1970 18HP Evinrude: At WOT, RPMs vary greatly

[Toddp01]

I own a 1970 18HP Evinrude with manual start, which I purchased a year ago. I purchased it from a man who does outboard repair from his garage, who has many years of experience working on them. The motor starts with ease, idles smoothly, and appears to run well at low speeds. But starting last month, something is wrong at wide open throttle. I first noticed that the RPMS were way too low at WOT, and the motor pushed my boat at less than half the speed it used to. Since then, I've learned that it will do this consistently for up to 30 minutes. But a small portion of the time, it will suddenly begin to increase in RPMs and power until my boat is flying like it used to. It will do this for 2-10 minutes. Then, it will die back to about half power and RPMs. All this happens while my hand is on the tiller, holding the throttle wide open.

Any suggestions for what's causing this strange behavior?

 

[kbait]

You are losing power from one cylinder, likely spark issue. Could be a fouled plug, dirty pointset, bad condenser, bad coil, bad plug wire..

 

[Toddp01]

Thanks much, kbait. I'll start eliminating causes, starting with the easiest. Any explanation why both cylinders run a small portion of the time, but 1 cylinder most of the time?

 

[JDusza]

Likely a spark issue. Might be fuel but verify ignition before you move on to fuel delivery or you will spin in and crash.
Verify ignition with a timing light on each spark plug wire. Remark your flywheel for good reads of both TDCs. You'll see if good, bad or ugly. With engine capable of running, this is an easy test.
As per kbait, you'll have to work your way through components.
Check components under the flywheel for wear and fraying and wire routing. Have to verify stator coil continuity and resistance. Measure resistance across the coil and compare to spec value. Put a meter across the coil leads and jog the flywheel passed. You should get only two signals that correspond to magnet locations in the flywheel.
Refresh points surfaces by dragging emery cloth between the contacts and regap.
Throw out the condensers and spark plugs as they are cheap. Get them out of the diagnostic loop.
Run the engine at night looking for sparks jumping randomly to grounds.
J

 

[Toddp01]

Am reporting back, asking for more feedback because the problem has not been resolved. Over the past month, a mechanic has done the following, in this order:

1. Tested compression.
2. Installed new spark plugs.
3. Tested spark.
4. Tested a new fuel pump.
5. Carefully examined all wires on the stator plate and from the spark plugs to the stator plate.
6. Used a multimeter to test components on the stator plate (coils, wires, condensers).
7. Tested new coils.
8. Cleaned and re-gapped the points on the condensers.
9. Replaced fuel hose connections in the motor (fuel pump, carb).
10. Removed and cleaned the carb.
11. Tested a different gas tank and fuel line.
12. Installed new condensers.
13. Again examined and tested stator plate, including its components and related wires.
14. Tested different carb

I have taken the boat out and tested it a half-dozen times during this process, as more than one change was done at a time. There was little or change until after Step 10 (removed and cleaned carb).

After Step 10, with throttle held at max, the motor would run half speed for 30 secs to a minute, then increase to normal speed. It would stay at normal speed unless you reduced throttle. If you then twisted to full throttle again, it would again run half speed for 15-30 seconds before returning to full speed. It continued to behave this way until after Step 14.

In Step 14, we installed a different carb from another 18hp Evinrude of similar year. With that in, at full throttle, the motor ran full speed 3/4s of the time, with intervals when the RPMS would suddenly drop to half or less or 5-10 secs before returning to full speed. It continued to do this for as long as we tested it (5 minutes).

We then re-installed my original carb. Now it's running as it did before Step 1! Yesterday I went fishing with it, and ran it 30 minutes to the spot and 30 minutes back. The entire time, it ran half speed at full throttle. It still starts easily, idles normally, and runs normally at low speed. Past half throttle or so, the RPMs don't increase. I think it sounds bogged down, like it's getting too much fuel. But that's just an impression.

This is driving me (and the mechanic whose been charging me only for parts) crazy. Any suggestions? The only suggestion from Jdusza I've not yet carried out is to run it at night and look for stray sparks to ground.

 

[Toddp01]

Likely a spark issue. Might be fuel but verify ignition before you move on to fuel delivery or you will spin in and crash.
Verify ignition with a timing light on each spark plug wire. Remark your flywheel for good reads of both TDCs. You'll see if good, bad or ugly. With engine capable of running, this is an easy test.
As per kbait, you'll have to work your way through components.
Check components under the flywheel for wear and fraying and wire routing. Have to verify stator coil continuity and resistance. Measure resistance across the coil and compare to spec value. Put a meter across the coil leads and jog the flywheel passed. You should get only two signals that correspond to magnet locations in the flywheel.
Refresh points surfaces by dragging emery cloth between the contacts and regap.
Throw out the condensers and spark plugs as they are cheap. Get them out of the diagnostic loop.
Run the engine at night looking for sparks jumping randomly to grounds.
J

Thank you. Good suggestions. Have carried out all but the last. Will do so tonight.

 

[oldboat1]

might check the bolt at the front of the lower carb. Make sure it's snug (high speed jet underneath, and rpm will be a little crazy if the bolt rattles around).

 

[ryan 98]

The fact that it still does it with either carb and all this work makes me still think spark. I've probably had 10 or more motors of the same omc design over the years and they are really deciving on 1 cyl. They sound just bogged down, not like half the motor isint running. Points are truly spotless and gapped at .020? Nothing to bind up there? Now your motor kinda viciously drags the spark plug leads across the block when the mag advances, never liked that design much. There's even the magnito advance arm there to pinch them if somethings out of whack. I'd look super carefully in to that area for a bad lead arking out. Had that that few times right in that area, a really light spring can kinda help keep them from rubbing every time you advance throttle. But that would possibly tie a miss to throttle position. Finaly if it's a situation where it does it when it warms up look at coils and condensers. If a condenser is bad you will probably see it in a hot spot on one of the point sets. I'm assuming the green Sierra coils are already in yours? but don't count them out since their new. I had one go bad in the exact same way at only 1 season old. it wrecked my head trying to figure that out last year. Heats up and breaks down. Pretty simple critters but that magneto is kinda delicate and needs to be set just right to run good. Does it sit at 50% power but randomly just jump up on power and just about send you out of your seat for a second? Maybe surge like it's trying to gain power? In my experience that's always a sign of ignition on these motors.

 

[ryan 98]

And yeh, whatever fiber washers they used for gland packing on the needles of these carbs all seemed to break down. Causes a air leak and the motor to not want to idle or tune off the needles. I wouldn't think that would cause your issue, but might be an issue still. Oddly enough a coleman lantern graphite seal is what I put in mine, just trimmed down short first. If you leave it too long the threads for the needle are in the jamb nut and it will hold the needle too far out off the seat. Works reall nice and holds the needle steady so It doesn't vibrate around. I know they sell the actual packing new. But being canada it's hard to order suff like that, but coleman parts come easy. Just a thought if yours needs them and their hard to order.

 

[oldboat1]

need to check spark using an open air adjustable device (auto parts store). Tests magneto function.

 

[Toddp01]

Thank you, Oldboat1 and Ryan 98. I will follow your suggestions and report back.
I have more info. I went out on the river this morning and did more testing. I brought a new purchase: a hand-held visual tachometer (reads flash from piece of reflective tape you put on the flywheel).
First 15 minutes:

• At WOT, steady at about 3000 (+/- 200) rpms.
• Did drop test. While running at WOT, pulled rubber hood off top plug: Dropped to and ran at 1850 rpms.
• Did same with bottom plug: Dropped to and ran at 2150 rpms.
• While running at WOT, squeezed fuel hose rubber bulb until hard: No change in rpms.
• Removed cap on fuel tank: No change in rpms.

After 15 minutes,

• At WOT, motor increased to about 4200 (+/-200) rpms. Boat speed increased along with it.
• RPMs then went up and down every 5-15 secs, alternating between a low of about 2000 (+/- 200) and a high of 4200 (+/- 200).
• Never exceeded 4400 rpms.
• While running at WOT, squeezed fuel hose rubber bulb until hard: RPMs dropped to about 2000, then slowly recovered to about 4200. Did this twice and reacted same both times.
• Removed gas cap: No difference.

This motor is pushing a Lowe 1448 jon boat. Flat bottom, flat bow. 14 feet long, 4 feet wide at the bottom, increasing to 6 feet at the gunwales. So my guess is that the 4400 rpms is this motor's normal max when it's pushing my boat. I may be wrong.

This motor has clear fuel hoses inside the cover (connector to fuel pump; fuel pump to carb). The fuel running in these hoses always includes a lot of air. Is that normal? It's always there no matter how my motor's running, so I assume so, but I thought I would mention it.

Any different or additional suggestions, given the new info? I appreciate your help.

 

[oldboat1]

Probably appropriate to ask about condition of the hull. If you have water moving around under the floor, you might see the conditions you describe. Waterlogged foam an issue too, but think that would not yield any variation.