'75 Johnson 15hp missing

[chilifriedpork]

I'm new here. I have experience in repairing motorbikes and vehicles, outboards are a little new to me...<br /><br />I bought this motor rescently and the prev. owner had it to a marine shop to do the impeller, replace carb with a re-built one, and replace a coil. (not sure about a compression check)<br /><br />Today, the motor was running fine. Kind of a rough idle though (that's a 2 stroke for ya) Slow speed mixture setting 1.5 turns out.<br /> Now the motor starts missing every second or so. But in a pattern, not randomly. It runs for a sec, misses, looses rpm's, gains the rpm's then misses again. <br /><br />Possibly the other coil needs replacing? I am going to get a repair manual promptly (any suggestions on brand? clymer etc.) (I need to get a puller for the flyweel and inspect the points and condenser as well)<br /><br />I am also curious about the thermostat's condition(I hear newer ones perform better), and the BIG thing is the motor has 45 pounds of compression!(while the engine is hot) Mind you it's even in both cylinders. So unless my comp guage if faulty, I think it's time for some rings. (possibly new pistons depending on the sides of the cylinder and piston.<br /><br />Any possible suggestions or faults the motor may be experiencing that I haven't mentioned? (besides the compression. And the last owner said it had LOW hours on it!)<br /><br />Thanks a lot!<br />Spencer

 

[Hooty]

Re: '75 Johnson 15hp missing

Welcome to iboats cartoper, the board where we get'um runnin' and keep 'um runnin' (sometimes).<br />I'm gonna give ya a website for the factory service manual for your engine. Before ya do any re-ringing, us the search function and read up on de-carboning (decarb) your engine. Good chance the rings are just stuck and a decarb will take care of that but I'd get the manual before I did anything.<br /><br /> Service Manual <br /><br />g'luck & c/6<br /><br />Hooty

 

[chilifriedpork]

Re: '75 Johnson 15hp missing

I start the motor again, about 5 hours later, and no missing. It runs suprisingly well, besides the vast amount of smoke.<br /><br />Could the excess carbon build-up that may be lowering my compression 45psi! be easily removed? I have heard of decarbonising spray(foam) that the whole combustion chamber, and gets rid of the carbon around the rings too. Is it worth a shot before I tear down the powerhead?<br /><br />-The weird thing is, the motor starts suprisingly well. When warm, just a li'll pull and she fires up. You would never know about the lack of compression, mind you it hasn't been for a water trial yet...<br /><br />Thanks for the help<br />Cartoper

 

[Boatist]

Re: '75 Johnson 15hp missing

CarToper<br />Make sure your are running the motor at 50 to 1 oil mix. I also have a 1975 15 Johnson with lots of hours on it. Last year I ran a half can of Johnson engine tuner thru engine and raised the compression for each cylinder by about 7 lbs. Now one top is 127 and bottom is 120. This was the first time ever did any decarb. Gap a old plug to 7/16 and ground plug with a wire or hold against the block with insulated pliers. If you get a good spark at this gap then points condenser and coil should be ok. <br />Do the decarb thing by the way they suggest one can per cylinder. 45 PSI is very low. Adjusted the low speed idle carburetor adjustment by turning the knob on front of engine. turn just a little and Allow engine at least 30 sec to change. The miss you decribe sounds like a running a little too lean. Also pull each spark plug wire off while running to make sure fireing on both cylinders. I know my motor sounds almost the same if one cylinder is not fireing. Running on the muff with no load the skip you have is also very normal. Out on the water in gear may be completely different.<br />I also would suggest you get the factory manual by far the best.<br />You can pull the flywheel with out a puller if you are careful. Before I made a puller I would just put two pry bars under the flywheel and with the nut loose to the top of the threads give it a little tap.<br /><br /> http://www.pugetsoundanglers.org/Fishing Info/Johnson 9.9_15.htm

 

[chilifriedpork]

Re: '75 Johnson 15hp missing

Boatist, thanks for your help. I have already read the link you added. Suggested 80 - 100 psi minimum. And if you only got about 7 psi out of carb cleaner, then I think I gotta tear it down, replace the rings etc. <br /><br />Would a Johnson dealer/service center sell factory manuals? I'm not sure if I want to go through all the trouble, cost and time of shipping a book across the country(s)<br /><br />Any ideas about adding an overboard water indicator? My 1975 doesn't have one...

 

[wayne h]

Re: '75 Johnson 15hp missing

cartoper try the decarb spray first before you rip into it.i have a 9.5 that i just bought and it had 85 psi on both cylinders and a miss.i decarb it with some "tune up" spray new plugs and clean the points.i did anothercompression test after i took it out on a lake and ran the snot out of it. it has about 110 psi now.its worth a try for a few bucks you might be able to finish the season with it and rip into it over the winter.are you doing your compression test with the throttle wide open? also look on ebay for a service manual just for your engine. i tryed the other brands and they cover too many engines.you are better off with one just for that engine you have.fresh 50:1 mix. and take it for a run it might clear up for you