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1980's Johnson/Evinrude 20/25hp conversion to 30hp with pics
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<blockquote data-quote="Edstrossner" data-source="post: 373688" data-attributes="member: 15774"><p>We had a beautiful day here in Georgia and I took the Lowe out for a spin . I just finished modifying the head so it was time to see if the mod did anything. The answer is a definite Yes!</p><p>Starting with the 1990 model, the cylinder head was changed to a low compression head. Apparently OMC was selling these motors to areas of the world that have poor quality gas. Their manual states that the minimum octane for the 9.9,15,25 and 30hp motors is 67 octane.</p><p>So, the stock head compression is around 100 psi. After my mod, it's at 145 psi.</p><p>Stock head with an 11 pitch prop and a 70hp carb......5800rpm and 25mph gps.</p><p>Modified head with the same prop and carb hit the pack limit of 6100 rpm so I switched to a 4 blade, 14 pitch prop.....5400rpm and 29mph.</p><p>I then tried a run with the hood removed. Hit 5800 rpm and 30mph. I suspected that the hood does not have enough air intake space.</p><p>I'll be adding some air holes to the hood next. The space just below the grab handle is a good place to start. After that it will be time to raise the motor.</p></blockquote><p></p>
[QUOTE="Edstrossner, post: 373688, member: 15774"] We had a beautiful day here in Georgia and I took the Lowe out for a spin . I just finished modifying the head so it was time to see if the mod did anything. The answer is a definite Yes! Starting with the 1990 model, the cylinder head was changed to a low compression head. Apparently OMC was selling these motors to areas of the world that have poor quality gas. Their manual states that the minimum octane for the 9.9,15,25 and 30hp motors is 67 octane. So, the stock head compression is around 100 psi. After my mod, it's at 145 psi. Stock head with an 11 pitch prop and a 70hp carb......5800rpm and 25mph gps. Modified head with the same prop and carb hit the pack limit of 6100 rpm so I switched to a 4 blade, 14 pitch prop.....5400rpm and 29mph. I then tried a run with the hood removed. Hit 5800 rpm and 30mph. I suspected that the hood does not have enough air intake space. I'll be adding some air holes to the hood next. The space just below the grab handle is a good place to start. After that it will be time to raise the motor. [/QUOTE]
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1980's Johnson/Evinrude 20/25hp conversion to 30hp with pics
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