Question - Need some advice. bogging down at wot - 1975 Johnson 25hp s/s rope start

[Doubledame]

I've been diagnosing a bog down at wot for a while now and can't track it down. idle to about 1/2 throttle is good. Researched it thoroughly online and could use some advice. Not new to outboards and this one has me stumped. Lots of un-burned residue left behind

Here's what I've done that checks out OK. Carb rebuild (completely clean), fuel lines, compression (140 per cylinder), clean strong spark, new points and condensers - gapped correctly, rebuilt fuel pump, timing is right on with cam follower mark to carb roller, grounds are good including a kill switch test, dropped lower unit checked for blockage and ran without l/u and there was no change. checked the exhaust gasket at the power head and looks good, no air or vacuum leaks I could find, tested plug wires and bulb tested it running in case fuel delivery was still an issue. No binding in the armature plate or excessive play

Have not replaced coils yet...they're a bit pricey for this year and before I dropped the dough, I can't locate the specs online, but when I test the primary and secondary windings they both give the same 12.5K ohm reading. I un-screwed the plug wire from coils and tested the coil from the terminal inside and the primary wire to get my reading. Only thing I can find online indicates coil resistance should be between 1K ohm to .2K ohm and honestly I'm not sure if this is accurate. Not sure if these are out of spec.

any thoughts. Thanks a ton!

 

[racerone]

Answer this question.----Does spark jump a gap of 5/16" or more , yes or no ?------Have you run with a timing light hooked up to see if there are spark issues when throttling up ?-----Reed valves checked ?

 

[F_R]

Bogging down is kind of a vague term. Exactly what does it do at full throttle? Quit running? Keep running but lacks power, like 1/2 power? Race at high RPM but not go anywhere?

 

[Doubledame]

Answer this question.----Does spark jump a gap of 5/16" or more , yes or no ?------Have you run with a timing light hooked up to see if there are spark issues when throttling up ?-----Reed valves checked ?

I've checked the other stuff, but no timing light check. I'm not familiar with light testing on OB's. I could give it a shot.

 

[Doubledame]

Bogging down is kind of a vague term. Exactly what does it do at full throttle? Quit running? Keep running but lacks power, like 1/2 power? Race at high RPM but not go anywhere?

Here's more detail...First 1/4 throttle to 1/2 throttle under load seems about normal, but once you get over that it hesitates and stutters/misses until you back it down to half throttle or below again. It stays running the entire time and It does the same thing when in neutral or in a tank test... revs to about 1/2 throttle & starts to miss and stutter again. it's not backfiring, but is shuttering from missing

 

[racerone]

Does spark jump a gap of 5/16" or more.----You may not realize it but when you answer the questions we can help you with YOUR motor.

 

[F_R]

A spark plug can cause that. OR...are you sure it is missing? Is it a violent shaking / grinding? A totally worn out clutch dog in the lower unit can do that. You would be able to feel it in the shifter handle.

 

[Doubledame]

Does spark jump a gap of 5/16" or more.----You may not realize it but when you answer the questions we can help you with YOUR motor.

Sorry, I'll be more clear on answers. Yes, spark test - good. Light test - I hooked up the timing light today and found the timing hash on the flywheel remains mostly on the left side of the two hashes marks on the plate. The light consistency became a little erratic when I throttled up to the point where it shutters and sputters. I haven't pulled the reed valves...I'm not sure what I'm looking for in there.

Tested it again with new coils today and without the lower unit it initially was smooth and fast through to the top, but after installing the lower unit with some back pressure It was doing the same thing, but I noticed more popping than it did before. meaning... It would sometimes start to rev up and then pop and cough hard enough to kill. still idles fine, but the left over residue coming out of the lower unit is like black tar.

 

[F_R]

Now you are starting to get a bit scary. Are you telling us that you are doing all this out of the water? Revving up with no load is a no-no.

 

[Tim Frank]

You ran it without the lower unit? Why?

 

[Tim Frank]

Is this a new problem, or have you recently acquired this motor and it came with the problem?