Yamaha 784cc into welded mod-v hull


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Looks like great progress!

If your welds don't need to be waterproof, then making a bunch of short ones may be easier. I had the problem of heat getting head of me & I would burn a big hole after about 4 inches.

Regarding running the engine out of water, it behaves drastically different with no load, won't tell you much. I tested mine by tying boat to traiier extra well & running engine with trailer backed down ramp. Stirs up some mud but much safer than being out in the current.

I had problems with th 'pop-off' springs, made it run bad at midrange. There is a pressure test for pop off you might want to do.

Good luck.
quick update, finally got the carburetors back together, and am installing them back into the waverunner. There's a hose that runs to the top of the bearing in the propshaft, but it's dangling into the bilge. What's it supposed to connect to?

The reed valves look to be in good shape too, so that's a plus. Hope to get the donor waverunner water tested this coming weekend. Will likely rip around with it for a couple weekends to learn to drive it before tearing it apart. What's it cost to have the driveshaft/propshaft shortened and re-splined?

Hoping to get this on the water soon.

Green hulk website is a great resource to research your hose question.

Getting a shaft resplined will be very expensive. Why do you need to do that? Some brands have different shaft lengths (2 vs 3 person).

Good luck
CedarRiverScooter said:
Sorry to burst the bubble but no way will a 110V machine do aluminum.

It takes over 150 A into the wire to get penetration.

Sent from my SM-G955U using Tapatalk
Do yourself a favor as I'm sure you will use it plenty on future work, and just pony up the money for a decent 220v welder. Being a hobbiest welder like me you don't need a top notch unit, but it's worth buying a decent one that will last. I bought this exact package around 2 or 3 years ago it's a great little welder I've done a few aluminum projects with the spool gun, and plenty of steel ones with the regular mig gun. I did a whole right up on when I made a transom extension for my boat to go to an outboard jet, and when I added in an additional cross support to my trailer. All of that was done with the little hobart along with lots of other little projects in the garage since. You can't beat it for the price. Sure there are cheaper units, and better ones, but I don't think there are better ones for the money.


promo code for 20 bucks off


It's a little heavy being a transformer powered unit vs an inverter powered unit, but I preferred that. Way I saw it my transformer powered lincoln square wave tig machine is still working great, and I bought it used. It's probably around 15-20 years old. Being that you seem like a garage and project junky like all of us I don't think you will regret getting a better welder. I know I haven't regretted having that little hobart mig, wish I had bought it sooner. Way I see it, it should last me a long time one of those buy once cry once things. Maybe I'm weird I have a hard time buying a new phone that cost 700 plus bucks that may last four years before it's crap or obsoleted. The welder was the same money and will probably last me a long time saving me lots of money I would have had to pay for another person to do welding for me.
Prowelder said:
CedarRiverScooter said:
Sorry to burst the bubble but no way will a 110V machine do aluminum.

It takes over 150 A into the wire to get penetration.

Sent from my SM-G955U using Tapatalk

Forgive me, I'm not an experienced welder by any measure, but would you explain what is wrong with the weld in the OP's practice attempt with 90 degree pieces which appear to my untrained eye to be firmly joined.

I've got a 110v setup I was hoping to use on a framing project using 1/8" materials which I was going to try heating the backside of the metal with a torch and welding on the cleaned surface on the opposite side from where the torch flame made contact.

I think if I'd made joints with a bead laid down looking half as well as his did I'd be ecstatic, and yet you're saying they aren't penetrated far enough?
He made some good welds I wasn't knocking them at all. All I did was suggest a welder that would be much more well suited for the job. I'm not a fan of 110 welders outside of sheet metal you usually find their limits pretty quick, the op had found the limits it seems as he has stated he is pretty well maxed it out to weld what he has. It's working but I imagine he would be happier with a 220v machine as has already been looking for one, and I just made a suggestion for a 220v welder that I have, and have been very happy with.
At max capacity you will overheat the machine quickly and eventually cause damage

Sent from my SM-G955U using Tapatalk
Guys, I do appreciate all of the tips, and the encouragement.

Project is going much more slowly than I had hoped. Ski doesn't run properly yet, but it does run on both cylinders now. Didn't realize what it was supposed to sound like. More like a motorcycle than a 1cyl chainsaw. lol. When I put it in the water in mid april, it was running on just the rear cylinder. But lacking any jet ski experience, I didn't realize that. I can tell now by the sound :O pulling a plug wire to the front cylinder confirmed that it wasn't firing. So I pulled the carburetor off and took it apart. And put it back together again, and it runs on both cylinders now. I've learned so much, and am realizing how much I have yet to learn.

The engine doesn't idle properly yet, and midrange has a lot to be desired. But full throttle over 4000 rpms and it's in a happy place. Put a big smile on my face. Engine feels strong up top. So mechanically it's sound. Just needs the carbies fixed properly.

The hose going to the driveshaft bearing is a grease hose, and was originally secured to a grease fitting on the expansion chamber of the exhaust. So I put it back where it belongs, and pumped a bunch of grease into it.

Once it's running properly, I'll take the engine out and start the layout process. I haven't purchased my sheet of aluminum quite yet. First, have to lay out, then build the water inlet and grille/strainer. Then order the aluminum, and build a jig/fixture on wheels. Still a ways to go. I'm still hopeful that I can weld it together at home. But haven't really practiced with bigger pieces of aluminum yet.

See you on the water!

handyandy said:
Do yourself a favor as I'm sure you will use it plenty on future work, and just pony up the money for a decent 220v welder.

This is actually a very serious consideration. Because I miss having a welder for steel, and I think it would be better to have a second unit for aluminum. Just need to see with my own eyes that the sp125 won't do the job. Then I'll respool it for sheet steel.

I suppose I'll need a 220v outlet in the garage too. Unfortunately I'm eyeing up the miller synchrowaves...

One step at a time.
If you want to spend the money on the synchrowave go ahead they're great machines. I couldn't justify that much money for a welder as a hobbiest home shop welder. I've been really pleased with the hobart 190 I have. The spool gun works great did a great job welding the 1/8" thick aluminum for my transom extension have also welded some 3/16" thick with it. It does very well on steel as well. When I first got it I was worried I wouldn't like that it doesn't have infinitely variable voltage settings, but honestly it's been great I haven't encountered anything I couldn't weld well on one of the settings. If you can afford better, you won't regret it, and I'm the sort of person 220 is a requirement in my garage between air compressor, lift, welders, and plasma cutters you can be so much more capable with 220v in a garage.
I have both tig and mig machines I wouldn't give up either, for new aluminum that is .090 or thicker I prefer my hobart with a spool gun any day over my tig. You can burn in a good weld so much quicker, but it doesn't replace a tig. The tig is great for welding anything, especially repairing old thin jon boats for people the mig just can't weld the thin varying thickness of old thin aluminum. It's also great for welding cast stuff assuming it's a decent casting of good material. Some castings are just garbage and near impossible to weld. One of the syncrowaves would be awesome 115v and 220v capable, AC or DC, mig, tig, and arc all in one package would be awesome. I have my tig and mig set up on one cart with both an argon and C25 bottle I call it my poorman multi process machine.

This would be awesome to have.
And, because I finally got back in the garage to pull the carbs for the Nth time, I took pics of the grease hose. Just because it confused me for a while. Unfortunately I'm becoming well versed with pulling the exhaust manifold.

The grease hose to the support bearing. Which, of course, has never had grease in it, for the first 18 years (56h) of it's life.


The location on the exhaust manifold to apply grease with your grease gun. After every rip on the water.


And where the hose connects to the manifold, secured with a tie wrap (or ziptie)

CedarRiverScooter said:
I had problems with th 'pop-off' springs, made it run bad at midrange. There is a pressure test foir pop off you might want to do.

Good luck.

Ahh, struck gold here. Will check the pop off once I figure out how to secure my bicycle shock pump to the fuel inlet. Cheers, first round is on me if we ever meet in person.

- Brian
Glad you are making progress.

Another failure mode I discovered (the hard way). If you over pump grease into the bearing carrier, it pushes out the seal. Then you get to pull the pump & shaft yada yada
I found the mikuni service manuals here:

So according to what I could glean on the web, popoff pressure is supposed to be set at 45-55psi.
edit: stock needle seat is 1.2mm, and stock popoff is 75psi.

Well I don't have a pop off gauge. But I have a few bicycle shock pumps. As far as I can tell, a popoff gauge is a bicycle shock pump with a hose on the end of it. And the typical $60 popoff gauge is about 2x the price of a typical bicycle shock pump. Pics on the web show a similar type of pump. So my ghetto rig should work exactly the same as the special factory tools.

My bike pump threads onto a schraeder valve (tire stem). So I took a tire stem, shaved the rubber a bit to fit a piece of hose. Took the valve core out, and then threaded the adapter onto the hose of my shock pump to make a pop off gauge. Yes, a 0-60psi gauge would have been ideal. But my gauge should be repeatable enough to get both carburetors to the same setting.



Well, I learned more stuff about carburetors. Turns out that I'm not running the SBN44 but rather a BN46i which is the carburetor that's specifically tuned for emissions. Not a big deal on my end, but that means the popoff results aren't what I was expecting. Also means that I will not be modifying this engine at all so that I don't have to deal with attempting to performance tune the carburetor. Because that would be an uphill battle. Searching on greenhulk got me the deets.

fyi. fuel pressure is 1-6psi depending on engine speed and load.
stock needle seat is 1.2mm. When I looked in there, it looked a lot smaller than the 1.5 hole I was expecting.
stock popoff is 75psi.

So, my bicycle shock pump would have worked fine for a normal carburetor where the popoff is 45psi or lower. But in my case, the fuel pump body leaks at 40psi. Which isn't a problem, because fuel pressure is <10psi. So I'm good to go. Would have been nice to have the experience to know this, but hey, experience is what I've gotten, right after I needed it most. Haha.

I ended up using a tire filler with gauge to check popoff pressure. I simply put my adapter hose into the tire pressure gauge, and slowly added compressed air until the needle popped at 75psi. I ignored all the air leaks on the pump side. And managed to wrinkle the clear pump diaphragm sheet. Had I known that would have happened, I would have used a rubber tip blow gun to apply pressure directly to where the little fuel filter screen mounts to avoid damage. It's not cracked, so likely will still be functional enough for now.

After I tested popoff (thanks scooter) and confirmed that both carb needles are popping at the same pressure, I decided to explore the insides of the carburetor a bit more, and checked the jets by spraying some carb cleaner through the jet to confirm that the passages to the carb throat were clear. Which they were. I also examined the anti-siphon valve flap, which was fine on the rear carb, but not sitting flush on the front one due to debris/varnish. Removing the flap shows that it had a permanent bend.



Hopefully this valve was the cause of my running issues. It does start easier now, and revs nicely on land. Can't wait to water test it again.

If you want to make your own popoff gauge, a yoga ball fill adapter will fit into a tire fill gun or bike shock pump. Add a piece of hose and you're good to go.

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