72 johnson runs better with no cowl

[FIRE34]

I have a 1972 johnson 20 hp. It runs about 17 mph with the cowl on at wot. And 23 with the cowl off at wot. i know that it is obviously sucking in its own exhaust fumes. Just wanted to know how to fix it. The seal that seats the cowl to the motor is good.

 

[kbait]

Re: 72 johnson runs better with no cowl

Never heard of that happening w/that motor. Can you see any leakage from the powerhead base gasket? It would have to be leaking some serious exhaust to lose 5mph.
Somewhere under the carb there should be a pretty large opening for intake air.. is it covered over or something?

Does your cowl weigh 92lbs and you left it on the dock for the test? haha

The cover itself may be pushing on a plug wire or pinching a fuel line, or in the way of carb linkage keeping it from opening all the way? Just guessing..

 

[FIRE34]

Re: 72 johnson runs better with no cowl

lol. nothing leaks or is being touched. im just going to cut a huge hole in the front for a vent. lol. jk.

 

[samo_ott]

Re: 72 johnson runs better with no cowl

Check the exhaust housing gasket but I also have not heard f it on that engine.

 

[kenmyfam]

Re: 72 johnson runs better with no cowl

Plug wires could be grounding out on the cowl. Run it in the datk with the cowl off and see if you get a light show. If so replace plug wires.

 

[samo_ott]

Re: 72 johnson runs better with no cowl

Plug wires could be grounding out on the cowl. Run it in the datk with the cowl off and see if you get a light show. If so replace plug wires.

They'd have to be grounding out with each other as the cowling is fibreglass.

 

[kenmyfam]

Re: 72 johnson runs better with no cowl

They'd have to be grounding out with each other as the cowling is fibreglass.

They can ground out in damp conditions though if there is an insulation issue with them.