Johnson 20R73 Surging at full throttle and running hot

Thanks to all the help I got from this forum I recently finished getting a 1973 Johnson 20 HP running again after it sat in a shed for a couple of decades. Took it out for a trial run on the lake and overall it ran well. Started easily, idled smoothly, shifted into forward and reverse and pumped water out the exhaust. Pushed my 1448 riveted aluminum flat bottom boat about 18 mph with 2 guys, a trolling motor and battery. However, I had 2 issues:

1. When I ran it at full throttle, it developed a rhythmic surging. The motor would slow a bit and then speed up. Backing down to about ¾ throttle fixed the issue and it would run fine and idle fine after doing this.
2. It runs a bit hot after running full throttle. I brought an infrared thermometer gun with me. After idling and running around at around half throttle for a while, temps were reading around 150-155F at the cylinder head. After running it for a few minutes at full throttle (with the surging) temps read about 179F at the cylinder head. According to the factory service manual it should not run hotter than about 165F.

List of repairs completed recently:
-carb clean/rebuild
-new fuel pump
-all new fuel/vent lines
-new fuel tank/line/bulb
-new thermostat
-new water pump and housing
-head gasket replaced
-ignition rebuild with new points, condensers and coils. Timing set using the factory timing tool
-lower unit re-sealed

I wonder if it is starving for fuel at high rpm? I cleaned and rebuilt the carb and set the float level about even with the carb body when held upside down. Fuel pump is new. All fuel and vent lines are new. The fuel tank/line/primer bulb are all new. I cleaned the water passage ways in the cylinder head when I replaced the head gasket. The ethanol free fuel is new and mixed at 50:1. If it is running lean would that cause it to run hot since the fuel is also the lubricant for the pistons?

I plan to take the carb off and double check float level before I take the boat out again next weekend. Any other ideas about what might be causing the surging and overheating?

Hold your carb upside down then, Set your float to be level to the carb body or a hair beyond.

Thanks Flying Scott I think that's where I set it when I rebuilt the carb but I'm going to double check. I've also seen a few posts on this forum where people accidentally put the float in upside down so I'll make sure I didn't do that. I wonder if there's anything else I should check before I head out to the lake?

Could this be a symptom of a wobbly armature plate? I don't feel or see a significant wobble in the plate but I'm not exactly sure what I'm looking for either.

Or possibly bad crankshaft seals causing it to run lean? Is there a way to test the crankshaft seals?

Does having someone constantly pumping the fuel primer bulb, acting as a manual fuel pump, make a difference?

If the armature plate has just a normal wobble of a couple thousands, that's average. If it wobbles a 1/8" or so, yeah, that's going to cause a point setting change as throttle is increased.

The thermostat opens at about 143 degrees I believe so the engine will be running slightly higher... but... 179 degrees does seem a little high. The cavitation plate of that engine should be parallel with the bottom of the boat and approximately 3/4" below the keel at the transom (no lower tan 1"). Is it?

Sometimes, a slight lean setting of a carburetor slow speed needle valve will result in a small surge. Check as follows:
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(Carburetor Adjustment - Single S/S Adjustable Needle Valve)
(J. Reeves)

Initial setting is: Slow speed = seat gently, then open 1-1/2 turns.

Start engine and set the rpms to where it just stays running. In segments of 1/8 turns, start to turn the S/S needle valve in. Wait a few seconds for the engine to respond. As you turn the valve in, the rpms will increase. Lower the rpms again to where the engine will just stay running.

Eventually you'll hit the point where the engine wants to die out or it will spit back (sounds like a mild backfire). At that point, back out the valve 1/4 turn. Within that 1/4 turn, you'll find the smoothest slow speed setting.

When you have finished the above adjustment, you will have no reason to move them again unless the carburetor fouls/gums up from sitting, in which case you would be required to remove, clean, and rebuild the carburetor anyway.
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(Carburetor Float Setting)
(J. Reeves)

With the carburetor body held upside down, the float being viewed from the side, adjust the float so that the free end of the float (the end opposite the hinge pin) is ever so slightly higher (just ever so slightly off level) than the other end. And when viewed from the end, make sure it is not cocked.

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Joereeves-
-I didn't think to try squeezing the primer bulb but will definitely try it when I get the boat on the water this weekend.

-I don't think I have that much wobble in my armature plate but if nothing else works I may look into that.

- The cavitation plate is sitting a couple of inches lower than the bottom of the boat. Can it impact the cooking capability if the motor is to low? I will mount it even with the bottom for my next outing.

-I set the low speed needle using your procedure before I took it out. However, it is entirely possible I have it a bit too lean due to inexperience with this motor and 2 strokes in general. I'll try richening up the mix a bit to see if that cures the surging.

The cavitation plate being a couple inches (2") below the keel could be giving you a problem of some kind... what kind?... Could be anything... not a clue. I just know where it belongs.

Too far below, like having a long shaft engine mounted on a short shaft transom (5" diff)... that'll create a bad drag and kick up a huge rooster tail spray when underway.

Having a plate even with the keel could possible cause the pump to draw air, or cause the prop to ventilate, maybe not... worth a try.

Thanks Joereeves. I'll get it mounted per your recommendations.

Had infrared temperature guns even been invented yet in 1973? If they had been, I sure as hang never had one. Besides, I'm sure lake water temperature has a bearing on full throttle operation. What I'm questioning here is are we sure there is a problem?

fdrgator said:

Had infrared temperature guns even been invented yet in 1973? If they had been, I sure as hang never had one. Besides, I'm sure lake water temperature has a bearing on full throttle operation. What I'm questioning here is are we sure there is a problem?

Click to expand...

No, not likely in 1973 but he's speaking of 06/10/2019 I'm pretty sure. However, yeah, that thought (no problem) crossed my mind also. We shall see.

According to my 1973 shop manual, the specified method of checking temperature was to use Markal Thermomelt Stiks. Something like a wax crayon. You mark the cylinder head with the 125F stik and the 163F stik. The 125F was supposed to melt and the 163F was not supposed to melt.

So if the correct operating temp is around 160F, I don't think the 179F readings I got would be catastrophic but not ideal either. I also have no way to calibrate my Infrared thermometer to make sure it is accurate. The motor didn't have that hot metal smell that overheated motors get and was never difficult to restart after I shut it off.

I know that when my 4 stroke motorcycle was running slightly lean it also ran a bit hotter. I don't know if the same thing happens with 2 stroke outboards.

I don't think my reply was understood. Yeah, we did have thermal sticks back in "The Day". Rather inaccurate, in my opinion. Most of us checked temperature by drizzleing a bit of water on the head, not accurate at all. My point is we didn't have accurate methods back then. So what's acceptable or normal when it comes down to precise measurements? I admit I don't know. Trying to be more scientific as to lean mix etc?? I suspect load on the engine and lake temp has more of an effect.

BTW, right now the local Gulf Water temperature here is 86 degrees. Gets up to about 96 in Summer. What is it in Lake Superior---up to 40 yet?

Fdrgator- I was just down in the Gulf last weekend doing some fishing with a buddy around Ft Walton Beach. You're right the water was just a tad cooler than bath water. And I think I get your point. People didn't have Infrared thermometers (most still don't) and somehow they managed to keep their outbards running. Only reason I have one is that the air conditioning repair guy left one at my house last summer. I called his office to let them know and he never made it back over to get it.
Being inexperienced with outboards I'm probably just overly nervous. I cooked a perfectly good chainsaw once because I had it tuned too lean and I'm trying not to repeat that mistake. I appreciate your advice. I can't afford to buy a new outboard or to pay someone to work on it every time it has a problem. Without advice from the guys on this forum boating wouldn't be an option for me.

Joereeves and Fdrgator- PROBLEM SOLVED.. Finally got some free time to work on the outboard and take it to the lake for testing. I had compiled a list of things to check based on your suggestions and things I found by searching the forum. Gathered up some tools and headed out to the lake.

While searching the forums I read about a guy having similar surging. Turned out he had installed his carb float upside down. I removed my carb and sure enough I had done the same. When I rebuilt the carb I installed the float the same way the previous owner installed it and didn't check the manual. My mistake. With the float installed correctly and adjusted per Joereeves instructions, it ran perfectly. Revved out to full throttle and ran with no surging. Seemed to have more power on the top end too. Pushed my 1448 jon boat about 21 mph.

I didn't check temp because the battery in my Infrared thermometer was dead. But I ran it at all rpm ranges including a lot of wide open throttle for about 3 hours with no problems. I could hold my hand on the cylinder head for a few seconds.

Thanks again for all your help. This old Johnson is gonna get many more years of use now that I know how to keep it running.

Joereeves- I do have one question. I put an 1 1/4" block of wood between the transom and asked the tilt pin so that the cavitation plate would be as you suggested. It is parallel with the keel and 3/4" below the keel. As I was running at higher speeds, there were times when I'd hit some wake and get the boat bouncing and my propeller would ventilate a bit and get louder. Is that normal? I just don't have enough experience with boats to know either way.

Yes... Jumping a wake with a small craft at a high speed would easily cause the prop and exhaust outlet to break the surface of the water. It happens often.