NSF4A won't run above 1/3 throttle

[YN2(SS)]

I have recently got this outboard running. I:
Changed the lower unit oil.
Replaced the complete water pump.
Changed the engine oil.
Lubed all the grease zerts.
Cleaned and gapped the spark plug.
Drained and flushed the gas tank and
rebuilt the carb.

So we get it on the lake the other day and after a few pulls it started. Then I set the idle RPM and mixture and then put it into gear and set the idle RPM and mixture under load. Got it idling great in neutral, fwd and rev.

It, however, won't run above 1/3 throttle. When the throttle is twisted to that point, the engine backfires and dies. Doesn't matter if I twist the throttle slow or fast. If I ease off the throttle quick enough, I might be able to keep it from dying.

My experience backfires are a lean mixture problem. I have not:
1) replaced the fuel filter or
2) rebuilt the fuel pump.
3) Is my carb float sticking?

Any thoughts? PNAVP?

 

[YN2(SS)]

Re: NSF4A won't run above 1/3 throttle

My apologies, I meant to type PVANP. I mentioned your name because you have been a great help in the past.

 

[pvanv]

Re: NSF4A won't run above 1/3 throttle

Above about 1/4 throttle, you are running on the main circuit of the carb; Below that, the idle circuit is doing most of the fuel delivery. You are correct in thinking that a carb backfire is often due to a lean condition.

Odds are you have a clog in the main circuit of the carb. The 4/5/6 is setup very lean for emissions purposes, and the passages can easily get varnished up.

The cool thing about the 4 hp model is the dual fuel tank capability. Late models have the external fuel connector standard, but the A version would require a few parts to get that capability, so you probably have only the integral tank -- unless the PO added the external-tank parts. If you are running on the integral tank, the fuel should make it to the carb OK, even if the pump isn't delivering well, so that's probably not an issue. Likewise, while it's prudent to change the fuel filter, if there's fuel in the carb bowl, you should at least be able to throttle up for a brief time, even if fuel delivery is slow (due to weak pump and/or slightly clogged filter).

I would pull the carb, disassemble it (including the jet, nozzle, etc, and soak it in a good cleaner such as Tyme, then blow out with clean compressed air, and reassemble, verifying float and needle/seat settings.

While the carb is off to the side soaking, check the fuel delivery. Opening the gas **** should allow gas to dribble out of the end of the fuel hose where the carb connects, as long as the fuel level in the tank is above the carb level. (Use a small cup to catch the gas, of course). If so, then give a few pulls on the recoil starter, and you should see pulses of fuel coming out, proving out the fuel pump. If this proves out OK, then after you finish a proper soaking of the carb, you should be good to go.

In the event you can't get the carb right, you may have to buy a new one. That can be the case if there is oxidation (corrosion) in the passages. If so, consider that all 4/5/6 motors can use the carb for the 6, giving you 50% more power.

 

[YN2(SS)]

Re: NSF4A won't run above 1/3 throttle

PVANV,

I always appreciate your insight. I was happy to get the motor running this well and the owner gave me my first sailing lesson. Very nice!! I had soaked/cleaned/blew out the carb already but will go after the main circuit again and see if I can find the culprit.

Just a thought, have outboard motors for sailboats ever been designed like this to prevent too much power? I was surprsed how fast this sailboat (28 Catalina) went with as little power as this 4 horse was able to produce.

Thanks, John

 

[pvanv]

Re: NSF4A won't run above 1/3 throttle

Your 4 hp motor may already have a 6 hp carb on it, making it a 6 in 4's clothing. You can check the casting number on the carb to verify that. The 4/5/6 does have a lot of thrust capability for its size.

Yes, OB manufacturers produce "families" of motors, based on the same blocks, but setup differently, in order to get a broad range of output power ratings. In 4-stroke Tohatsu Land, the 2.5/3.5 is one family, as is the 4/5/6, and then the 8/9.8, followed by the 15/20 and finally the 25/30.

I usually spec an 8 or 9.8 for that size Catalina, and save the 4/5/6 for the Catalina 22's. I also usually spec a high-thrust prop for sailboats, in order to get low-speed control and better reverse.

 

[YN2(SS)]

Re: NSF4A won't run above 1/3 throttle

The owner took the motor to a shop, she needed it running. The main jet was clogged. Either I missed it on the soak and rebuild or it got something in it. Problem solved. Case closed. Thanks all for your help.