70HP Johnson- weak spark on one cylinder

[turtle1173]

Hi all,
I've got a mid to late '80's 70HP Johnson. Serial plate is gone so I don't have a number. It was a VRO motor but has been disabled. Motor had been sitting for several years. When it ran, it had just had a new ignition system installed (stator, power pack, but not sure on the coils).

I cleaned the carbs and replaced all fuel lines. And also rebuilt the fuel pump.

The motor started right up and seems to idle good. I was getting ready to take it out for a test run. What held me back was that a couple times, the rpms would really increase. Then it would go back down. It dawned on me that with the idle lever all the way up, it really wasn't idling all that fast. A couple of checks and I found that the top plug spark was very weak. The other two were "knock ya down" blue. Ouch!

I took the top coil and swapped it with the middle coil. The weak spark moved to the middle.

My question is... Can I reasonably conclude that the coil is the problem, since swapping it out changed the problem cylinder? I don't have any electrical testing equipment but I also don't want to throw money at it hoping it's the answer.

Thanks!

Turtle1173

 

[ronward]

Absolutely... If u switched the coils and the problem followed then that coil is more than likely your problem.

 

[turtle1173]

Great! Thank you!

 

[WrenchHead]

Nice job troubleshooting the problem.

 

[turtle1173]

Well, unfortunately this didn't solve the problem. It ran better but now it's really obvious that it's missing.

Now it has bad hiccups. It will hiccup and for a split millisecond even the water will stop pumping!

I was by myself, so I was limited to what I could do. With the bottom plug wire grounded, the motor wouldn't even start. With the middle wire grounded, it would run but not well. With the top plug wire grounded, the motor ran better.

So I don't know. My new coil was put on the middle cylinder. I'm wondering if the bottom cylinder is doing most of the work.

Any suggestions? I'm not even sure where to start now.

Thanks!

 

[Bosunsmate]

check top cylinders timing, it shouldnt run better if grounded unless its out of time

 

[emdsapmgr]

You need to confirm that you still have a spark problem. An inductive timing light put on each plugwire while running-will show the quality of the spark. You can rent one from an auto parts store, or borrow one from a friend. Once the engine is well warmed up, check the spark on each plugwire. The flashes from the gun will clearly show weak spark, intermittent spark and no spark problems. It's possible that your "hiccup" problem is fuel related. If the tiny idle fuel passages inside the carbs get restricted with dried fuel gelatin (it happens on older engines) you can get a "lean sneeze". This is an occasional miss every few rpm's-while at idle (only). That is a carb issue, not an ignition issue. It's possible you've got two different, unrelated problems going on.

 

[turtle1173]

Well I think I might have narrowed a few things down today. First, I verified that there is good spark at all 3 cylinders. I found that the fuel bulb had no clamps. Putting those on made the motor not run quite as bad. After some various tries on seeing which cylinder was doing what, I noticed a bunch of fuel coming out of the primer solenoid whenever I pushed the key in. That led to pushing the key in while the motor was running. This is where it gets interesting. Pushing the key in while the motor was running, made the RPMs increase. The motor ran better.

So that told me that it is running too lean. Next was seeing how each carb reacted when placing my hand over the carb throat. The bottom cylinder had no noticeable difference when covered. Weird. Covering middle carb resulted in the motor speed increasing. Ok, too lean. Covering top carb resulted in the motor almost dying. Hey, that's what it should do!

So, wow, it's all over the place. I took all 3 carbs off and am going to open them up again. These carbs don't have an adjustable needle valve, so it shouldn't be too difficult to get straight.

Anyone have any suggestions or anything that I'm not reasoning correctly?

Also, what position is the red thing on the primer solenoid supposed to be in? What is its purpose? It pulls right out and has an o-ring on it. I'm assuming it isn't supposed to leak while the key is pushed in. Would this be an air leak consistently or just when depressed? Should I replce the o-ring?

Thanks for all feedback and input!

 

[racerone]

The red lever is for manual operation of the valve.---Used for starting the motor with a rope should you say have a dead battery.------Replacing the o-ring will stop the leak.

 

[turtle1173]

Any idea on the size of that o-ring? It would be nice if I could just pick it up at a hardware store.

I opened the carbs back up last night. The top and bottom carbs both had the little wire that connects the needle valve to the float laying in the bowl. I don't know if that would make much of a difference on the running of the motor or not. Also the same two carbs had the float drop distance as too much.

Other than that, things looked fine. I cleaned them thoroughly with carb cleaner and blew them out with compressed air. Hopefully I'll have time in a couple days to put them back on.

I hope that solves my troubles.