78 johnson 75hp missing (updated with video)

[78ImperialT]

75ELR78C model number

carbs are perfect...all fuel lines/connectors/filters replaced...new spark plugs...new head gasket...compression within 3lbs of each cylinder

the motor cranks like a dream....then wants to run great...however, seems like ever third or fourth cycle a cylinder misses. at low idle the miss sometimes kills the motor and makes it difficult to restart. when throttled up it never really opens up. a couple times it did run WOT and then back to just dragging.

when connected with the muffs at home at low idle, every time it misses, what appears to be white smoke blows out the exhaust.

any ideas? i really dont want to take it to the shop. big $$$ that i dont have right now.

this video might help

 

[Daviet]

Re: 78 johnson 75hp missing (updated with video)

It almost sounds like a lean miss. You say carbs are perfect, do you mean you overhauled them, removed all the jets and cleaned them? What type of plugs are you running, surface gap or the center electrode type?

 

[78ImperialT]

Re: 78 johnson 75hp missing (updated with video)

i overhauled the carbs...but dont know if the settings are correct. as far as the plugs go...im using the suggested QL77J4C Champion. thats what the sticker on the coil says to use.

 

[Daviet]

Re: 78 johnson 75hp missing (updated with video)

When you overhauled your carbs, did you remove the jets and make sure they were clean? There are no adjustments on the carbs, both high and low speed jets are fixed. The holes in the jets are very small and easy to plug.

 

[tashasdaddy]

Re: 78 johnson 75hp missing (updated with video)

as Daviet asked and you did not answer, did you remove all the jets, with the special tool? read carb cleaning, and ethanol here >>> http://forums.iboats.com/showthread.php?t=299680

also a decarb would probably help it.
Decarb, take a can of seafoam put 3/4 of it in the gas tank, with only 1 gallon of premixed gas. put the rest in a spray bottle. start the engine, and let it come up to temperature. then remove plugs, and them some real good shot of seafoam into the cylinders, replace plugs, let sit 15 minutes. restart, and spray the rest of the seafoam into the carbs, so the the motor almost stalls, wait and repeat until the seafoam is gone.then take for a wide open spin. then put in new plugs, ad premixed gas to the tank, and take it for a wide open throttle spin. it is going to smoke like a house on fire, during this process.

afterwards compression.recheck

 

[78ImperialT]

Re: 78 johnson 75hp missing (updated with video)

i didnt remove the jets...just blew them out.

 

[78ImperialT]

Re: 78 johnson 75hp missing (updated with video)

could it be a coil or some other electrical problem?

 

[Daviet]

Re: 78 johnson 75hp missing (updated with video)

Could be any number of things. It sounds like a lean miss. the low speed jets can be removed without removing carbs. Remove air silencer to expose front of carbs,on the front of the carb bowel at the bottom you will see a 7/16 slotted bolt. Remove that bolt and the low speed jet is inside that passage. It takes a special tool to remove it, I made one by grinding a screw driver to just fit inside the passage, needs to be a tight fit so it does not wobble around. It is probably 1 to 2 inches inside the carb. be careful as the jet is made of brass and is soft. Take the jet all the way out and hold it up to the light, it has a very small hole and does not take much to restrict it. The bottom carb may have a clearence issue, I solved this by making a stubby tool.

 

[tashasdaddy]

Re: 78 johnson 75hp missing (updated with video)

do the carbs correctly, replace the fuel lines, and i will bet 90% of your problems go away. all the directions are in the link i gave your.

 

[78ImperialT]

Re: 78 johnson 75hp missing (updated with video)

i will get on that this eve or tom. i guess i shouldnt have gotten in a hurry. however, ALL fuel lines have been replaced and the insides of the bowls didnt have any residue inside them.

 

[seasprite]

Re: 78 johnson 75hp missing (updated with video)

I had a miss like that once turned out to be a bad spark plug boot look for carbon tracking around spark plugs or run it after dark and see if you can see any spark leakage.

 

[78ImperialT]

Re: 78 johnson 75hp missing (updated with video)

spark leakage???

two of the plug boots do look pretty deteriorated. can i swap just the boot or do i have to swap the coil and all?

 

[tashasdaddy]

Re: 78 johnson 75hp missing (updated with video)

looks like you can change boot. really depends on what coil you have.