Spark Plug Analysis

Vorden72

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I pulled the surface gap plugs on my 1972 85hp Chrysler outboard and saw black dry carbon covering top two plugs and shiny new looking surface on bottom plug. Interested to hear from you on possible causes. Could this explain unable to accelerate motor past idle?
 

Jiggz

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A shiny clean plug indicates two things. No fire on the said cylinder or there's water intrusion causing steam cleaning. Since acceleration is severely affected, my money is on the, NOT firing cylinder problem.

In fact, you can do a quick "Drop cylinder" test by pulling the plug wire while idling and listen for engine rpm change. If there is no change, then said cylinder is not firing. The question, is it compression, fuel or electrical.?
 

jerryjerry05

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The first step in analysis is a compression then a spark test.
If it starts? then the comp is probably good(maybe).
​The clean plug: no spark or fuel.
Follow what Jiggz suggested.
 

Vorden72

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Jiggz, your comment on steam cleaning makes sense as when I tried to run the motor hard earlier yesterday I thought I detected more vaporized water than usual from the exhaust. Additionally did check firing of that cylinder and there was noticeable difference when pulled that plug. So.....
If water getting in bottom cylinder does that mean leaking head gasket? And if so is there a fix that does not involve removing the head and replacing entire gasket?
 

Jiggz

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Before going any further, do a compression test first. You can rent one from auto parts store for minimum cost. Make sure you do all cylinders for comparison purposes. If the compression is good, it is most likely the intrusion is in the exhaust gasket. If the compression is bad then the intrusion is most likely in the head gasket.

After many years of work on these motors, you will be more than happy if the intrusion is due to a torn head gasket than an exhaust gasket. Unlike 4-cycle engine which has multiple parts mounted on the cylinder head, i.e. cam shaft, timing gear, valves, etc, on these 2 cycle motor there's barely none except maybe for the spark plugs and T-stat.

On the otherhand, if the leak is on the exhaust plate or gasket it's not about the parts mounted on it but the pain and hard work to remove the bolts that are exposed to heat and water. Yup, the bolts are heavily corroded and most breaks easily making removing the left over broken stud a real PITA to work on.
 

Vorden72

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Boat and motor are more than hour drive from my house so expect to do compression check later this week and will post results and go from there
 

jerryjerry05

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Any water on that plug when removed?
​And pray for a bad head gasket compared to an exhaust gasket.

​One of my 88/85's had a miss.
I did a comp test and it was 145 on all 3.
​Then I ran it and found the trigger lead was broken under the shrink wrap on one cyl.
​Another time a miss developed, did the comp test and found 145 all 3
Then noticed a tiny dot of water on one plug.
​The head gasket had developed a tiny hole about 1/64"" it allowed just enough water to stop it from firing.
 

Vorden72

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When I pulled the bottom plug I was so surprised to see shiny metal surface I did not notice any water. Does not mean it was not there but not obvious. And as I mentioned in earlier post I did the drop cylinder test and it was firing. But as you and Jiggz said Jerry I am keeping fingers crossed for breach in head gasket and not a more complicad issue with exhaust gasket.
 

Vorden72

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I checked compression and found 155psi on all three cylinders. My intention is to finish cleaning of carbs and reinstall and switch plugs on cylinders and check results and if similar results as last time am I then looking at dealing with replacing exhaust gasket? No idea what is involved and may be beyond my capacity. suggestions?
 

Jiggz

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With compression test being good. The next step is to verify if water is really getting into the cylinder (although steam cleaning is already indicated by the clean plug) through the exhaust ports. To do this, use water paste on the face of the spark plug. Disconnect the plug wire so the said plug will not fire. Try running the motor without the said plug firing for a few seconds. Thereafter, remove the plug and check for water contamination on the plug's face. If it's positive, it is time to check the exhaust plate for water intrusion.

However, if there is no indication of water contamination from the face of the plug, the cause could be just lean fuel on the said cylinder. Remember, the bottom cylinder is fed solely by its own carb the same way with #1 and #2. If this is the case, then all you need to do is to increase the air screw setting to 1 1/2 turn out from lightly seated.
 

jerryjerry05

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The float might be stuck?
Dirt in the float bowl?
​Install a filter between the pump and first carb. Fram G2/3/12

A bad plug?
 

Vorden72

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Thanks for suggestions on next steps to take. Expect to be able to do so next Monday after which I will report on findings. Keeping fingers crossed.
 

Vorden72

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Follow up on latest suggestions and comments. Water indicating paste not readily available but on drive to boat persuaded gas station attendant to sell me ($1.00) small portion of their supply used for dipping their fuel tanks. Followed steps put forward by Jiggz and when pulled and inspected plug face did not see evidence of water. Further confirmed by dipping plug face in fresh water and saw immediate change in paste colour. White to violet. Also elaborated on test by starting motor with only one plug and cylinder connected to ignition with results that able to start in each instance, ie top, middle and bottom cylinder. Which suggests to me that individually each cylinder has compression, fuel and spark sufficient to run......but when trying to power up collectively cannot advance. Stumped. Where to go next?
 

jerryjerry05

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Stator.
outboardignitiondotcom has test procedures for some Chrysler motors.
​Otherwise you'll need a factory service manual.

Any model#s or serial#s on the motor?
I might have the original manual if the #s match?
 
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