Re: 1976/77 OMC 9.9 & 15 carb needle valve bearing

[johnyrude200]

Now THIS, is a topic right up my alley.

I have fixed about 100 of these over the last 3 years. Not as many as some of the gurus of this forum, but to add my $0.02.

I agree, the carb change WITHOUT the low speed needle bearing seems to show up in ’79. HOWEVER, I JUST NOTICED that depending on the production line between 1974-1978, you MAY have a carb with a smaller mouth on the back of the carb. I only noticed this on the 9.9 carbs….haven’t handled enough 15hp carbs to notice a difference. The 1974-1978 motors seem most sensitive to the carb configuration.

Some production models have an AFT carb barrel with a smaller diameter. Did anyone know this?

My conclusion as to why later (as in, post ’78) carbs across the board DON’T need the low speed needle bearing has to do with the the orifice plug. These change depending on year.

NOW, granted, the ORIFICE PLUGS primarily affect the lionshare of fuel for high speed (which I surmise, affect top end power), we all know that the main jet controls the lionshare of fuel, and perhaps the low speed circuit, sort of, provides fuel in concert based on this orifice plug…? I would assume this mirrors when they introduced leaf valve shims, and a tuned exhaust.

But I’ve got about 50 9.9/15 carbs here, and there are a lot of variations of these carbs, despite what M.E. parts lists indicate.

I have had to transplant so many different combinations of 15hp carbs on so many different 9.9/15hp powerheads, that I don’t bother to use service manuals on initial test tank calibrations just to get the motor running. For instance, the old service books say a 9.9 uses 1 turn out, a 15hp uses 7/8th. Well, try putting an ’89 carb on a ’77 model or vice versa. Watch what happens. There is no rhyme or reason, in many instances, the low speed needle needs to be 1.5-2.5 turns out. Now the low speed needle TAPER seems to be the same across all years. Forget about post ’92/’93 models (totally different).

WHAT I LOOK FOR is if the motor starts easily or does it run in an odd manner (i.e. , inconsistent starting, bogging, bad idling, etc.). 99.9% of the time, REGARDLESS of what ‘generation’ of powerhead/exhaust housing combination you have, and whatever carb you are putting on it (in the 15hp variety), if the motor is running odd, it’s probably because the carb is missing that low speed needle bearing.

KEEP IN MIND, I am talking about if your ignition system & fuel system are fully rehabbed (which they are before I try anything), and there aren’t any bizarre situations going (i.e. air leaks somewhere). AND timing is set correct in terms of a typical link and sync (more difficult on the old metal cam follower style motors…plastic cam followers, while not as durable, are far easier to calibrate).