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1978 Johnson 100 single cylinder not firing

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  • #1

    1978 Johnson 100 single cylinder not firing

    Hi guys, I've got a 1978 Johnson 100 that does not fire on the lower port cylinder. The compression on the cylinder is about 115-118 and the other 3 cyls are 125 -128. The motors runs very rough, and pulling the plug wire from that cyl while running cause NO drop in idle. Rebuilt the carbs last year. Spark is snappy in that cyl, although oddly enough the spark was intermittent earlier due to a bad connection. Spark is now nearly 1/2" jumping from wire to ground. I give up. Squirting ether into the lower carb on that side has no effect, andactually causethe other cyl to drop in rpm. Anyone have any ideas?

    Frank
    Tags: 100 hp, 1978, not firing, spark
  • #2
    Spraying fuel mixture (Don't use ether) into a carb throat and having the rpm drop indicates that the cylinder is flooding out.

    If you have compression and spark as you state, that cylinder should fire UNLESS the high speed jet is missing, carb gaskets faulty, that sort of thing.

    What does that s/plug look like as compared to the other three? Have you tried that plug in a different cylinder? (bad plug?..it happens).

    Comment

    • #3
      The plug is moderately dirty with minor carbon. The plugs are new, and I've tried swapping them. The other plugs show the typical look of about 5 minutes of running. This was also thecase last year, and very randomly, for about 1 minute at WOT the motor fired on all 4, and it ran really well. When I rebuilt thecarbstbey were fiiied with varnish, they're clean now. I definitely didn't leave anything out on the rebuild.

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      • #4
        When cleaning the carburetors.......... did you carefully and manually clean the four high speed jets with a piece of single strand steel wire? Solvent alone doesn't clean those hidden jets properly.

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        • #5
          Absolutely. I even pried the pressed caps off to clean things out. Those carbs are spotless. Btw, I goofed, its a 1979. The model number is 100ML79S. This is bizarre. The cyl should fire, unless the coil generates just enough spark to look good when outside, but not enough for the compressed cyl. But given the look of the spatk I think thsts unlikely. Is it possible the timing is off(electronically, that is) such that the spark on that sylinder is too late? Would that imply a bad ignition module? Could the reed valvle be stuck open for thst cyl?

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          • #6
            Double check the spark.... it should jump a 7/16" gap with a strong blue lightning like flame... a real SNAP! Nothing else will do!

            You can use a medium size philips screwdriver (#2 I believe) inserted into the spark plug boot spring connector, then hold the screwdriver shank approximately 7/16" away from the block to check the spark.

            A reed plate leaf valve stuck open? Possible but not probable. If that was the case, squirting ether into the carburetor would have no effect.

            A coil can be tested by switching the orange wire that powers a good coil to the coil in question.

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            • #7
              I've already done all that. Tomorrow I'll try a spray bottle of gas into the lower left venturi. I just can't imagine anything else.

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              • #8
                Spark plugs should be Champion QL77JC4 plugs gaped at .040 (strong spark). When all is well, the gap can be dropped to .030 (longer life).

                And yes... If you have compression, spark, and fuel, that cylinder should fire.

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                • #9
                  The plugs are gapped properly. Maybe I should swap out a coil?

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                  • #10
                    Originally posted by hung09126
                    Thu Mua Ipad CÅ© Giá Cao, cữa hÃ*ng thu mua ipad pro, thu mua ipad air, thu mua ipad mini,…. thu mua tất cả các dòng ipad cÅ©, ipad má»›i giá cao nhất tại tphcm
                    Whoever you are.... nobody here is able to understand what you're saying. Get your computer or your setup with your IP straightened out. Whatever the language problem is, it needs to be corrected!

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                    • #11
                      Ok, heres an update.
                      Went out this morning and fired it up. Briefly the cyl started firing, pulled the wire and the rpms dropped. Hmm.. after that it stopped firing again. Rewired the top and bottom coil to switch them and it had no effect. Bottom still not firing. Could it be fuel? The carbs are rebuilt. Should I switch the top and bottom carb and see?

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                      • #12
                        Originally posted by Ffarrell View Post
                        Ok, heres an update.
                        Went out this morning and fired it up. Briefly the cyl started firing, pulled the wire and the rpms dropped. Hmm.. after that it stopped firing again. Rewired the top and bottom coil to switch them and it had no effect. Bottom still not firing. Could it be fuel? The carbs are rebuilt. Should I switch the top and bottom carb and see?
                        Whoa!.... The bottom coil was firing, then you pulled the plug wire and the cylinder dropped rpm (normal), then the bottom coil wasn't firing.... you switched the wire firing the top coil to the bottom coil.... the bottom coil still isn't firing? That indicates that the bottom coil is bad!
                        Last edited by Joe Reeves; 07-23-2016, 04:11 PM.

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                        • #13
                          No... the bottom coil fired on initial startup. So I to verify the coil I moved the plug wire to the top and the top wire to the bottom, and swithed the 2 orange trigger wires from yhe ignition module. The top(previously bottom) worked fine and the bottom(prev. Top) didnt, indicating that the coils were not the problem, and the fault was either carb(rebuilt) or somerhing else. Now what?

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                          • #14
                            Originally posted by Ffarrell View Post
                            No... the bottom coil fired on initial startup. So I to verify the coil I moved the plug wire to the top and the top wire to the bottom, and switched the 2 orange trigger wires from yhe ignition module. The top(previously bottom) worked fine and the bottom(prev. Top) didn't, indicating that the coils were not the problem, and the fault was either carb(rebuilt) or something else. Now what?
                            Not likely a carburetor problem as the cylinder was firing until you pulled the HT plug wire off then put it back on. I'd suspect a intermittent break in a wire, perhaps from the timer base as those wires do get moved back and forth. Also it could be a timing sensor on that timer base. Check the manual to see what the ohm reading should be.

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                            • #15
                              In general, that cyl didnt fire, thats the issue. By swapping the coil I verified the coilnwas not the problem. Swapping the carb would verify that waent the problem.

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