97ish 350 SBC, thru-hub exhaust (Y-pipes)

Problem: VERY little water spitting out of the exhaust bellows (switch from earmuffs to barrel of water to ensure enough flow)

Things I've done to troubleshoot:

1) New lower unit impeller (professionally installed after I wanted to verify my work)
2) swapped thermostat housing (w/ new thermostat) with the other engine - problem did not follow
3) removed risers and exhaust manifolds - verified no clogs with easy flow of water thru with a hose
4) stuffed the hose in the top of the Y-pipes and verified it poured out of the exhaust bellows with solid flow
5) removed raw water hose at the thermostat housing and verified water was pumping in - like a good garden hose
6) replaced the water circulating pump with a known good pump (old one didn't look bad)

Only thing I have left in my plan of tests is to pull the two hoses at the thermostat housing which exit the intake manifold and feed the exhaust manifolds from underneath. Was just about to try it and it started pouring rain here! If that shows little to no flow thru the engine, what could it be? If that flows nicely, I give up.

Any ideas?

Couple of questions..
1) Is this something that you just noticed and before it use to pump a lot of water out of exhaust?
2) What is water temp at? Both on hose and under load?
3) Do you know how much water pressure you are running?

1) I replaced the intake manifold and carburetor (both engines). Starting it the first time this season, it started fine, then I noticed no water coming out and it started to get warm (~175 deg) so I shut it down. After dropping the lower unit I found a bunch of grass - a nest in there. I wondered if blocking the exhaust could prevent water flow. That wasn't it. I then noticed it was hydrolocked after that. I believe when I shut it down, the hose was still running and got in the cylinders. I pulled the new plugs, cranked it over to expel the water, put the plugs back and it ran well. Checked the oil for water and didn't notice any. Put it in a lake for the first time out and it started to get hot - 190+ deg so I shut it down. Found milky oil too. On my second oil change to fix that. expecting to have to do 3. The water flows strong out of the other engine.

2) see above for temps. I don't use a hose anymore - I found a plastic wheel barrow that fits nice - fill it with water and leave the hose in it running.

3) No idea on water pressure. Seemed like a good flow coming in to the thermostat housing.

Rain stopped - heading out to check the flow out of the engine at the thermostat housing.

Thanks.

7) Pulled the two hoses at the thermostat housing which exit the intake manifold and feed the exhaust manifolds from underneath. No water! DRY!

8) Repeated test #5 (above) - this time, NO WATER!!

9) Inserted the garden hose into the raw water input at the thermostat housing, water came out from the hoses in #7 above (the two hoses at the thermostat housing which exit the intake manifold and feed the exhaust manifolds from underneath.)

So now it looks like the brand new pump in the lower unit isn't working - BUT IT DID 3 DAYS AGO. This is CRAZY!!!!

OK work your way back toward the lower unit impeller. Next step take the hose off at the oil cooler below the port side of the motor. Run the motor briefly & see what kind of water you get there. if still none you could pull the hose that enters the transom behind the drive. Although those are a bear to reinstall.
What's weird is you had good flow at the thermostat inlet in an earlier check. My guess is your wheelbarrow got too shallow and allowed the impeller in the leg to fry. I'd reinspect the impeller assembly at this point.

I suppose you could swap drives. But when you do, use a 2 sided muff or a barrel that deeply submerges the pickup. While the drive is off you could blow or rinse backward through the cooling system just in case there's any debris in there. And don't rev it too high, just let it come to temp and open the stat. Once warm keep it below 1500 rpm. Don't run the motor with the drive tilted above the trim limit in the trailering range.
That's all I can think of, hope it turns out to be something easy and cheap!

Thanks for the help. I forgot to mention, I did put the garden hose in at the t-stat end of the hose that runs to the lower unit to check for blockage. Water came out of the pickups on the lower units...

unfortunately, sometimes the impeller can come apart with only a very short time when run dry... not always, but often.

I had an issue with a Perko Flush kit that had an internal check valve that would close each time you put water pressure to the flush fitting. After flushing engine one time the check valve stayed closed, sorta glued into place by a very thin coating of slime/algae. Started engine, and the suction of the sea water pump was not enough to unstick the valve and after 10 seconds ruined a good impeller. Thought I had a bad impeller, not drawing water so replaced it with new one, housing was perfect, got all the pieces out so I thought I was good to go. Started engine, 15 seconds later ruined the brand new impeller. SO....after checking all the lines to the pump for blockage finally pulled the flushing fitting. Used a long screwdriver to push the valve open and that is when I discovered it "glued stuck", removed valve from both engines and never looked back. I was upset to learn how fast an impeller could be destroyed running dry!! For my rig it is only 10 to 15 seconds!!!!

replace the impeller again,then take boat to water,forget about the muffs,they dont move much water.the impeller can fry in under 30 seconds if not WET.

Take a look at the hose that connects to the transom fitting.
Water comes in from the drive through a hose connects to the gimbal housing and then another hose connects to the gimbal housing fitting inside of the boat. On some units they used a plastic sleeve that is wedged inside of the hose. This is often where you get corrosion and the inside diameter of the hose is reduced substantially. Inspect the inside of the hoses and the fittings, could be a possibility. Let us know your solution.
I had the problem and the motor would run fine up to 3500 then start to get hot, bring the rpms down and it would cool down. All becaued of corrosion inside of the hose. I would guess the hole diameter was reduced to 3/16 of an inch.
I;m not sure but I think Mercury actually had some type of a recall on the hose connection ??
Good Luck
Brian