1976 75 HP Evinrude runs good but won't run full throttle.

Strahm

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1976 75 hp
starts pretty good. idles good, drives half throttle good. once you push the throttle to full the engine continues at its half speed and seems bogged. it does run, just won't open up.

when I bought it, I noticed the main fuel line came in the bottom carb. and the gas hose bib was so close to the bottom cowling that the fuel line was pinched. I moved the bottom carb to the top to help this pinch issue. that made no change in the peformance.

I have taken all three carbs apart and cleaned them, although they didn't seem dirty or varnished.

I have squeezed the gas primer bulb while it was bogged down, and there was no change in performance. So I am sure its not the fuel pump.

I have changed the spark plugs, bottom of the three seemed more dirty than the others but nothing too crazy.

I have also replaced all the fuel lines on the carbs. and the fuel tank/hose is new

What should I do next?
 

Joe Reeves

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Re: 1976 75 HP Evinrude runs good but won't run full throttle.

(Hesitation/Bogging On Applying Throttle)
(J. Reeves)

This is a common problem and is usually caused by a sticking timer base under the flywheel or fouled, clogged carburetors, however... also check the carbutretor linkages, compression, spark, and the timer base as follows.

Carburetor Linkages: Check the carburetor throttle butterfly linkages that lead from one carburetor to the another to see if they're adjusted properly. They should be set so that at idle, all butterflies are absolutely closed. You do not want to have one slightly open at idle while the others are closed. Also, all throttle butterflies should close and open at the same time.

Compression: Normally have all of the spark plugs removed BUT if the bendix gear kicks out of the flywheel constantly install all of the s/plugs excepting the cylinder you're checking at the time. Let us know what the individual psi readings are.

Spark: With all of the spark plugs removed, rig a spark tester so that the spark has a 7/16" gap to jump. The spark should jump that 7/16" gap with a strong blue lightning like flame... a real SNAP! Does it?

Timer Base: With the engine NOT RUNNING but in forward gear, have someone advance the throttle slowly from the idle position to full throttle while you observe the timer base. It should advance smoothly throughout that travel range. If it sticks, find out why and correct that restriction.

If all of the four area above are okay, and especially if the engine has been sitting for some time, remove, clean, and rebuild the carburetors using complete carburetor kits. Even if you've rebuilt them yesterday, it's possible that you've overlooked something.

Pay particulay attention to the fixed brass high speed jets that are located horizontially in the bottom center portion of the float chambers (way in back of the drain plug). Make sure that they are absolutely clean. I recommend that they be cleaned manually with a piece of single strand steel wire as soaking in cleaning solvent really doesn't do the job properly at times.
 
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boobie

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Re: 1976 75 HP Evinrude runs good but won't run full throttle.

Do a cylinder drop test on it when running it to see which cylinder isn't firing.
 

coolbri70

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Re: 1976 75 HP Evinrude runs good but won't run full throttle.

I had the same issue and it was the linkage to the timing base was broken, a plastic ball socket that turns the timer base
 

Strahm

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Re: 1976 75 HP Evinrude runs good but won't run full throttle.

Thanks for the help. I have been busy clearing snow and working. But plan on working on the boat tomorrow. Trouble is, the only time it has problems is in the water with the load of the boat on it at full throttle and all the water is frozen. So I guess I'll run through everything you guys stated and then wait for open water to test it. I have tried a trashcan with water, but as soon as I put it in gear all the water gets blown out. Don't really feel like filling a stock tank for it.
 

Strahm

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Re: 1976 75 HP Evinrude runs good but won't run full throttle.

did a compression check with our old tester. It uses a rubber plug pressed against the spark plug hole, and the rubber wasn't very supple anymore. That being said all three cylinders read 100 PSI.

I have removed the carbs, and am going to atleast do a deep clean. Might get a rebuild kit, but prefer not to spend $60 on a motor if that wasn't the problem in the first place.
Also when I first got the motor, the main fuel line came in the bottom carb and split from there to the other two carbs. well the way it was routed the line was being pinched. and after looking at another similiar motor I notice the main fuel goes to the middle carb and splits from there to the other carbs. So I will also run the main fuel to the middle carb.

I have one high speed orfice that I cannot get to come out. I think the slot for the screwdriver is worn out. Any tips on getting it out?

Are the round plugs that are over screws, or jets supposed to be replaced when rebuilding a carb? or are they just there for the casting process?
 

Joe Reeves

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Re: 1976 75 HP Evinrude runs good but won't run full throttle.

No need to remove that jet... clean it where it sits with a piece of single strand steel wire.
 

Strahm

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Re: 1976 75 HP Evinrude runs good but won't run full throttle.

No need to remove that jet... clean it where it sits with a piece of single strand steel wire.

Ok thank you.

What is the need for the carb kit? if the gaskets are fine, the needles are not worn out seems like a thorough cleaningwould be adequate?

Also forgot to mention the timer base is not restricted. the two plastic ball and socket joints look fine.

also checked that each spark plug had a crisp blue spark.
 

Joe Reeves

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Re: 1976 75 HP Evinrude runs good but won't run full throttle.

If you don't need a carburetor kit.... all of the gaskets etc are okay... that's your decision to make.

There is a high speed jet located in a horizontal position in the center botom portion og the float chamber... use a piece of single strand wire to carefully clean it thoroughly. A clogged jet would cause a problem such as you describe.
 

Strahm

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Re: 1976 75 HP Evinrude runs good but won't run full throttle.

well I have cleaned the carbs extremely well. Soaked them for 30 minutes in carb cleaner. The kind that comes in a one gallon can with a strainer basket. used a piece of wire for the high speed orfices. Per instructions on the can I washed everything with water, then to keep water deposits from forming I sprayed it off with canned carb cleaner, didn't think it would have to be washed off, since it evaporates so quickly.
I then repeated all of this the next day just to be absolutely sure they were clean.

I was able to remove two of the three high speed orfices, both of which were spotless from the beginning. they both said 55d, which I believe is correct for a 1976 75hp evinrude?

So after I get the carbs back on, what is my next step if the problem still persists? I can't imagine the high speed orfices were clogged, since they looked so clean and I could look though them and see daylight shining on all edges.
 

boobie

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Re: 1976 75 HP Evinrude runs good but won't run full throttle.

Did the timer base move when you advanced the throttle ??
 

Strahm

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Re: 1976 75 HP Evinrude runs good but won't run full throttle.

UPDATE.... same problem persists.

next question is what to check next?
carbs are spotless and the floats were all set to parallel, nothing changes when I pump the primer bulb so I don't think its the fuel pump, timer base moves freely, spark plugs all spark clean crisp blue spark.

one thing I did notice while checking the timer base. It moves as I increase the throttle, but as soon as it gets to 3/4 throttle, it seems that the timer base hits the rubber bump stop. This 3/4 throttle mark is real close to where I start to lose power.
How do I check to see if this rubber bump is in the right place? Am I on to something here?
 

Strahm

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Re: 1976 75 HP Evinrude runs good but won't run full throttle.

So if I understand it correctly to set the timing, the plugs must be grounded or set to spark a gap.
The throttle should be set all the way up. Make sure the timer base is against the bump stop.
Then use the light to see if it hits 4 degrees under factory spec.
If it doesn't then I adjust the bump stop to correct it ?
Is this correct ?

If the bump stop truly is how you adjust it. Can I just go out on the water and adjust the bump stop until the motor runs correct?
 

racerone

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Re: 1976 75 HP Evinrude runs good but won't run full throttle.

Please read your instructions again.---------The rubber stop is for maximum full throttle timing advance ( around 20 degrees or so ) and rarely needs adjusting for the life of the motor.--------The 4 degree timing is where the throttle plates start to open.----That then would be a carburetor linkage adjustment.----Do not start adjusting if you are unsure of the results !!
 

Strahm

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Re: 1976 75 HP Evinrude runs good but won't run full throttle.

Please read your instructions again.---------The rubber stop is for maximum full throttle timing advance ( around 20 degrees or so ) and rarely needs adjusting for the life of the motor.--------The 4 degree timing is where the throttle plates start to open.----That then would be a carburetor linkage adjustment.----Do not start adjusting if you are unsure of the results !!

I thought i read it correctly. The article stated to time it 4 degrees under factory spec, because when the engine is running the ignition components make up for that 4 degrees. It seems odd to me to time the engine without having it running. Im thinking maybe just because it is easier, so you don't have to find a water source. I think I will time mine while it is running.

Another thing I forgot to mention when cleaning the carbs. there was a gasket in the float bowl, about the size of a quarter, that goes around the fuel supply, heading up into the body of the carb. Only one carb had this gasket while the others did not. I could not determine what exactly it was sealing since it was in the internal part of the carb.
Would this have a major effect like I am having with my engine?
 

Joe Reeves

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Re: 1976 75 HP Evinrude runs good but won't run full throttle.

The 1976 70hp had problem with its carburetors necessitating the modification of all carburetors. I've had a few boater that swear they have done everything that could be done on the 1976 75hp models and wanted to modify their carburetors also... which apparently cured theie hesitation problem. Should you want to look over the OMC bulletin (3 pages) to decide if you need to go that route, send me a PM with your email address and I'll get it to you. Just remind me as to what the PM is for.
 

Strahm

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Re: 1976 75 HP Evinrude runs good but won't run full throttle.

next update.
I put the motor in a tub of water. started it.
removed top spark plug boot, engine rpm goes down slightly.
middle spark plug when removed kills it immediately
bottom spark plug had no effect on motor.

So I started looking into if the bottom cylinder was getting fuel or not. I didn't know if each carb independently fed each cylinder or not, so for my learning process I clamped the fuel line going to the top and middle carb, and that made the engine die down, after the fuel in the bowl was used up.
so then I clamped the bottom carb, motor only slightly bogged down, not near as much as the top two. BUT after releasing the line the rpm's picked up, and then I went and pulled the plug on the bottom spark plug, and it now has an effect on the engine performance.

Is this just a fluke or what? When pulling the boot off the plug, i can hear the spark jumping to the plug, and I have made sure the spark plug has a blue lightning snap to it.


right now the main fuel goes in the bottom of the middle carb then comes out the side of that carb and splits to the other two carbs. Would it be better to split the main line, and feed all the carbs direct from the main line?
 
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