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1979 Evinrude 55 hp will not run full speed without choke.

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Aeroflyer

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My 1979 Evinrude 55 hp outboard engine will throttle up to about half throttle and then start bogging down. If the choke is activated it will then gladly run full speed when advanced to full throttle. It revs up nicely when not in gear. Overhauling the carburetors did not help. I'm using different fuel tank and fuel line than the previous owner. I found out the previous owner had the same problem but never discovered the benefit of using the choke. Why isn't the engine getting enough fuel to run up to full speed?
 
I would say that the high speed jet in one carburetor bowl is plugged.------Do not run this motor until you sort this out.----But post the compression numbers here.----Good for future reference.
 
Aeroflyer said:
My 1979 Evinrude 55 hp outboard engine will throttle up to about half throttle and then start bogging down. If the choke is activated it will then gladly run full speed when advanced to full throttle. It revs up nicely when not in gear. Overhauling the carburetors did not help. I'm using different fuel tank and fuel line than the previous owner. I found out the previous owner had the same problem but never discovered the benefit of using the choke. Why isn't the engine getting enough fuel to run up to full speed?
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Next time that you're out open the fuel tank ( safely) just a crack when it starts to bog. Was on a fishing trip and one of the boats was doing the same thing, buddy told me to do this, took right off lol. If it opens up with the choke on that means that it can handle the fuel flow so not timing issue/firing issue or carb issue( if high speed was partially plugged wouldn't open up to full speed even with choke - constricted flow. Fuel pump seems like only thing left, good luck. Keep us posted.
 
Thanks for your input on this engine problem. It has been a chronic issue with this engine. Various fuel tanks have been used with equal results so I expect that it is not a venting problem. I will, however, check that. I had a shop go through the carburetors, so I would hope they would have made sure the high-speed jets were clean. However, I will check that also. The shop may have reassembled the carburetors, after cleaning, like they found them. If there was a problem with the emulsion tube(s), they may not have noticed. After discovering the ability of the boat to achieve full throttle with the choke on, I spoke with the son of the previous owner who said the boat had never run that fast. The previous owner had bought the 1979 Evinrude 55hp, model 55975c new. The compression numbers were in the 90s, if I remember correctly. Both cylinders were basically the same. I'll recheck the compressions and post them within the next week. Thanks again.
 
Aeroflyer said:
Thanks for your input on this engine problem. It has been a chronic issue with this engine. Various fuel tanks have been used with equal results so I expect that it is not a venting problem. I will, however, check that. I had a shop go through the carburetors, so I would hope they would have made sure the high-speed jets were clean. However, I will check that also. The shop may have reassembled the carburetors, after cleaning, like they found them. If there was a problem with the emulsion tube(s), they may not have noticed. After discovering the ability of the boat to achieve full throttle with the choke on, I spoke with the son of the previous owner who said the boat had never run that fast. The previous owner had bought the 1979 Evinrude 55hp, model 55975c new. The compression numbers were in the 90s, if I remember correctly. Both cylinders were basically the same. I'll recheck the compressions and post them within the next week. Thanks again.
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If you run it wide open without using the choke and then pull the spark plugs out how do they look ? are they oily and fouled or dry and brown and are ALL 3 looking the same ? Could identify if only 1 cylinder is causing the problem.You could also use a piece of clear fuel line and see if the problem is fuel "feed" without the choke on , just a thought.
 
have you checked that the carb floats are adjusted to the right height- very important-if not could have low fuel level in carb bowl when under demand-easy to adjust.
 
Next time you take it out push choke in when it does it and see what it does. It could be a number of things, but may be as simple as a simple carb clean.
 
Thanks for all the input. Yesterday after removing the starter, I noticed that the first section of fuel line is only secured by a wire tie. The same section of fuel line has a clamp where it attaches to the fuel pump. It would swivel easily but would not pull off the tank fuel line connector. That seems like a potential air leak. However, the parts diagram shows the fuel line secured only with a wire tie strap. I replaced the wire tie with a clamp. I am waiting a part and then will get everything reassembled and check the compressions. I hope to get the boat in the water for a test run on Saturday. Thanks to all.
 
The compressions on the 1979 Evinrude 55975c were 140 psi on both cylinders. That was a warm engine. I plan to get the boat in the river tomorrow for a full power check out. I haven't done anything on the fuel issue except a clamp on the fuel line where a wire strap had been. The man at the shop that went through the carburetors said he suspects a weak fuel pump. The only known problem with this engine is that it will only accelerate to half throttle and then needs full choke to accelerate to full throttle. We will see how it runs tomorrow.
 
KyBullet said:
Next time you take it out push choke in when it does it and see what it does. It could be a number of things, but may be as simple as a simple carb clean.
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Today on the river the engine started and ran well with the exception it bogs down around half throttle. When I push the key to engage the choke it accelerates to nicely to full throttle. Approximately 30 mph which is all the speed i expect for a 17.5 fiberglass boat. I think the carbs are clean.
 
I would say it clearly indicates that carburetors need attention.-----Orifice plugs confirmed clean.----Found in the bottom of the float bowl.
 
These fuel pimps are elegantly simple.----Not sure how it can be weak.----But a new diaphragm is easy to install.----How does the motor respond when your assistant operates the manual pump ?
 
Its a carb problem. If not OEM kits the gaskets can be too thin to seal emulsion tubes (Serria kits do have problems)
 
i think carbs has an intermediate and idle orifice that works around the speed you are describing. be sure to remove the screw to both and be sure they arent swapped in the wrong hole. should be #37 in front (idle) and #44 on top stbd side (intermediate).
 
I have not taken the carbs off this week as I had planned. I hope to on Monday. I will try to look closely at the items mentioned above. I wonder at what point the carbs go from depending on the off-idle jet to the high-speed jet. My acceleration issue actually becomes apparent somewhat before half throttle.
 
Aeroflyer said:
I have not taken the carbs off this week as I had planned. I hope to on Monday. I will try to look closely at the items mentioned above. I wonder at what point the carbs go from depending on the off-idle jet to the high-speed jet. My acceleration issue actually becomes apparent somewhat before half throttle.
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as i mentioned it has an INTERMEDIATE orifice. Someone more knowledgeable about this type can chime in, but again i'd think the intermediate combines/kicks in around 1500-1800rpm or higher with the idle, then the high speed takes over completely
 
Yes, it has an idle jet, an off-idle jet and a high-speed jet. I've assumed they work together and keep "adding in". It sounds like the high-speed jet works alone above the approximate 1500-1800 rpm range.
 
I removed the carburetors on my 1979 55 hp Evinrude 55975c outboard. I inspected the jets. All were perfectly clean and looked good to my untrained eye. The idle jet is a 45 in the top carb, and an unreadable number (looked like VV) in the lower carb. The off-idle jet is a 37 in the top carb, and a 36 in the lower carb. The high-speed jet is a 57D in both carburetors. I was unsure as to how to remove the emulsion tubes. I assume they unscrew but they didn't seem to want to turn, and I didn't want to do damage. The emulsion tubes seem fragile. I need to remove them and see the condition of the washer/seal, which Faztbullet mentioned. I do not know what everything should look like. I'm mainly looking for contamination but am not seeing any. This is a long-time problem with this engine.
 
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