71 Johnson 3 cylinder miss at 3/4 throttle and above

[mokanwarrior]

After extensive repairs to ignition system I feel Im very close to getting this thing finished. However I have a miss/jerk at 3/4 throttle and above. I suspect a distributor or spark plug wire issue but before I spend $300+ on parts I may not need I wanted to get some more experienced opinions. It has a pretty nice idle quality and seems to want to accelerate but it just seems like it cant get "cleaned out". Carbs are spotless and Ive ran it at night and dont see any sparks with hood off. Any thoughts?

 

[Daviet]

Re: 71 Johnson 3 cylinder miss at 3/4 throttle and above

Use a timing light and check each plug wire when it is acting up. If you have a constant and steady spark on each cylinder, your problem is most likely a fuel problem.
If you find the spark is OK, remove the silencer cover and squirt fuel directly into each cylinder, see if you have a cylinder pick up and run properly, if so you have found the problem.

 

[mokanwarrior]

Re: 71 Johnson 3 cylinder miss at 3/4 throttle and above

I've never thought of using a timing light to check for spark. Thats a great idea! I dont suspect fuel due to the fact when it starts acting up it's very similar to car with a wet distributor. Plus I've pulled the plugs and the all appear to be burning the same. Thanks for the tip!

 

[boobie]

Re: 71 Johnson 3 cylinder miss at 3/4 throttle and above

I vote for the ignition miss.

 

[mokanwarrior]

Re: 71 Johnson 3 cylinder miss at 3/4 throttle and above

In order to duplicate the miss the motor has to be under a load, hence under way. I tried the running engine in a tank and blasted my garage full of water! Yeah really impressed the wife! Anyway I've decided to replace the distributor cap as its pretty much the only componenet I havent replaced. I've found several on flea bay and new ones are around $300. Any ideas on where to get one? Thanks again.

 

[Daviet]

Re: 71 Johnson 3 cylinder miss at 3/4 throttle and above

Before I spent that much money on a guess, I would put the boat in the water and run it. Sometimes I just back the trailer in the water, leave the boat tied down and test run them that way.

 

[boattechlm]

Re: 71 Johnson 3 cylinder miss at 3/4 throttle and above

under the dist. there is a rotor on the crank , these models also had a spring that clipped over the crank under, or on top of the rotor.
sometimes the spring would wear and cause it to move around and short out the rotor ( ignition ) . the purpose of the spring was so the motor would not start backwards. if the spring is still there you can inspect it or even remove it. they have given me problems in the past , even more so at high speeds when crank is turning faster rpm....
you can run the motor without the spring.
those dist. caps are very easy to inspect, clean it up good and look for long burns or cracks they are pretty durable. also inspect the rotor
most of the time when these caps go bad they leak spark to the flywheel. if you run the motor with the cover off you can hear the spark jumping when the miss happens or even see it...
they have very strong spark and can be herd or seen if coming from dist.
hope that helps

 

[mokanwarrior]

Re: 71 Johnson 3 cylinder miss at 3/4 throttle and above

I've removed the anti-reverse spring and I've inspected the engine running on muffs after dark and dont see any sparks but the cap does show some signs of sparking under it however I'm not sue if thats normal or not.. I have located a donor engine but it's an Evinrude of same year/model to use for testing purposes. Would this work? I've heard these are the same motor.

 

[boattechlm]

Re: 71 Johnson 3 cylinder miss at 3/4 throttle and above

yes some signs of sparking is normal , as long as it is on or the copper strip going to the plug wire. the spark on these engines is very hot spark and does cause some normal burns, however if one is excessive then it would be suspect.
the evinrude parts should be the same.

 

[mokanwarrior]

Re: 71 Johnson 3 cylinder miss at 3/4 throttle and above

I bought a used distributor cap off of flea bay and it should arrive in a few days. Ill report back once its installed. Im certain mine has an issue. Thanks again all.

 

[mokanwarrior]

Re: 71 Johnson 3 cylinder miss at 3/4 throttle and above

Well the saga continues. Ends up the one I received was damaged in shipping and so now back to square one. Im going to test the integrity of my plug wires. Whats the easiest way to crimp the brass fittings on the spark plug wire ends so the will be solid and secure?

 

[mokanwarrior]

Re: 71 Johnson 3 cylinder miss at 3/4 throttle and above

I purchased a new distributor cap and an nos rotor and still have the same issue. The local (reputable) boat shop recommened this also after paying their diagnostic fee. Still same issue. I've invested close to a $1000 dollars to revive this engine aand I'm not any closer than what I was. Thanks for your help but I'm sinking this piece of s**t. It amazes me that something this simple can't be diagnosed....

 

[Daviet]

Re: 71 Johnson 3 cylinder miss at 3/4 throttle and above

Did you back the boat in the water and run it using a timing light to check for spark while it is missing, what was the results?

 

[mokanwarrior]

Re: 71 Johnson 3 cylinder miss at 3/4 throttle and above

Did you back the boat in the water and run it using a timing light to check for spark while it is missing, what was the results?

I have not and the reason being whenever it starts acting up it doesnt appear rhythmic it's more sporadic. I begining to question a points issue. This particular motor has two sets. I have very little experience with points and my manual just shows adjustment procedure, not theory of operation. Does one set work on low rpm and the other on high?

 

[mokanwarrior]

Re: 71 Johnson 3 cylinder miss at 3/4 throttle and above

I checked the points adjustment again and it appeared that the rear set was gapped too large. I'm not sure how big of an impact that would make. It didnt seem excessive. Before I drive to the lake again does anyone have any thoughts on that? Thanks again.

 

[Daviet]

Re: 71 Johnson 3 cylinder miss at 3/4 throttle and above

How big was the gap, what did you set it to?

 

[mokanwarrior]

Re: 71 Johnson 3 cylinder miss at 3/4 throttle and above

I set it at .10. It may have been .15, maybe...

 

[mokanwarrior]

Re: 71 Johnson 3 cylinder miss at 3/4 throttle and above

I previously replaced the old power pack with a new amplifier/coil assembly from CDI Electronics. I suspect the new coil that came with it maybe faulty??? I've contacted their tech support and have yet to hear back from them.

 

[mokanwarrior]

Re: 71 Johnson 3 cylinder miss at 3/4 throttle and above

I put the boat in the water last night and ran it on the trailer. I hooked up a timing light to #2 plug wire and ran it at full throttle and watched the light. It was nice and steady up until about 1/2 to 3/4 throttle and when the miss started the light became unsteady. I tried it on #3 wire with the same results. I still have yet hear from from CDI.