johnson 1967 20hp wont start and backfires

[fisherman15]

i have just aqquired a johnson 20hp 1967, that has been sitting in the back of a garage for 10yrs, it pulls over well has got good sparks on both cylinders, but when i try to start it all it does is trys to start and konks out straight away, and when trying to start it it backfires on me, i have changed the leads around but still does it , could it be carb thanx

 

[ondarvr]

Does it backfire out the exhaust, or sneeze out of the carb?

The common problem with a backfire out of the exhaust is a sheared flywheel key.

​A sneeze out of the carb is from running too lean.

 

[82rude]

Sitting for 10 years it could have built up varnish in the carb hence running to lean as ondarvr has mentioned.I would reccomend a rebuild which is cheap and easy to do or at the very min a good thourough cleaning of said carb.Also it wouldnt hurt to pop the flywheel ,make sure the key is ok and check the coils as if original they are proably toast and will let you down at the worst moment possible.

 

[F_R]

First place I would go is under the flywheel. Expect to find bad (cracked) coils. And yes the key if somebody has been messing with it.

 

[boobie]

What's the compression on it ??

 

[fisherman15]

hi thanx for the info top cylinder is 85 the lower is 80 will take the top off and the carb and clean out with comp air, i will put new plugs and leads in ,and the coils are v good and read right with the tester , will check the points are clean as well , where can i get a timing fixture plate for it thanx paul.

 

[ondarvr]

Coils can read good but fail when the motor is running, so I don't put much faith in testing them. Although if they test bad they normally are.

 

[racerone]

Fact----The flywheel has timing marks on it and can be used to set the points for maximum spark.----The timing fixtures ( i have them too ) are nothing but a " dummy flywheel." and are used to save time in a shop.----It takes more time lifting the actual flywheel on and off.----Results are the same.

 

[oldboat1]

Set the point gap at .020 after cleaning up contacts with acetone or something similar. I usually just set them when the rubbing arm is at the high point on the cam. You can then check the spark by dropping the flywheel back over the key, and briskly turning the flywheel by hand (plugs out). Need an adjustable spark tester -- look for 1/4 to 3/8" spark. Might start with a half inch setting, and close the gap until you find strong spark. Assuming coils are working, weak spark could be a condenser issue is well (I usually replace points and condensers together).

If carb cleaning is called for, compressed air isn't enough. Need something to dissolve varnish -- good idea to disassemble and soak, then use carb spray on all openings (top of carb as well under the little domed cap if it's not idling well).