dead cylinder

[jwhitchard1986]

I have a 140 hp Johnson I have great compression in all cylinders (120psi) and great fire. But I have a dead miss in one cylinder also when I pull the plug after running it is drenched with gas. What could cause this? TIA

 

[boobie]

Model # ??

 

[jwhitchard1986]

I don't know the model number but I believe it's a 77-79 model motor

 

[jwhitchard1986]

Is the model # relevant to this situation

 

[boobie]

Yes. With the correct model # nobody has to guess with their answers.

 

[jakedaawg]

You can find model number on the transom clamp or on the welch plugs on the top of the block.

 

[jakedaawg]

You can find model number on the transom clamp or on the welch plugs on the top of the block.

 

[emdsapmgr]

Likely the wet spark plug is not firing. Check all 4 spark plugs for ignition fire. You can use the cyl drop test or use an inductive timing light to show plugwire firing. Assuming that you confirm you have no fire on that plug, you will need to troubleshoot the ignition system. Unlikely it's a coil, but it's easy to swap the dual power packs to see if the dead plug situation moves to the other head. If so, you need a new power pack. If not, you can find further excellent ignition troubleshooting at this website: cdielectronics.com

 

[jwhitchard1986]

The thing is when I pull the plug out on that cylinder the check the fire it has a great blue spark. You can choke it out a little on the water and it will take of fine but as soon as u let the choke go it bogs back down

 

[racerone]

Maybe the fuel pump diaphragm has a hole in it.

 

[jakedaawg]

How are you checking spark? An open air gap tester is the only way to know.

Are you running with the cowling on or off? Sometimes these old ones ground out the plug on the cowling latch when installed, careful routing of plug wire is necessary .

Post #8 is the proper place to start here.

 

[jwhitchard1986]

I pulled the plug out and checked the fire by plugging it back in to the coil. Fire doesn't seem to be the problem. Ive run it with the cowl off and it does the same thing. I have done everything but rebuild the carbs on it. Ive swapped coil packs with one another and still the dead miss stays on the same cylinder

 

[jakedaawg]

Swap the power packs

 

[emdsapmgr]

The two bottom plugwires are both usually close to the lower cowling latches. Sometimes the (old) wire's close proximity will cause them to short to ground on the cowling latches. Make sure they are clamped with enough distance from the two lower cowl latches. Two easy ways to check for spark when the engine is running, the cyl drop test and by using an inductive timing light on each plugwire-then view the flashes from the gun.

 

[racerone]

So far I see " that cylinder "---So tell us which cylinder is misfiring / dead.

 

[jwhitchard1986]

I think it is cylinder 3. It's the bottom right when standing at the back of the boat.

 

[jwhitchard1986]

It's the bottom right cylinder when standing at the back of the boat.

 

[racerone]

You need 4 things for the engine to run properly.---Compression in the cylinder.---Spark at the right time.---Fuel in the air.---And you need crankcase compression as well.---So does # 3 have spark that will jump a gap of 7/16" or more , yes or no on that question ?