Crank mount pump

I have a 1975 350 OMC motor that the water pump finally gave out and pours water out at the seals, nobody services it and impellors were difficult to get so decided to put a Johnson crank mount pump on. I have 1.25" hose from seacock to pump and the 1.25" reduced to 1" (how the old pump was) because that is what size the transmission cooler is. Got everything rigged up and thought all was well till I ran it at the lake and the first thing that happened is it blew the 1.25 out line on the pump down and sprayed water everywhere, I thought it was over and she was going down till I figured out what happened. I then tightened the hell out of the hose clamp and it stays on but actually blew a hole in the line out of the trans cooler next, maybe the line needs replacing but it looked good. I assume necking the pump down and increased volume and pressure with new pump is doing all this. I am wondering if anybody else has done this and had to neck it down to 1" after the raw water pump, I do not like how tight I have to put hose clamp on the bib of the output side of the pump, seems like something is going to give. Then during all this at low idle my transmission is now making funky noise in forward and reverse, no water in the transmission fluid.   Not a good start for the year.

I reduced it thru a PVC reducer, as smooth a transition as I could find. The hose that blew is old and is not correct hose, PO had used white PVC hose that is not very strong. Idle is low on motor as for some reason it is idling low. I purchased the boat last year so it could be the dampner or maybe to low an idle. Guess I will adjust my idle and then see what I have.

Go to hose clamp for soft hose and tube

"An extended tail acts as a liner to protect soft hose and tube, such as silicone, from being cut by the slots in the band"

I had the exact same situation, Chevy, OMC, crapped out pump, went to crank mount. I didn't have any problems like you describe, I am thinking faulty-weak connections and faulty-weak hose, tho you said it seemed fine. That barbed fitting above look ideal for the reductions.
Here's my set up.

1 1/4 (or is it 1 1/8?) out of the through hull



Since both in and out hoses to the pump terminated short because the crank mount is farther from the hoses than the pulley mount was, I got some 1 inch hose, by digging through all the variety of radiator hose at the auto part store till I found two with the right bends. I used copper connectors for the reduction connection, the radiator hose had enough flex to stretch over the larger diameter. Conventional hose clamps.



One inch into pump



From pump outlet to tranny cooler, same connection, back up to 1 1/8 (or 1 1/4) as this is what the fittings on the cooler are.



1 1/8 into cooler.



I never really noticed that the 1 1/8 out of the cooler had been reduced to 1 on its way to the thermostat, but this connection has always been here.

    

That is different than mine. Tranny cooler back to t-stat. No other hoses entering manifolds other than the two up front that come out of the t-stat housing. I have a 305 not a 350 but I would think all the fittings would be the same.   Same manifolds, just different end caps. Odd.

Just pulled the other riser and there is no hole so for sure riser is bad.

Here are some pics, still waiting on FRP elbows to connect exhaust. I guess because I am a Marine surveyor by trade everything has to be to ABYC standards, though I did use PVC in the water line for the elbow. PVC is not supposed to be used on any below water line connections but I figured since it was freshwater and could not find bronze and hate galvanized fittings that I would use it. Total change in plumbing of the motor. By the way I have the old manifolds for sale if anybody needs parts, listed them in the for sale section.