Tach reading........Could this possibly be right??

[Likesspace]

Okay, I have a new prop on order to replace the very banged up one that came with my boat. The current prop doesn't just have a few nicks.....it has places where the blades are "folded" towards the center, the size of a nickel. On top of that there are also numerous nicks and rough places on all three of the blades.

Also, this prop looks very small compared to some of the other props that I've looked at in our marina.
For instance there is an 18' Bass Buggy beside us that has a 60 HP Bigfoot and their prop looks much bigger than the one that is on our boat. Not only does it appear to be a larger diameter but the blades themselves seem much wider than ours. Now granted my motor is only a 50 HP Bigfoot but I'm thinking that the prop that is on our boat is WAY undersized.

Keep in mind that my boat is a 1986 model while the motor is a 1999 model. That makes me think that this motor was imply transferred off of another boat, onto ours. I'm thinking that if this motor came off of a 16 foot john boat they simply moved it over to the pontoon without any consideration as to whether or not it was suited for a 20' pontoon.

So here's my question/dilemma:

Tonight I went and installed a cheap version of a tiny tach. Basically it's a digital tach that has a pickup wire that you wrap around a spark plug wire. It's basically picking up the pulses from the wire and giving a digital readout. I feel confident that it's fairly accurate since last night I hooked it to my zero turn mower and it gave a WOT reading of 3190 to 3200 on a motor that was spec'd at 3200.

Well tonight I took the boat out and it gave some pretty stupid readings. At what was barely above idol I was getting readings of 2300 rpm. At WOT I was getting readings of a little over 10,000 rpm. Is that really possible??

There are various settings to make the tach read correctly. The different settings are as follows:

Setting 1: 2 strokes, one cylinder
Setting 2: 1 stroke per cylinder
Setting 3: 1 stroke, two cylinder

I had it on setting 2 since my motor is a four stroke. It made sense to me that one stroke per cylinder made sense. Also that was the setting that I used on my mower and like I said.....it gave a very accurate reading on that application.

So, is it possible that with an extremely undersized and damaged prop that I'm actually turning over 10,000 rpm at WOT? And if so, would it also read such a high rpm rating at idle? This is all new to me and I'm really hoping that this new prop will bring the rpm readings back into line.

Thanks again for your help and ideas.

Dave

[teecro]

No something is up with those readings...

Idle RPM should be around 600 RPM and WOT should be in the range of 5 to 6 thousand....

Many older multi cylinder engines for cars to motorcycles use what was known as "wasted spark" to make the ignition system easy and cheap to manf. Perhaps your older Mercury used this technology? I simply don't know. But that would explain what looks like 2X readings as their was a spark every time the piston came to TDC no matter compression or exhaust.; hence the term wasted spark...
With only 2 other setting try them out and see what readings you get...

[Likesspace]

I thought the same, Teecro......

So, did I select the correct setting on the tach?

Does a four stroke motor make 1 stroke per cylinder or does it make 2 strokes per cylinder?
After giving it some thought I'm thinking that a four stroke would make one intake and one exhaust stroke per revolution, per cylinder. Is that correct? The only setting that I didn't try was 2 strokes per cylinder. I was thinking that meant a 2 cycle engine. I don't know......I'm kind of confused right now.

[teecro]

I thought the same, Teecro......

So, did I select the correct setting on the tach?

Does a four stroke motor make 1 stroke per cylinder or does it make 2 strokes per cylinder?
After giving it some thought I'm thinking that a four stroke would make one intake and one exhaust stroke per revolution, per cylinder. Is that correct? The only setting that I didn't try was 2 strokes per cylinder. I was thinking that meant a 2 cycle engine. I don't know......I'm kind of confused right now.

Seems to have been the right setting for your lawn-mower but no way was it right for your outboard. Like I said I don't know the type of ignition used on your Mercury much less your lawn-mower but it matters not if your confused as with only 3 settings to try, you try them all and pick the one that sounds the closest... There was NO WAY your motor was turning 10K it has a rev limiter somewhere around 6,200 to 6,500 if not lower.... Another thought is if your "tiny tach" was made in China who knows how well the translation of directions went...

"A four-stroke cycle engine is an internal combustion engine that utilizes four distinct piston strokes (intake, compression, power, and exhaust) to complete one operating cycle. The piston make two complete passes in the cylinder to complete one operating cycle."

[Likesspace]

Well looking at the four stroke explanation you just posted I'd say that the one setting that I didn't try is probably the correct one. If a four stroke makes two complete passes to complete an operating cycle.....well it makes sense that option one: Two strokes, one cylinder, might be the correct one.
Now I'm wondering if I might have had it set to option one when I tested it, last night.

I will say that the instructions are written in "Chinglish" which makes it a little difficult to understand. I didn't want to drop $50.00 on a real "tiny tach" if I could get my prop dialed in with a $13.00 "chiny tach". Hopefully I can get some accurate readings by playing with the various settings.

Tomorrow or Friday I'll get the new prop installed and let you know what I find. Thanks again for your help and advice.