Sears Gamefisher 7.5hp - Weak Spark, Marginal Compression

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J. Fenn

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Seems I may have gotten too big for my britches. I am working on a Sears Gamefisher 7.5hp and I cannot even get a bark out of it.

I worked a deal with a new acquaintance to get some sailing lessons. His kicker engine is a Sears Gamefisher 7.5hp, but he said it had not been used for 5 years. (He has been using an electric trolling motor.) After learning that the engine was an air-cooled Tecumseh 2 stroke, I undertook to get it running.

He already had a new carb kit for it. I gave the carb a thorough cleaning (including clearing all the tiny passages on these wonderful Tecumseh carburetors), cleaned the tank, replaced the fuel lines and inline filter, and installed the new kit. Gas on the spark plug indicates the engine is drawing in fuel.

The problem I now have is weak spark. In my preliminary test, my inline spark tester would not light up when connected inline between the coil lead wire and spark plug. However, if I connected the coil lead wire to the inline tester and the inline tester to ground, I got a strong flash when I turned the engine over. I had previously tested the spark plug in another working engine.

The points look brand new. Gap is set at .020". The condenser looks like it has some age on it.

I reasoned that since I had spark when connected to ground, the problem was a weak coil and not the condenser. I replaced the coil with a new one, but still have no light or barely visible light on the inline tester when connected to the spark plug. The new coil produces a strong flash on the tester when connected to ground.

To compound my error, at the outset I began working on it without first testing compression, because it "felt like" it had good compression when pulling the rope. I have since tested it with my gauge and it is 80 psi. I do not know if this engine is equipped with compression release.

The only two things I can think of that could possibly be the cause of weak spark is 1. the condenser, or 2. weak flywheel magnets. I realize it may not run even if I can get a good spark out of it, but I feel like gambling another $5 on a condenser.

I would appreciate any comments from anyone familiar with these Tecumseh 2 strokes or anyone who has successfully repaired 2 stroke engines with the same issue.
 

J. Fenn

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Turns out the poor compression was due to a blown head gasket. So now all I am dealing with is weak spark.
 

J. Fenn

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An update...

Compression is back to 120 psi after head gasket replacement. In case you were wondering, the torque specs for the head screws (yes, they are screws) is 80 to 100 inch pounds.

The wrong spark plug had previously been installed. A Champion RJ12YC is called for in the Sears parts list. After replacement, I have been able to get it running. However, the spark still seems weak to me, and I am wondering if it is due to weak magnets in the flywheel. If any of you have encountered a weak spark due to weak magnets, please post and tell me how you rectified the problem.

As I said, I have been able to get it running, but not without some effort. It seems the carb is providing too much fuel. I am wondering if I installed the pump diaphragm correctly. The float needle seems to be holding. (It is new. Came with the carb rebuild kit.)
 

tomhath

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Even if the points look prefect, use an ohm meter to verify the electrical contact is perfect. The tiniest bit of lint or oxidation can cause problems.
 

J. Fenn

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So that would be done by closing them and checking that there is no impedance between them?
 

J. Fenn

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I also replaced the plug with a non-resistor plug and am getting a better spark now.
 

J. Fenn

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Seems there is a procedure for setting engine timing to the carb throttle. If anyone has a pdf of the procedure, please pm me.
 

brim_buster

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I sent you a PM about the manual. It does show how to set timing on all the models
 

J. Fenn

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Just a follow-up to report that the engine is starting and running well now after having performed the following:
1. Replaced coil and condenser. This may not have been necessary as I had weak spark both before and after these parts were replaced.
2. Replaced blown head gasket. This restored compression to 120 psi.
3. Replaced impeller. Old impeller had two of six vanes broken.
4. Replaced the resistor spark plug that was installed with a non-resistor plug. I believe this was key to obtaining consistent spark. The ignition system was designed for a non-resistor plug. The manual calls for a J13C I believe. I think the output was weak due to loss of magnetism of the flywheel magnets and it could not consistently deliver spark through the higher impedance resistor plug.
5. Thanks to brim_buster I was able to get carb into specs (float level, 45 degree orientation of that red rubber piece that provides vacuum for pump). I also re-used the older thinner gasket between carb and reed valve assembly instead of the new thicker gasket that came with the carb kit.
6. This engine pulls fuel so well that when cold it only needs one pull with the choke engaged. After that, disengage choke and it starts on first or second pull.
 

brim_buster

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Glad everything worked out. Now you have a great little motor that will last a good long while.
 

mirrorcraft1970

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Just a follow-up to report that the engine is starting and running well now after having performed the following:
1. Replaced coil and condenser. This may not have been necessary as I had weak spark both before and after these parts were replaced.
2. Replaced blown head gasket. This restored compression to 120 psi.
3. Replaced impeller. Old impeller had two of six vanes broken.
4. Replaced the resistor spark plug that was installed with a non-resistor plug. I believe this was key to obtaining consistent spark. The ignition system was designed for a non-resistor plug. The manual calls for a J13C I believe. I think the output was weak due to loss of magnetism of the flywheel magnets and it could not consistently deliver spark through the higher impedance resistor plug.
5. Thanks to brim_buster I was able to get carb into specs (float level, 45 degree orientation of that red rubber piece that provides vacuum for pump). I also re-used the older thinner gasket between carb and reed valve assembly instead of the new thicker gasket that came with the carb kit.
6. This engine pulls fuel so well that when cold it only needs one pull with the choke engaged. After that, disengage choke and it starts on first or second pull.
Hello J
I just tore into one of these engines myself, found a non-running 5.0 HP. Just finished replacing the head gasket and rebuilding the carb, and about to replace the ignition components. I haven't re-installed the carb yet, and was curious about your decision to re-use the old gasket on the reed block vs the new gasket in the kit? Also, Did you notice the impeller not pumping water correctly or just inspect and replace before you ran the motor?
Thanks in advance!
 
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