1988 Johnson 8hp bogging at full throttle

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davesko
Posts: 135
Joined: Thu Nov 10, 2016 9:48 am
Location: Hershey Pa

1988 Johnson 8hp bogging at full throttle

Postby davesko » Thu May 17, 2018 7:29 pm

My son was recently given a 1988 Johnson 8hp that his father in law gave up on after $550.00 and three trips to the marina. So far this is what I know. The cdi box, both coils, plugs and fuel pump have been replaced and the carb rebuilt(according to receipts). I have done the following: checked spark, it jumps a 5/8 gap and rebuilt the carb again and checked Lync and sync. I had it out Sunday and it still bogs down. The only thing I can pinpoint is it has the wrong air box on it and when I partially block the carb throat it takes right off and runs well. Is the proper air box critical or do I need to check elsewhere. I’m not sure how to check the timing and my manual is very vague. The motor runs very nicely until half throttle. Thanks in advance

Michael
Posts: 50
Joined: Wed Mar 18, 2015 5:12 pm

Re: 1988 Johnson 8hp bogging at full throttle

Postby Michael » Thu May 17, 2018 7:55 pm

Think you need the correct air box and gasket for it to seal up against carb. I normally push the air box hard up against the carb, then snug the 2 screws down.

davesko
Posts: 135
Joined: Thu Nov 10, 2016 9:48 am
Location: Hershey Pa

Re: 1988 Johnson 8hp bogging at full throttle

Postby davesko » Fri May 18, 2018 1:58 pm

I thought I had heard they ran lean without one so with the wrong one it stands to reason it wouldn’t work. I hope this cures it, enough money and time have already been poured into this motor

Michael
Posts: 50
Joined: Wed Mar 18, 2015 5:12 pm

Re: 1988 Johnson 8hp bogging at full throttle

Postby Michael » Fri May 18, 2018 2:33 pm

I have the correct air box if needed.

davesko
Posts: 135
Joined: Thu Nov 10, 2016 9:48 am
Location: Hershey Pa

Re: 1988 Johnson 8hp bogging at full throttle

Postby davesko » Fri May 18, 2018 5:00 pm

That would be great. Email me please davesko69@yahoo.com Thank you

fleetwin
Posts: 2219
Joined: Mon Mar 09, 2015 4:27 pm
Location: Rhode Island

Re: 1988 Johnson 8hp bogging at full throttle

Postby fleetwin » Fri May 18, 2018 5:38 pm

Yes, the correct air box is absolutely necessary, along with the stupid little expensive sponge seal between the carb and air box. The fuel system on this engine is way "over engineered" to the point of being stupid....So yes, the engine will run lean at high speeds without the correct air box/seal properly installed.
You can just try something to be sure we are on the right track though...When the engine starts to falter at high speeds, simply pull the primer knob out to the middle "enrichment position" to see if this helps solve the problem. If so, the engine definitely is lean...The wrong air box can surely be the culprit, but there are a few other pesky cheesy carb parts that cause trouble as well. The high speed jet is housed in a plastic housing that fits up into the carb nozzle. The jet or brass tube could be partially plugged. There is a flimsy rubber hose that connects from the plastic housing to the carb body as well. These early hoses were known for dissolving depositing rubbery debris everywhere in the carb. Early model hoses were black, the improved hoses were tan in color. There is also a rubber gasket that fits between the bottom of the plastic housing and carb bowl, don't forget that or the engine will run too rich at high speeds...Does the engine idle OK? If so, the rubber hose is probably OK, the jet could still be dirty though...
In any event, let's start with the correct air box and seal. Would love to see a picture of whatever someone has rigged up in there now...

davesko
Posts: 135
Joined: Thu Nov 10, 2016 9:48 am
Location: Hershey Pa

Re: 1988 Johnson 8hp bogging at full throttle

Postby davesko » Fri May 18, 2018 7:20 pm

Hi Dan, thanks for the reply. I carefully cleaned the high speed nozzle including the brass tube and orifice. I also replaced the little plastic tube. New top gasket and bowl gasket. The air box is off a 1985 johnson 6 according to part number. I suspected it from the start but wanted to cover all my bases, I’m growing to hate these plastic topped carbs. I will get a proper air box and hopefully he will have a good motor. We are having some flooding in our river this weekend so no test runs unfortunately. Thank you! Dave

Michael
Posts: 50
Joined: Wed Mar 18, 2015 5:12 pm

Re: 1988 Johnson 8hp bogging at full throttle

Postby Michael » Fri May 18, 2018 7:39 pm

I would have thought the 1985 airbox would work fine if it has the sponge foam gasket and is mounted snug up against the carb. The 1985 one is the two part one correct?
BTW i think these are great little motors when the run correctly. I habe an 8hp one that screams and performs way better than every other one I have had

davesko
Posts: 135
Joined: Thu Nov 10, 2016 9:48 am
Location: Hershey Pa

Re: 1988 Johnson 8hp bogging at full throttle

Postby davesko » Fri May 18, 2018 8:05 pm

The 1985 one looks nothing like the correct one. It is one piece with the sponge but it has 8 vent holes in the back and the correct one only has one and has a pass through for the primer.

davesko
Posts: 135
Joined: Thu Nov 10, 2016 9:48 am
Location: Hershey Pa

Re: 1988 Johnson 8hp bogging at full throttle

Postby davesko » Fri May 18, 2018 8:09 pm

The correct air box part number is 396564.


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