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1974 40 hp issues

P

Patrickstar74

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Originally this motor was pushing my 15 ft fiberglass tri-hull about 7 mph. Found a bad plug wire. Cleaned carb. Changed plugs. Also swapped props(believe old one was slipping). Compression is approximately 120 in both cylinders. Throttle does open fully on water, in gear. Took back out yesterday, ran better then first time. Top speed was approximately 12. I moved the trim to all 4 positions and found what I believe worked best and left it. The motor that was on this when I bought it was a 55 Chrysler and it flew. Till it blew up lol. Ran about 25-26. Planed out great. I didn’t think there would a huge difference in 15 horsepower. So anyway, the prop that the motor calls for is a 10 3/8 x11 1/2. Coincidentally the same that was on the Chrysler. What’s on it is a junk 10 1/2 x 13. Could this be a major factor? It basically seems like if this was a car it had 5:13 gears in it and your trying to cruise 100 on the highway. This is my first boat and boating experience but I believe this thing should perform much better. Especially when I seen a similar boat on the lake yesterday with a 9.9 and he gapped me like I was in reverse. Everything so far has been an improvement so I need a little insight from someone with some experience.
 
Post your compression numbers.-----Then check for strong spark.----Spark must jump a gap of 5/16" on a test device.---Check fuel pump diaphragm.------Sounds like it is running on 1 cylinder.---
 
racerone said:
Post your compression numbers.-----Then check for strong spark.----Spark must jump a gap of 5/16" on a test device.---Check fuel pump diaphragm.------Sounds like it is running on 1 cylinder.---
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compression numbers are posted. Both are around 120 each. I did test spark and yes it does jump the 5/16 gap on the test. Nice and blue. Both cylinders. This is how I found the bad plug wire. It’s clearly firing on both. I also had the fuel pump apart as well and it looked good.
 
racerone said:
Sounds like you need to adjust the carburetor then.
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so nothing I’ve asked about the prop is of any relevance here? And as far as carb adjustment, the sticker on the hood says initial setting is all the way in and 3/4 out and go from there. It idles great. Doesn’t seem rich or lean. No sneeze. So what more exactly can I do with it?
 
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The best you can get out of it is 12MPH? Something is sick here. I don't think a prop is going to fix it. Unless of course it is slipping. Is the motor racing but not going anywhere?
 
racerone said:
The poster wanted some input on a prop.----Assumption is that there is nothing wrong with his motor.
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Assumption was correct. Motor is fine as I suspected. Changed the prop and ran it tonight. Came up on plane beautiful. Motor sounded better then ever and it topped out at 23 mph with 2 200 lb adults in it. The previous shifting issues have also been resolved. This little $200 motor finally performed like I had hoped.
 
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