1974 Johnson 40 Won't Rev Above 1500-2000 ;(

[SC64]

I am restoring a 1959 Cherokee Aluminum - weighs all of 300 pounds - maybe. Purchased a 1974 Johnson 40 Twin that was rarely used - I would say it appears to have less than 50 hours on it if that. When it was last operated was unknown - but it looks brand new.

- I did not actually check the compression other than to say trying the rope start qualitatively said compression was good if not great.
- Checked spark - fine
- I did buy a new impeller kit (installed) - and re oiled the lower unit with new seals etc
- I did buy a carb kit but decided to see if it would start.

Motor started right up...idled well...so I put it on the boat and in the water. First run around, starts, idles...but won't go above 1800 -2000 rpm (guessing). All the linkages seem to work well - so I figure the carb needs rebuild. I take the carb off, clean, new float, new gaskets, clean, set low speed idle...not much to these things...put back on....and exact same performance - except the low speed idle went from good to perfect - thing idles very smoothly - and starts at first crank without a choke.

have rebuilt carbs before so pretty sure I am not missing anything (also have shop manual).

The gentlemen I bough it from is a member of the classic outboard society and lives across the country - but we have been trading texts - he believes it is the high speed jet (not getting gas). The high speed jet couldn't be simpler in these carbs and it is clean as is the little brass insert at the bottom of the bowl that feeds the jet.

The motor doesn't sound like it is "bogging" - just gets to 1/3 speed and that is it. The linkages are good and the throttle opens 100 percent.

Timing? - I gotta belive it is something simple....

 

[oldboat1]

running on both cylinders? Do a drop test -- or hook up a timing gun and check for flashing/no flashing. no flash = no spark.

 

[jimmbo]

Just because the jet looks clean doesn't mean the passages downstream of it are not plugged with varnish/gum/dirt.
Does the engine respond if the choke is temporarily closed, then partially opened?
A dead cylinder comes to mind. Have you done a compression test? Coil under the flywheel in good condition? Points, condition and are both opening and closing?

 

[SC64]

Just because the jet looks clean doesn't mean the passages downstream of it are not plugged with varnish/gum/dirt.
Does the engine respond if the choke is temporarily closed, then partially opened?
A dead cylinder comes to mind. Have you done a compression test? Coil under the flywheel in good condition? Points, condition and are both opening and closing?

Thanks - AFA carb dirty - there just isn't anything to these carbs - low and high speed and they are clean...

I will fiddle with choke and see what happens
By dead cylinder do you me no compression or no spark?

If there was no spark in one cylinder - wouldn't it run like crap - especially at idle? Runs well at idle...

Will check spark and compression in both cylinders....

 

[jimmbo]

"Thanks - AFA carb dirty - there just isn't anything to these carbs - low and high speed and they are clean.."

All passages clean? Gasket/Oring to seal float bowl where main nozzle passes thru?.

"I will fiddle with choke and see what happens
By dead cylinder do you me no compression or no spark?"

Either

"If there was no spark in one cylinder - wouldn't it run like crap - especially at idle? Runs well at idle..."

Doesn't mean both cylinders are still firing at speeds above idle

"Will check spark and compression in both cylinders...."

With the plugs out check for spark with the timing advancing