

350 to 400 degrees is a common cylinder head temperature.
I would expect the mixture to be at least that when the exhaust valve opens on a missed spark cycle.

Not a big jump to hit a sharp edge with temp higher than that.
 
Flame front and propagation is good to know also.

Wikipedia is not a good engineering resource, by the way.
 
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Thanks for the list and resource suggestions.
As EC said "been there, done that, got the Tee-shirt and use it for a rag"
At least we agree that the coil rev limiter works by causing misfires and not detonation or pre-ignition.
 
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