Compression and Spark Good....I think!?

[mtkinzel]

Model number/tag no longer on my motor. Bought used. With the help of others narrowed it down to a 1991 or so 25/30HP Johnson, tiller, manual start, short shaft. Recent issues. Running WOT it cut back one day to about 50-70% power. Limped back in. I had new ignition coils that matched and swapped them out. Did a compression check (100 and 105) and a spark test (Harbor Freight tool) and had the light firing on both cylinders with engine running.

TEST RUN.....WOT = 50-70% still.
In driveway on ears idles fine......backpressure under load , in gear = problem.

A.) Do I need to spark test again with tester (under load/in a barrel) and put in gear? Remove tester and just ground it to block and look for blue spark ?
B.) Should I carb clean and kit it? (Thinking: Compression, Spark, Fuel)
C.) Pull flywheel and check power pack?
Just not sure what my next diagnostic path is. I am a novice but can read.
Your advice is greatly appreciated. NO I HAVEN'T bought a manual as it was TODAY we determined the age range of motor! But it's in the works.
Thanks to all for responding.

 

[jbjennings]

Re: Compression and Spark Good....I think!?

It's possible that you could have something clogging up your high speed jet. I'm not sure about your spark tester. You need to know if the spark will jump at least a 1/4 inch air gap. If not, it's not good enough. When it starts acting up, you might try grounding out one of the plug wires (being careful not to get shocked) and seeing if each plug wire removal has an affect on the engine speed. If pulling one wire doesn't change how the engine runs, that cylinder isn't firing either due to lack of spark or water intrusion.
Good luck,
JBJ

 

[AlTn]

Re: Compression and Spark Good....I think!?

rule out spark problem first with an air gap spark tester set at 7/16""...the "light tester" will show the presence of a spark...not the quality or strength...air gap tester is ~$10 or so at most auto parts joints...if this test is good...I'd follow JBJ's advice...take an inductive timing light with you and test on the water at the speed your problem occurs...test each plug wire with the timing light for spark presence, consistent firing, and strength of flash

 

[Cofe]

Re: Compression and Spark Good....I think!?

Could be your high speed jet clogging.
Remove fuel line on front lower fuel bowl, the high speed jet is straight threw that fitting. Peel the plastic off a bread tie and poke it into the jet to clean it out. If this clears up the problem, plan on removing, cleaning, and rekit in the near future.
The recommendations given by the other posters are good also.....

 

[Solittle]

Re: Compression and Spark Good....I think!?

Possible overheat?

 

[mtkinzel]

Re: Compression and Spark Good....I think!?

Thanks for the input........Not certain is overheating due to condition happening right after take off, plenty of water through engine and out tell tale.

I bought today a "gap" spark tester and will set it and test it asap. I do not have a timing light (can't even say I know what one looks like or how to use it either). I can pull the wires while running to see how the motor responds.

I will also check the high speed jet and follow the bread tie advice.

Unfortunately I just got a call I am off to Florida at four a.m. due to a family illness. These tests will be completed upon my return. I will keep you informed.
Thanks again.