1976(?) Johnson 55hp - problems at full throttle. (With Video! :D)

[Aces_Up08093]

I posted about this issue before, but there was a forum problem and it was deleted along with a bunch of other folk's posts. Someone had given me some advice, but I don't remember what it was, so I'm reposting in hopes I can get a starting point to diagnose this issue and hopefully fix it.

I believe I have a 76 johnson 55hp - I'm not sure because there is no visible serial number I can find on the motor. To my knowledge the carbs have no high-speed adjustment, only idle needles. When I first purchased the boat I was having problems with it stalling at idle when shifting. It ran great at idle, but as soon as you shifted it would die. I rectified this by turning the idle up a little and adjusting the idle valves until it ran perfect at idle while in gear.

Now, it runs great at idle and mid-throttle (up to about 80%). When I try to crank it up to full throttle it starts to randomly lose power and seem as if its stuttering. The longer I keep it there, it seems the issue gets worse, but as soon as I bring it back to about 80% throttle or less, it runs perfect again.

I've uploaded the video to my Facebook, here's the link: https://www.facebook.com/photo.php?v=678984482175548&set=vb.100001920080448&type=3

It's just a short video I took of the problem with audio so maybe someone can advise me based on actually hearing what it's doing for themselves.

Thanks for any advice

BTW, What I've already done:

Cleaned the fuel filter out.
Checked for good fuel pressure (The bulb is always hard)
replaced the plugs
checked compression (140 on both)

 

[Aces_Up08093]

bump

 

[hardwater fisherman]

maybe you should try cleaning and rebuilding the carbs. from the look of your motor in the video i think it is more likely an early 70s model.

 

[M Strawn]

Have you checked to see that the carb butterfly is not past the horizontal position when the binnacle is fully advanced? Could just be a cable adjustment needed.

 

[BonairII]

Have you checked the quality of spark you're getting on both cylinders? New plugs doesn't mean good spark. Coils may be weak and/or misfiring. Make sure your timing is advancing all the way(not getting stuck) when you throttle up.

 

[Aces_Up08093]

Have you checked the quality of spark you're getting on both cylinders? New plugs doesn't mean good spark. Coils may be weak and/or misfiring. Make sure your timing is advancing all the way(not getting stuck) when you throttle up.

I've verified it to be a 76. The model number is 55el76e.

I have not checked the quality of spark, mostly because I don't know how I'd go about doing that. How big of a gap should it be capable of jumping?

Also, I honestly think it is a spark advance issue. I've been searching around for a guide to setting the advance / timing, but I've come up empty. Could you maybe point me to a benchmark for setting correct timing / advance?

 

[racerone]

Timing normally does not need adjustment for the life of the motor.-It just does not go out of adjustment except for a sheared flywheel key.------Spark should easily jump a gap of 7/16" or more.

 

[BonairII]

Get yourself a spark tester at an auto parts store etc. Pull the plug wire that you're not checking and ground it to the motor. Plug the wire being tested into the spark tester and clip the clip to the engine block(good ground). Have someone crank the motor for a second or two...visually check for bright blue "snappy" spark and confirm that it jumps the 7/16ths gap.