300/440/550 300 rebuild

[yamarider]

This is my js300. You might have seen it in my previous posts. It had always had one of those rock and roll type motors that when running look like they are ready to jump off the motor mounts. The bad vibrations were due mostly to the little end of the rod being a bit worn out and having some freeplay in it.

Recently the freeplay got worse resulting in the piston slapping the cases each time it came down and I rode it untill it was unrideable due to the motor cutting out over half throttle.

I wasnt too upset because i had known that the motor was due for a rebuild for some time but i was gutted to find that the vibration from the piston hitting the cases had cracked the casting on the barrel making air/oil bubbles come out from around the bottom of the transfer ports. I liked that barrel cause it had a really nice port job and where it had cracked it cant be welded. Ill get some pictures of the cracks soon.

So I began the rebuild I want to make a nice job of it cause this was my first ever jetski and i have heaps of memories attached to it.

 

[yamarider]

View attachment 158431

I pulled the motor and cleaned the hull up all nice and then cut out of a gym mat some squares of foam to make a flat surface for the gas tank to sit on later on. I also cut a pad out of 25mm thick foam for the waterbox to sit on and glued everything in there with contact glue.

I used paint stripper on the engine mounts to remove all the paint then sand blasted and clear coated them.

So got a new rod kit and bearings for the motor it and the crank has been rebuilt. Got another barrel for it and had it ported the same as the last one that got cracked and had it bored 1mm oversize with new art piston and rings. It all looks nice and shiney!!

I have been busy painting and sandblasting engine parts im going for metallic blue and clear coated aluminium.

 

[yamarider]

Here are some of the parts I have been painting. The barrel still needs a clear coat. The cases are ready to go back together with the rebuilt crank. The reed block, front cover, flywheel cover and intake manifold have been stripped and now are clear coated aluminium.

I have decided to scrap the yellowing gas tank and go for a freestyle type tank setup just so I can have a nice new tank! I have ordered some new fuel lines as well and have decided to replace the stock waterbox which i wasn't happy with' with a supertrapp muffler. Suprizingly the supertrap fits nice with the js300 stock side exhaust I wondered if it would only suit a 550 exhaust.

 

[Joker]

i got a mariner water box and a kerker wet pipe that does really nice.

 

[yamarider]

The motor is all finished and back together im hoping that the new rod will sort my vibration issues out for awhile! I had the top of the head machined to look pretty. Ive got this cool westcoast driveshaft support bolted in now and a bilge pump bracket glued in since my venturi line is used for dual cooling.

 

[dbattersby20]

That's one clean looking 300 you have three. I was given one over the winter and all I did was get it to run for other people to ride until it breaks (then I was gonna throw it in a dumpster), but I might have to copy you if mine breaks. I kinda want it to break now...

 

[yamarider]

The mighty 300 is finally complete and I cant wait to get it wet!

 

[armymen0268]

View attachment 158608View attachment 158609

replace the stock waterbox which i wasn't happy with' with a supertrapp muffler. Suprizingly the supertrap fits nice with the js300 stock side exhaust I wondered if it would only suit a 550 exhaust.

Be sure to make the ring around the blue rubber piece and the metal not too tight, she'll rip and you'll be pissed. I've ripped two, the first one to tight and the second want was hardly on the exhaust. So just a fore warning that it might rip on you.

 

[yamarider]

Thanks for the advise i tightened it right up!. How tight is too tight i dont want it to leak?

 

[dbattersby20]

what tank is that?

 

[armymen0268]

Thanks for the advise i tightened it right up!. How tight is too tight i dont want it to leak?


See that's what im not sure of! i had it pretty lose at one time. And then i bought a new one and had it pretty tight and they both ripped. My ski was bored out and both times they had like 16 of the plates inside the waterbox, how many are you running in yours?

 

[Motofro26]

what carb are you running on this?

 

[Jetpilot106]

what tank is that?

I need a tank like that for mine

 

[yamarider]

Hi sorry for the late reply the gas tank is an Arrow 9 litre Go-Kart tank.

 

[Sti9mm]

Awesome build im doing the same with my js300. But I can't fined a head pipe an I'm looking Mt make one do you have any pic or info on how you made yours cheers