# 1978 Johnson 70 HP no power/bogging



## bnt5 (Jun 10, 2014)

I recently acquired a 1978 Johnson 70 hp outboard with a boat I purchased, the model number is 70EL78C. It originally would try to start then die for the previous owner. When I got it home I cleaned gas tank and fuel pickup emptied float bowls and put new gas mixed 50:1 and it started right away and idled fine on the muffs. Just to satisfy myself I tested compression and it read 120,119,119 on all three cylinders needless to say I was very relieved! Well I loaded the boys in the truck and took it down to the river to run it around. At first it was hard to start, then I couldn't get it above idle speed without the engine bogging and dying on me, I could push the electric choke when it was dying and it would come back to life...so time for a good carb clean. To make a long story short I soaked carbs and cleaned everything thoroughly, adjusted linkage and it fired right up and I was able to accelerate the throttle on the muffs...so back to the river I go. This time not much problems getting it started, idled fine and went into gear fine however it now will only go about a quarter speed even with throttle pushed to fast. If I push the throttle to fast it bogs and dies, I can still play with the key choke to get it back to life but it is waaaay underpowered, we are talking like 9 mph at full throttle and this is a 70 hp engine on a 14' boat! So basically it's not running full out and bogging down. I pumped the tank bulb while running and it did nothing...

I am stumped as to what to do now, how should I proceed to diagnose and resolve issue? 

much thanks,
Bill


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## kofkorn (Jun 11, 2014)

Still sounds like a Carb problem to me. It may be that you need a full teardown and rebuild. I would definitely pull the jets and idle screw out and flush them really well with a compressed carb cleaner. 

When you are revving on the muffs, you are still only barely cracking the throttle, and probably aren't even getting into the main jet much. I would start there first.

Good luck!


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## ccm (Jun 11, 2014)

I suggest using a barrel or testing tank instead of muffs because with muffs the motor is not experiencing real life back pressure. A carb kit is probably needed too.


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## bnt5 (Jun 11, 2014)

It was a complete carb teardown clean and replace. The odd thing about these older carbs is that there isn't any adjusting anything other than linkage. The 2 jets are just brass holes and they were perfectly clean, matter of fact the carbs were already perfectly clean when I pulled them, however I went through the overnight soak in berrymans anyhow to rule them out. I am going to replace the fuel hoses and make sure I don't have any leakage anywhere. I did notice that my primer bulb from gas tank isn't "rock hard" when pumped up, it seems I can still pump it at all times...I cant squeeze it fully but it is a little spongy?? 

After going through fuel system, I am going to make sure the timing cam is moving with the throttle arm, I read somewhere that they like to stick and don't advance timing based upon throttle movement...maybe, who knows???!...LOL

I'll keep plugging away at it until my hair is completely gray and pulled out. :shock:


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## bnt5 (Jun 11, 2014)

Found that my fuel pump gasket was leaking just a little, ordering a new one. I kinda doubt that a teeny pinhole leak at fuel pump would cause so much issues? maybe just a piece of the puzzle.


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## bnt5 (Jun 12, 2014)

Update to those that may have this motor:

The high speed jet is located in the bottom of the float bowl behind the drain screw, and it needs to cleaned manually. After soaking in berrymans there was still significant blockage in 2 of 3 high speed jets. I cleaned these and put everything back together and will test run when I get the other parts in the mail.


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## kofkorn (Jun 13, 2014)

That sounds about right. I hope that this fixes it for you. As an FYI, usually you can unscrew these jets pretty easily for cleaning. I've ground down the sides of a flat head screwdriver so that it just fits inside the hole. This way I don't damage the jet when removing. 

Good Luck!


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## bnt5 (Jun 13, 2014)

I was thinking about doing just that but I figured I might as well take everything apart one more time on the off chance I missed something else. I have been waiting for a gasket in the mail and the suspense is killing me! I really am anxious to find out if these repairs have fixed the motor. Too bad no one around here sells outboard parts anymore, makes every repair take 3 times as long.


Does anyone know the correct spark plug gap for a 1978 Johnson 70 HP (70EL78C)???


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## SumDumGuy (Jun 13, 2014)

[url=https://www.tinboats.net/forum/viewtopic.php?p=355637#p355637 said:


> bnt5 » 13 Jun 2014, 05:50[/url]"]...
> 
> Does anyone know the correct spark plug gap for a 1978 Johnson 70 HP (70EL78C)???



The "correct" gap is 0.040". But you can play with them between 0.030" and 0.040" to find the best gap for your particular use.


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## Siguz (Jun 13, 2014)

I have a similar problem.....

My 1989 evinrude 20HP (mod to 25) was running fine up until 2-3 days ago.
Now it won't hold an idle without the choke/primer pulled out partially.
Runs perfectly fine at WOT and everywhere in between.

When I rev it up I get a ton of white smoke...... still has lots of power on the water and is performing normally when run hard.
It's the low speed that I can't make work right.

Any ideas?


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## bnt5 (Jun 13, 2014)

Might want to check your carbs/linkage to make sure there are no issues. At the very least a thorough carb cleaning would be a start.


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## Siguz (Jun 23, 2014)

Turns out that one of the idle jets was clogged at the top of the carb. All good now. 
Thank you for the advice.


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