"1978 Johnson 35hp coughs, bac
"1978 Johnson 35hp coughs, backfires, and bucks mid to low RPM
I have a 1978 35hp Johnny that I bought with a 24ft pontoon a few weeks back on the cheap. It wasn’t running when I bought it but I got it running fairly quick by changing the plugs and using fresh gas (50-1). The last owner had replaced the powerpack and head gasket among other things trying to cure the same issue I am experiencing before it quit on him. This beast has had many owners and many 'mechanics' over the years.
After I changed the plugs and fuel I got it running fine at mid to high RPM (the tachometer is nonfunctional) but it would cough buck or backfire at lower RPMs. When it does it’s bucking thing it DOES NOT blow anything out of the carb. Puffs of smoke do come up from the 3 inch diameter hole right under the carb and at the water line.
I scanned a lot of online motor sites and played with the low speed idle screw (all the way in then out 1 ½ turns to start with). I ended up getting the motor to run pretty darn smooth at idle by eventually backing out the low speed idle screw a total of SIX turns or so! It ran fine for about 5-6 days then started to develop its bucking thing again getting worse and then better . . . then worse.
I took advantage of a cool cloudy day and bought a manual and carb rebuild kit and rebuilt the carb. All new gaskets, new float, needle, seat, core plugs, etc. Popped out the core plugs, soaked, then sprayed out all orifices and tubes etc with Gunk carb spray and air. I also cleaned and rebuilt the leaf plate assembly and the low speed needle valve looked good. The carb didn’t look gunky on the inside even before I soaked the parts overnight in a gallon of Gunk Carb & Part cleaner.
When I put it all together it ran NO BETTER than when I started the project.
One weird thing it seems that the good days seem to have been all hot days (85-95 degrees F) and the bad running days are (70-85F) though the water temp in the lake is always about 81-84F. Maybe its just a coincidence (or thermostat??).
Also if I can get it into gear it seems to run a bit better under load (pushing the pontoon).
After running it for about 15 minutes at mid RPM; two minutes later I had no problem touching the cylinder head and removing the spark plugs (black ends).
Outside of maybe checking the thermostat my next task is to locate a timing light and check the timing. I must say that I am confused by the timing procedure because there is nothing anywhere near the timing stop on the inside of the starter bracket and you set the timing at full throttle (my issues are at mid to low RPMs).
I have basic mechanical skills and hand tools and have more spare time than cash so I will be working on this myself and I would appreciate any suggestions.
Thanks in advance!"
"1978 Johnson 35hp coughs, backfires, and bucks mid to low RPM
I have a 1978 35hp Johnny that I bought with a 24ft pontoon a few weeks back on the cheap. It wasn’t running when I bought it but I got it running fairly quick by changing the plugs and using fresh gas (50-1). The last owner had replaced the powerpack and head gasket among other things trying to cure the same issue I am experiencing before it quit on him. This beast has had many owners and many 'mechanics' over the years.
After I changed the plugs and fuel I got it running fine at mid to high RPM (the tachometer is nonfunctional) but it would cough buck or backfire at lower RPMs. When it does it’s bucking thing it DOES NOT blow anything out of the carb. Puffs of smoke do come up from the 3 inch diameter hole right under the carb and at the water line.
I scanned a lot of online motor sites and played with the low speed idle screw (all the way in then out 1 ½ turns to start with). I ended up getting the motor to run pretty darn smooth at idle by eventually backing out the low speed idle screw a total of SIX turns or so! It ran fine for about 5-6 days then started to develop its bucking thing again getting worse and then better . . . then worse.
I took advantage of a cool cloudy day and bought a manual and carb rebuild kit and rebuilt the carb. All new gaskets, new float, needle, seat, core plugs, etc. Popped out the core plugs, soaked, then sprayed out all orifices and tubes etc with Gunk carb spray and air. I also cleaned and rebuilt the leaf plate assembly and the low speed needle valve looked good. The carb didn’t look gunky on the inside even before I soaked the parts overnight in a gallon of Gunk Carb & Part cleaner.
When I put it all together it ran NO BETTER than when I started the project.
One weird thing it seems that the good days seem to have been all hot days (85-95 degrees F) and the bad running days are (70-85F) though the water temp in the lake is always about 81-84F. Maybe its just a coincidence (or thermostat??).
Also if I can get it into gear it seems to run a bit better under load (pushing the pontoon).
After running it for about 15 minutes at mid RPM; two minutes later I had no problem touching the cylinder head and removing the spark plugs (black ends).
Outside of maybe checking the thermostat my next task is to locate a timing light and check the timing. I must say that I am confused by the timing procedure because there is nothing anywhere near the timing stop on the inside of the starter bracket and you set the timing at full throttle (my issues are at mid to low RPMs).
I have basic mechanical skills and hand tools and have more spare time than cash so I will be working on this myself and I would appreciate any suggestions.
Thanks in advance!"
