Alumacraft Jet Jon Project

[CedarRiverScooter]

Shakedown cruise completed!

Results:

Carbs still not right, but at least not too lean so not damaging pistons, Low priority.

Getting cavitation, but could work through it. Going to have to add spoon. Will be looking at PSG's posts for sure. Medium priority.

Hate the fore-aft stick steering. Ran it for 15 minutes & never got used to it, have to think about which way it will go. Not safe IMO.
Redesign pending. Highest priority.

Didn't get a top speed due to carb issues, but got to high 20's. It was really fast enough already, so I'm happy with the speed, esp since there was no bow rise at all (not used to that).

The big PIA (such as it is) is that every fisherman that came to the ramp while I was there wanted to check it out. Got a bunch of compliments already. What a cross to bear

Later guys . . .

 

[PSG-1]

Shakedown cruise completed!


Getting cavitation, but could work through it. Going to have to add spoon. Will be looking at PSG's posts for sure. Medium priority.

My boat doesn't have a spoon. With a 12 degree deadrise, I got lucky, with minimal cavitation. For building a spoon, Ranchero50 is the guy to consult for this.

 

[amk]

IM actually thinking of going to a stick set up like yours and ditching my center console where did you get the chairs and stick controls from?

Its probably unnatural to you now but you will get used to it. Airboats are all stick stear and they do alright its just a learning curve.

 

[CedarRiverScooter]

Update:

Prototype spoon. Made from plywood pancaked up 3 levels, wrapped in vinyl & injected with canned foam. Deck screws holding it on with some goop adhesive. It just has to last for 15 minutes of testing.

Another change is that I reversed my stick steer action. I had it working opposite natural body english & I think that is why it was difficult to use. Now it steers the same way you turn to look. Hoping for the best there.

 

[Ranchero50]

Definitely needs seat belts.

 

[amk]

https://www.ebay.com/bhp/seadoo-intake-grate

probably all you need I have one on my smaller jet boat works great there is an after market one for every ski made most likely

 

[Tinny's Dad]

Love the boat, but also jealous of that workshop, looks great

 

[Ranchero50]

Did you change your pics?

I don't think the above spoon is going to perform very well? It's awfully abrupt at both ends and doesn't seem to have a smooth transition to the hull.

Here's a pic of mine for reference. I might be 1/2" deeper than the original keel strake. I welded a 1" x .125" length of angle from one end to the other on my keel and the spoon just barely covers it.

 

[CedarRiverScooter]

Update - Tested the prototype spoon today, no good. Cavitates worse than before. I peeled off 2 of the 3 layers of plywood & not much improvement.

Ranchero, I am sure you are right about the abrupt transitions. It is not easy to feather it in with the glue-on approach I used. If I had a Phd in hydrodynamics I would know the exact shape to cast in fiberglass, but the trial & error approach could be time consuming.

My new grate will be here next week, got my fingers crossed that it works.

In the meantime, I think I will need to change pop-off springs in the carbs. Not getting anywhere close to running right.

I think I need to have the impeller repitched too.

Got to do some bat-turns today, woohoo! It's going to be a blast once I work out the bugs.

 

[dimecovers3]

That is by far the most stone cold cool jet build ion planet Earth.

 

[Flat_Bottum]

Well done on gettin her out on the water. You should be every bit of high 30's in your boat once you get it running right. If you dont mind me asking, what is the boat doing that you think the pop off is wrong? The new grate should help you out a lot, I know that it was horrid in my boat on its first run but once I added the little scoop to my grate, it helped out a lot. I lost a few MPH's but hook up was much much better. That was with my intake even with the boats bottom though, so I am hoping that by sinking it down (intake about 1.5-2" below boat bottom now) I can run a regular grate. Keep up the good work!

 

[CedarRiverScooter]

The carb problem I have is that the throttle response is very flat (no response from throttle untill you hit 3/4 & then it jumps up). I have tried all combinations of lo/hi speed adjuster settings & I can't tune it out.

I have read different Greenhulk posts & also mikuni manual about this & decided to try different pop off springs (they are only $1.80 each from iboats).

I am also going to repitch the impeller lower to handle the additional load, I think I'll do that first, maybe that is part of the problem.

Another upgrade I decided on is on the thru hull bearing. The one that came with the ski was the wrong part (maybe is a yammi or kaw) & the bushing was loose to boot. I got the repair kit but the bushing wouldn't fit. So I went to bearing store & got a bronze oilite bushing & turned it to fit. Locktited it & pressed it in, it is there to stay. Fast forward, I must have the clearance too tight because it is getting hot.

So instead of continuing on that path I bought this from watcon, should be good now:

https://www.watcon.com/jet-pump-drive-line/drive-shaft-bearing-carrier/drive-shaft-bearing-carrier-1

 

[Flat_Bottum]

Well I hope you get it figured all out. Are you running a triple pump or the factory single? Also, what year of ski did your 750 come out of?

 

[ColtonS]

If your carbs have not been rebuilt in the recent past I would HIGHLY suggest doing so and upgrading to a triple outlet fuel pump if you haven't already. Before you do all of that I would check each cylinders compression or pull the head caps off and take a visual look because the lack fuel to the front carb is known for burning the piston up in these motors.

 

[CedarRiverScooter]

Hi Colton - on top of that. Have triple FP & just rebuilt carbs myself (that may be the problem). I have been monitoring piston wash after every run - it looks to be too rich. I have higher PSI pop off springs coming, which is supposed to lean it out at midrange.

 

[ColtonS]

If you put all new parts (including pop-off springs) in the carbs when you rebuilt them it should not run poorly.

I would double check to make sure everything is within spec...fuel mixture/pop-off/return line pressure/jets/carb timing/needle and seat size.

Once I knew all of this is correct I would then question my reeds.

 

[CedarRiverScooter]

Thanks for the advice Colton. I'll open thrm bwck up. Got to admit I didn't measure pop off pressure (but did put new black springs in). The return restrictor orifice is in place. One thing I have been wondering about - I have the gas tank open vented to atmosphere (out the stern). I didn't reuse the in-out check valves. I wouldn't think this is the problem, but you never know.

Are you running the stock impeller on yours? I am thinking I should repitch my impeller down. When it did hook up & go hard I wasn't getting anywhere close to 6800 RPM, more like 5800.

I have been readingf the how-to on DIY repitching, would like to give it a shot.

 

[Ranchero50]

I would run what you have for a season before playing with the pitch. You have no idea how it's going to run once you get some stuff dialed in and load the boat down with toys.

You also need to get the carbs working correctly before something dumb happens. #-o

 

[Flat_Bottum]

Open venting your tank shouldn't matter, my tank is vented to atmosphere without any issues. One thing to check, and I know this sounds dumb, but check your spark plug wires to make sure they are all 3 securely connected to their boots. When I first dropped my 750 into my boat and fired it up, it would start and idle but just didn't seem to be right. Come to find out, the PTO cylinder plug wire had just barely come loose inside its boot. Once I screwed the boot back on better and put on a new tie, it was a night and day difference.

 

[CedarRiverScooter]

Got the top loader grate today so just had to try it out (1 screw install).

1/2 way there. Got it to hook up well when going into the chop, but still cavitates going with the chop.

Engine is topping out at 5000 RPM, so I think I need the SLT impeller (lower pitch). With the good hookup I can nail the throttle so the mid range carb problem was not relevant. Piston wash says I'm rich so not melting any pistons.

I stared at the pump intake for a good 10 minutes & finally decided to try a 4 foot long spoon, made from treated 1x4. It will terminate at the junction of aluminum & ski hull (well before the intake). 1x4 will be on on each side of the center rib (tapered to fit tight) will be about same width as the intake. It will have the granddaddy of exit taper to match the flow of the hull. Hopefully this will direct the air outboard.

If I could start over I would plant the jet ski hull deeper & use it's keel as the spoon. It could have been so simple . . .