Johnson 5hp TD20 ignition timing facts & figures?

[legendre]

Howdy friends, glad to see the forum is back in the life!

Does anyone here have the correct ignition timing data for a late 40’s TD20? I’d prefer to know in inches / mm BTDC, but degrees would be fine too. Would at least need data for full advance, and full retard would be nice as well.. but really, any point of reference would be very welcome.

Here’s what’s cooking..

So I’m trying to straighten out this ’49 Johnson TD20 that came to me in genuine basket-case form.. just a pile of shrapnel. I’ve gotten it all re-assembled and compression is reading 65 psi. on both cylinders. My reliable ‘good runner’ TD20 reads 68 psi. (both) so I know that must be OK, at least for average running quality.

Trouble is, this second motor simply will NOT run below about half throttle – and I’m pretty confident that all of the other areas are in good order. Carb(s) are clean and working, spark is nice and hot, etc. So I’ve turned my attention to ignition timing.. and have come up empty-handed. Despite having the factory service manual, there is zero info on ignition timing.. nada! Is there another manual / document that has this info in it?

Any help sincerely appreciated.

[jim-moffatt]

Test the carb check valve. It should allow air to pass through one way but not the other,

[legendre]

quote Jim Moffatt:

Test the carb check valve. It should allow air to pass through one way but not the other,

Check valve seems fine, in that I’ve tried two different parts – one from the carb of my good runner. In fact, I tried the complete carb from the runner as well – same issue, all around.

Primer also works great – and it wouldn’t, with a poor check valve. Likewise, the slow-speed mixture needle has significant influence over the motor, when I’m running it at the lowest speeds possible. That’s the odd part – it displays almost what you’d expect for a ‘low’ idle, when the lever is just slightly below half-setting.. but it gets very rough, very fast, below those settings. Motor reliably dies 1-2 clicks below half-throttle..

[frankr]

The timing is fixed by design, and cannot be changed. At least not if you have all the right parts and assembled correctly. So think of something else being your problem.

Now that I’ve said that, there is another look-alike magnet that may have gotten put in there. Or maybe the magnet is upside down?

Have you got the slow speed needle set at 5/8 turn open? Perhaps the Y fitting is destroyed? Perhaps the piston skirts are scored? That is a third port engine with a rotary valve, and everything has to be in good condition if you expect it to run properly. Compression is only half the story.

Also: It has to be cooling properly. Because the carburetor is part of the exhaust manifold, it gets hot as the motor heats up. Too hot and the gas vaporizes inside the carburetor and it won’t run good at all.

[jim-moffatt]

I had forgotten this. If the throttle butterfly doesnt close properly and completely it will do just what you have experienced.
Sometimes the butterfly will create a lip in the air tube where it seats. The extra air causes it to die.
I dont think this is an ignition problem.

[legendre]

Howdy Frank,

quote FrankR:

The timing is fixed by design, and cannot be changed. At least not if you have all the right parts and assembled correctly. So think of something else being your problem.

Now that I’ve said that, there is another look-alike magnet that may have gotten put in there. Or maybe the magnet is upside down?

A magnet issue wouldn’t alter the timing, it would only impact the quality of the spark. On this motor, the spark timing cam is an integral part of the parent metal of the crankshaft; or at least it seems to be. And as I said, spark quality seems excellent – and if it can be easily pull-started (it can) then there’s plenty of spark at very low RPM.

quote :

Have you got the slow speed needle set at 5/8 turn open?

Yes, very close to that for best running.

quote :

Perhaps the Y fitting is destroyed?

Externally, it shows no evidence of abuse or damage, and the valve action works just as expected. Likewise the jet of fuel from the primer can be shut off or made quite fat by changing the needle setting. I’ve been all through the insides, and there are no blockages in the crank passages or the case that I could find..

quote :

Perhaps the piston skirts are scored? That is a third port engine with a rotary valve, and everything has to be in good condition if you expect it to run properly. Compression is only half the story.

The pistons were in reasonable shape, not perfect but not ruined by any stretch.

But now I’m curious – assuming compression is OK, how would scored pistons prevent this motor running at low throttle settings? Below half-throttle, the ‘main’ carb is completely shut-off, and all fuel & air are delivered via the rotary valve / low-speed carb system. Which does raise a question.. if the air / fuel settings are fixed at these low speeds, is engine speed controlled entirely by spark timing?

Think about it – you have no control over throttle position, the low-speed system is always ‘full open’ – so there’s nothing except spark timing to control speed, right? This is why I’m looking to verify ignition timing, as I don’t know what else can be so strongly influencing speed at these low throttle settings.

quote :

Also: It has to be cooling properly. Because the carburetor is part of the exhaust manifold, it gets hot as the motor heats up. Too hot and the gas vaporizes inside the carburetor and it won’t run good at all.

Sure – but it behaves as it does from cold start.. it’s not like it’s faltering once things get hot. Also the manifold has a new gasket + sealer, and the water pump works great.

So.. what about a head gasket? It’s not leaking externally, but it could be leaking internally.. how would that affect this?

[legendre]

quote Jim Moffatt:

I had forgotten this. If the throttle butterfly doesnt close properly and completely it will do just what you have experienced.
Sometimes the butterfly will create a lip in the air tube where it seats. The extra air causes it to die.
I dont think this is an ignition problem.

The throttle linkage on this motor was broken, the longer of the two shafts snapped-off near the middle drilling. I made a dang-near perfect copy using 1/8" brass rod and the throttle rod from my good runner as a pattern. As far as I can tell, the main butterfly opens & closes when it should – and no, there’s no notch worn in the carb bore.

I did have some major air leaks between the manifold & block, and also between carb & manifold. But I believe I’ve corrected these – we milled / lapped-flat the manifold and carb flange surfaces. Also, both got brand new gaskets (made them myself from 1/32" Fel-Pro black rubber-fiber material).

Correcting these leaks did make a notable improvement in overall running, but the low-throttle setting issue was almost totally unaffected.

Candidly, I don’t think it’s ignition either, but I want to rule that out for sure before I chase more exotic things..

[legendre]

The reason I’m keen to learn the correct timing figures, is that when I measured the ‘problem’ motor, it seems to be firing at about 30′ ATDC at the lowest throttle setting. My main experience with 2-stroke motors is in motorbikres, and in most cases, older 2-stroke bikes are spec’d to fire at TDC, or with some advance.

Most 2-stroke bikes have no advance mechanism, though, and this outboard does – albeit manual via the speed control lever. But that said, I have never in my life encountered any engine that timed at 30′ ATDC at any speed.. the most retard I’ve ever seen was a very old 4-stroke engine that timed 2-4′ ATDC at full retard.

[frankr]

First, it seems like you are pretty wise concerning this stuff, but I’ll attempt to address a couple of the things anyway. Yes, it is true that the magnet does not affect timing. That is controlled by when the points open and cannot be changed except by points’ gap, to some extent. And certainly not what you are looking for here.

Piston and cylinder condition affects the crankcase compression. Especially important on a 3rd port engine. Doubly important on these touchy little HD’s

Yes, on virtually all outboards, idle speed is controlled by spark advance/retard, and on most, the carburetor butterfly is closed at idle speeds. You probably are familiar with other motors where the carb has to be synchronized to start opening at a mark on the cam that opens it. I don’t remember exactly when it opens on a HD.

I tossed in the bit about cooling because that was a lesson I learned the hard way….after spending days trying to get one to run properly when I was doing this for a living. Lost my shirt on that job.

Head gasket?? I don’t know. Possibly. Like I commented, "touchy" motors.

30 degrees ATDC? Well, that does seem unreasonable. I don’t have any input on that.

EDIT: Sorry, I got my motors mixed up and was thinking HD instead of TD. But most of what I said still applies. The rest, you can store somewhere.

[legendre]

Howdy Frank,

I figured you were referring to the TD, but as you point out, virtually everything you said would apply to the HD line as well. The present service manual covers numerous older models, and isn’t all that well organized – so over the last few weeks I’ve become better acquainted with the TD’s friends & relatives than I’d initially intended.

The bulk of my work experience is with 2- and 4-stroke motorcycles, mostly the latter, though I’ve had my hands on and in lots of other stuff over the years. But outside of these two TD20s and a Mercury 4.5 (CDI model), I’ve never had much call (or desire) to work on an outboard motor. As you note, they are a bit touchy – but not any more so than a lot of the items I’ve had the pleasure of dealing with.

Speaking of losing your shirt on a job.. if you were an obm / small engine tech, I’m sure you’ve known the deep desperation that is the good-as-gold but thoroughly dispirited teenage kid who shows up with their French or Czechoslovak-made moped? Talk to me about touchy machinery.. 😉

Thanks for all of your input on this, it’s sincerely appreciated. I’ll get back after I’ve had a chance to measure the timing on the ‘good’ TD and compare the two – need to satisfy my mind. Weather went to heck again, and I’ve got these set-up outdoors.. so set it aside for a day.

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