I think my '82 OMC tach is lame

[interalian]

Pretty sure I'm passing 4400 rpm, but the tach seems to disagree. Thoughts??

https://youtu.be/36-azXGY-0g

 

[Fed]

Sticky needle.

100% agree, it sure sounds more than 4400 and you can see it's not immediately responding to the motor rpm.

 

[interalian]

It's really the first time since the rebuild I've had it even close to WOT. If the tach is working at all, the rectifier should be doing its job, right? Not as if somehow it hits a wall?

When I put it back together, I dialed the timing back a couple of degree but have been running premium 91. Standard timing is 28 degrees. Where's the best place to run it on today's 91?

 

[emdsapmgr]

When rectifiers start to go, they can fail in a number of different modes. The tach can indicate an inaccuracy in 2 or 3 ways. 1. Tach runs fine to 3000, then goes to zero. 2. Tach reads off, somewhat like yours is doing. 3. Tach won't make any movement. So, based on my limited experience, the rectifier could still be the culprit.

 

[interalian]

Well, I do have a spare rectifier off the parts motor, and another spare ('82 original IIRC). The one that's on now, I believe, was replaced a few years back. I'll swap it out for grins.

 

[interalian]

Just thinking that if I switched to a different pulse setting, I could prove whether the tach itself hits a 'wall' at 4400. Standard setting is 6 pulse/rev, so setting it to 3 should make it read twice as high for a given engine speed. I'll try that first.

 

[Fed]

Souns like a valid test.

 

[emdsapmgr]

When having tach issues, it is good to clean off the terminals of the pulse set screw on the back of older tachs. When the switch terminals get old, dirty, the tach can act up. So it is a valid test to move the tach setting screw on the back of the tach and then return it to the normal "6" setting every year or two, as a sort of maintenance type item. That small movement will "clean" the set screw terminal faces. Good idea, it may work and does not cost anything!

 

[interalian]

I already did the pulse switch setting sweep to clean when I first started the motor. The tach appeared dead then. Moving the switch back and forth then back to 6 didn't help. Tapping the gauge face did - had to do that for the first few starts after, but it's been OK since.

Too bad these are welded together - probably some old electrolytic capacitors inside that could stand changing after 33 years.

 

[interalian]

I found a good scan of the original brochure from 1982 and noticed the "Approved RPM Range" is 5000 max for the 140 but 5500 for the 115 and 90. Any thoughts as to why, or a typo??

https://issuu.com/liquidnirvana/docs/1982---evinrude-outboards-v2--x---wm-

 

[emdsapmgr]

I don't have anything this old, but do have an 82 factory sales brochure. My brochure says all three engines have the same rpm range of 4500-5500. They all are hp rated at 5000 rpm's. Can't believe a 140 would be recommended up to 5000 by the factory. Must be a misprint, or maybe not printed by the factory.

 

[interalian]

I can't imagine why there'd be any difference in working RPM range since the rotating bits are all the same among the 90/115/140. Probably a typo - scan appears to be legitimate OMC bumf from 1981 though. Once I have the tach straightened out (tomorrow AM for another test), it's time to start playing with propellers.

 

[interalian]

So, tested today. Regardless of where the switch is set, the tach reads the same. This makes zero sense...

However, when stuck at 4400, tapping the gauge face freed it up. (See earlier post about having to do that to wake it up in the first place.) Since tapping, I had it as high as 5500 and was still accelerating. Gauge now passes 5000 every time. :boink: Guess the needle just needed some exercise??

Anyway, now that I know I'm capable of running higher than max RPM at WOT with just me on board, suggestions for prop?

Or should I dial the timing in first? I backed off the timing when I put it together, so it's a few degrees retarded. If I use premium 91 (R+M)/2 fuel, should I go the full 28 degrees on these hi-comp heads? I would expect advancing the timing will also add a few HP.

There's a shop locally with an outboard dyno. Not sure I want to spend much more $$$ this year though.

 

[Fed]

Regardless of where the switch is set, the tach reads the same. This makes zero sense...

That makes sense if the needle is mechanically sticking at 4400, oil it :lol:.
Try it out with all the crew & toys before thinking about props.
Can't help with the timing, hi comp frankenmotors are above my pay level.

 

[interalian]

That makes sense if the needle is mechanically sticking at 4400, oil it :lol:.
Try it out with all the crew & toys before thinking about props.
Can't help with the timing, hi comp frankenmotors are above my pay level.

Well, the tach body is glued/sealed - I'll have to cut it open to do that. I've tried to find a NOS tach from the early '80s but don't even know the part number. I'm willing to cut it...

Had a full load the other weekend and it pulls out of the hole like a rocket even then. Didn't have enough hours on it to crank it up though. I suspect I'm under-propped regardless of load.

Don't know the max timing for the '79 140 donor - the label on the silencer was mangled beyond reading. It was a complete motor, and the high-comp heads came with it - part numbers match for the year anyway. Anybody know max timing for this piggy??

 

[rob 135]

take dremel and lightly grind lip of tach face off except for 2 spots . One on each side then bend up a lip and pop face off tach . Bend lip back over when put back together Very lightly pry up on needle. Should be fine after. I take many tachs apart . Have converted auto tach to boat ..

 

[interalian]

take dremel and lightly grind lip of tach face off except for 2 spots . One on each side then bend up a lip and pop face off tach . Bend lip back over when put back together Very lightly pry up on needle. Should be fine after. I take many tachs apart . Have converted auto tach to boat ..

I've done lots of old speedometers and tachs off cars. This one is sonically welded plastic - black bezel fused to green plastic body. Rather than fuss with it during boating season, I'll leave it for a winter project since it works OK in the RPM range I typically use. Probably a jewelled movement and it's either dirty or tight.