Help with Johnson 48hp

Curapa · #1 06-25-2011, 11:43 AM

My buddy has an early 90's Johnson 48hp that is having some problems. He purchased it several months back and hasn't ran right since. It seems to run well on the hose but not so in the water.

Past two trips symtoms:

Runs decent for the first couple of minutes. At WOT the motor cuts in and out, you can drop back the RPMs and it will straighten out. After a few times the RPMs drop and stay down until the engine shuts off. Then the only way to get the boat to run is by holding the choke in. We had to limp back to the boat ramp both trips by playing with both the throttle and choke. It will intermittently run good and then back to crappy.

We took it to the shop:

Mechanic replaced broken reed valve
Rebuilt carbs
new plugs and wires
Tightened some bolts he said was previously loose around the intake
Told us to ditch the aluminum tank for a plastic one due to condensation

We bought a new plastic tank and took it to the river. The motor runs in the same condition as when we dropped it off with the mechanic.

Any thoughts?

Destroyer · #2 06-25-2011, 01:13 PM

Quote:

Originally Posted by Curapa

My buddy has an early 90's Johnson 48hp that is having some problems. He purchased it several months back and hasn't ran right since. It seems to run well on the hose but not so in the water.

Past two trips symtoms:

Runs decent for the first couple of minutes. At WOT the motor cuts in and out, you can drop back the RPMs and it will straighten out. After a few times the RPMs drop and stay down until the engine shuts off. Then the only way to get the boat to run is by holding the choke in. We had to limp back to the boat ramp both trips by playing with both the throttle and choke. It will intermittently run good and then back to crappy.

We took it to the shop:

Mechanic replaced broken reed valve
Rebuilt carbs
new plugs and wires
Tightened some bolts he said was previously loose around the intake
Told us to ditch the aluminum tank for a plastic one due to condensation

We bought a new plastic tank and took it to the river. The motor runs in the same condition as when we dropped it off with the mechanic.

Any thoughts?

My first guess would be to look at your coils. You've ruled out fuel as a problem by rebuilding/replacing just about everything, so it seems to me you have an electrical problem. Since the engine runs good while it's cool and then starts to act up when it warms up I think you have a cracked/bad coil somewhere. Of course, that's just an opinion and I don't have nearly the expertise of some of the other guys here, so wait and see what they have to say before you pull anything apart. (But if it were my engine that's where I would look first) In all cases I think your problem is electrical, not fuel related.

Curapa · #3 06-25-2011, 02:48 PM

Quote:

Originally Posted by Destroyer

My first guess would be to look at your coils. You've ruled out fuel as a problem by rebuilding/replacing just about everything, so it seems to me you have an electrical problem. Since the engine runs good while it's cool and then starts to act up when it warms up I think you have a cracked/bad coil somewhere. Of course, that's just an opinion and I don't have nearly the expertise of some of the other guys here, so wait and see what they have to say before you pull anything apart. (But if it were my engine that's where I would look first) In all cases I think your problem is electrical, not fuel related.

Thanks,

I actually had the opposite thought. Seeing as it will stay somewhat running with the choke held I figured it to still ba a fuel issue.

I'm not sure warming up helps or hurts, it seems to act up when WOT is reached and goes down hill from there.

Curt

tsubaki · #4 06-25-2011, 02:54 PM

When y'all swapped tanks, did you change to a different hose and bubble too?

Curapa · #5 06-25-2011, 09:46 PM

Quote:

Originally Posted by tsubaki

When y'all swapped tanks, did you change to a different hose and bubble too?

Hose and bubble stayed the same. Brand new only been used about 5 trips.

Destroyer · #6 06-26-2011, 02:40 AM

Like I said, you've just about fixed or replaced the entire fuel system.. that's why I went electrical.. and if you think about it... when you're on the muffs at home you're not really heating the coils up ... not stressing them the way you do when you're at WOT. I've seen many a coil that went south as soon as it got really hot.. and then seemed fine again when it cooled down.. Anyways, it's still my best guess...

tsubaki · #7 06-26-2011, 07:28 AM

We have a similar situation going on with a Mariner and have narrowed it down to the new fuel fitting on the tank or the new bubble. Fuel tank has been drained, cleaned, inspected, all the way up to the new outlet fitting. Vent too.
When we swap to a remote tank and different hose/bubble, the problem goes away.

On the 48 horse I'd be next thinking like Destroyer, look at changing the coils.

phatdaddy · #8 06-26-2011, 08:39 AM

probably a long shot, when it starts runing bad, does taking the cowling off affect it. had a old 7.5 honda that dirt daubers built a nest in the cowling vents and it was starving for air. didn't act up on the hose because i usually had the cowling off

Curapa · #9 06-26-2011, 06:54 PM

We are going to get a fuel pump rebuild kit from Burns tomorrow. That is the last part besides the fuel line and primer bulb that I can think of to replace or repair.

If no luck we will look at the electrical side.

Cowling on or off makes no difference.

Curapa · #10 06-28-2011, 03:27 PM

Just an update...

We disassembled the fuel pump and it was clean as can be. The diaphragm looked great so we put it back together. My buddy dropped the boat back at the shop today. We will see.