How are rpm's measured on a 2 cycle Lawn Boy?

[Gene123]

Can rpm's on a 2 cycle Lawn Boy model 5247 be measured with an automobile tach/dwell meter?

 

[Mad Mackie]

Probably not with a typical tach/dwell meter as they read contact point opening and closing and contact point closed duration(dwell). A tach with an inductive pickup and a two cycle or DIS position on the RPM selection will work.
The most universal tach is a non contact IR digital tach. It only needs a piece of reflective tape stuck to a rotating object to read the RPM. They are very reasonably priced however made in China. I have two in my test equipment box and they were under $20 each.
Mad Mackie in CT

 

[Gene123]

Probably not with a typical tach/dwell meter as they read contact point opening and closing and contact point closed duration(dwell). A tach with an inductive pickup and a two cycle or DIS position on the RPM selection will work.
The most universal tach is a non contact IR digital tach. It only needs a piece of reflective tape stuck to a rotating object to read the RPM. They are very reasonably priced however made in China. I have two in my test equipment box and they were under $20 each.
Mad Mackie in CT

Thanks for the great answer, Mad Mackie. Some of it I didn't understand, though. :confused2: What is "DIS position"? Where would I stick reflective tape on an old Lawn Boy model 5247? Maybe the flywheel? It's covered by the plastic top shroud which has a gas tank built in. Removing it to get to the flywheel requires removing the gas line, the primer line, and two coil wires. If all that is removed, I couldn't start the mower --- unless there's a trick I don't know. :smile: I like the idea of a $20 tester. What brand is it? Where can I purchase one? Thanks, again.

 

[Mad Mackie]

Hi Gene,
DIS is distributorless ignition system which is the same positon on the tach that would be used for two stroke engines, spark at every engine revolution. Been a while since I've dealt with two stroke Lawn Boys.
I have several types of tachs in my test equipment box. A vibration tach marketed by B&S and made in Germany, a wireless tach that reads thru an antenna and has a digital readout, and the two IR tachs that I have mentioned. With the IR tachs, if there is no access to the rotating part then they won't work. I use the wireless tach on small engines like chainsaws, weed trimmers, etc where there is no visable access to the rotating parts like the flywheel so this type would work fine on your Lawn Boy providing the spark voltage is high enough for the tester to pick it up. The wireless tach is the most expensive of the group, mine was $250 but it has an additional pickup that is used on diesel engine injector lines. I will have to look at the vibration tach instructions to see if it will work on two stroke engines, I do know that the vibe tach gives false readings when used on a V twin engine.
Inductive pickup type tachs have been around for years, I retired mine several years ago and sold it to an automotive test equipment collector, I bought it in 1974 along with a high speed timing light.
I've added some pics of my test equipment. I don't have a pic of my second IR tach but it is a different brand than the Neiko and a lot smaller.
Mad Mackie in CT

 

[Gene123]

Hi Gene,
DIS is distributorless ignition system which is the same positon on the tach that would be used for two stroke engines, spark at every engine revolution. Been a while since I've dealt with two stroke Lawn Boys.
I have several types of tachs in my test equipment box. A vibration tach marketed by B&S and made in Germany, a wireless tach that reads thru an antenna and has a digital readout, and the two IR tachs that I have mentioned. With the IR tachs, if there is no access to the rotating part then they won't work. I use the wireless tach on small engines like chainsaws, weed trimmers, etc where there is no visable access to the rotating parts like the flywheel so this type would work fine on your Lawn Boy providing the spark voltage is high enough for the tester to pick it up. The wireless tach is the most expensive of the group, mine was $250 but it has an additional pickup that is used on diesel engine injector lines. I will have to look at the vibration tach instructions to see if it will work on two stroke engines, I do know that the vibe tach gives false readings when used on a V twin engine.
Inductive pickup type tachs have been around for years, I retired mine several years ago and sold it to an automotive test equipment collector, I bought it in 1974 along with a high speed timing light.
I've added some pics of my test equipment. I don't have a pic of my second IR tach but it is a different brand than the Neiko and a lot smaller.
Mad Mackie in CT

Thanks again, Mad Mackie. Your advice and the pictures have been very helpful.
Gene