Topic: 1960 40 HP – Bottom cylinder misfire – Page 2

This topic has 20 replies, 7 voices, and was last updated 10 hours, 5 minutes ago by Don.

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October 28, 2026 at 6:10 pm #330654
Don
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TJ Brandon wrote:

I will test the compression when I get it put back together.

I am sure it is the bottom cylinder because I have spark testers. Top is nice and consistent and strong while the bottom cuts out. I swap testers and the problem stays with the bottom cylinder. When the bottom does decide to fire it is nice and bright as well.

For the cut out switch, i disconnected from the switch itself. There is a nut on a stud the ignition wire connects to. I removed that.

A lower crank case seal can lead to poor ignition? How does that happen and how can I check? Upper crank seal I have replaced before and is easy to diagnose because of the mess it leaves on the magneto place

OK, disconnecting the lead from the vacuum switch is not the most accurate way of ruling out issues. The two black leads come out from under the flywheel and have those pesky “knife connections” with the chinese style torture tube sleeves. I would disconnect the leads at that point and tape them up to be sure they don’t short to ground. Again, something as simple as a lousy plug can cause the secondary voltage to drop enough so it won’t light the spark test light. Are you testing the ignition while the engine is running??

And no, a faulty lower crank seal would not cause the lower cylinder to lose spark.
October 28, 2026 at 8:45 pm #330662
JOSEPH BRINCAT
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This may be “out there” but check the side to side play in the crank shaft top bearing. We had a problem similar to what you are experiencing on a 1968 Evinrude 9.5 HP… it turned out the the lateral motion of the crank shaft and the attached breaker cam was seriously affecting the point gap.The strange thing, this only affected the lower cylinder. why only that cylinder??? can’t answer that mystery. You can read about the entire ordeal in an article I wrote in the July 2025 “Outboarder” page 18

Joe B
October 28, 2026 at 10:07 pm #330664
Tubs
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Should have re-read your initial post before responding. A failing crankshaft seal can result in erratic crankcase pressure / vacuum of a cylinder resulting in fuel starvation causing a miss. Nothing to do with the ignition.

A "Boathouse Repair" is one thats done without having tools or the skills to do it properly.
October 28, 2026 at 11:07 pm #330667
JOSEPH BRINCAT
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I should have asked , have you checked the spark and timing using a timing strobe light? That motor should have the timing marks on the mag plate and corresponding marks on the flywheel. See if the timing remains consistent and steady on both cylinders up through the speed range. Referencing my previous post about the 9.5 hp ,, a dramatically fluctuating timing indication on the lower cylinder was the clue to diagnose the ultimate problem.

Joe B
October 29, 2026 at 9:13 am #330674
TJ Brandon
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Once again, thanks everyone for the replies.

Don – I did the test as you mentioned, same results. I have scoured that wire to make sure it is not exposed or grounding anywhere!

Joseph- To try and find out what you are referring to, I have the flywheel off and put the but back on the crankshaft and tried to see if there was any movement if I pushed at it in all directions, is that a sufficient test? For the timing, no I have not done a timing light test, I don’t own one.

I have a NOS breaker coming from Ebay today. I’ll try that again…
October 29, 2026 at 10:16 am #330677
JOSEPH BRINCAT
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Going back to what we found on the 9.5 … I used a dial indicator to , found that there was .013 lateral movement . Consider that the point gap is only .020, if the cam in moving .013 sideways that will change the gap from .007 to .033 .

As for the timing light, it’s just the old fashioned flashing light that one used to set timing on cars back in the days of distributors. Obsolete technology like dwell meters… Common at garage sales for a couple of bucks.

There may be a video somewhere on using the light to set / check point gap ( timing ) on your outboard.

If you have the July 2025 issue , read the story , better explanation of what we found. And it remains a mystery as to why only affects the lower cylinder.

Joe B
October 29, 2026 at 10:18 am #330678
Don
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TJ It seems like you have done most everything, only to get the same result. Unfortunately, that means we are missing something here. I understand your frustration, we have all been through situations like this….Sometimes, it is best to step away from a stubborn project like this for a bit, giving your head and chance to clear and start over again.

You have replaced the plug wire, but I am still not convinced that a short does not exist between the cut out wire and the mag plate. Those darn wires are really crammed in tight under there, to the point of just about being pinched. I guess if it were me, I would disconnect the darn cut out leads from both breaker point posts and tuck them away/tape them off…Then retest.

You have already had a few sets of points in there, so I don’t know if a new set will make any difference. Keep in mind that BRP had issues with some OEM points that were not manufactured correctly…So, if you can not set the gap correctly using the offset adjustment screw, please know you are not going nuts.

Joe makes a good point about a wobbly mag plate, or perhaps a loose upper crank bearing that can create a variation in the point gap. Have you tried to “wobble” the mag plate from side to side? With one set of points open and the flywheel nut (only) installed, try to wobble the crank from side to side in the block to check for a big variation in point gap. How about the post coming out of the mag plate, is that somehow loose allowing the points to wobble around. Not likely I’m sure, but a possibility.
October 29, 2026 at 10:18 am #330679
JACQUES
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well in theory the problem is intermittent.. likely induced by either vibration or heat … I know you swapped things around but warmed up condensers can be finiky and under full operating conditons they can fail. VOM testing isn’t sufficientto measure internal leakage.

done any ohms comparative tests ??? post results

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October 29, 2026 at 2:40 pm #330695
TJ Brandon
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I just got a NOS breaker point and so far the barrel test in the driveway looks like it is promising. I can’t get to the lake until Saturday morning though. I really don’t know why this set of points would have worked.
October 29, 2026 at 5:38 pm #330705
JACQUES
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if you match a suspect point to a NOS you may see a difference in the physical size of the rub arm

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