9.8hp only running on one cylinder

[britabroad]

Hi all, I'm running out of ideas here and getting fairly frustrated.

I have a Merc 180cc Sailpower (basically a 9.8hp from approx 1985), which is only running on its top cylinder.

I've put about 100 hours on it over the last couple of years, and it's always been reliable, until a long 8 hour run recently, when it lost power about half way.

I did some tests and found that the bottom cylinder wasn't contributing at all (pulling the spark plug lead made no difference). This is what I have done to date.

- rebuilt the carb with new gaskets and integrated fuel pump diaphragms

- taken off the exhaust cover and replaced the gaskets (thought the mess on the bottom cyl spark plug might be watery) - also replaced the exhaust baffle as the old one was warped

- tested the compression - both cyl 120psi (when cold)

- checked for spark on both plugs (good),

- replaced the spark plugs

- swapped the ignition coils (just in case one was weaker).

- replaced the bottom cowl to powerhead gasket (collateral damage when taking off the powerhead).


I'm not sure what's left to try. Reed valves? Is it time to give up and get another engine?

 

[dajohnson53]

Re: 9.8hp only running on one cylinder

Hi all, I'm running out of ideas here and getting fairly frustrated.

I have a Merc 180cc Sailpower (basically a 9.8hp from approx 1985), which is only running on its top cylinder.

I've put about 100 hours on it over the last couple of years, and it's always been reliable, until a long 8 hour run recently, when it lost power about half way.

I did some tests and found that the bottom cylinder wasn't contributing at all (pulling the spark plug lead made no difference). This is what I have done to date.

- rebuilt the carb with new gaskets and integrated fuel pump diaphragms

- taken off the exhaust cover and replaced the gaskets (thought the mess on the bottom cyl spark plug might be watery) - also replaced the exhaust baffle as the old one was warped

- tested the compression - both cyl 120psi (when cold)

- checked for spark on both plugs (good),

- replaced the spark plugs

- swapped the ignition coils (just in case one was weaker).

- replaced the bottom cowl to powerhead gasket (collateral damage when taking off the powerhead).


I'm not sure what's left to try. Reed valves? Is it time to give up and get another engine?

Another similar problem I had with my 1974 9.8 hp Merc 110. It couild be the switchbox. This might help:

First - you need to make sure it's not a spark plug or spark plug wire problem.

Second - what exactly happened when you swapped the coils? Did the same cylinder not contribute? In other words, before you swapped the coils, one cylinder was "bad" - the engine didn't change tempo when you pulled the spark plug lead off that cylinder, but when you replaced that wire and removed the other, the engine quit entirely ... correct? Then, after you swapped the coils, the same thing happened, same cylinders... the "problem" cylinder did NOT follow the coil?

Third, exactly how did you test the spark? Did you use a adjustable gap spark plug tester like this: http://www.etoolcart.com/sparktester-th-404.aspx

Or did you just check to see if it was jumping the plug's gap? You need to use the adjustable tester set at (IIRC) 3/8' or 7/16" and make sure the spark is bright blue, strong without any irregularity. Remove both plugs and ground the one you're not testing.

I had the problem with these symptoms: Good compression, good plugs, good spark plug wires, all connections solid and clean, apparently good spark, but one cylinder was not contributing and that bad cylinder did not follow the coils when they were switched. The thing that tricked me was that until I watched pretty closely, I didn't notice that the spark was inconsistent using the spark plug tester. It was kind of a sporadic thing.

Turned out it was a bad switchbox. The good news is that - on my engine - it was very easy to replace. The bad news is that - on my engine - it is an expensive part. I replaced it and the engine has been flawless since.

Hopefully someone will chime in and correct me if I'm wrohg, but I think a bad cylinder spark (judged by the type of test you did - pulling one spark plug wire at a time) which doesn't follow the coils indicates a bad switchbox.

There might be an actual test for the switchbox, but like I said, the above tests - I believe - are the thing to do.

 

[britabroad]

Re: 9.8hp only running on one cylinder

Another similar problem I had with my 1974 9.8 hp Merc 110. It couild be the switchbox. This might help:

First - you need to make sure it's not a spark plug or spark plug wire problem.

Second - what exactly happened when you swapped the coils? Did the same cylinder not contribute? In other words, before you swapped the coils, one cylinder was "bad" - the engine didn't change tempo when you pulled the spark plug lead off that cylinder, but when you replaced that wire and removed the other, the engine quit entirely ... correct? Then, after you swapped the coils, the same thing happened, same cylinders... the "problem" cylinder did NOT follow the coil?

Third, exactly how did you test the spark? Did you use a adjustable gap spark plug tester like this: http://www.etoolcart.com/sparktester-th-404.aspx

Or did you just check to see if it was jumping the plug's gap? You need to use the adjustable tester set at (IIRC) 3/8' or 7/16" and make sure the spark is bright blue, strong without any irregularity. Remove both plugs and ground the one you're not testing.

I had the problem with these symptoms: Good compression, good plugs, good spark plug wires, all connections solid and clean, apparently good spark, but one cylinder was not contributing and that bad cylinder did not follow the coils when they were switched. The thing that tricked me was that until I watched pretty closely, I didn't notice that the spark was inconsistent using the spark plug tester. It was kind of a sporadic thing.

Turned out it was a bad switchbox. The good news is that - on my engine - it was very easy to replace. The bad news is that - on my engine - it is an expensive part. I replaced it and the engine has been flawless since.

Hopefully someone will chime in and correct me if I'm wrohg, but I think a bad cylinder spark (judged by the type of test you did - pulling one spark plug wire at a time) which doesn't follow the coils indicates a bad switchbox.

There might be an actual test for the switchbox, but like I said, the above tests - I believe - are the thing to do.

1. Plugs are brand new, and swapping the plugs made no difference (same cylinder inactive). Plug wires (or at least insulation) definitely aging - I got shocked when pulling the wire while the engine running - blue arcs everywhere!

2. I didn't actually swap the coils completely - I simply ran the coil from the inactive cylinder connected to the good cylinder instead (it was a quick swap, and since the top coil plug wire doesn't reach the bottom plug I only ran on the one cylinder). Bottom line was though the top cylinder ran happily no matter which coil was powering it (obviously I swapped the switchbox wiring around too).

3. Didn't use a spark tester - simply looked for a nice spark.


I saw your post about the switchbox, am slightly gulping though at the thought of the 200+ dollar part!

Fundamentally I think it is either the switchbox or the reed valves, though getting inside the crank case doesn't fill me with glee either.

How do you use the tester? Do you run the engine, or just manually crank it?

Ever heard of a switchbox tester?

Thanks for your help.

In terms of replacements - did you use sierra or CDI? My existing part no. is 339-6222A4 (superceded by 339-6222A10). Neither are available now.

Anyone have anything bad to say about the Sierra or CDI replacement components?

 

[britabroad]

Re: 9.8hp only running on one cylinder

OK - I've tested the spark using an old plug with the top contact removed, so the spark has to jump right to the rim.

I can't tell the difference between the sparks on either cylinder.

I suppose it is possible that there is a slight differnce in intensity that I can't detect, but if so I'll have to take it on faith.

Supposing the switchbox is ok, are the reeds all that are left?

 

[britabroad]

Re: 9.8hp only running on one cylinder

Bump - anyone?

 

[britabroad]

Re: 9.8hp only running on one cylinder

Bump again.

Would a bad lower crankshaft seal cause the same symptoms? (No running on bottom cylinder?) Any easy way to check the bottom crank seal?

 

[britabroad]

Re: 9.8hp only running on one cylinder

Am I asking foolish questions here?

 

[Motor Boater Bill]

Re: 9.8hp only running on one cylinder

Not foolish questions, but the long pause that comes when everyone gets stumped. I am inclined to still suspect spark, even though you can see fire on the plugs. I would suggest new plug wires if they are arcing all over as you describe. Also, I have found that the inline spark testers that a small bulb inside that lights when the plug fires can be useful in this kind of problem. They cost about $6 each at Harbor Freight. Put one on each plug, you can run the motor with them in place, and run it in the dark so you can see them fire, or not fire. This may not give you a definitive answer, but it does provide an option for diagnosis before you go for the new switchbox.

If you had water on the bottom plug, the bottom crank seal is also suspect. If it's not ignition, you are getting into difficult territory.

Perhaps Laddies will show up and post some better diagnostic information. These CDI boxes can sometimes be checked out. Also you can sometimes find them used on eBay--no guarantees, of course.

 

[d.boat]

Re: 9.8hp only running on one cylinder

Am I asking foolish questions here?

From what you wrote about testing the spark strength, I'm still not sure if you tested it properly. I don't understand what you mean when you wrote:

I've tested the spark using an old plug with the top contact removed, so the spark has to jump right to the rim.

You really need to get an adjustable spark plug tester and look carefully at the spark with the tester set at 3/8-7/16 and watch carefully for any sporadic misses, or weak spark across that gap. You need to go through quite a few cycles (pulling the cord, watching carefully) in a dark place. I can't tell from what you wrote exactly what gap you were having it jump or how you were watching it.

I say this because - based on my limited experience - it really sounds like a spark problem, but I really don't know anything about symptoms of bad reeds, etc.

Oh by the way - are you using any sort of service manual? You really should get a Merc. service manual or at the very least an aftermarket kind like Seloc.

 

[britabroad]

Re: 9.8hp only running on one cylinder

OK - finally plumped for the expensive new switchbox - and it's made no difference. I guess the lesson is that if the spark looks really strong, it probably is really strong.

I tore the engine down again to check out the bottom crankshaft seal - and lo, I can hear and feel the engine "breathing" when I turn the crank. So it seems I have totally lost vacuum at the bottom crankshaft seal.

So - experts out there, before I tear into this even further and split the crankcase, is it worth me spending any more time on this engine?

Is the bottom crank seal fairly easy to replace? Are there likely to be other issues that I discover when I get in there that make the exercise pointless?

Many thanks for all your help so far - this site is great.