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Thread: 75hp Johnson Stinger low Compression PLEASE HELP

  1. #11
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    if its a 20'leg they are everywhere thus y they are refured to as fishing engines common as a brick
    15'legs are more rare n have the same powerhead as a 20'
    I have several of both engines
    Untill you pull the head and exhaust plate u wont know if anything has been done to the exhaust ports
    I can show you some if you like
    Whats ur email
    Who aint addicted to fast boats.....

  2. #12
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    Quote Originally Posted by camohunter View Post
    I didn't pull the welch plugs. I'm sure that's sound advice. I did pull all the brass orfices and made sure the passages were open. The "cam follower" is there and seems to be in good shape.
    The welch plug on top covers a chamber. That chamber is fed gasoline based on the idle air orifice... In turn, the chamber "drains" into the venturi above the butterfly plate through TINY!!! holes that need to be clean & clear. This affects your idle quality alot. Lean condition causes a crankcase backfire that you may be calling a "mist out of the carburetor".

    You have to clean them right, then replace ALL the fuel lines between the fuel filter and the carbs prior to reassembling the carbs to the engine. Otherwise, debris in the ancient fuel lines that is falling off will clog your new carb rebuild job.

    Tom

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    Quote Originally Posted by 75luva View Post
    if its a 20'leg they are everywhere thus y they are refured to as fishing engines common as a brick
    15'legs are more rare n have the same powerhead as a 20'
    I have several of both engines
    Untill you pull the head and exhaust plate u wont know if anything has been done to the exhaust ports
    I can show you some if you like
    Whats ur email
    I've got several of the 20"ers and the parts to build a short. I have a clean looking 49 powerhead for my short but I'm thinking about getting a 56 to go on it. 56s are $ harder to come by here. I'm going to put it on an aluminum duck boat. Id like to see what you have. Id really like to see the difference on the ports inside. camohunter1981 at gmail. Thanks

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    Quote Originally Posted by 25XS View Post
    The welch plug on top covers a chamber. That chamber is fed gasoline based on the idle air orifice... In turn, the chamber "drains" into the venturi above the butterfly plate through TINY!!! holes that need to be clean & clear. This affects your idle quality alot. Lean condition causes a crankcase backfire that you may be calling a "mist out of the carburetor".

    You have to clean them right, then replace ALL the fuel lines between the fuel filter and the carbs prior to reassembling the carbs to the engine. Otherwise, debris in the ancient fuel lines that is falling off will clog your new carb rebuild job.

    Tom
    I didn't know it was common to pull the welch plugs but I'll try that because the crankcase backfire is what's happening

  5. #15
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    I think you have more then one problem with your motor, for one thing your compression is way, way to low, having said that I would try another compression tester just to make sure, on stock heads with a fresh motor you should be around 130-135lbs., shave the head .050 in. and that should bring you to 150lbs.

  6. #16
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    Low compression will cause starting and idle problems as well as reeds having issues but I have run engines with low compression (couldn't afford rebuild in the young years and made due) and they ran great at high rpm but never went slow so it didn't matter To test rings/bore you could add some outboard oil in the cylinders spin over a dozen times and recheck the compression, if a substantial increase in psi the rings/bore are suspect. As for the compression gage just use it as a relative psi measure and always use the same gage (important to see all cylinders near the same as well as not to low). As stated somebody could have raised (and squared?) the exhaust ports and that will lower the rope over compression. Could also have carbon build up and sticky rings so try some carbon remover like Seafoam, Merc #2 Quickleen, etc. I would do these simple things before a tear down and unnecessary $$$.
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    Quote Originally Posted by 25XS View Post
    The welch plug on top covers a chamber. That chamber is fed gasoline based on the idle air orifice... In turn, the chamber "drains" into the venturi above the butterfly plate through TINY!!! holes that need to be clean & clear. This affects your idle quality alot. Lean condition causes a crankcase backfire that you may be calling a "mist out of the carburetor".

    You have to clean them right, then replace ALL the fuel lines between the fuel filter and the carbs prior to reassembling the carbs to the engine. Otherwise, debris in the ancient fuel lines that is falling off will clog your new carb rebuild job.

    Tom
    Ok I'll pull the welch plugs on the carbs if the compression checks out right. I've got to find another tester to verify my readings.

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    Thanks everyone for the good advice. I'm going to try rechecking the compression first. My $15 eBay tester may not be accurate. I think pulling the welch plugs on the carbs is a must also. I did change the fuel lines, but I'm going to change them again and install a fuel filter. I've got a much cleaner 49ci powerhead in the shop, so if the compression fails I guess I'll be swapping powerheads.

    What's the best way to pull the welch plugs on the carbs? Do I drill them out or is there a better way?

  9. #19
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    Quote Originally Posted by camohunter View Post
    What's the best way to pull the welch plugs on the carbs? Do I drill them out or is there a better way?
    Using a drill press on your first time is a very good idea. You have much better "depth control" with a drill press. Once the drill drops thru the welch plug, you don't want to plunge into the bottom of the chamber and damage the carburetor main body.

    I use an 1/8th inch drill and pop a couple holes about 1/8th inch in from the edge. Then I use a very strong "scratch awl" (booger pick, o-ring pick, sharpened screwdriver, etc...) to pry out the welch plug.

    A factory carburetor rebuild kit will include the new welch plugs. Installation does not require any sealant, you simply lay the new welch plug into the hole, support the carb body solidly and use a 1/4" drive socket extenstion and give it a sharp smack with a ball peen hammer to "set" the new welch plug in the hole. You'll get the feel after a couple of them. You don't SMASH IT... Just a small depression in the center will set the new plug.

    Tom

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    Smile 75 HP Stinger

    Quote Originally Posted by 25XS View Post
    Using a drill press on your first time is a very good idea. You have much better "depth control" with a drill press. Once the drill drops thru the welch plug, you don't want to plunge into the bottom of the chamber and damage the carburetor main body.

    I use an 1/8th inch drill and pop a couple holes about 1/8th inch in from the edge. Then I use a very strong "scratch awl" (booger pick, o-ring pick, sharpened screwdriver, etc...) to pry out the welch plug.

    A factory carburetor rebuild kit will include the new welch plugs. Installation does not require any sealant, you simply lay the new welch plug into the hole, support the carb body solidly and use a 1/4" drive socket extenstion and give it a sharp smack with a ball peen hammer to "set" the new welch plug in the hole. You'll get the feel after a couple of them. You don't SMASH IT... Just a small depression in the center will set the new plug.

    Tom
    Did you check the reeds as suggested earlier? That will definitely cause poor idle and mist coming out the intake of the carb. The backfire is caused by a lean condition, which the reeds can create if chipped or broken. Also, contrary to other opinions, the bad reeds will reduce compression readings. On the piston down stroke, you create crankcase pressure which drives the intake charge into the cylinder. If reeds are bad, that pressure comes out the carb inlet, rather than going to the combustion chamber. The compression should be 125-135 psi. Also, the port timing is not different than the stock 70 hp. However, the factory did chamfer the upper lip of the exhaust ports to improve top end performance. All other ports are stock.

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