I need a diagram for a circuit to make a 4cyl engine tachometer read correct on 1cyl

I know I've asked this before but I am still not able to figure this one out. I have a 1973 8hp briggs and stratton lawn tractor engine with the old mechanical points and condencer. the factory magnito was broken and after comparing prices I found out it was cheaper to go buy a new stock chevy style 12v coil and ballist resister plus then I got a higher voltage spark which means a cleaner burn of the air/fuel mix and a better running engine. but my problem is now the engine wants to run a higher rpm than the factory maxium rpm's so I want to hook up a tachometer, but all the automotive tachometers in my price range only work with 4, 6, and 8cyl engines. So I need a circuit to make the tachometer think the one spark per one full rotation output of the engine is acutally 4 sparks per one full rotation.

Can any one help me by making a diagram for a circuit that will work for my needs, Please?

Thanks in advance,

John

Isn't it one spark every other revolution?

Two edge-triggered D-FlipFlops wired as a Johnson counter to divide by four should do it.

You will need the circuit I posted to this thread to make a clock for the flip-flops.

Mike, he needs to multiply, not divide.

However, all the Briggs and Stratton engines that I have seen are timed from the crankshaft, so you get a spark per revolution, not one for every other revolution. That means that the rev counter for a 4 cylinder engine will read half what it should when connected to the B&S engine.

Ok, then you use the posted circuit in the other thread as a clock, feed it to one input of an EXOR gate. Delay the clock using an RC, and feed that to the other input of the EXOR. That will cause it to output a pulse train at 2X the rate...

Not to dampen your thread but a tach will not help you with the RPM issue other than telling you what you apparently already know.

There should be an air vane governor arm you can adjust on the Briggs and Scrap Iron motor.

It may be outside your budget, but how about a clamp-on tachometer that works with any number of cylinders such as this.

the 8hp B & S engines have mechanical governors driven from inside the engine. Usually they get a bit worn and need to be tightened up or the linkage needs tightening. Spark power has nothing to do with engine speed control on them.

Also the ignition coils from most any other B & S are interchangeable between them to some degree. The other flywheel diameters are still close enough to the 8 Hp that a coil from a 3 -5 Hp push mower will still work on them or from a 10 -14 Hp will as well. Plus most push mower engines have the self contained electronic ignition coils so no points and condenser connection is needed under the flywheel.

thanks everyone for your responses. the real reason I want to do this is because I am building a go-kart out of this engine and I want to be able to see what the RPMs of the soon to be modified 8hp briggs. I have an old lawn tractor transaxle that I beefing up internaly. Once I get the money, I'm gonna beef up the engine with aftermarket lawn mower racing parts, port and polish the engine block, ect. My Brother-In-Law told me that when he raced Jr. dragsters as a kid, the engines were running 7000RPMs with a factory briggs 5hp engine block, crankshaft, and flywheel (the factory block was modified with a bigger piston, bigger valves, ported and polished, ect. but was still an original cast aluminum briggs and stratton block) and that was 15-20 years ago. So I'm pretty sure with the right parts I can get 7000 RPMs out of a 8hp briggs because small engine technolgy has advanced a lot in the past 15-20 years.

Hi again circuitman, sounds like an interesting project. your engine will be governed to 3500rpm max to over come this you will need to remove the govner from inside the engine, this will give you upto about 6000-7000rpm but it cannot be sustained for too long, practically you will have a top end of 5000rpm with a red line of 6000rpm.

A tacho for this sort of setup is straight forward, i have as you already know built my own iginition unit for a similar engine (Honda 9hp)

If you want to run this engine as speed such as this you will need a good quality oil that is fairly thin, this aids in cooling the engine.

The factory B&S engines tend to shatter the connecting rod at around 4000+ RPM.

The internal governor will work to much higher speeds just by putting a stiffer governor spring on it. They are really not that complicated of engine to work on.


I have played around with the modified B&S engines a number of times myself. I have a 3 HP push mower engine on my nieces go cart with a 3.5 HP connecting rod and the lightened 3 HP piston. It seems to have no trouble up to around 5000 RPM where the valves start to float (but she doesn't need to know that it can go that fast yet.)

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