Ok – so back to the motor problem. Since I typed that long post yesterday, I took another look at the 9.8 carb vs the 7.5 carb and found that in addition to the high speed jet being different size, now upon taking out the brass “nozzle” that goes from the center of the float bowl to the plastic venturi in the carb throat, that nozzle was a lot different! The hole up the middle was same size, but the 7.5 nozzle is much shorter with fewer side holes, so that explains why it ran bad. All other carb parts except the nozzle and the jet appear to be identical. I expected the carb throat to be different diameter somewhere along its length – but no it’s not. Extracting the nozzle from the 7.5 carb was a challenge because it hung up on a small burr where the threaded hole for the high speed jet comes through, and I couldn’t get any tools in there to pull it out. I didn’t want to file the burr with a small round file because I was afraid of damaging the threads for the center bolt that fastens the bowl on. I finally managed to thread a very small sheet metal screw part way into the nozzle and pry on the screw head to pull it out.

I’ll try out the motor tomorrow with the 7.5 nozzle and jet in the 9.8 carb. Still a mystery why the 7.5 motor ran too rich to idle good with it’s original carb, but I don’t care any more – if it runs OK now, I’ll have it out of my workshop! I’ve learned a lot about these dark side carbs! I’ll let you know how it runs.
Dave

Before I put the carb back on, I took one last look and found that the throttle butterfly was different from the 7.5 to the 9.8 also. The three small holes in it that let air through at idle are slightly larger on the 9.8, so I swapped that part too – easy – it took about a minute. That’s why its minimum idle speed was faster with the 9.8 part in place and wouldn’t shut off with the throttle.

I put the motor on a boat and it runs great at all speeds above idle. I adjusted the low speed needle for best running and with everything I’ve done, the idle is much improved, but still not the best at slow trolling speed, but I can idle it down much more before it starts missing occasionally, and I think it will be fine for my neighbor to duck hunt with. It idles slightly better without standard plugs instead of surface gap.

It has a starting quirk. When it’s warm, it doesn’t want to start easy unless the low speed jet is adjusted richer than where it will idle fairly good, but choking the warm engine starts it right up. Given that, as you may expect, if I stop it with the choke instead of turning the throttle way low, it starts right away without the choke on again, because it pulled in a shot of gas as it shut down. It just seems to need to have a bit more gas in the system to start. The stop switch is disintegrated so I disconnected it.

I’m thinking the really low idle issue and the starting quirk is either reeds or crankcase seals, and I have no desire to pull and split the engine block to get at these parts. I did that several years ago to change a crankshaft bearing and a piston on a similarly constructed Mercury and don’t want to do that again, so I’ll explain the situation to my neighbor, and if his nephew has trouble living with it, I’ll loan him a motor for this year’s duck season.

I put more time into this than I wanted to, but I did learn a lot about the differences in the 7.5 and 9.8 carbs.
Thanks for all the suggestions!
Dave

• This reply was modified 4 years, 10 months ago by outbdnut2.

I noticed another typo inmy post above and am posting here so I don’t risk it going away. I meant to say it idles better with standard spark plugs, instead of surface gap, not without std plugs.
I just ran it for my neighbor and he’s sure his nephew will be happy with it for duck hunting.
Dave