View Full Version : Johnson performance issues

03-18-2007, 10:05 PM
"I have a late 70's johnson 140 hp outboard. After running for about 20 minutes, the engine will not rev over 2000 rpm. When the throttle is increased, the rpm's drop. Maybe the fuel mixture is wrong? I'm mixing the oil at 50:1. Any help will be appreciated."

03-18-2007, 10:21 PM
mixture wrong will not cause those problems.50:1 is correct.Check the compression and check that you have a " strong " spark at each cylinder.

03-18-2007, 11:58 PM
"Overheating may be causing the engine to seize somewhat. Check the overheat warning horn. Engine not running, key in ON position, ground out the TAN wire that is protruding from the cylinder head. This will complete the electrical circuit leading to the warning horn. The horn should sound. If it does not find out why as that's the only warning you'll receive if the engine overheats.

Also, you may have a fuel restriction. Check the fuel bulb valves and also check the following.

(Fuel Anti Siphon Valve)
(J. Reeves)

Many of the later OMC V/6 engines incorporate a fuel restriction warning via a vaccum device attached to the powerhead. If the engine overheats, or if you have a fuel restriction, the warning is the same.... a steady constant beep.

NOTE... Only the V/6 & V/8 engines have the above "Fuel Restriction Warning". The warning horn will not sound on the other models.

The fact that a engine is not overheating, but the warning horn sounds off with a constant steady beep, and that the rpms drop drastically would indicate that the engine is starving for fuel due to a fuel restriction. Check the built in fuel tank where the rubber fuel line attaches to the tank fitting. That fitting is in all probability a "Anti Siphon" valve which is notorious for sticking in a semi closed position. It will be aluminum, about 2" long, and the insides of it will consist of a spring, a ball, and a ball seat. If this valve exists, remove it, knock out those inner components which will convert it to a straight through fitting, then re-install it. Hopefully that cures the problem.

The above procedure will cure a restriction problem with the anti siphon valve as stated. BUT, it may also allow fuel to drain backwards to the fuel tank when the engine is not running (siphoning backwards) due to the fact that the carburetors/fuel pump etc are higher than the fuel tank. This condition is not an absolute as the valves in the fuel primer bulb usually prevent this backwards siphoning problem. However.... if this does take place, the cure would be to install a new anti siphon valve.

http://stores.ebay.com/Evinrude-Johnson-Outboard-Parts-etc?refid=store ("")"

03-19-2007, 12:19 AM
"To Joe's above checklist, add the tank vent hose. It must run from the tank out through the vent without any 'drop down' that may create a waterlock. Simply open the fill cap and do a test. If engine runs, check the hose and vent for blockage/'waterlock'."

03-22-2007, 02:42 PM
"Without a doubt,the 77-78 140,was the most overhauled engine my shops saw in the early 80's.We at the time,were prob doing 2-3 of those over hauls ,a month,for prob 3 summers. in my area.This was due to the piston ring pins,and 'high-ring'pistons'.I would suggest,running a compression test on this motor for sure,but i would expect it has been overhauled,at least once ,since late 70's."

05-02-2007, 10:33 PM
now i have an early 70's engine that was doing simmalar things. it happened that i had a bad rectifier. im not sure if you have the same type of thing but make sure the fly wheel isnt hotter than it usually is after running for a while. also make sure your battery is charging while engine is running