NS3.5A not pumping much

I have an old NS3.5A that starts and runs beautifully. I've found that if I start it at idle, it won't pump any water for as much as 30 seconds. I shut it down at that point. There is what appears to be exhaust coming out of the rubber outlet hose at start-up. If I open the throttle, it does start to pump but not much. I have replaced the impeller and head gasket (metal rim). Ran a wire into the water outlet. Checked the seal at the base of the water tube. The water tube itself appears to be OK (at least the bottom). I cleaned all the salt and sand out of the water jacket around the cylinder. I'm running out of ideas!! Has anyone seen this?? The season is fast approaching.....

If you are getting exhaust from the pee tube, you are not pumping well enough. The 3.5A has the pump on the prop shaft; no replaceable cup or wear plate -- turns slower than the pumps that are on the drive shaft, hence the bigger impeller for more volume. Too much side clearance in the pump could do that. Is there any grooving or wear in the housing? Did you install an OEM impeller, or an aftermarket one? You may have a worn pump housing, 309S650101, WATER PUMP CASE, $19.55, and or 309S601011, HEAD, GEAR CASE, $12.09, available from any dealer, including me.

The gear case head is indeed worn a bit....maybe a few thousands of an inch.....is that enough to cause trouble?
It is certainly easy to replace. I can't tell if there is any wear to the water pump case. Of course the seal and
bearing need to be installed in it....requires special service tools?? Will a Nissan Marine service place do that if
I pay for the parts?? The impeller I put in is a genuine Tohatsu.

I put a straight edge across the gear-case head....the wear may be more than a few thousands on an inch. We'll see what happens when I replace it......

Well......I replaced the water pump case and the gear-case head. That helped a bit, but it's still not pumping like it did when it was new. The impeller looks fine. I ran a wire up the outlet tube again. I'm afraid to take the cylinder cap off again....the bolts might break (they did before). Is there anything I could add to the water in my trash-can test tank that might help clear the cooling water passages???? Is the water tube seal a common source of leaks? Should I try compressed air up the outlet tube?

If you think it's pumping well, but the flow is restricted, you could add salt-away (expensive) of a mild acid. Some have had good results using vinegar in the tub, and then running for several warm-up/cool-down cycles.

Wow!.......I gave it a shot of compressed air up the rubber outlet tube. It's pumping better than it has in years. There is a steady stream....even at idle. Only wish I had tried that first!
END-OF-PROBLEM.

Believe it or not.....here we are 3 years later.....it happens again!
Cooling water coming out the rubber outlet tube slowed to a dribble.
Gave it a shot of compressed air into the outlet tube....it's pumping
like it hasn't in years. You don't need an air compressor...just get
a can of "dust off", make sure it is sealed tightly to the rubber outlet
hose and give it a good blast!

Believe it or not....here we are 1+ years later.....it happens again!
Cooling water coming out of the rubber outlet tube is just a dribble.
The compressed air didn't work this time. So, I decided to take the
outlet hose off and twisted a 5/64 inch drill bit into the hole using
my fingers only. There was a lot of stuff that came out. It's pumping
very well, again! I'm guessing this is a very common problem...easily
SOLVED!!