Simple electro-petro-mechanical maths question?

[nev]

Trying to measure accurately the rpm on my bike engine.
I have a multimeter that measures rpm via the coil (-ve?) but it has settings for 8, 6 and 4 cylinders but not for two or one.
Meter instructions on page 29 here

The engine is a V twin, a Honda VT500C.
It has one crank sensor, 2 cdi's and two coils. Each cylinder/ coil has two spark plugs/ leads. It works as a wasted spark system so sparks (uselessly)on the exhaust stroke too.
So if I set the meter to 4 cylinder do I need to multiply, divide or what to the given figure to get the correct figure?

You may now pick up your pencils

 

[Racers]

I would connect it up and see what setting gives you nearest/multiple of 1000 rpm at tickover.

Does it not have a tachometer?

Pete

 

[nev]

I would connect it up and see what setting gives you nearest/multiple of 1000 rpm at tickover.

Does it not have a tachometer?

Pete

Not any more.
You are probably right though, connect it up and see where tickover reads, mines somewhere between 800 and 1200 I think, it should be at around 1k.

 

[Racers]

Oscilloscope and a signal generator set in xy mode is another way lissajous figures.

Pete

 

[MikeG.]

Well, you could set up your multimeter for 8 cyclinders (on your bike engine), and take the reading with the engine on tickover. Then turn it to the 4 cylinder setting and take the reading. Assuming linearity, you might then expect that whatever the relationship between 8 and 4 was to be replicated by the relationship between 4 and 2. A simple bit of arithmetic should give you your figure. However, if the realtionship isn't linear, but, say, logarithmic, then your brain is going to hurt.

 

[sunnybob]

On motorcycle engines some manufacturers use a "wasted spark" system, where the spark plug fires every stroke. you need to find out if the honda does that because it will screw with your results.
Assuming its the old fashioned one spark every other stroke then its as simple as suggested. If you get silly readings on idle like 4000, or at the other end 50, just keep switching till you get the closest to what it sounds.
1200 on a twin is a fast idle. 600 is a low idle, barely achieving it.

 

[CHJ]

On motorcycle engines some manufacturers use a "wasted spark" system, where the spark plug fires every stroke. you need to find out if the honda does that because it will screw with your results.
..

Think that was answered in Nev's opening question.

…..The engine is a V twin, a Honda VT500C.
It has one crank sensor, 2 cdi's and two coils. Each cylinder/ coil has two spark plugs/ leads. It works as a wasted spark system so sparks (uselessly)on the exhaust stroke too.
...

 

[Eric The Viking]

I would connect it up and see what setting gives you nearest/multiple of 1000 rpm at tickover.

Wot ee said. ^^^

Are you setting the timing up (advance)?

I had a CB175* - same wasted spark system. It was a bit of a mare to sort out, as the timing adjustment involved removing the flywheel, IIRC - it was decades ago tho.

E.

*much nicer than the CD version - twin carbs and an extra gear.

 

[nev]

Thanks for all the replies gents.
I am or will be attempting to fine tune the air mixtures , which involves starting at point X and then being able to see a 50 rpm drop or rise as one fiddles. I have my doubts as to the validity of the instructions provided by Monsieur Clymer but willing to give it a go as it costa nowt and I might learn something which is always a bonus.

 

[Eric The Viking]

Whatever happened to checking the colour of the plugs after thrashing it?

Feelin' very old

E.