1978 70 Hp Johnson Missing Problem

[karlp]

I have a 1978 Johnson 70 HP that has started to miss after about one hour after new spark plugs are installed. When I remove the plugs the lower plug is black with a lot of carbon while the other two are clean. I thought it might be the ignition coil for #3 cylinder so I swapped the #3 coil for the #1 coil and the same problem resulted so it does not appear to be the #3 ignition coil. I have yet to check compression but since the engine starts and runs fine for awile until the plug fouls I feel it must be the ignition system or possibly something to do with the #3 carburetor. Any suggestions? Thanks

 

[ezeke]

Re: 1978 70 Hp Johnson Missing Problem

Check your choke and throttle plates for synchronization.

 

[karlp]

Re: 1978 70 Hp Johnson Missing Problem

Don't understand exactly what you mean by choke and throttle plates.

 

[ezeke]

Re: 1978 70 Hp Johnson Missing Problem

When you look into the carburetor throats on your engine, you will find that they have two plates each. The forward one is a choke plate which reduces the air flow to enrich the fuel/air mix for a cold start. The rear plate is the throttle plate (as in wide open throttle or WOT), which controls the fuel/air flow during normal operation of the engine.

The plates need to be synchronized so that all three of each type are moving together at exactly the same time and opening and closing the exact same amount.

 

[karlp]

Re: 1978 70 Hp Johnson Missing Problem

Thanks for the advice. I have since checked the compression and all three cylinders are the same with 50 psi on the first stroke and 125 psi after three
after strokes. I will check the carburetor plate alignments today per your suggestion. It appears that something is making the third cylinder run rich and the choke plate could do it. If the plates look good I may have to check the float level on #3 carb. Thanks again. karlp

 

[jay_merrill]

Re: 1978 70 Hp Johnson Missing Problem

Start with the spark plug - just because its new doesn't mean that it can't have a problem. Check the coiled lead inside of the spark plug wire too, to make sure that it has remained clipped into the wire.

 

[karlp]

Re: 1978 70 Hp Johnson Missing Problem

I switched ignition coils between 1 and 3 cylinder and the problem stayed with 3 cylinder which means the miss can't be associated with the ignition coil or plug wire. However there are three other components in the ignition system so the problem could be one of these other components and I do not have the test equipment to check these other components out.

A new plug gets black with carbon in about 30 minutes on #3 cylinder so too much fuel may be the cause. I checked the throttle and choke plates as suggested earlier and all are opening and closing properly. My next effort will be to rebuild carb #3 and see if that solves the problem.

Thanks for your suggestion. karlp

 

[jay_merrill]

Re: 1978 70 Hp Johnson Missing Problem

This one is still hitting me as an ignition problem.

From your description, I'm not sure if you have done multiple plug changes, or just one. If its just one, I would swap the #3 plug with another one, just to rule out problems with it.

If the plug isn't the culprit, I would start looking at the powerpack and/or timer base, because both will sometimes do what you are describing. If you have an OEM manual, I suggest doing the tests detailed for troubleshooting each of these components.

 

[karlp]

Re: 1978 70 Hp Johnson Missing Problem

I have put new plugs in all cylinders several times and each time I get the same problem with #3 cylinder so it isn't a bad plug. I do have the manual and will look up the tests you mention but from previous reading of the manual I think it requires special testing equipment which I do not have. All I have is a simple volt/ohm meter and an induction timing light. I will recheck the manual and see what I can do. Thanks for the advice.

karlp

 

[jay_merrill]

Re: 1978 70 Hp Johnson Missing Problem

Check your service manual, look in the ignition section towards the end of the part that details the various tests that you can do. There may be a couple of paragraphs on doing ohms tests.

 

[willamettejeff]

Re: 1978 70 Hp Johnson Missing Problem

Although maybe not the problem in your particular case, make sure your engine is getting up to proper temperature range as running too cold will cause spark plugs to foul. I believe the #3 cylinder runs the coolest on that motor and would then show the problem first. Replace the thermostat and bypass valves if this is the case.

 

[jay_merrill]

Re: 1978 70 Hp Johnson Missing Problem

One thing that I neglected to mention, is that there is a very easy way to determine if the problem in the #3 cylinder is ignition related, or not. By using an inductive timing light while the engine is running, you can easily see if you are getting the appropriate electrical impulses to the spark plug.

When I want to check one of my motors under various conditions, I just tape the trigger closed on the timing light, hook it up as needed, and then lay it on the deck facing the helm. I then go run the boat and alternate between steering and glancing back at the light. This procedure will usually point out ignition problems very quickly. Obviously a dead cylinder is immediately noticeable, but even an intermittant ignition will be obvious, simply because the normal flicker won't maintain a consistent pattern.

 

[ezeke]

Re: 1978 70 Hp Johnson Missing Problem

I think that you eliminated the synchronization possibility, so it is most likely the carburetor that is causing you to run rich. Be sure to check the jets to be sure that you have the correct ones on that carburetor.

 

[karlp]

Re: 1978 70 Hp Johnson Missing Problem

Thanks for your response. I was thinking of doing what you describe but I am not sure the cables are long enough. However there is no qusetion that the cylinder misses as you can feel it. Let me describe the problem again. First it is a new problem. I have two engines on my boat and they have run very well until this problem started. They problem started after running them for 45 minutes or so on our second trip of this season when one engine started to miss and the engine rpm and the boat slowed. Then the engine will stop missing for a few seconds and the boat will speed up. This keeps repeating. When I get back to the dock and check the plugs only the #3 plug is badly carboned. I think the high level of carbon is why it misses. Whether the plug fouls because of poor ignition or whether the plug becomes fouled due to a rich mixture is the question. The other engine runs fine.

 

[jay_merrill]

Re: 1978 70 Hp Johnson Missing Problem

That's good info because it confirms what I thought you were saying - that it is a definate miss, rather than a matter of a plug being subjected to a rich mixture and eventually getting so gummed up that it doesn't want to fire at all. While you could have a bit of junk in the carb, I'm still thinking ignition.

Based on what you have just described, I'd bet you a beer that the problem is the #3 sensor in your timer base. Check the ignition chapter in your service manual and see if there is an ohms test for your timer base - there probably is. On my 1972 Johnson 65 hp, which is essentially the same engine as yours, there are two ohms tests. Both use the same type of CD ignition, but your timer base part number is different, so I don't know if the expected values are the same. That said, here's the procedure for the 65hp ...


CHECKING SENSOR COIL

a. Discinnect sensor leads from the power pack terminals #8, #9, #10 and #11, common lead. See Figure 4-16. Connect ohmmeter (LO ohms scale) to each sensor coil lead (white with black stripe). Connect other lead to the common lead (black with white stripe), meter should read 8.5 +- 1.0 ohms.

b. With sensor leads still disconnected from power pack, connect ohmmeter (HI ohms scale) alternately between each sensor lead and ground. Infinity reading on each sensor indicates good sensor. Zero reading indicates leads or sensor is shorted to ground and should be replaced.

c. The sensor coil and timer base are serviced as a unit.


If you have one of the original, tin timer bases, the sensor coils are actually adjustable, but you will only buy a little time by trying to do that. Instead, I would just buy a new one. The shop.evinrude.com site says that they are still available for your motor at $225.00, but CDI will most likely carry one for much less.

 

[jay_merrill]

Re: 1978 70 Hp Johnson Missing Problem

FOLLOW UP

I did a little research on the timer base in your motor. The part number for it is 581251. In finding aftermarket parts for this timer base, the first thing that I discovered is that the part number is a replacement number for part number 580970, which is the timer base in a 1972 OMC 65hp motor. What this means is that they are the same part and it should mean that the ohms test information that I gave you in my previous post, is valid for your motor.

 

[karlp]

Re: 1978 70 Hp Johnson Missing Problem

Per your suggestion I measured the Charge Coil and the three Sensor Coils with an ohm meter and everything was at the specified level per the manual. I had checked out the Ignition Coils earlier and they were also ok. This was an ohm meter and not a test meter so it may not have picked up the problem.

Since the miss starts after 15 to 45 minutes of running I am thinking more strongly that it is a rich mixture fouling the plug. My next step will to rebuild carb #3.

karlp

 

[jay_merrill]

Re: 1978 70 Hp Johnson Missing Problem

OK, that sounds reasonable. Let us know the result.

 

[karlp]

Re: 1978 70 Hp Johnson Missing Problem

Jay, this is what I did. I took off #3 carb and put in a new float and needle. The carb was very clean so I did not drill out all of the passages. But I did blow them out with compressed air. I put in new plugs and connected the motors to fresh 50:1 gas in the aux tank. I went out to the ocean at 4500 rpm and kept this speed up for 45 minutes. No miss. When I returned to the dock I checked #3 plug and it was clean so one of these changes was effective. Since I made a number of changes I will have to sort out which one was effective. I need to find out what will happen if I run the motors only at 2500 rpm. Only cleaning the #3 carb was a change only for the problem cylinder so maybe the float or needle valve was defective since they were 31 years old.

None of the plugs show carbon but they do appear oily. I think that unless one runs the motor at max rpm a 50:1 mixture has too much oil in it. But that is what the manual recommends.

I would like to thank you and the other members for their advice.

karlp

 

[Sandhammaren05]

Re: 1978 70 Hp Johnson Missing Problem

Stuck float on nr. 3 carb?

I have a 1978 Johnson 70 HP that has started to miss after about one hour after new spark plugs are installed. When I remove the plugs the lower plug is black with a lot of carbon while the other two are clean. I thought it might be the ignition coil for #3 cylinder so I swapped the #3 coil for the #1 coil and the same problem resulted so it does not appear to be the #3 ignition coil. I have yet to check compression but since the engine starts and runs fine for awile until the plug fouls I feel it must be the ignition system or possibly something to do with the #3 carburetor. Any suggestions? Thanks