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Early 70's Johnson 70 HP outboard no power on max throttle

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redfisher

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Hi All, I have an early 70's Johnson 70 Hp outboard that runs well at idle and up to 1/4 throttle but when I go to full throttle the boat does not increase power. At full throttle it runs smooth and does not die and when I check the throttle cable and carburetor linkages they are at max but no power. The compression on the motor is good, 120 psi on all cylinders and I have good spark. I have read postings and it appears I may have a fuel problem on full throttle. I have ordered a new fuel pump and will be replacing some of the fuel lines, the 3 carburetors have small fuel that are tee'd together, should I replace them too ? the top and bottom are premoulded 90's so I don't think replacement fuel lines would work without kinking them or should I just remove them and clean. Any help would be appreciated.
 
Well that was a brain fart on my part!! Put new hoses on the carbs you can either make a big enough loop on the hose so they don't kink. Or use fuel resistant Ts and elbows. Are the carbs opening all the way? Is the timer base moving to the stop smoothly and correctly.
 
Thanks, johnson fuel pump.pngIts pretty tight in there so I will try and find some fuel resistant 90's, anyone have an idea what size those fuel lines would be. They look very small 1/4, 5/16 ??? (lines 8 & 13)The attached picture is similar to my setup, except a larger line off the fuel pump goes to the middle carb and a smaller line comes off the middle carb bowl and tee's to the upper and lower carb.

flyingscott said:
Well that was a brain fart on my part!! Put new hoses on the carbs you can either make a big enough loop on the hose so they don't kink. Or use fuel resistant Ts and elbows. Are the carbs opening all the way? Is the timer base moving to the stop smoothly and correctly.
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Replaced all my fuel lines and now I have discovered the lowest out of the 3 stacked carbs has no fuel in the bowl when I prime the hand bulb. When I checked a week ago all carbs had good fuel in the bowls so I am assuming the float or needled is stuck closed. I have sprayed seafoam in the carb and in bowl drain plug with no luck. Is there anything else I can try to free the float up before removal of the carb ? Thanks in advance
 
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Well--- the carburetors are also the " oil pump " for your motor.----You have to make sure they are clean.----So take the carburetor apart for inspection.----No fuel gong into one cylinder also means no oil !
 
redfisher said:
Replaced all my fuel lines and now I have discovered the lowest out of the 3 stacked carbs has no fuel in the bowl when I prime the hand bulb. When I checked a week ago all carbs had good fuel in the bowls so I am assuming the float or needled is stuck closed. I have sprayed seafoam in the carb and in bowl drain plug with no luck. Is there anything else I can try to free the float up before removal of the carb ? Thanks in advance
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How did you resolve the molded hose problem? You sure the bottom one is open?
 
I did have enough room to make the 90 bends longer so they didn't kink. After replacing all the lines I primed the lines by squeezing the gas tank bulb, then I opened the drain plug on the carbs and I have good flow / pressure when squeezing the bulb on the middle and top carb but no gas at all in the bottom carb so I am assuming the float is stuck closed. Shouldnt the float drop down and open the needle valve if no gas is in the bowl ??? The strange thing is I did have fuel in all bowls last weekend when I was inspecting the fuel lines.
 
Best to take that carb off and pull bowl for inspection and cleaning. It's possible there is something clogging the fuel passageway or the bowl is sticking
 
Took the bottom carb off and opened it up. Super clean, before I opened it I could hear the float moving when I tipped it upside down. I cleaned everything with carb cleaner and reinstalled. Now it runs like a charm.....Very strange. I tried to upload pictures but they are not attaching.
 
You are correct, brain fade on that. Either the float was stuck in the up position or the needle was stuck in it's seat.
 
Not a problem, just wanted to make sure I had the function correct. The big test will be when I drop the boat in the water this weekend.

PhilSch said:
You are correct, brain fade on that. Either the float was stuck in the up position or the needle was stuck in it's seat.
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I put the boat in the water last weekend and happy to report that the motor is running like a bat out of hell, boat planes almost instantly and will do over 30mph easily. Just to recap, I Replaced all fuel lines including the tank line, removed all carbs, opened and cleaned. Thanks for all the help.
 
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