RPM drops on 2hp 1977 Evinrude

[Plankwalker]

The carb was just fully serviced and the water impeller replaced. Gas tank was cleaned and fuel line replaced. Filter was rinsed with solvent. It starts fine and after a bit of warm-up, I adjust the high-speed needle and the engine is running fine at top speed. Then, after only a few minutes, the RPM drop. Further adjustment of the high speed needle do nothing. The engine will continue to run without problem, just with lower RPM. I check the vent and it is fully open.
I am suspecting it is the float that needs adjustment to allow for more fuel in the bowl.
Any ideas?

 

[Crosbyman]

remove the fuel cap and retry if ok recheck venting in the cap
seems affected by heat possibly have you service the electricals ?
points condensers plug wires etc...
measure secondary resistance ( between ground and inner clip in the boot)

report back the value.. should be 4-7 kohms

if not twist turn the plug wire to see it you get resistance values changing from OPEN to 4Kohms or more ... if yes... bad wire needs replacement

use proper oil fuel mix for you engine

did you clean and blow the central HS post in the carb and install the fat gasket at the bottom of the carb ?

grease up the HS needle shaft nut as a temporary test to block any chance of air being sucked in along the HS needle . If ok repack the HS needle

 

[Plankwalker]

remove the fuel cap and retry if ok recheck venting in the cap
seems affected by heat possibly have you service the electricals ?
points condensers plug wires etc...
measure secondary resistance ( between ground and inner clip in the boot)

report back the value.. should be 4-7 kohms

if not twist turn the plug wire to see it you get resistance values changing from OPEN to 4Kohms or more ... if yes... bad wire needs replacement

use proper oil fuel mix for you engine

did you clean and blow the central HS post in the carb and install the fat gasket at the bottom of the carb ?

grease up the HS needle shaft nut as a temporary test to block any chance of air being sucked in along the HS needle . If ok repack the HS needle

Some things are done such as a full cleaning of the carb and proper gaskets (the fat at the bottom of the post) are in place. Fuel is brand new, high octane, no ethanol, with proper oil mixture. I will try removing the cap to see if it is my vent hole closing up, but I doubt it.

I will try other options you mention, but it is tough to test in the bucket at high RPM for extended periods and field testing is about once or twice a week.

Thanks for the ideas,

Paul

 

[racerone]

Possible filter screen is partially plugged on tank valve assembly.----Pull the flywheel and inspect the magneto.----Possibly a cracked coil.

 

[Plankwalker]

Possible filter screen is partially plugged on tank valve assembly.----Pull the flywheel and inspect the magneto.----Possibly a cracked coil.

Filter screen is clean as a whistle. Coil is intact. I do a test run tomorrow morning. I did a test in the tank a few days ago. I suspect the only thing wrong was the loose nut on the handle.

 

[Plankwalker]

Most of the problem turned out to be fuel needle adjustment. As the engine warms up, it likes to run really lean, but then starts to starve. I just needed to back off the needle half a turn and it seemed to run fine.