I purchased a 1973 18hp Evinrude in good physical shape a few months ago. The motor had a rebuilt carb and water pump. The motor started but hesitated on acceleration. After a few seconds at more than half throttle things clear up and the motor runs great with lots of power. Alot of unburned oil and carbon was coming out of the exhaust at lower speeds though. I did the following to the motor:
- Set carb float height
- Replaced points and condensers
- Set timing
- Synchronized timing advance and carb
- Replaced plugs
- Replaced sensor coil
- Replaced 1 of the 2 high tension coils (they seem OK)
- Replaced thermostat
- Performed decarb procedure
- Resealed lower unit
After the above was done and running on OMC oil and premium gas the motor runs much smoother but the hesitation on acceleration persists. The new thermostat seems to have solved the oil and carbon coming out of the exhaust so I think the motor was running too cool. The exhaust outlet water is hot to the touch and a infrared thermometer shows about 150 degrees on the cylinder head. After using the motor a bit I observe that when running half throttle or more the hesitation goes away after a few seconds and the motor runs great. When slowing down below half throttle the motor idles down nicely but after 30 seconds it sounds like it is missing and it gets more smoky. If I speed up again it hesitates for a few seconds and then clears out. The motor starts with a 1/4 pull when warm but when cold it will start when choked but it always stalls and requires several pulls to get going. My next thought was rings so I got a compression tester and each cylinder shows 105 psi. If I put a teaspoon of oil in and retest the results are 120 psi on each cylinder. If I peer into the cylinder through the spark plug holes I observe some discolorization on the cylinder walls by the exhaust ports but it is hard to tell if it is scoring. The motor is in good shape, after a touch up paint job it looks great. From looking at the wear on the driveshaft, propshaft and water pump housing I believe it has not been abused but has hours typical of a 1973 motor.
I purchased the service manual and went through all troubleshooting procedures. My next step would be to pull the head and consider replacing rings and maybe reboring the block. Some questions:
- Does the compression test and motor behavior suggest worn or stuck rings?
- Any other suggestions for the hesitation and missing behavior below half throttle?
I only paid $450 for the motor so I would consider investing a few hundred dollars in it. It has been a fun learning project so far. Thanks for any advice.[b]