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Author Topic:   Runs Rough at Slow Speed

Alex15 posted 06-26-2005 11:02 PM ET (US)
I have a 97' Johnson 48 on my 87' 15' Sport CC. I bought the whole rig used, ran great for one season, trouble ever since. My mechanic, who has a good local reputation, seems to be able to tune the thing to run well at high speed, as it does now, or at low speed, but not both. It starts up ok, runs rough at idle and low speed, gradually smoother as speed increases. Is this a fuel mixture problem? Running too rich? Advice appreciated.
Baseline posted 06-27-2005 02:27 PM ET (US)
It sounds the ignition timing is not set or is not advancing correctly to me.
Most of the modern carburetors do not have adjustments to change the fuel mixture. To change the fuel mixture, you have to re-jet the carburetor. You can still have flooding and fuel starvation issues if the float is out of adjustment, but your symptoms do not sound like that is the issue.
The carburetors and ignition timing are related and mechanically linked. Normally the carburetors are set to assure that the butterflies are opening and closing fully. There are adjustments on the carburetors, linkage and throttle cables that can affect the carburetors setting. To set the ignition timing correctly, it needs to be done with the motor running and at full throttle under load. This motor needs to be fitted with a test wheel; a propeller that places a motor under load but doesn’t push the boat forward. The motor needs to be mounted on a test tank, or on a boat at a dock or on a trailer to run the motors at full speed. It sprays lots of water and makes lots of noise, but it’s the only way to get it right.
I have heard of more than one mechanic setting the timing, with the motor running on a hose with ear muffs; It doesn’t work, the motor has to be under load to get the correct setting.
Good luck
Alex15 posted 06-28-2005 11:07 PM ET (US)
Thanks, baseline. The most recent repair my mechanic did was to check the carbs during full throttle load against a dock. He took off the air hood and sprayed raw fuel into each carb. He diagnosed the lack of high speed power as a dirty carb, which he cleaned out, but seemed satisfied with the low speed roughness. I am not. I know this guy doesn't have a big test tank to run it at low speed under load. My carbs do have a fuel mixture adjustment, so I may try leaning it out a bit, see if it helps. Maybe I need a new mechanic.

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Author	Topic: Runs Rough at Slow Speed
Alex15	posted 06-26-2005 11:02 PM ET (US) I have a 97' Johnson 48 on my 87' 15' Sport CC. I bought the whole rig used, ran great for one season, trouble ever since. My mechanic, who has a good local reputation, seems to be able to tune the thing to run well at high speed, as it does now, or at low speed, but not both. It starts up ok, runs rough at idle and low speed, gradually smoother as speed increases. Is this a fuel mixture problem? Running too rich? Advice appreciated.
Baseline	posted 06-27-2005 02:27 PM ET (US) It sounds the ignition timing is not set or is not advancing correctly to me. Most of the modern carburetors do not have adjustments to change the fuel mixture. To change the fuel mixture, you have to re-jet the carburetor. You can still have flooding and fuel starvation issues if the float is out of adjustment, but your symptoms do not sound like that is the issue. The carburetors and ignition timing are related and mechanically linked. Normally the carburetors are set to assure that the butterflies are opening and closing fully. There are adjustments on the carburetors, linkage and throttle cables that can affect the carburetors setting. To set the ignition timing correctly, it needs to be done with the motor running and at full throttle under load. This motor needs to be fitted with a test wheel; a propeller that places a motor under load but doesn’t push the boat forward. The motor needs to be mounted on a test tank, or on a boat at a dock or on a trailer to run the motors at full speed. It sprays lots of water and makes lots of noise, but it’s the only way to get it right. I have heard of more than one mechanic setting the timing, with the motor running on a hose with ear muffs; It doesn’t work, the motor has to be under load to get the correct setting. Good luck
Alex15	posted 06-28-2005 11:07 PM ET (US) Thanks, baseline. The most recent repair my mechanic did was to check the carbs during full throttle load against a dock. He took off the air hood and sprayed raw fuel into each carb. He diagnosed the lack of high speed power as a dirty carb, which he cleaned out, but seemed satisfied with the low speed roughness. I am not. I know this guy doesn't have a big test tank to run it at low speed under load. My carbs do have a fuel mixture adjustment, so I may try leaning it out a bit, see if it helps. Maybe I need a new mechanic.