69 Johnson 9.5 miss-fire.

I have a 9.5, dumster dive. I got it to run with a minimum of work last year, new points and condenser, water pump, lower unit seals.

it would not idle for long periods of time, it starts to slow down then misfires and dies. if i give it full throttle right away it starts right up, otherwise i need to go through the chock process.

I decided to change out the coils, wires, plugs this year. I also pulled the head and cleane out some carbon, replaced the gasket. the clyinders are and even 70 psi at 4 to five pulls cold. it now has a full time miss. when i pull the top cylinder wire it dies, when i pull the bottom cylinder wire it runs smoother.

any ideas what i did to it.

Re: 69 Johnson 9.5 miss-fire.

You may have broken the crankshaft key at the flywheel. Did you torque the flywheel? It needs to be 40-45 ft lbs.

If the engine speeds up when you add a little choke, you may be running lean, which has to be adjusted at the dial on the front. Follow the instructions by Joe Reeves in the Engine FAQ forum to set that correctly.

Re: 69 Johnson 9.5 miss-fire.

Now im no expert by any stretch but I have a '68 9.5 that does the same thing at a low idle and ezeke is right on the mark as for my motor, that is the lean/rich adjustment. Just yesterday I had it running in a barrel and at lower idles I actually had to go a bit leaner and it purred like a kitten, Good luck

Re: 69 Johnson 9.5 miss-fire.

thanks for the replies.
I have checked the key, it is fine. Adjusting the fuel is making some difference however it is going so for out of spec that it would seem it is trying to make up for something else. it seems to be lean in only the lower cyinder? I have been wondering if there is a pluged port somewhere?
the spark is a nice bright blue. one mechanic suggested trying to set the points for that cylinder at 18 or 16. I have not tryed that yet.
could it be a carboned pluged port or ring on that cylinder? or maybe a leaky gasket on that half?

I'm not a super machanic just an old farmer to stuburn to throw it away and go by one that runs for less than i will have into this one when i am done!

Re: 69 Johnson 9.5 miss-fire.

The 9.5 motors have been around a long time and they were best sellers in the day. Noone ever had to set the breaker point gap at anything but .020 that I have heard, [Edit] except for new points which were .022. I have heard of using J6C (hotter) plugs on 9.5 engines with low compression.

If you have not decarbed the motor yet, I highly recommend that you do it, and run the sequence several times.

The motor will stay cleaner longer and run better if you use synthetic oil and make sure that the 143? thermostat is working properly. Since there is no difference in appearance between the 105? and the 143? thermostat, it pays to take them out and see what is stamped on them.

BTW, the Mercury 143 thermostat # MM75692 works, fits and does the same job for half the price.

Re: 69 Johnson 9.5 miss-fire.

thank you all for your help, i think i have discovered a great fix.
Craigslist, a 1967 johnson 9.5, looks new, smells new and runs perrrfect, even responds to lean - rich adjustments, even *sqiurts* a lot of water.
used once a year on opening day. everything on it is tight.


upon really close examination of the 69 the plate the coils mounted to when at idel wobbled quite badly. it evened out at higer rpms.
even after fixing those probems it would still be a low compression engine that needed new rubber mounts.

love the new one.