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  1. K

    4 stroke v 2 stroke. Outboard

    I hope it's ok to revive an old thread. I have had my hands in old two strokes recently. Re-building them. Testing them on my 3.3M SIB. Two are late 70's, one is a 1981 and another is a 1990. All two strokes 7.5, 8.0, and 9.8 hp model Mercs. My son owns a 9.8 HP 4 stroke. 2018 year. He also has...
  2. K

    1981 Mercury 7.5 2 stroke

    Following up to wrap up this thread. I had this rebuilt Merc 7.5 outboard out on the lake today. I broke it in and ran it for another 2.5 hours. It ran very well. It has 7” pitch triple prop on it. Nice and quick to get on a plane. Top speed was about 14.5 MPH. Great little engine.
  3. K

    Stubborn To Start 1979 9.8

    I feel like I finally broke thru with this 9.8. Seems like I’ve solved all the issues it thew at me. No more need to give it a shot of raw gas at initial cold start. I’ve worked on many carbs. Rebuilds, jetting, tuning. This Merc carb is super finicky. It’s running nicely now at 80 psi...
  4. K

    Stubborn To Start 1979 9.8

    Some more tinkering today. My 9.8 is running the best now, since I acquired it. Yet it still wants a raw squirt of gas when stone cold, to initial start. Researching some more. I’m thinking maybe the part that my Merc manual calls it an “idle restriction tube” is suspect. I took a spare carb...
  5. K

    Stubborn To Start 1979 9.8

    I’m all-in now. L-R: 1991 8, 1976 9.8, 1977 9.8, 1979 9.8, 1981 7.5
  6. K

    Stubborn To Start 1979 9.8

    Coming home with 2 more 9.8 Mercs must have been intimidating for this problem child. Running great this afternoon. What did I do to solve this? I’m not 100% sure. I did swap over the flywheel from the ‘74. The two appear identical. I’m not convinced that solved it. I also unbolted the 3...
  7. K

    Stubborn To Start 1979 9.8

    Ya but I’m having fun and I will beat this. Lol. The two I just picked up both measure 105 psi. Both offer lots of spare parts. Both have perfect 2 blade props. Worth the price just for the props. I also have a 7.5 that can use all the parts as well. Its a win!
  8. K

    Stubborn To Start 1979 9.8

    Well here it is, the spark easily jumping a ¾” gap. I’m laughing at this point. It’s becoming comical. I really thought we were onto something with the spark that may not jump the gap under compression conditions inside the cylinders. I have fuel in the combustion chamber I have 80 psi in both...
  9. K

    Stubborn To Start 1979 9.8

    @Nordin I think you are on to something there. Under compression the spark may not be firing. What you say makes sense and is possible. Everything else I’ve tried isn’t improving things nor computing in my head. I don’t have a spark gap tester. I have ideas to try though. I have 2 spare coils...
  10. K

    Stubborn To Start 1979 9.8

    The Ignition system looks ample to me. I need to comfirm it’s occuring at the right time. These engines are pretty basic, fool proof. I am a bigger fool than average. Lol.
  11. K

    Stubborn To Start 1979 9.8

    Reviewing my outboard technical textbooks there are 8 different Mercury ignition systems that they used. You pretty-well need to be an electrical engineer to decipher them all. Lol. Seems as the magnets pass the trigger they do their own separate induction magic and send its own separate...
  12. K

    Stubborn To Start 1979 9.8

    Tried swapping the spark plug coil wire positions. No improvement. Also coils (2 qty) tested in spec. Flywheel keyway is in place. As mentioned in my post, the cut in ‘74 flywheel magnet seen here. Wonder if this sets the trigger coil to collapse?
  13. K

    Stubborn To Start 1979 9.8

    Are you using the Choke? Yes. With or without the choke. Made no difference. If I remember, the Fuel Pump is integral to the Carb, and there are 2 brands of Carbs used on the 9.8. Did you get the correct Overhaul Kit? I think I had the right carb kit. I ran around on a local lake for 2 hours...
  14. K

    Stubborn To Start 1979 9.8

    I just studied my Mercury shop manual here again to confirm. There are no published tolerances for piston clearance. No compression specs. No piston measurement specs.
  15. K

    Stubborn To Start 1979 9.8

    To clean the piston grooves I broke off a piece of the removed piston rings. I used it to scrape out any carbon. Honestly the engine was pretty clean in general when I tore it down. If extra scrubbing was needed I used Some steel wool and scotch-brite abrasive pads. Washed and blow-dried the...
  16. K

    Stubborn To Start 1979 9.8

    I used a 3 finger, sprung, straight-style home. I performed a quick hone. Not over-doing it. The carb float was set as per factory service manual. The engine does not have a decompression system. For break-in I warmed up the engine then worked it at various speeds. No full throttle for an...
  17. K

    Stubborn To Start 1979 9.8

    Someone told me they were low compression. I am doubting that. I have since seen folks on-line measuring 110 psi. The break-in exercise on the lake yesterday, it wouldn’t start. I squirted some raw fuel in the carb. It flashed up then. High idle and up, it seemed to run quite good. Wide open...
  18. K

    Stubborn To Start 1979 9.8

    I spent 2.5 hours out on the water today. Breaking-in the engine. When I got home I measured 75 psi still. Seems odd to me after a light hone and new rings. I measured bores and also used a dial bore gage. The cylinders measured to be well within specs. I re-used the pistons. There is no...
  19. K

    Stubborn To Start 1979 9.8

    Thanks for your responses folks. I feel like I solved it. It was indeed low compression. I lightly honed the cylinders and installed new rings. Compression test dry, netted 75 psi. I pulled the plugs and put some oil into the cyliders. I pulled it over a few times to coat everything. All the...
  20. K

    Stubborn To Start 1979 9.8

    The issue appears to be compression. I put oil in the cylinders and raised compression to 110 psi. She’ll run. I’ll see if seating the new piston rings will improve it. The cylinder taper and out of round was well within spec. Pistons looked great. Stock bore. New rings and honed cylinders...
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