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1975 15hp johnson wont run

T

tiller1

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i am new to boat motors and am having problems with my 1975 15hp johnson (15R75C). i bought it last winter, took a little while to bit it started but i took it out the first time and it ran ok. i took it out the next time and it died and will not start now. I had a friend who has worked on a few motors. We replaced the spark plugs, condensors, fine tuned the new points, carb was cleaned by a mechanic. I also replaced the fuel pump and hoses. It has great spark, 105 compression in top cylinder, 103 in bottom. Mechanic told me reed valves where good, not sure if he checked though as he was very busy. We did unplug the kill switch and nothing changed. Fuel gets to the carb fine but it acts like it is not being pulled in. We did dump a little fuel into the spark plug hole and it still doesn't seem to pop. I was wondering if anyone had some ideas. The three of us are at a loss.
 
We did set it to spec. when we pulled the flywheel key out it appeared fine but i will check it again. Also my spark plugs don't seem wet. if the timing was off wouldn't they still be sprayed with fuel? Also we tried putting a small amount of gas in the sparkplug hole and it still doesn't pop. Could there be a bad gasket or something if I still have compression over 100 psi
 
Did you do an external spark test to see if it can jump a 3/8" gap and that both cylinders are firing. If you put gas in the cylinders and still didn't get a pop, I would have to assume that you either have no spark or it is timed wrong. Are the plugs going to the correct cylinders or the clips going to the right cylinders. If either one (coil clips or plugs) is wrong, your sparks will be firing at bottom dead center and you can pull the cord till the cows come home and get nothing.

My other thought is how well did you clean your points. I think you said something about new points but whether new or old they need to be cleaned good with laquer thinner to remove oils and residues. You can always test continuity with a multi-meter. In any event the spark test above will confirm if they are working or not.
 
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Had a similiar problem that was not getting any gas to plugs. But had good spark. Although my compression was 60psi - but suppose to be adequate for 6hp. Someone told me the trick to verify was fill up thru carb with fogging oil and pull and see if plugs get wet at all. Reed valves were good and carb was rebuilt - but nothing to the plugs. I pulled motor apart and rebuilt - only think I can think was bad seal between casing.
 
To run you need good spark, compression, timing, and proper fuel mixture. You have good compression (150 and 130). I would eliminate timing by checking the flywheel key, if it is okay your timing is good. 2 parts done. Next test spark, it has to be strong enough to jump a 3/8" gap and that the correct wires go to the correct cylinder. If that is in order then you have a fuel problem. You can then focus on the area giving you the problem. Could be a number of carb things, but eliminate the easy things first so you aren't jumping all over the place trying to diagnose problems that don't exist.
 
i did use an external tester and it has a beautiful blue spark and i also cleaned the points before i put the flywheel back on. I have spark and compression so it is a problem between the fuel and it getting to the plugs. we also tested continuity and it has the plugs hooked up right because it ran before, but i did switch them and try it to make sure. The carb does work on another engine, has been cleaned, and appears in good condition so therefore i believe that is also not the problem. i was told the reed valves are ok by a mechanic, but i will open it up this weekend and check myself. Is there anything in particular i should be looking for?
 
Well the only thing between your carb and the combustion chamber are your reed valves and the vacuum to suck it in. Reeds rarely give one a problem and since you have reeds for both cylinders, both would need to be defective for the motor not to run. The other thing is crankcase pressures. You do have two crankcase seals (one for the upper cylinder and one for the lower cylinder) but again both would need to be defective, at the same time to get a "no go" from the motor. Usually bad reeds and seels just leave you with a crappy running motor, not a dead one.

The other thought is the spark plugs themselves. What plugs are you using. It calls for Champion plugs but I know mine run a lot better on the NGK plugs (B7HS). Again the champions would still run, but in mine, they run like crap (surging and bogging).

I am still perplexed by the fact that you have good spark and compression, but when you manually put a little gas in the spark plug holes you got nothing. Have you tried squirting some pre-mix gas into the carb throat and pulling the starter cord. Also, is the choke closing properly when you pull the knob?
 
What I heard and I may be all wet (not pun intended) is that that even though you may have still have upper good compression the lower seals may be bad and not drawing fuel. I'm sure somebody will correct if wrong - lot of smart guys belong to this forum. I had same situation tried to find my old post but it looks like they archive them after a certain point. I could not see any thing obvious when tearning apart. Although it would be hard to tell if seals are bad visually. I really did not expect the problem to be corrected by rebuilding. In any case after rebuild started right up. SInce I redid all electronics, carb, check reeds, etc. before rebuild and still did not start or draw fuel I'm pretty sure it was the seals.
 
hmm, seal failure can prevent fuel delivery from getting to combustion chamber but if squirting fuel directly into cylinders does not at least make engine fire once or twice then you still have issues elsewhere.
 
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