After spending 3 years on my 76 here are my thoughts on these older carbs..I have had to set the float level slightly higher with the aftermarket plastic floats for the idle to be right on these first carbs. It would lean stall when throttling back. I wonder if the cork and plastic floats are actually equivalent. I got this tip from a member here I think.. The following story is for my 15hp but might be of some help..

I took out some cam lift from the beginning to about 3/4 or just where the cam starts to curve up. Not sure what cam you have on that 9.9..I can’t see how there can be a smooth transition from fully closed to just opened. So I set the roller at the vary beginning of the cam to just have the carb barely opened. I soldered the 2 holes closed on the butterfly and filed a flat on the top of the butterfly as the later ones have. It now runs better than ever. Even the wife commented about how smooth the motor runs when docking and such.

When I first got this motor 3 years ago all apart in a box it had a piss poor carb setup with it. I have spent hours on that carb..looking at ebay pics of carbs and looking at every carb exploded view I could for these motors. I have had it apart I betcha 20 times in 3 years and have made many changes that were a step back..

I believe the motor your working on doesn’t have points so you can’t use a timing fixture on my buddies 1961 50hp Mercury when the throttle started to open the points needed to open or it would stall not really your problem but interesting about timing of ignition verses carb. I was thinking what if you took off the cam roller on carb and adjusted timing to see if you could get a good idle if you find it then you would know when butterfly needs to open. I think one of mine I actually had the low speed screw turned out like 4.5 turns letting more fuel in at idle. Just wonder if that might help.

Also when I slow down an outboard I do it slowly and let everything catch-up since its all mechanical. I am sure you do also but I have had to show folks that when operating an outboard.

Joe

When I was young I didn’t even know an outboard could run well at low speeds. I thought we were flying if it would go well at high speeds lol. As I get older I appreciate knowing that they also run well at low speeds when everything is working together in harmony. A lot more goes into that and I think its a great challenge but can be very frustrating to get there. But when you do its an awesome feeling. Nothing like it.

Not many reed valves worth checking to see if they might not be sealing correctly to rule it out or rule it in.

Joe

Someone here might elaborate on this more about spark plugs.

One thing I did do to the 1975 this past winter was a trick I learned in a Scott Atwater manual to the NGK spark plugs I had in it. It was called old timers tricks in a manual written in 1950. 1 trick that I applied was this Champion would call their plugs like J6J or J6C the J at the end meant the side electrode was ground half-way back to where it was only covering half way over the center electrode making the fire jump in a sideways motion. I filed my NGK plugs to that patern and it seemed to help at idle making plug less likely to foul.

The other old timer trick was to mark the spark plug where the insulator curved tip is on outside of plug and when you install plug make sure the mark is in the down position even if you have to put in a second washer this helps a cylinder that is fouling. It also mentioned widening the plug gap on that cylinder also a tad.

Just something to ponder,
Joe

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I have learned a lot on this thread especially page 4 . I have tried to index the spark plugs also on my ’76 9.9. So i should have the ground electrode at the bottom,with its point facing up to the center electrode?
thanks,Jim PSB