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My 70hp Johnson lacks power

R

RoiRat

New member
Hi Folks, I'm new to this forum and boating. I just bought a 16ft Fiberform with a 1975 70hp Johnson outboard model 70ESL75E. My first time out I found the boat wouldn't get up on plane. The previous owner assured me the motor had plenty of power, but admitted replacing the fuel tank, tank lines, fuel lines to carbs and rebuilding the carbs.
Compression is 124/130/130. I pulled the carbs to inspect and found the top carburetor's drain screw plug was very loose and leaking fuel. My question is, could this be the cause of lack of power? My service manual indicates it's the difference in pressure within the carb that moves the fuel, so a poor seal on the carb would make sense. Looking for some confirmation. I really appreciate any thoughts you might have. Anxious to take the kids out on the lake. Thanks
 
The leaking problem could be it, if it's leaking faster than the pump can fill the bowl...however, I feel that if it was leaking that bad, you would've noticed before this. Seal up that drain and try it out, if not...

My vote is ignition. These engines are notorious for running very well on 2 of the 3 cylinders(I have one that did for a while when i got it). However, while 2 of 3 might putt around the lake, it won't get you up on plane.

Get an air gap ignition tester and set it to 1/2 inch. Good spark will jump this gap with a hot blue lightning like spark with a loud snap. This will most likely show that one of your cylinders isn't firing under load. Once you find it, you need to test the coil, do this my swapping a good coil and the bad coil. If the weak spark follows the coil, replace the coil. If it stays with the same cylinder it's either the power pack, trigger, or possibly part of the charging system. Sometimes you can tap on the power pack with the handle of a screw driver while it's running and it might pick up the third cylinder. If this happens, you know a problem lies in the power pack.

If you DO have good spark on all three cylinders, it's most likely a carb problem at that point.
 
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Another problem with these motors is the varnish cracks on the coils and allows moisture in...sooner or later the coil will fail in high humidity or when it gets wet...even if you dont have a problem now if the varnish is cracked i would reseal the coils with varnish...if i saw that i would buy a spare coil and put in the tool box... they are cheap...
 
Thanks for your quick responses folks. Update...further inspected top carb and obviously was recently serviced. Seems clean and new float assembly. Although, when I removed the float I found the needle valve was stuck in the up or closed position and I had to remove it by hand. I would have expected it to fall out freely on it's own. Perhaps carbs aren't as clean as they appear.
Also, coil number 2 has a slight crack in it's plastic case. My service manual has a good section on testing the coils, I'll try.
For Father's Day, my 16 year old son helped me drop the lower unit. We replaced the water pump, although it also was recently serviced. The gear oil looked and smelled old, but no sign of water in it.
Off to town to get tool for removing high/low speed jets, and pick up a gap ignition tester. Thanks again for all your advice. I'll keep you posed on how things progress.
 
Update...finished rebuilding carbs, but nothing obvious. Did find the fuel line from the pump to the middle carb was severely cracked, with evidence of leaking.
Then, while removing the old gasket material from the intake manifold in preparation for installing my rebuilt carbs, I was inspecting the leaf plate assemblies. The top assembly (1) is oily and wet while the middle and bottom assemblies (2 and 3) are clean and dry. I seriously misspoke compression readings in my initial posting. Cylinder 1 compression is 107, not 124 (not sure why I had that number in my head!). So, can we all agree I'm screwed. I'm thinking there's no point reinstalling the carbs. Rather, I'm rebuilding my powerhead.
If anyone has advice or thoughts, I'm listening. Thanks.
 
I'd be the first to admit i'm no boat mechanic but have owned a 70 hp with less compression than yours and could plane my little tri-hul and skier. Are you sure you are running on all cylinders?
 
No, I'm not sure if cylinder 1 is firing. In fact, because of the buildup of oil on the leaf springs and spark plug, I suspect it's not firing. I'm going to go ahead and reinstall the carbs and check it out. Earlier, eClark53520 posted that these motors are notorious for running seemingly good on 2 of the 3 cylinders. I'll try and verify if this cylinder is actually getting good spark or not.
Man, all I wanted to do was go out crabbing...I'll post an update this afternoon. Thanks.
 
Update. Tested on the lake today and although a little rough, I do have much better power now. I suspect that leaky fuel line was the real problem. Now need to fine tune it. I think I need to lube the choke linkage, seems a little sticky. May need new solenoid to linkage spring. Should the solenoid stem be lubed or dry?
Thank you all for working with me on this. Now it's time to build a crab davit.
 
Leave the solenoid stem dry. Lubing it will just get stuff stuck to it, and it won't work. Good luck with the crabbing!!
 
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