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1981 Johnson 70 popping & backfire

J

Jake1ekaj

New member
This is my parents boat that I'm trying to get running reliably to sell it. It has a 1981 Johnson 70 hp. The last time it ran was 13 years ago.

Performed compression check and from top to bottom it was 125, 125, 120sih.

Initially, there was no spark and that was isolated to the power pack. Replaced and that resolved the spark issue.

Carburetors were cleaned with the exception of the bottom jet (didn't know it was in there).

Starts, but ran poorly. Lots of missing and back firing.
-- Did continuity check of plug wires & two were failing. Repinned the ends and now the continuity is good.
-- Found the bottom jet and cleaned and unclogged the lower jets.

At this point, it had a pop. Then it developed into a pop with a backfire. (sometimes with a nice smoke ring)
-- Re-cleaned the carbs and replaced needle and seat.
-- Replaced the plug wires.

All this and no effect.

Rebuilt all the carbs. Removed welch plugs, replaced all gaskets, made absolutely positive that all passages including upper jets are clear.
-- NO effect, still pops.

While the last time it ran, I pulled the plug wires to see what is up with each cylinder.
-- Upper wire pulled. No change in running condition.
-- Middle wire pulled. Engine died.
-- Lower wire pulled. no change in running condition.

When to store and bought a spark gap tester, set at 7/16".
-- All coils produce nice blue spark across tester.

Checked coils with multi-meter and then all test with consistent numbers between primary and secondary coils.

So back to the removing the plug wires when the the engine is running. Any thoughts?

I'm kind of at a loss here with this thing and tired of replacing stuff without any effect. Tomorrow I'm going to swap the 1 & 2 coils and see what is up.

Any other things that I should be doing? I'm getting pretty frustrated with this dad gummed thing. Most posts I see about issues like this, there is no resolution posted about the issue.

Thanks in advance.


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Backfire generally means spark is happening at the wrong time in the revolution of the engine.

First thing I would check is that the primary wires to the coils (orange w/stripe) are on the correct coils.

Orange/blue is top, Orange is middle, Orange/green is bottom.

First thing to check.
 
OK then to eliminate totally the possibility of an ignition timing problem, I would index the flywheel.
Find TDC for #1 (given), mark, find TDC for #2, and then find TDC for #3.
I mark #1 with while nail polish with one hash mark, #2 with two hash marks, #3 with 3 hash marks.

Put a timing light on #1, crank engine without starting. You should see the single hash mark only.
Repeat with #2 looking for 2 hash marks only, and then #3 as well.

Simple to do, and you know for sure the ignition sparking is happening at the right time.

All this is of course if you are truly backfiring. Your words. Does it sound like a shotgun going off? That is a backfire.
 
You didn't answer my question. Is this a true backfire...a shotgun blast out the prop hub....
If not, and you are lean sneezing, all the above will not apply.
 
Most of the time it would make a popping noise but occasionally, it would back fire.

I am running it on muffs.

Today, I did mark the TDC points on the flywheel and then used the timing light to confirm the firing point and they seemed to be good.

It seems that it is only running on the middle cylinder. If you place your hand over the carb barrel on 1 & 3 if keeps on running. Cover #2 and it dies.

The compression is good on all cylinders.

The carbs are good.

It has good spark on all cylinders.

So why would it only run on the middle cylinder? Crank seals?
 
All the plug wires are new and hooked to their corresponding cylinder. If you are talking about cyl 1 to coil 3 and cyl 3 to coil 1, the wires are too short.

Yesterday, the firing points for each cylinder were verified to the TDC point marked on the flywheel using a timing light.
 
Is there any possibility that someone could have moved wires in the harness plugs between the timer base and the powerpack?
I.E. pulled a wire out of the plug and swapped it's position? It would have to have been two wires....

Look the connectors over carefully and see if the wire colors are consistent on each side of the connector assy.
 
Daselbee, the boat was put into garage 13 years ago in running condition. I was hoping that it would have come out that way (with a little tune up of course.). In no time in between was the motor messed with. I did unplug and reseat all connectors, but the coil connectors are keyed to the power pack in a rubber connector so its essentially mistake poof. As far as the orange/blue - top, orange- middle and orange/green - bottom, they are arranged accordingly.

This morning, I did swap the #2 & 3 coils and then ran the motor for a bit. The motor continues to only run on the #2 cylinder.

It has good spark on all cylinders.

I has good compression on all cylinders.

The carburetors were moved about to see if it could be related to them. 2 & 3 were swapped. It still only ran on # 2. (I did put them back)

If the crank seals were shot, would it affect the motor like this? I filled the recess on the top seal with PB blaster and then rotated the motor. I didn't see and bubbling when turning the motor though. I did run it again spraying a shot of starting spray under the flywheel to see it would change the motors running condition, no change.

I'm at a loss with this thing.
 
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Yes I know how the rubber connectors are "mistake proof". Trust me, it would not surprise me to find swapped wires.
Seen it before.

But with all you have done, it appears that the spark is hitting at the proper time.

That leaves fuel. Can you tell if the reeds are stuck shut? Or even broken and therefore open at all times?

If the CRANKCASE cannot build pressure, the engine will not pull in fuel/air mix.
Open reeds will not allow crankcase pressure to build.

Reeds stuck shut (weird one) will not allow the engine to suck new fuel/air in....

Are the two carbs involved full of fuel in the bowls?

Maybe check these things.....
 
Saturday, we pulled the reed block of to see it there were any "oddities" and everything appeared normal.

I just switched the pin position of the orange wires in relation to their corresponding pins connecting to orange/blue and orange/green.

It does seem to have smoothed out the motor noticeably. It ran without popping with a smooth idle. However, when you cover the middle cylinder, it kills the motor again. I know its possible, but is it probably that CDI reversed those wires? Its a new power pack.

Placing your palm over the barrels on 1 & 3, it has a very strong draw. I wish I had a IR thermometer to check those cylinder temps. If I can find one, what would be an ideal temperature on the water jacket or head?
 
connector.jpg
So now, the orange/blue is going to the bottom and orange/green is going to the top cylinders. The blue rubber boot is connected to a new CDI power pack. Is the O/Gr next to black/yellow? I guess i can go look at the old one. Anyway, it runs without popping like this but dies when you put your hand over the middle carb.

What is the cylinder order? Is 1 on top or the bottom?
 
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About all I can suggest at this point is make sure you have gas in the affected cyls carbs, and that at idle the fuel is being drawn into the cyl.

Try using pre-mix gas in a spray bottle and spray into the throat of the two dead cyls. See if engine picks up.

If it does, the carbs affected need to be cleaned thoroughly. Pay special attention to the small brass tube sticking down from the upper carb body into the bowl.
If totally blocked, the engine will not get idle fuel.
 
Ok, today I talked with a boat mechanic and told him everything that I had done. He said, "sounds like you bought a bad power pack." So I went home and took the old power pack that has some sort of intermittent issue. Some days the old pack would create good spark after re-seating all the connectors and then the next day, it would be dead. Anyway, I put the old pack on and it fired right up and ran pretty dammed smooth. I even pulled the middle cylinder wire and it ran just fine.

With the CDI power pack, it was only firing on the middle cylinder. I've read on other posts where guys had said you can buy bad power packs. I guess I got one. CDI seems to have a good warranty and I filled that out their paperwork. Hopefully, I can get this resolved quickly with them.

Thanks to all that suggested fixes and what not. I appreciate it.
 
HUH????
You said in post 8 that you indexed the flywheel, and all cyls sparked at the proper time. Or to be more precise, you said they "seemed" to spark right.

Something aint right here.
 
I don't like to mix electrical components. I don't like comebacks. If stators and triggers are not thoroughly tested, they will damage a pack or switch box. CDI will tell you the same. I have been using their components for years and years without a failure. My customers will spring for all suspect components,or they can try to fix it themselves, no warranty. I give a 1 year warranty on all my repairs and if I skimp on electronics, it may sting me.
 
I went to my local Mercury dealer once, and I was specifically discussing the "difficulty" in diagnosing the defective component in a Merc (or any outboard) ignition system.
I was absolutely shocked when he told me he never diagnoses a problem, he just shotguns the whole system...replaces switch box, trigger, stator, etc. as one whole package.
He says this reduces repair time, increases profit, and reduces customer comebacks.

But, to the tune of 1000 or more, that is an expensive way to fix something.

The only thing I can figure from this OP's post is that maybe the pack was double firing....
Or we got bogus testing and debug info....

I don't know....
 
Yes,the value and condition of the boat/motor for sure has to be considered before any expensive work. New motors are costly and heavier and old motors can bite you. You all know the definition of a boat, don't you?
 
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