Top Cylinder won't fire

[Puncheon50]

1981 70 hp Johnson. Top cylinder stopped firing. Compression: Top 135(cold because it wont fire, Mid cylinder(warm) 139, bottom 134(warm). Spark good, as close as I can guess to the 7/16 but my cheap spark tester broke. Plugs firing against bolt on cylinder head. I see no water on plugs but top one is oily. Carbs are clean and rebuilt. Top carb sucks air but doesnt fire.

History: Been sitting in a barn for 20+ years. Rebuilt carbs and replaced fuel pump and lines. When first purchased, only top cylinder would fire. after carb rebuild all three good to go and ran fine on the lake. Last outing ran for a couple hours, I shut it down to drift jig and would not restart. limped home and when I put the engine down, sludge dumped out telling me a cylinder wasnt firing which turned out to be the top cylinder.

What little I have seen on the internet says it might be the coils overheated that caused the no start, or the power pack. I am not understanding, if I have compression, spark, and fuel and air, how or why it could be the coil or power pack. Could this be somehow timing related. I also would like to verify that I can check coils by swapping 1 for 2. How do I check if the power pack is at fault. Thanks

 

[Bosunsmate]

Yes check them by swapping
Have you done a drop test on the top cylinder by pulling out the ht wire with insulated pliers while its running?

 

[boobie]

Agree with the above. Do a cylinder drop test on each cylinder.

 

[Puncheon50]

Yes , I have pulled the plug wires on all 3 cyl while running. Top one gets no response but if I try to reconnect the top wire, while running, it acts like Itjolts the electrical system.

 

[Puncheon50]

Guess I will go througH everything with a multi meter and the manual I have. Just having a tough time understanding how this could be electrical unless it is the timing side of electrical

 

[racerone]

Timing does NOT go out of adjustment .----Unless of course it is tampered with by an owner doing some guess work.----Note there are 3 sensor coils and 3 ignition coils.-----Also some bits in the powerpack that may have failed.

 

[Bosunsmate]

If powerpack or timing base is faulty that top cylinder could go out of time, so yes you should check that as it seems youve checked everything else

 

[Puncheon50]

I have gone through everything with a multimeter and all the ohm readings check out. I ordered a DVAdapter to check voltage outputs. I ohm tested the rectifier and it checked out. Readings in one direction. I read that most overheating power pack problems are from the rectifier. Should I check voltage output from the rectifier and is that the red wire, or should I check voltage to the battery cables? This Seloc manual has nothing about the rectifier. Also, this manual says you need a special tool to measure voltage to the stator. Can I use the DVA to get a stable AC reading from the Stator, or will that fry the DVA?

 

[racerone]

The rectifier is part of the battery charging system.----The ignition system does not need the battery to run.----Check for issues with carburetor / reed valves / crankcase compression on the top cylinder.----And did you remove the high speed jet on the top carburetor.------It is about 5/8" long with 1/4-20NC thread.----Screwed horizontally into the very bottom of the bowl.

 

[Puncheon50]

The rectifier is part of the battery charging system.----The ignition system does not need the battery to run.----Check for issues with carburetor / reed valves / crankcase compression on the top cylinder.----And did you remove the high speed jet on the top carburetor.------It is about 5/8" long with 1/4-20NC thread.----Screwed horizontally into the very bottom of the bowl.

Yea, I have had that carburetor off like 3 times and checked both idle and High speed orafices. float looks good. The sludge I had come out is from the top cylinder not firing and running the unburnt gas and oil through the exhaust so that is one way I am sure the carb is working unless it is totally flooding the top cylinder. reed valves look good when I had the carb off. i have checked compression and spark more than a few times. latest numbers are in OP. I guess it wont hurt to check the carb again.

I still think I have a timing issue. I have'nt had a chance to get out on the lake to properly check it. I checked it in my driveway at idle and not 100% sure of the results. I read somewhere that if the powerpack is goes bad, the idle timing will read 1 degree as opposed to the 4+/-1. is this correct. Also, i wanted to check the voltage outputs on everything before I start messing with the timing.

 

[Bosunsmate]

With engine off does primer bulb stay firm?
Dont mess with timing, just check it using Joe Reeves method, if its out enough to cause no fire when the cylinders on its power stroke you will see this clearly

 

[Puncheon50]

With engine off does primer bulb stay firm?
Dont mess with timing, just check it using Joe Reeves method, if its out enough to cause no fire when the cylinders on its power stroke you will see this clearly

Primer bulb stays firm. It is my understanding that all 3 carbs are different. The Top and bottom carbs look pretty close to alike. Is it possible for temporarily swap the 2 carbs and run for no more than a minute or will it create a no start?

 

[racerone]

The 3 carburetors all work the same !!----Difference in part numbers is due to fittings / linkages on them.

 

[Bosunsmate]

yes do the swap, good idea

 

[Puncheon50]

I received my DVA today and was going to check out the power pack. I went back through to double check the resistance tests and clean the connections with some wd40. Anyway slot B from my sensor coil is testing 12 to 17 points too high. A and C are coming back at 17.5 ohms and B is coming in at 33. I guess the sensor coil needs replaced but could someone explain why that channel is higher, and is it worthwhile checking the voltage output from the power pack until the sensor coil is replaced? If the sensor coil is bad, will that make the powerpack readings meaningless?

 

[Bosunsmate]

Which cylinder is slot B firing?
Id replace the wire first, see if that improves it, it maybe corroded internally
You could check the voltage outputs to see if any difference

 

[Puncheon50]

looking at the wiring diagram, looks like the Middle cylinder. The 4 sensor wires are female that connect to a male plug that goes to power pack. Can the plug//wire assembly be purchased somewhere or is there a method to change an individual wire?

NVM, I see Crowley Marine has it if iboats doesnt

 

[Puncheon50]

Also, how to I check the AC coming from the sensor coil. My manual says there is some special measuring instrument required. I am guessing a multi meter cant handle it or give an acurate reading?

 

[Bosunsmate]

You can on most plugs release the wire by inserting a tiny screwdriver from the back.
If it fires your middle cylinder i dont think its your problem

 

[Bosunsmate]

Also, how to I check the AC coming from the sensor coil. My manual says there is some special measuring instrument required. I am guessing a multi meter cant handle it or give an acurate reading?

Ive never done this.
Id be leaning towards the powerpack at this point but id check Voltage from sensor first