Outboard Wont Run With Cowling On

[blacky]

G'Day raiders ,

I have recently bought a boat with a 1967 70hp johnson seahorse outboard . I have had it checked out by a

mechanic and all seems to be good . Great compression in all 3 cyl, fuel pump is good, plenty of spark. But it

only runs well with the cowling off.It idles with the cowling on but when you increase the revs it wont rev

more than 250rpm. I have had the carbies serviced as well. ( air box is clean ) .

Any ideas would be greatly appreciated !!

Many thanks raiders and a Merry Christmas to you all !!!

Blacky

[austral]

G'Day raiders ,

I have recently bought a boat with a 1967 70hp johnson seahorse outboard . I have had it checked out by a

mechanic and all seems to be good . Great compression in all 3 cyl, fuel pump is good, plenty of spark. But it

only runs well with the cowling off.It idles with the cowling on but when you increase the revs it wont rev

more than 250rpm. I have had the carbies serviced as well. ( air box is clean ) .

Any ideas would be greatly appreciated !!

Many thanks raiders and a Merry Christmas to you all !!!

Blacky

just leave the coweling off.. seriously..

[mudcrab]

I had the same issue with a '70's model johnno years ago.

It was traced back to a small hole in the exhaust pipe about 1/3 the way down the leg.

Motor ran fine at the mechanics (cowl allways off)., then like a dog everywhere else. Take the cowl off - she'd go again. It was oxygen starvation from the motor trying to eat it's own exhaust fumes.

I dare say you won't get the leg off that model engine so 'keyhole' surgery may be required (cut hole in lower cowl/leg casing to gain access then patch later.

If you can drop the leg, have a look for pinholes near where the exhaust pipes join.

Hope this is of some use.

Edited December 22, 2009 by mudcrab

[a boat]

Hi, as mentioned you have to be certain that the engine is not breathing its own exhaust fumes and not hard to see if this is happening. Also be certain that the engine cowl is not shorting one of the spark plug leads when put back on.

Cheers,

Huey.