What else? - rough idle

[jdsgrog]

I have a '67 Johnson 33hp motor, model RXL-15 (maybe RXE, can't quite remember). First, the motor does run and starts fine when the throttle is up halfway, BUT when I put the throttle all the way down, it shakes, sputters and dies. However, if I idle it just a bit faster, it stays running. I've tried everything, and can't keep the motor running on a low idle. Here is what I have done and checked:

1. Compression on both cylinders good at 100psi each
2. Both cylinders have spark and are definitely firing up after pulling plugs and checking them.
3. Carb has been cleaned inside and out. Soaked in carb cleaner for a couple of days. Ran compressed air through all the jets and "nooks and crannies." Rebuilt the carb.
4. Tuned up and synchronized. New points, condensers, and coils. Points gapped at .020. Carb linkage synced as per shop manual.
5. Adjusted needle valves as per FAQ in this forum.

Here are a couple of observations:
1. When pulling the plugs, they seem very slightly wet. The plug electrodes are a blackish color. The electrodes seem relatively dry. Using J4C's.
2. When I increase the throttle to a higher idle (a little less than center), the motor runs pretty smoothly. When I lower the throttle to 1/4, it shakes quite a bit, but stays running. When I put the throttle all the way down, it sputters and dies.

Any idea what might be going on? I'm thinking on changing the ignition wires as they are a bit worn. I've only ran the motor on muffs as the lake is over an hour away. Also, I don't have a tach, so I don't know what RPM it's at when it stays running. Plan to get a tach soon. Any other suggestions would be helpful.

Thanks in advance.

 

[KenMcG]

Re: What else? - rough idle

I know that this is going to sound stupidly simple. I had the same situation. The minimum, all the way down, throttle limit screw has mushroomed with age. Crank it in some, so that when you turn the throttle all the way down, the engine idles reasonably without dying. Play back and forth with this screw and the idle mixture screw until you cannot kill the engine by going minimum throttle. Good Luck! Ken

 

[KenMcG]

Re: What else? - rough idle

On my engine, this screw is at the complete rear of the tiller assy on the side of the engine case.

 

[tashasdaddy]

Re: What else? - rough idle

does the cam follower roller have a groove cut into it, from years of use? this throws link n sinc off.

 

[jdsgrog]

Re: What else? - rough idle

Thanks for the replies. I've already messed around with the throttle limit screw. I'm trying to get the idle down to a normal idle speed, which I have read in other posts to be around 750 rpm. Now I don't know how fast the idle is right now, but it sounds faster than what it used to idle at. My guess is about 1000-1200 rpm

The cam roller looks to be all right. One thing that I noticed the other day was that when I try to initially start the motor when it has sit for a bit, it seems to backfire or cough once. But after the initial turnover of the flywheel, I don't get this problem. Would this help diagnose this problem?

 

[Randybeall]

Re: What else? - rough idle

It is not common but you may have a reed problem. The back fire indicates either excessive lean mixture or reed not sealing. When they do not seal properly idle is the first problem, at higher engine speeds mixture inertia helps to hide the problem. Check the carb base gaskets and even go through the idle circuits in the carbs again. That failing you must look further into the intake.

 

[jdsgrog]

Re: What else? - rough idle

"Idle circuits"? What do you mean by this? Are these the needle valves? Concerning the base gaskets, I'm assuming you are talking about the gasket where the carb connects to the powerhead. This gasket is new. Actually all carb gaskets are new when I rebuilt the carb. If it's a reed problem, would I need to replace them?

 

[KenMcG]

Re: What else? - rough idle

I have a '67 Johnson 33hp motor, model RXL-15 (maybe RXE, can't quite remember). First, the motor does run and starts fine when the throttle is up halfway, BUT when I put the throttle all the way down, it shakes, sputters and dies. However, if I idle it just a bit faster, it stays running. I've tried everything, and can't keep the motor running on a low idle. Here is what I have done and checked:

1. Compression on both cylinders good at 100psi each
2. Both cylinders have spark and are definitely firing up after pulling plugs and checking them.
3. Carb has been cleaned inside and out. Soaked in carb cleaner for a couple of days. Ran compressed air through all the jets and "nooks and crannies." Rebuilt the carb.
4. Tuned up and synchronized. New points, condensers, and coils. Points gapped at .020. Carb linkage synced as per shop manual.
5. Adjusted needle valves as per FAQ in this forum.

Here are a couple of observations:
1. When pulling the plugs, they seem very slightly wet. The plug electrodes are a blackish color. The electrodes seem relatively dry. Using J4C's.
2. When I increase the throttle to a higher idle (a little less than center), the motor runs pretty smoothly. When I lower the throttle to 1/4, it shakes quite a bit, but stays running. When I put the throttle all the way down, it sputters and dies.

Any idea what might be going on? I'm thinking on changing the ignition wires as they are a bit worn. I've only ran the motor on muffs as the lake is over an hour away. Also, I don't have a tach, so I don't know what RPM it's at when it stays running. Plan to get a tach soon. Any other suggestions would be helpful.

Thanks in advance.

One other thing to consider, and I am talking from personal experience: could you take the spark plugs out and see if the engine spins clockwise smoothly by hand? You never want to rotate the engine in reverse direction and stress the water pump impeller fins.

My point is this: I went through my engine everything to try for smooth idle and then the lower unit threw a gear through the side of the case right behind propeller without warning. I had drained and changed the lower unit oil every year.

Just some food for thought. This may not be your problem at all.

 

[jdsgrog]

Re: What else? - rough idle

Thanks KenMcG for the thoughts, but the motor turns fine with the plugs out.

Wondering if I might need to decarb the motor. Never did that with the motor and I'm curious if carbon build up would cause difficulty in the idle?

I've ordered the tach and will see what RPM it stays running. Also, I'm waiting on new plugs and wires (Can't find J4c's anywhere here in Chicago).

Let me know if there is any other thoughts.

 

[matt9923]

Re: What else? - rough idle

Hi, i just got my motor going and it does the same thing. I have it in a barrel maybe its not warm? I don't know what it is either. I played with the low speed needle but no luck once you let down the throttle it dies and shakes.
Let me know if you figure it out because im very eager to get on the water even know they drained the river for a few weeks right when i got it running.
Good luck Matt

 

[jdsgrog]

Re: What else? - rough idle

reply is a little late, but i did find the issue. it was pretty much the timing of the motor. I adjusted the cam roller arm. Needs to be adjusted just right. The motor idled down fine after that.