HELP! 1979 115 HP Johnson fuel issue???

[Iowaboatguy]

Motor is a 1979 115 hp Johnson. The problem I'm having is that when I have it on the muffs it idles very strong, shifts into lowest gear perfect and then throttles all the way up perfect as well. Runs like a champ on muffs.
When I put it in the water, it doesn't idle as high, almost sounds like it wants to die. If it happens to stay in idle, once I shift into drive, it dies. The only way I can get it to run is to have the fast idle lever all the way up then slowly ease it down 3/4 of the way and then slam it into full throttle all the way and put the fast idle lever down and drive that way. If I slow down to lowest drive speed it neutral, it eventually dies.
Here's the catch, I've done alot to trouble shoot:
Cleaned carburetor, repalace all 4 spark plugs, replaced the gas tank, replaced the gas line twice, replaced battery, replaced all the hoses from the intake/fuel pump/fuel filter, checked the fuel pump and filter, manually increased the idle screw to maximum.
I don't know what the problem is to save my life. It runs fine if I slam it in full throttle, but anything lower and it dies.
Any ideas?

 

[interalian]

Your idle speed is set too low. On muffs there's no exhaust backpressure so the idle is higher. Idle is set in gear, tied to dock - about 600rpm. On muffs it would be around 1200.

Setting idle requires the throttle cable be removed and then run engine, set idle using screw on mid/upper face of throttle arm. When properly adjusted, fit the throttle cable again, adjusting the trunnion so the cable pulls the throttle back to the new idle stop position with a little tension.

And don't rev the motor on muffs - bad things can happen.

 

[Iowaboatguy]

That makes sense on the difference then. Any opinions on what my problem might be?

 

[interalian]

I was editing my post while you replied. Look up to post 2.

 

[interalian]

Other questions:

Is this a 'new to you' motor? Have you done the basic compression and spark tests? Spark should jump 7/16" when cranking with a fat blue spark.

 

[racerone]

They all run " great " on a hose.----Make lots of noise and no work for the propeller to do.----Even in gear on the trailer it is really still in neutral.----Will sound good running on 3 of 4 cylinders.----Do the compression test and post the numbers.