1976 70hp Johnson lack of power

General Lee I · #1 (**permalink**) 07-27-2011, 02:17 PM

Hello, first time forum user here.
I have a 1976 70hp Johnson that we use to be able to tube and ski, now it has a hard time getting on plane.
First it would not accelerate without hitting the choke lever on take off, after take off it sounds like its not reaching full RPM. I removed and rebuilt all 3 carbs with rebuild kit and set to manual specs, the carbs seemed very clean when disassembled. I checked the fuel filter, it was clean and replaced fuel lines.
Now it will accelerate without hitting the choke lever, but still lacks take off power. new prop as of last year. Fresh oil in lower unit. I checked spark plugs and they were a little wet with fuel and water drops. Could this cause power loss? I havent got a chance to inspect the ignition yet, but that will be my next step, ignition coils firing at a 7/16 gap, RPM's, Timing light to check spark at WOT

Any advise
Thanks in advance

General Lee I · #2 (**permalink**) 07-27-2011, 07:14 PM

When I got home this evening I checked spark at 7/16" gap, has good spark. After running the engine on muffs for about 10 minuts I noticed the engine was not hot, the water coming out was like bath water, not over 100 degrees. I removed the t-stat cover and found the t-stat was missing. Could this cause any kind of power loss, maybe not burning fuel properly?

Thanks for the replies

General Lee I · #3 (**permalink**) 07-29-2011, 09:59 AM

Anyone???? Thanks

mwood1129 · #4 (**permalink**) 07-31-2011, 05:15 PM

Does it run smooth but lacks power ? I had a boat with that problem one time. The inner hull was full of water.

lk2ds0n · #5 (**permalink**) 08-03-2011, 07:24 PM

I don't claim to be a marine mechanic by any stretch, but I sure would be curious as to why the thermostat was missing. I know that there is an "ideal operating temperature" range for the motor. And, I know that the motor doesn't quite run at its best until it's warmed up, or at it's "ideal operating temperature".

One of the first things I'd do is absolutely make sure that the water impeller is in good shape, and that there are no restrictions in the cooling system. I'd sure be curious why the thermostat was taken out and not put back. And I'd definitely get the right 'stat for it. I'd also be curious about those components that function as a result of vacuum or temperature. For the sake of saying I did so... I'd probably check the compression on the motor. For no other reason than to rule out anything potentially major going on. Just for peace of mind to determine if the motor was worth putting a few bucks into to get fixed.

Good luck!!

General Lee I · #6 (**permalink**) 08-03-2011, 10:14 PM

I'm curious also about why its missing, I'v only took it to one guy one time and my grandpa owned it before me. Unfortunatly the guy I took it to has passed away.
My cylinder compression is 135psi on all.
I did buy a new t-stat for it but then relized there are 2 pressure control valves that go beside the t-stat that are missing also. Do you know what the pressure control valves are for?

Thanks