Gear Ratio for Older Johnson 20?

[Makaw]

I'm trying to figure out the easiest way to do some rpm measurements on my 1967 Johnson FD-21. It appears I have roughly ZERO prop options for my motor, so I'd like to test and really fine-tune my jackplate and trim settings so I know where that outboard will continue to live a long, happy life. Published OEM specs say idle speed should be 550 rpm and 20 hp output is at 4500 rpm. I've added weight to the boat and this can obviously have a significant influence on rpm range. I don't have a wired tachometer and the reading I've done seems like it can be fairly tricky to install with questionable accuracy.

I was thinking the easiest way might be to use a handheld laser tach and measure flywheel rpm. I'm assuming the lower unit gearbox is not 1:1, but I can't find this info anywhere and I've never actually heard anyone discuss this as a means to measure rpm. Does anyone know the flywheel to prop ratio on this unit or similar older outboards?

Second question is: is there a general acceptable rpm tolerance from published specs that is best to stay within at WOT? For instance, if I'm running at 4050 rpm at WOT (10% below published), is that acceptable?

Thanks in advance!

 

[DaleH]

A Tiny Tach wired to one of the spark plug leads is easiest and cheapest, $22 from eBay:


FWIW if you do a lot of engines, this "wireless" handheld one is da nuts! But yes more expensive! But a BONUS is that you can wrap a wire around a plug lead, route the wire outside the engine cowl and just lay the handheld tach on the wire and it will read! Just confirm the RPM reading by pointing the tach at a plug (with the cowling ON) from 4 to 15" away. I sealed both ends of the 16 AWG wire I used with heatshrink and just left it dangling in place. So anytime I need to confirm the RPMs on that motor ... PRESTO, instant reading!


FWIW a OEM Service Manual for 1968 for the FD-22 20hp says the gear ratios is 12:21 so I would presume your's is the same - but confirm before you rely on that value.


And go to the Marineengine.com website, as they list 2 props for your old 'pinned' and 'non-thru-hub exhaust' propeller, a 9x10 and 9x9. Usually with an OB running in cleaner water and mounted higher you can go UP or steeper in prop pitch. Some guys tweak such old props to add cupping, Maybe seek out a good prop shop? But I'd get the tach to confirm specifications/RPMs.

Link = https://www.marineengine.com/parts/johnson-evinrude-parts.php?year=1967&hp=20&model=FD21
Hopefully Pappy comes along, as he is DA MAN for older OMCs!

 

[Makaw]

Thank you @DaleH! Great info! I had not found (or maybe overlooked) that OEM spec page, but it sounds like a wired tach would be the best option anyway.

 

[mbullen]

Both Solas and Michigan Wheel offer a variety.
9, 9.25, and 10in diameters with pitches varying 7-12.

 

[LaqueRatt]

I put a Solas on my '65 Fastwin 18, but it took significant modification. Hey, where there's a will, there's a way!

 

[Makaw]

Thanks @mbullen for the info. For some reason I was thinking Solas discontinued their pin drive options. And I wasn’t familiar with Michigan Wheel but I like what I see. I thought I’d be relegated to used OEM spares, but I’m glad I was assuming incorrectly.

 

[LaqueRatt]

I'd like to emphasize that the Solas prop I bought which said would fit my motor, did not! I spent several hours of quality time with my Dremel and used up every small grindstone I had to make it fit. I'm happy with it now, but figure the average person would have been stuck. The prop was also NON-returnable. Maybe your experience will be better, but this was the only option for my Fastwin so I had to make it work. I searched everywhere, called and stopped at marinas and prop shops and nobody had a hub or a prop for my motor. NOBODY. You gotta be pretty doggone handy to keep these old motors going. Good luck to you.

 

[Makaw]

@LaqueRatt thanks for clarifying. Solas doesn’t get the best reviews on this site and with your experience I think if I try a different pitch it will be with a Michigan Wheel product.