I have a freshly restored 1975 Merc 402 40hp Thuderbolt. I ran it on the water for two days no problem but the first days were short 10 min ride runs.
On the 3rd day out it started missing but only after running more than 10 minutes. Shut it down for an hour and go back out and it runs great for 5-10 minutes and then starts missing and running rough. Sometimes so rough I cant get on plane, other times not as bad but still running terrible.
If I cruise along at 1/3rd throttle or less it seems to run fine except for the occasional light surge in rpm about 100-200 rpm. If I can get it on plane and get up to near full power it runs smoother but still misses.
These things combined make me think that the carb is set to give it too much fuel. What I am thinking is that when the engine is cold the more dense air burns the fuel completely but when the engine warms up and the the hot=less dense air is being used it cant burn all the fuel completely. Thats my theory at least. I just need to find out how to adjust the carb so I can try it. I did not do the restoration myself and the guy that did the work probably tuned it without ever getting the motor up to full temp which is why it runs great cold.
I have drained and put in new fuel and the motor has new hoses throughout and new wiring etc. I have tried running with slightly more and slightly less spark advance and both seemed to make the engine run worse. So I think that leaves it to the ignition and fuel. Since it runs great cold I doubt its the ignition.
Whatcha guys think? Thanks.
Chris
www.vupilot.blogspot.com
On the 3rd day out it started missing but only after running more than 10 minutes. Shut it down for an hour and go back out and it runs great for 5-10 minutes and then starts missing and running rough. Sometimes so rough I cant get on plane, other times not as bad but still running terrible.
If I cruise along at 1/3rd throttle or less it seems to run fine except for the occasional light surge in rpm about 100-200 rpm. If I can get it on plane and get up to near full power it runs smoother but still misses.
These things combined make me think that the carb is set to give it too much fuel. What I am thinking is that when the engine is cold the more dense air burns the fuel completely but when the engine warms up and the the hot=less dense air is being used it cant burn all the fuel completely. Thats my theory at least. I just need to find out how to adjust the carb so I can try it. I did not do the restoration myself and the guy that did the work probably tuned it without ever getting the motor up to full temp which is why it runs great cold.
I have drained and put in new fuel and the motor has new hoses throughout and new wiring etc. I have tried running with slightly more and slightly less spark advance and both seemed to make the engine run worse. So I think that leaves it to the ignition and fuel. Since it runs great cold I doubt its the ignition.
Whatcha guys think? Thanks.
Chris
www.vupilot.blogspot.com