Need guidance on Prop pitch and Power, 78 Johnson 55hp 55EL78S

jimmbo

jimmbo

Supreme Mariner
Joined
May 24, 2004
Messages
13,446
Since no one on this Board, can actually see or touch your Outboard, any and all suggestions are based on the Info you provide. If the info is incomplete, or more Info is added, a new reply, seemly in contradiction to earlier replies, may appear.

An engine needs 3 things, in the proper amount, and at the proper time to run:

Compression, on a 2 stroke, both Crankcase and Cylinder.
Fuel
Spark.

Take away any of the above and it doesn't work, or work well

The carbs might be adjustable for elevation, but no info is shown as to what parts maybe required. Your carbs are fixed Jets for both Hi and Low speed, not my favorite choice, but it does prevent users/mechanics from setting things too Lean on one or more carbs
 
B

brodmann

Chief Petty Officer
Joined
Jun 17, 2008
Messages
426
check the plugs and see if they look different. Is one getting flooded and the other look fine. The spark gap tester is a great test of ignition system. I had a 15' dual console Cobia and it ran about 30 MPH with a 55 HP evinrude on it. No idea what pitch the prop was, but pretty sure the prop was original to the motor. Worn out looking aluminum prop. If you want to go to work on it, I'd start by pulling up a blown up picture of the carb and go to work on cleaning them up with emphasis on the idle jets. Also watch a youtube video on what to clean, and make sure you set the float levels where they need to be. That might be the cause of your flooding. I've attached website that might help. #9 is your idle jet and #35 is your high speed jet. You'll need carb kits because you'll be removing some plugs to get to the passages that need to be cleaned and blown out with forced air. Making sure spark is strong and that fuel delivery is what it should be is a great starting point.
 
jimmbo

jimmbo

Supreme Mariner
Joined
May 24, 2004
Messages
13,446
For some reason Part #7 is often forgotten to be installed when a carb is torn apart for cleaning
 
M

mduncan

Cadet
Joined
Dec 24, 2022
Messages
12
brodmann said:
check the plugs and see if they look different. Is one getting flooded and the other look fine. The spark gap tester is a great test of ignition system. I had a 15' dual console Cobia and it ran about 30 MPH with a 55 HP evinrude on it. No idea what pitch the prop was, but pretty sure the prop was original to the motor. Worn out looking aluminum prop. If you want to go to work on it, I'd start by pulling up a blown up picture of the carb and go to work on cleaning them up with emphasis on the idle jets. Also watch a youtube video on what to clean, and make sure you set the float levels where they need to be. That might be the cause of your flooding. I've attached website that might help. #9 is your idle jet and #35 is your high speed jet. You'll need carb kits because you'll be removing some plugs to get to the passages that need to be cleaned and blown out with forced air. Making sure spark is strong and that fuel delivery is what it should be is a great starting point.
Click to expand...
Thanks, Im gonna start checking all that out very soon. Local Napa didn't have a gap test tool so I got one coming. I dug out my compression gauge but it's ancient and was a cheep one anyway. So I got to pick up a new one. Also found the original Johnson service manual specific to that engine on eBay. My first impression on this from the beginning has been carburetion. But we will see. I am curious about what the compression is going to be. Thanks for all the info. I'll post my findings.
 
M

mduncan

Cadet
Joined
Dec 24, 2022
Messages
12
I'm back! I am happy to report that the compression is 128 and 130. I checked it with two different gauges and multiple times so I was pretty happy about that.
I started with the gap tester. At idle it jumped a half Inch gap. But it was cutting out at higher rpms and seamed rich. So I started looking around and found the three pin connector to the coils was corroded green and the black wire was hanging on by a thread. The black wire I believe is to the kill switch and the others to the coils, It was a mess so I wired in a weather pack connector. New coils were pretty cheap so I put in a new pair. After that it seemed to run much better out of the water. I believe it must have been missing intermittently. Oh yeah, also new plugs. Anyway, on the water it did real well. I don't know the speed I was going but it was faster than I wanted to go. Never having driven a boat before. However, it did seem to lag at high rpms. And never reached WOT. And that is with the 11 pitch prop still on it. It actually runs like I would expect an engine to run at high elevation that was not adjusted for it. But apparently there is no adjustment. I am planning on doing the carbs this winter when the snow is 4 feet deep.
Which brings me to this question. Jimmbo sent listings for the Jets that had the following options. Can someone please explain what this means? I thought transoms were measured in inches. Excuse my ignorance.
MODELS WITH 20FT.FT. TRANSOM
MODELS WITH 15FT.FT. TRANSOM
 
M

mduncan

Cadet
Joined
Dec 24, 2022
Messages
12
Yes I know you did. And I found that listing also. That had me confused. Thanks Jimmbo.
 
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