I have a 1978/1981 evinrude 70 HP that was my grandpa's that I just moved to my 15'4" bass boat while I rebuild my 140v4.
The motor is a 1978 that had been sittings since the mid 90's. It was on a 16' fantail that he used to pull a shrimp troll so it has a lot of low speed hours on it. Compression on all 3 cylinders is 125psi+ and within 3 psi of each other. I did a full cleaning on all 3 carbs, link-n-sync, new plugs gaped @ 0.040", new water pump kit, changed gear case lube, new thermostat kit with poppets and springs.
On my first run I determined I was not getting fire to cylinder #2, after investigation, I traced it to a faulty power pack.
I had a 1981 that had water get to the lower crank bearing as a spare parts motor. It had a brand new power pack (plug in style). I transferred all eleectronics (timer base, stator, PP, coils, rectifier) and the fly wheel and harness over to 78. I verified all electronics and set the WOT timing in accordance with the cranking speed -4 deg BTDC instructions which meant 15deg at cranking speed.
On my run today (unloaded with only myself on board) I noticed it took forever to get on plane. Trimmed all the way down, I would throttle up, and it would hang @ 3-4K RPM for a little while then slowly make its way up on plane. Once on plane it act normally. I had a 17 pitch prop on it and my max RPM wold push almost 6K. Holding 5,500 RPM and trimmed all the way out, I was only about 3/4 throttle and running 32mph.
I switched to a 15 pitch to see if it would improve the hole shot. It did not, same issues getting on plane. Max RPM was well over 6500, and holding 5500RPM, was 2/3 throttle, could not trim up past half way or would blow out, and running 29mph.
I have a 21 and 23 pitch I could try but have a feeling that will make my hole shot even worse. I am thinking it is more of an engine issue rather than a prop issue. If I still have room to go on the top end, that would mean I am under propped, but my hole shot is horrible. What am i missing here?
My only idea at this point is a more thorough decarb procedure. With the many years this motor spent pulling a troll at slow speeds, I am sure the exhaust system is fairly carboned up. Would this restriction be what is keeping it from making it through the mid band quick enough?
I did notice after pulling a buddy's boat back 5 miles (engine loaded down @ 3K RPM), I made some WOT passes after I cut him loose and my WOT RPM went from just shy of 6K to 6200 RPM (I didnt hold it there). I am guessing restrictive carbon deposits broke loose and made their way out under load. This gives me more faith in my carbon theory.
The motor is a 1978 that had been sittings since the mid 90's. It was on a 16' fantail that he used to pull a shrimp troll so it has a lot of low speed hours on it. Compression on all 3 cylinders is 125psi+ and within 3 psi of each other. I did a full cleaning on all 3 carbs, link-n-sync, new plugs gaped @ 0.040", new water pump kit, changed gear case lube, new thermostat kit with poppets and springs.
On my first run I determined I was not getting fire to cylinder #2, after investigation, I traced it to a faulty power pack.
I had a 1981 that had water get to the lower crank bearing as a spare parts motor. It had a brand new power pack (plug in style). I transferred all eleectronics (timer base, stator, PP, coils, rectifier) and the fly wheel and harness over to 78. I verified all electronics and set the WOT timing in accordance with the cranking speed -4 deg BTDC instructions which meant 15deg at cranking speed.
On my run today (unloaded with only myself on board) I noticed it took forever to get on plane. Trimmed all the way down, I would throttle up, and it would hang @ 3-4K RPM for a little while then slowly make its way up on plane. Once on plane it act normally. I had a 17 pitch prop on it and my max RPM wold push almost 6K. Holding 5,500 RPM and trimmed all the way out, I was only about 3/4 throttle and running 32mph.
I switched to a 15 pitch to see if it would improve the hole shot. It did not, same issues getting on plane. Max RPM was well over 6500, and holding 5500RPM, was 2/3 throttle, could not trim up past half way or would blow out, and running 29mph.
I have a 21 and 23 pitch I could try but have a feeling that will make my hole shot even worse. I am thinking it is more of an engine issue rather than a prop issue. If I still have room to go on the top end, that would mean I am under propped, but my hole shot is horrible. What am i missing here?
My only idea at this point is a more thorough decarb procedure. With the many years this motor spent pulling a troll at slow speeds, I am sure the exhaust system is fairly carboned up. Would this restriction be what is keeping it from making it through the mid band quick enough?
I did notice after pulling a buddy's boat back 5 miles (engine loaded down @ 3K RPM), I made some WOT passes after I cut him loose and my WOT RPM went from just shy of 6K to 6200 RPM (I didnt hold it there). I am guessing restrictive carbon deposits broke loose and made their way out under load. This gives me more faith in my carbon theory.
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