1986 70 HP Johnson Intake Leak

midsized johnson

Petty Officer 2nd Class
Joined
Jul 28, 2005
Messages
155
My 70 has developed a strange issue. It idles and will run fine to near WOT, then bogs when the throttle is pushed all the way. We ran the boat with the cowling off and my buddy throttled it up by hand and it bogged, so it clearly is something happening in the fuel/air mix and not electrical. We sprayed carburetor cleaner on the intake between the carb and the intake and it kills the engine on the middle and lower carbs idling in the driveway. We checked the butterflies with the throttle pushed up to where it ran at 4500 or better, they looked barely open. I pulled the carbs and the intake off and the gaskets all look good and the carbs/intake appear to be in good shape with no visible cracks or pits on the gasket surfaces. Reeds are all in good shape as well.

Also, while at idle, if you put your hand over the front of the carbs, one by one, it will kill the engine. Don't know if that is meaningful to the question, but it occurs to me that if there were a significant air leak then that wouldn't kill the engine.

I've got new gaskets for both sides of the intake, going to put them on using OMC's gasket sealer stuff to see if that makes a difference. At this point, it doesn't seem to me that this is going to fix the problem.
Any thoughts for what could be happening? Am I missing something?
 

Bosunsmate

Admiral
Joined
Apr 7, 2012
Messages
6,129
Re: 1986 70 HP Johnson Intake Leak

Try doing a link and synch
Might be the fuel pump failing at high rpm, try priming bulb manually at high rpm to act as a manual fuel pump.
Changing those gaskets sounded like it was needed.
Redo that check with carb cleaner afterwards to make sure theres no further leaks as you dont want it running lean
 

midsized johnson

Petty Officer 2nd Class
Joined
Jul 28, 2005
Messages
155
Re: 1986 70 HP Johnson Intake Leak

I think the fuel pump is good, we tried the "squeeze test" and it didn't help it. I'm hoping new gaskets fix it, but I've got my doubts. Will definitely have to lync and sync since everything's apart.
Just thinking, what about a plugged high speed jet? I'm no mechanic, just backyarding it with the help of some friends that are. One of the fuel lines was coming apart internally and the fuel filter at the powerhead was restricted pretty bad. Changed lines and filter and it ran much better, just no upper end. Thinking maybe giving it full throttle pulls something in and restrict flow???
 

boobie

Supreme Mariner
Joined
Nov 5, 2009
Messages
20,826
Re: 1986 70 HP Johnson Intake Leak

You don't put any kind of sealer on intake gaskets !!!!!!!!
 

Bosunsmate

Admiral
Joined
Apr 7, 2012
Messages
6,129
Re: 1986 70 HP Johnson Intake Leak

I think the fuel pump is good, we tried the "squeeze test" and it didn't help it. I'm hoping new gaskets fix it, but I've got my doubts. Will definitely have to lync and sync since everything's apart.
Just thinking, what about a plugged high speed jet? I'm no mechanic, just backyarding it with the help of some friends that are. One of the fuel lines was coming apart internally and the fuel filter at the powerhead was restricted pretty bad. Changed lines and filter and it ran much better, just no upper end. Thinking maybe giving it full throttle pulls something in and restrict flow???
Yes clean those jets
 

racerone

Supreme Mariner
Joined
Dec 28, 2013
Messages
36,033
Re: 1986 70 HP Johnson Intake Leak

Do not run this motor if you suspect a plugged float valve or plugged high speed jet !----You risk serious damage to the engine.--------No fuel going into one cylinder also means no or little oil too.
 

midsized johnson

Petty Officer 2nd Class
Joined
Jul 28, 2005
Messages
155
Re: 1986 70 HP Johnson Intake Leak

Yes clean those jets
Will do! As for not running it with a plugged high speed jet, well, if that's the cause, it's too late. Gonna try and put her back together this weekend. If I learn anything, will update.
 
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