2004 Johnson 50 hp (J50PLSRS) Idle Speed

[Greecy]

After rebuilding the carbs and setting up the linkage and synchronization as per the service manual, the motor idles at 1,400 rpm with the throttle plates fully closed. The idle speed is supposed to be 850 rpm. The low speed timing is 1 deg after TDC.

According to the service manual, the initial setting for the low speed mixture screws is 3 1/2 turns. The engine will start but dies when the primer is shut off. When increased to 4 1/2 turns the engine starts and stays at high idle. The mixture on each carb is adjusted for max rpm then backed off slightly. This was done to each carb sequentially until the idle was as smooth as possible. There is still an erratic miss at idle which was present before rebuilding the carbs.

I don't have a test wheel and I have not put the boat in the water to see how it idles in forward gear. While fixing some other small issues, I ran a compression test and both cylinders are at 105 psi +/- 5 psi. I also put a new OMC OMS pump on.

My induction tac and the tac on my instrument panel agree so this is not a measurement problem.

I did not replace the primer solenoid. If it was leaking into the carbs, I would expect to need to reduce the flow through the low speed jets instead of increasing it (4 1/2 turns instead of 3 1/2).

Any ideas of why the idle is so high?

 

[racerone]

Retard the timing !

 

[oldboat1]

doesn't seem wildly high if running on muffs.

 

[Greecy]

Retard the timing !

Service manual stipulates 1 deg +/- 1 deg ATDC but that is at 800 rpm. I am checking the timing at 1,400 rpm. I re-read the Service Manual and it requires the measurement be made at 800 rpm in forward gear. Maybe 1,400 without a load is about right. Taking it out for a trial on the water later today and will tweak the timing and mixtures.

Thanks to both of you for your help.

 

[racerone]

But----Only the maximum timing advance is set.-----Idle timing is the point where throttle plates start to OPEN , so it is a linkage adjustment.-----Where the timing is with engine idling smoothly does not matter.-----Sorry , need to read and understand this concept.------Timing is not the same on these as with an older 350 Chevy !

 

[Greecy]

But----Only the maximum timing advance is set.-----Idle timing is the point where throttle plates start to OPEN , so it is a linkage adjustment.-----Where the timing is with engine idling smoothly does not matter.-----Sorry , need to read and understand this concept.------Timing is not the same on these as with an older 350 Chevy !

Understood. Screwing the idle adjustment screw in or out changes the low end timing without impacting the high end timing Changing the length of the linkage arm will change low and high speed timing. I set the high end by pulling off the plug wires, connecting a spark tester to number one wire, moving the control to WOT and turning the engine over. I adjusted the linkage arm to a length that gave me 15 deg BTDC. The spec at 5,000 rpm under load is 19 deg. I will test the high end on the water to confirm that the last 4 deg is added by the power pack.

I go back to small block Chevys that were 265s and 283s (kickin the crap out of Y block Ford engines). Now I'm totally puzzled when I open the hood on my F150. What does all that stuff do??? Showing my age here.

 

[Greecy]

Results are in. Idle rpm drops to 950 in gear while under way. I retarded it another degree which took it down to 850. There is still a random misfire at idle but it is less frequent. At 850, the misfire is enough to kill the motor now and then. I can live with that but it is annoying.

I was able to do a simple test on the primer solenoid. Pushed the key in while underway at 3000 rpm and it dropped the rpm to around 2000. This confirms that it is injecting a fuel-oil mix into the carbs.

Now I can focus on fishing instead of wrenching.