1995 28hp Evinrude - Engine Surging

[mrdopes]

Hello all, my apologies if the post is long.... as always, i appreciate all your help on here.

Looking for input on proper troubleshooting procedures/steps, so that i don't overlook something. I was running great at full throttle (caught a lot of fish!), then RPMs dropped and engine started to surge..... I don't have a tach, but it would drop down to 5-8 mph, then go up to full speed again, quickly, then back down again. It never stopped running or died out. I turned it off and used my kicker to get back to the dock.

This is what I've looked at so far -

1) Fuel lines are all good; even changed the line from the tank to the engine with a spare one i had.
2) Fuel vent and pick-up tube are both clear.
2) Compression is at 115 and 118.
3) Changed the fuel pump/filter with a new spare i kept on hand (also tested with vacuum gauge, was good)
4) Checked plug gap (was good, cleaned plugs and reinstalled)
5) Checked that spark jumped 7/16th gap on my adjusted tester (was good with a crisp snap)

Took it back out on the water, same issue...I sprayed some carb cleaner in the carb while is surged, but it didn't make a difference. i also noticed that it would slightly backfire from the exhaust, something it's never done before. So, i pulled one plug wire, and noticed a stumble when running on the bottom cylinder, and when i pulled the bottom, the top cylinder ran smooth. At this point, i turned the engine off, and restarted. When i restarted, the overheat alarm went off, but the telltale was pissing like normal, and head was not too hot that i could not touch it. Brought it back home....

Retested compression, it is still good. Started it in a water drum and the overheat alarm did not go off again.

I'm at loss as to where to go with it now....

In my parts bin, I've got a new coil pack, thermostat kit, and carb rebuild kit. I was thinking about buying a powerpack just in case, but i thought i would ask for some seasoned help before buying too many new parts, or replace parts, i may not need to replace/buy...

Any ideas?

 

[racerone]

You asked.------Use a timing light at full throttle and observe spark behavior when motor falters.----" back firing "------ to me that suggests checking the flywheel key.-----Check fuel pump diaphragm.-----Total money spent doing these inspections is ---ZERO $

 

[oldboat1]

^^Remove the flywheel to check the flywheel key as suggested (really to see if the flywheel location slipped on the crankshaft, probably shearing the key). Suspect torque was too light if this happened.

also could have been running low on gas, and/or the pickup was pulling some water from the bottom of the tank.

Check the linkages around the throttle lever assembly, throttle control rod adjustment slipping, etc.

 

[mrdopes]

oldboat1

"You asked.------Use a timing light at full throttle and observe spark behavior when motor falters.----" back firing "------ to me that suggests checking the flywheel key.-----Check fuel pump diaphragm.-----Total money spent doing these inspections is ---ZERO $"

Done. Fuel pump and diaphragm are good. Checked spark with a timing light, and there is definitely a miss on the bottom cylinder. I've got some photos of the key here for you to look at too, it looked "okay" to me. One question, when the spark jumps the 7/16th gap on the tester, should the entire spark be blue? It is blue at the tip, but becomes white and not as bright.

oldboat1

"Remove the flywheel to check the flywheel key as suggested (really to see if the flywheel location slipped on the crankshaft, probably shearing the key). Suspect torque was too light if this happened."

The key looked okay to me, i will post some pictures here for you to look at and tell me if i should change it out.

"also could have been running low on gas, and/or the pickup was pulling some water from the bottom of the tank"

I ran it off of a different tank today to be sure, same issue.

"Check the linkages around the throttle lever assembly, throttle control rod adjustment slipping, etc."

this all looked good to me, but take a look at this cam roller/follower thinga-ma-jig. I saw another thread somewhere where someone said it should be 1/4-1/2 inch away after the 2 timing points. Is this too far off? thank you both for your help with this.

https://photos.app.goo.gl/trEoCbHx5JzRAMR4A

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[Bosunsmate]

sounds like either timing base or powerpack, you can test the timing base/trigger basically with a cheap multimeter.
Id suspect powerpack as lead culprit

 

[Bosunsmate]

Hard to tell if the key has something partially left in the keyway on the flywheel.
It could be ok, some a bigger or smaller than others. Essentially its only there to line things up so so long as it assists with that its done its intended job

 

[mrdopes]

Bosunsmate

Thank you, sir. I will check those two things next!