1967 Evinrude Starflight-S 100 HP under load problem

Hello All, new to the forum and looking for an outboard expert. I have a 1967 Evinrude Starflight-S 100 HP on a 1969 Sidewinder hull. I am having an issue with it not wanting to run correctly under load. The motor will rev up to 5500 RPM in neutral, even after it won't run under load. It starts and idles find. Put it in gear and the problems start (both forward and reverse work fine). A few weeks ago when you hammered the throttle it would run up to 5500 RPM and 32 MPH. Now it is bogging down and when you first put it in will only go to 2000 RPM. After it runs a while it will slowly (very slowly) start to increase and make it up to about 4500 RPM and 25 MPH but go no faster. It will also cut out or drop the RPM down at times or surge up at other times. I did put in the wrong lower unit lube but drained it and put in the OMC Type C it calls for now. I thought that was my problem but it seems like it's not. This engine sat for 25 years and I rebuilt it: head gaskets, carbs, fuel pump, stater, fuel and water hoses were all replaced or rebuilt. This motor has never shown this type of "laying down" before as it used to be my uncles.

Any help or ideas would be greatly appreciated.

For starters do a compression test. Then do a cylinder drop test on it to see if it's running on all cylinders.

The compression was 105-110 for all 4 cylinders. I did check spark on all 4 cylinders and had good spark in all 4. How do you do a drop test?

Run the motor at idle and remove one plug lead at a time.-------Report on if any sound changes or not.---------If no change when pulling a lead then that cylinder is not working.

I'll hook up my batwing and try that.

Finally got around to do the drop test. Two cylinders are for sure running as it almost dies when you pull the plug wires. One starts to drop RPM but not as much effect and the forth doesn't seem to do anything to it. Where do I go from here. I know the cylinder is getting spark so that only leaves full and air but the plug was wet. Any suggestions?

Does the primer bulb go hard ???

How did you check your spark ? If it was just with the plug that won't flush. Spark should jump a 1/4 to 3/8 " open air gap on a tester.

It seems to have a hard primer bulb yes.

Slowly increasing the rpm points at a fuel problem.