25 horse johnson shudders when accelerating

[tfishingnut]

I have a 1975 25 horse johnson outboard that idles fine however it wants to vibrate really bad when accelerating from a stopped postion. could someone please give me some possible reasons for this rough running when accelerating. It seems to run ok once it has reach higher speeds.

 

[Bosunsmate]

Re: 25 horse johnson shudders when accelerating

does it vibrate in neutral?

 

[Bosunsmate]

Re: 25 horse johnson shudders when accelerating

Sometimes its a resonance between motor and the hull

 

[steelespike]

Re: 25 horse johnson shudders when accelerating

Its possible that the low speed needle valve is set too rich.
Should be open one turn from lightly seated,final mixture should be set idling(650) in forward gear.
Once you have smoothest setting turn very slightly toward rich,so to be sure its not too lean.
Also there may be a crankcase drain that is routed to the intake so it doesn't dump into the river.
If it has the drain it is said to cause rough idle.The drain may cause less trouble if your sure the idle mixture and speed
are set right.

 

[tfishingnut]

Re: 25 horse johnson shudders when accelerating

does it vibrate in neutral?

I have played around with the idle screw and got it to idle smooth, mind you I do not have a way to check rpms but just get it to where it sounds right, but now it doesn't want to accelerate without shuddering real bad. Seems like when it doesn't idle good then it will accelerate good. I am sure there is a lot going on with this old motor but was just wandering if it may be an issue with the timing or idle adjustment. I know I am supposed to check while under a load but shouldn't I atleast be able to find a happy median between idle and throtling up without having to adjust it under load.

 

[steelespike]

Re: 25 horse johnson shudders when accelerating

I think there is a timing stop that keeps the stator from advancing too far and of course the cam on the stator needs to pick up the carb linkage at the right point.(lync n sync).
If the fuel pump diaphram is leaking it could be flooding the crankcase.
If it does have a crankcase drain if it's plugged that could be flooding the crank case.