DT65 powerhead small crack

[Frazys]

Hi guys,
I have 2 problems and will start with the worst, I have a DT65 that I recently got running and today gave it a good run with the muffs on. After a few minutes the thermostat opened and water ran out the pee hole so I shut it down. I then noticed a small puddle of water on sitting on the cylinder head and a small crack on top of the powerhead.
I have a few options;
1. Try to weld/braze the Ali casting, I know castings are not great for welding, but hopefully a pro can do a good job.
2. Devcon, A friend suggested this which is an epoxy with aluminium fillings.
3. Full part replacement, man that’s a big job.
4. Wreck it lmao I’ve spent a bit of time on her and she’s almost running mint.

Attached is a photo of the crack and a video URL of the of the 2nd problem and a photo of the air pilot screw.
https://youtu.be/8BwpnvTwGsY

2nd problem is fairly minor considering. A few weeks ago I resolved a no spark issue which was a combination of a faulty emergency stop switch, a failed fuel pump and not turning on the ignition when testing with a remote starter. After resolving these issues I had her running for a few minutes and opened the throttle to WOT. When I pulled back the throttle, the engine stalled and wouldn’t start. I thought I would pull the carbies out one by one and give them a clean. The top was nice and clean, #2 had a little gunk and the 3rd had a lot of what I guess is old fuel and a fair amount of corrosion. After cleaning it still would not start so I tested the fuel pump (I had just tested the pump and it was running) which failed due to a split the diaphragm. I had be meaning to do a compression test which came back ok #1 80psi #2 105psi #3 80psi. The first pump had a failed spring so I used the good diaphragm and built one good pump, tested and passed. Engine still wouldn’t start, I knew it had spark (it ran a few days earlier), the rebuilt fuel pump worked and the bowl was filling but wouldn’t pass up the jet and through the Venturi. I squirted some “start ya bastard” in the carbies and she jumped into life and ran (anyone have any insight as to why it took the additive to make her start). After starting the engine would start without the additive.

Now the engine won’t idle, it runs nicely and will idle with the the choke open slightly. My guess is the damaged taper on the air pilot screw Is allowing to much air into the mixture, I’m going to try turning a new taper on the screw (I’m a machinist). It will make the screw seat a millimeter further, or will I have need to make new ones, otherwise i don’t think anyone has new ones in NZ so the next bet is to import from the states.

These are my thoughts and best practices but with input is appreciated.
Thanks

[mphelle8vld]

You appear to be at an expensive crossroads. I can't make out the crack with all the corrosion but it would seem a proper repair would require a complete teardown. It's hard to get that motor to run right unless the carbs are spotless, an ultrasonic bath works well. If the bottom carb is too badly corroded I have a set that I would be willing to donate but you didn't mention the model year. I believe the bigger problem to be those compression numbers, even my 85 had 120 in each cylinder. Your numbers would indicate possible scored cylinder walls, broken or stuck piston rings, corroded head gasket/sealing surface, really low cranking speed, perhaps even a bad gauge. A couple of other thoughts, you should have a strong stream from the pee hole even at idle, cold or hot. You shouldn't rev the motor over 2k rpm on muffs with no load, it can easily "run away" even running on one cylinder. Running on muffs won't give you any indication of the health of the motor, it will both idle and scream running on 1,2, or 3 cylinders. You will only know you have 3 healthy cylinders when it will turn 3 thru 6k revs pushing your boat. This looks like a fun learning project but I would find out the cost of a running replacement motor and write that number on the wall of your shop where you can see it while you are working on this one. Where are you located?