1975 Johnson 70hp

[Booberdoo]

Quick question if you don't mind:

I lost one cylinder yesterday (the uppermost one...forgive me but I forgot to look in the book to give its numerical designation). I have good compression, good fuel and there is spark, but I am still leaning towards the coil being bad.

Reason why; over the past two years I have had to replace two coils due to cracked housings resulting in arcing at the coil itself. This uppermost cylinder is still running on the original coil which makes it the only one I haven't replaced.

The book directs you to use a tester which I don't own, and I see no visible cracking or arcing on this one. When it gets dark I'll see if I can see something but I was wondering if there was a simple test I can do to check it...

When I pull the plug wire off the plug I get zapped, but it just doesn't seem as potent as I'm used to, and there is no difference in run quality so I know its not firing. I did swap plugs to check for a bad plug with no result.

Any quick ideas? I figure if the others went bad this one is probably on its way out the door as well.

Thanks

[HighTrim]

If it is original, it is likely bad, but check it nonetheless.

Swap that coil with one from a cylinder that is known to be firing. If the problem follows the coil, you know for sure that it needs replacing.

BTW, judging a coils spark effectiveness by the degree to which you get zapped is pretty medievel, not to mention a bit silly Get yourself an inline adjustable tester, set the gap to 7/16 of an inch. The spark should jump this gap with a bright blue ZAP!!

spark tester.jpg

[Booberdoo]

BTW, judging a coils spark effectiveness by the degree to which you get zapped is pretty medievel, not to mention a bit silly

Haha, well I didn't happen to have my tools at the house so hence I had no insulated pliers with which to pull the plug wires to find out which cylinder wasn't firing, so I did it by hand and of course I was zapped. I just know from experience that it didn't seem strong (no, I don't typically use this as a test)

I was in a hurry last night because it was such a nice day, now it looks like weather is coming in so I have some time to go get my diagnostic tools from my shop and properly test everything. Thanks for the thought on switching coils real quick...simple idea; I do tend to over think things from time to time and miss the simple solutions...

Cheers

[TommyA]

I tend to over think things myself. I just keep reminding myself to step back and look at the basics first and that usually helps.

[Booberdoo]

Okay, interestingly enough I found #1 (topmost) carb had a stuck needle & seat. Now I just rebuilt these things and it only sat for 1 day... why?

Per service tech's recomendations I am running a pre-mix of 32:1. I have seen a lot of talk about running at 50:1 but I have hesitations of running that lean as I did this with a Can-Am 350 motorcycle only to seize the piston, slide off the road and hit a tree at over 100 mph. I spent almost 6 months in a body cast and am so full of metal I need to carry a notorized doctor's certificate to get me through the metal detectors at the airport.

Running lean scares me, so tell me what the absolute correct ratio on a 1975 Johnson 70hp is please...

[steveo32]

50:1, I have the same motor, same year, I have not had a problem with 50:1 and I believe the service manual says to run 50:1 also but not for sure.

[Maj7860]

I have the same 1975/70 Johnson Seahorse and I am told to run it at 40:1, not sure if this is by the book though.

[borz170]

I have never used an in line tester, but I am going to look into it. I have always used a spark plug, plugged into the plug wire, grounded to the block. This will at least show you spark, and you should be able to compare the color to the other coils. You can hold onto the wire, as long as you don't touch the engine. This grounds you, and will light you up. Remember to use the same plug on all wires. This eliminates another varriable. So, did you swap coils? I bet your problem is with fuel, since you mentioned a stuck needle.

[Booberdoo]

Yes, pulling the needle out of the seat and cleaning it did the trick. I also use the plug out and grounded as an indicator of spark but this is not the most accurate way of doing it as it will tell you if you have spark but unless your experienced you cannot tell if it a GOOD spark are not. The testers allow you to check it properly.

I just looked in the manual and it does call for 50:1 so I guess I've been running a bit rich which is probably what is gumming up my carbs. Thanks for pointing in the right direction, I guess if all else fails read the directions

Funny thing is the plugs have good color and show no sign of fouling...

[mikesea]

Based on your post ,I would guess you have a coil /spark problem,wires are important as well.It dont hurt to treat you engine to a set .But,if you think you will be boating for yrs.to come.An n expensive compression guage is a good investment.(afriend with 1 is better).Under load your spark will find a path of least resistence,so ,you may not see arching at idle,but it can leak.Check the wires good,even a rust stain from the top of the plug can allow spark to travel.

[Booberdoo]

Based on your post ,I would guess you have a coil /spark problem,wires are important as well.It dont hurt to treat you engine to a set .But,if you think you will be boating for yrs.to come.An n expensive compression guage is a good investment.(afriend with 1 is better).Under load your spark will find a path of least resistence,so ,you may not see arching at idle,but it can leak.Check the wires good,even a rust stain from the top of the plug can allow spark to travel.

A simple check for arcing is to wait until it's dark then run it, you will see the arcing if present.

If you read above my problem is related to the fuel mixture ratio; the PO and a service tech both told me to run at 32:1 which is too rich. This is causing my carbs to gum up which in turn causes the needle to stick shut in the seat which of course stops the flow of fuel to the cylinder(s).

I run a race shop (part time) where we build both drag and circle track cars, I do this for my nephews. I do have quite an assortment of tools including compression testers and ignition testers anf the knowledge to use them but unfortunately for me the shop is 1.5 hours away so I have to make the run after work to retrieve things I need when diagnosing. What I do is do my tuning the old fashioned way first and make the run only if needed. Most of the time I can figure out the problem it's just that this year I did full rebuilds and calibrations on the carbs so It didn't occur to me that they could be the culprit. After swapping the coils around I found that they were performing just fine which led me to investigate the carbs and lo and behold, dry bowl on #1.

Thanks for the suggestions though as it is good info for everyone to read, sometimes a simple little statement can go a long way in helping someone.

[mikesea]

Yep,thats why I go into as much detail as I can the first time.Saves alot of back and forth,plus people do alot of searching,so I try and use some key words.Gives me something to do.Totally agree with the spark arch,BUT,as you know it does leak if it can.Glad you back on the water.Mike

[Booberdoo]

Yep,thats why I go into as much detail as I can the first time.Saves alot of back and forth,plus people do alot of searching,so I try and use some key words.Gives me something to do.Totally agree with the spark arch,BUT,as you know it does leak if it can.Glad you back on the water.Mike

Oh yes, sparks like to leak whenever they can, kind of akin to letting the factory installed smoke out of wires...

I built my first full blown race engine back when I was 15; it was a 455 Buick bored and stroked to 492 C.I. with a 14:71 B&M blower. I put it in a 1973 Firebird. Darn thing started acting weird, bucking and misfiring one day. Since they didn't have blow off valves in the blower manifold at the time I was justifiably concern about a blower explosion...

I farted around with that thing all day long to no avail; it was only when it started getting dark that I saw the pretty little blue lines traveling all around the coil. I grabbed another coil and off I went... Sometimes you just learn things in the strangest ways.

Another way to check for arcing on a coil or in the wires is to lightly squirt water on them while the engine is running.