Nissan 4/5/6 HP engines and revisions

[km1125]

I'm trying to understand the differences in revisions over the years for the NSF4/5/6 engines.

I have a 2003(ish?) NSF5B and a 1999 NSF4A. Other than the carburetors, what are the differences in the A/B/C versions of these engines? Are there parts that are NOT interchangeable through the years in these two engines (or others in this series)?

I had already upgraded the NSF5B to a 6HP with the carb swap, and I'm considering taking the old 5HP carb and putting it on the NSF4A. The carb is a 3R1COK2. Anyone see any problems with that swap?

Also, I'm thinking of taking the internal tank and cover from the NSF4A and putting them on the NSF5B and making it into a internal/external configuration and making the NSF4A just an external tank config. I have the parts I need on the way (I think)... are there any issues I should be aware of in making this swap?

I have three carbs... is there any way to check what these actually are for?
3H9BNI4 (I think was original on the NSF4A)
3R1COK2 (I believe a 5HP from my original NSF5B)
3R4AVA2 (a used carb that should be a 6HP.. but its not in great shape so I may not use it at all)

Specific to the carbs, the old NSF4 has a 70 main jet and the old NSF5 has a 65... would there be any value in using the 70 main jet in the NSF5 carb?

Also at the top of the water fill tube, where it curves and goes into the top of the drive shaft housing, I'm wondering if there is supposed to be a grommet or washer sealing that joint. It is loose in there. I'm not sure how you would even get that out, as there is the curve in the end of the pipe. Any insight to this?

 

[pvanv]

The 4/5/6 A was the first production run of these motors. The A2 was the next series, and is still the preferred carburetors, because they comply with an earlier EPA rating, which was not as restrictive as the current regulations. Any of these carbs will work on any of the motors.

3H9 carb is for 4 hp
3R1 is for 5 hp
3R4 is for 6 hp

You can NOT interchange jet sizes between different carbs. Bigger throated carbs with bigger venturis require bigger jets. More air requires more fuel for the same percentage of fuel in the mix... and produces more power. If you put a big jet in a small carb, it will be super rich and foul plugs like crazy.

Newer models use a single-piece ignitor, as opposed to the separate, discrete ignition components of the earlier models. There are also integral fuel tank changes to allow the front gas filler of the newer motors, and front shifter. That also means that the RC kit for the newer models is different from the earlier models. Original 5 hp versions did not have an integral fuel tank, except for special orders.

And of course, there were paint color and cowling changes over the years as well.

The upper end of the water pipe uses a square grommet with a round hole in the middle of it.

 

[km1125]

Paul, THANK YOU for your input. I have read MANY of your posts on here and you always have great info!

You can NOT interchange jet sizes between different carbs. Bigger throated carbs with bigger venturis require bigger jets. More air requires more fuel for the same percentage of fuel in the mix... and produces more power. If you put a big jet in a small carb, it will be super rich and foul plugs like crazy.

That all makes sense, but what I was confused about was that the older 4hp carb had a BIGGER main jet than the 5hp carb... or am I reading into that wrong? I was assuming a "70" meant a larger orifice than a "65". I was thinking perhaps the newer motors were running leaner due to emissions and perhaps I could go to a richer setting with the jet change. Not a big deal, but I thought if I had them available and apart, that would be the time to do it.

The upper end of the water pipe uses a square grommet with a round hole in the middle of it.

Can you just pull that water pipe up and out? I haven't tried yet but it just doesn't seem like you could with that bend at the top. I was wondering how they got it in there! I'll have to look closer at the parts diagrams to see if I can find that grommet. Thanks.

 

[Sea Rider]

To access upper water pipe and grommet must remove complete powerhead from base, once removed will see grommet located at extreme of water labyrinth. Probably will be better to remove lower leg too to disconnect water tube from water pump upper grommet and pull both faster.

BTW, is there a part code for upper water pipe grommet, can't find that part on service manual, seems the japs forgot all about it. Same grommet is mentioned with code 309-650311 on the 2.5/3.5 models which should fit perfectly.

Happy Boating

 

[km1125]

So I got the parts swapped around to get the internal tank on my 6HP motor and to convert the 4A to a 5HP with my old carb. I also had to change the lower housing, as the 4A engine I got had the cavitation plates and skeg busted off. I got a used housing, but I'm guessing it was a "B" or "C" model because it has slight changes compared to the 4A.

I noticed a few differences in the two engines.

The "A" had a round anode on the lower unit, the other one has a rectangular one.

The "A" unit had a tiny offset in the screw on the water intake screen. When I put that screen on the new housing, it was cocked about 5 degrees to one side.

The "A" motor has a straight nipple for the crankcase ventilation hose. The other motor had a 90 degree fitting which makes attaching the hose much easier.

Before I got the motors done, I took my old 5HP carb (the 3R1) apart and cleaned it out. I also drilled out the idle mixture screw cap so I could remove it and clean in there too. Then I took it out to the lake with my 6HP motor and did a few "speed" runs. Really, I just wanted to make sure the 5HP carb was running good before I tried it on the 4A motor, as I had no history on that motor.

While I was out, I did a run with my current carb (3R4), then swapped the 3R1. Then I took the main jet and from the 3H9 (4HP carb, but it has a #70 jet) and put it in the 3R1 to see what difference it would make. While I was swapping carbs, I noticed that the spacer between the carb and intake manifold was "backwards", which slightly restricted the opening, so I put it on the right way and re-ran the 3R4. These tests were on a 8'6" West Marine fold-up inflatable with a wood floor and inflatable keel. I had a cooler, 3gal gas tank, anchor and PFDs and my ~200lbs in the boat. The lake had about a 12 MPH breeze that I was running with and about a 1' chop. I got too beat up trying to run against the wind, but I had wanted to make passed both directions. I also am running the "elephant ears" prop.

3R4 6HP 16.2 MPH
3R1 5HP Stock 16.2 MPH
3R1 5HP w/#70 15.7 MPH
3R4 6HP (fixed gasket) 16.8 MPH

 

[km1125]

Oh, and Sea Rider, I did get that grommet and replace it. There was one in there that was so dirty I thought it was metal and couldn't figure out how to get the water tube out. Once I removed that base gasket it was obvious that the grommet just slides up and out with the tube.