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Old 16 December 2018, 08:03   #1
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Johnson 15hp four stroke wont start ( bad timing?? )

Got an early Johnson four stroke 15hp off a mate.

Not a bad looking engine but it refuses to start.

It was one he bought but couldn't get running.

All that it does is give the occasional big sneeze out the carb after pulling it.



Here's what I've tried...

1. Checked timing belt. pretty sure on these that the timing is Cam sprocket mark lined up with extrusion on pull start mount and the crank at the split on the port side ( 270 degrees if looking down from the pull start)

Found it to be a tooth out so took belt off and refitted.

2. Checked plug wires if they were the wrong way round. No sneeze out of carb but would not start.

3. checked spark with plug out, definately there as its sneezing occasionally.

Plug wires are in pretty bad shape so could do with replacing. Is there anything else I should be looking at? Could be a bent valve?
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Old 16 December 2018, 10:57   #2
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If you have spark do you have fuel. Plugs should be wet after you have tried turning over a few times
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Old 16 December 2018, 11:40   #3
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I tried easy start down the carb to eliminate the fuel delivery, I'll take the carb off it and check for varnish in the bowl next.
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Old 16 December 2018, 14:16   #4
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If you have spark and fuel then try compression next. If the timing is ok you have fuel a spark and air. Question now is there too much air compression will help with that
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Old 16 December 2018, 15:03   #5
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If you heat the spark plug up in the oven it will make startup easier. If that works its probably a weak spark
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Old 16 December 2018, 15:05   #6
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Are they an interference engine timing belt broke in its life bending valves belt was a tooth out?
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Old 16 December 2018, 15:50   #7
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yes it is interference. I played with the belt, on and off maybe 5 or 6 times and I could make valve to piston contact if I wanted.

I could maybe whip the head off and have a look on tuesday however a compression test should put that to bed.
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Old 16 December 2018, 17:14   #8
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Process of elimination. Air, fuel, spark.

4-strokes are the devil's work, but good news on checking the timing belt!

Check the fuel next. The primer bulb diaphragm can fail on any fuel supply. Quicksilver is as good as any. Assuming this is electric start, then check the wiring to the choke solenoid, and ensure it's actually opening the butterfly valve. Also check the dead man's lanyard.
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Old 16 December 2018, 17:28   #9
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lol you would think it has all of those things, but its a pull start with a manual choke.

Its a super super super simple engine for a fourstroke. I am actually very impressed with its design.

Tomorrow is halfords for feeler guages. I've had the suggestion of lashing valves which seems plausible
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Old 17 December 2018, 08:25   #10
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I'd do a compression test..would tell you lots...bent valves,broken rings, head gasket etc Nik
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