Super Jet 760 help

[superjetchet21]

Hey guys this may be a really stupid question and im getting mixed reviews but i have a 06 sj/62 t and im in the process of wanting to do a 760 big bore i already have the 84.500 pistons can I just use a stock 760 top end? or Is the porting completely different this was the set I was looking at

http://www.ebay.com/itm/YAMAHA-760-...75606174&pt=Personal_Watercraft_Parts&vxp=mtr

 

[ryan]

your better off with your 61x701 cylinder. just have it ported or do both a big bore and porting. you will have less power with a 760 cylinder swap then a good running 61x701 .

 

[superjetchet21]

okay, yeah im getting all mixed reviews. Im in the process of rebuliding the top end and it never got the right compression even with a fresh top end it has a ada head and 33 cc domes and isnt getting anywhere near what ada says it should be getting. The cylinder is ported and polished but I dont know what to much is or where it should be at since I bought the ski like this.

 

[ryan]

the porting of the 760 just doesnt work very well. does it run yes the mid power is very good but thats about it. lose thoses 44s if there stock yamaha there untuneable. a few will say they work but they most likey have never ridden a boat with a set of carbs that work.
im back to stock 38s and love them. i have used every carb made. 38sare good up to about 771 cc are the way togo.

 

[superjetchet21]

Yeah after I bought them I have been hearing they are very hard to dial in

 

[Nuklear6]

Believe me the 760 cylinder CAN have awsome power. There are some port timing changes (minor machine work) needed that are quite easy and cheap (way cheaper the BB sleeves and the required machine work). When set up correct a 84mm 760 will run just as good as any 61x big bore with 84mm bore for half the cost. You either cut the case or remove the sleeves from the cylinder and cut the bottom of cylinder and then cut the squish relief in the combustion chamber domes. A bit of piston pop up never hurt anything.

Cut cases (if you choose that route) $60
R&R sleeves, cut cylinder botttom, deck cylinder to clean up (if you choose that route): $120
machine domes for squish clearance (two domes): $60

so it either costs you: $120 (if you cut cases)
or $180 (if you cut cylinder)

61x big bore sleeves $130 x 2= $260
Bore cylinder casting, install sleeves, final bore sleeves, deck cylinder for clean up: $300

61x BB= $560
760 set up with same port timing for freestyle/ low end= $180 tops

Now both set ups still need to be ported and cases will need to be bored (if not 64x) but im just trying to show the difference

 

[WatrLogd]

Between porting and cutting the cylinder bottoms, head domes recut, and massaging the exhaust and boost/transfer ports it cost me about 400 total. I have yet to see the results but I couldnt beat the cost. From what ive read and been advised, the stock 760 jugs have thicker sleevesbut still need bb sleeves past 85.5mm. Decking the bottom of the cylinders helped drop the exhaust port to alter timing (all ports were lowered). The full porting job will be done by Buck out of Snow Racing (they are a prominent Kart Racing Team much like the reputation of Swedetech, both shops are local for me). Not sure how itll turn out but I couldnt beat the price, and Buck used to run Magnum Engines for Kartel Racing. Steve Slobodnick trained and taught him how to port, Slobodnick of Factory X. If all turns out well I'll PM you his contacts. Fingers crossed... On anther note, deleting your base gasket and using 1211 sealant instead will drop your ports...

 

[imperialgtr]

nuclear what do you sugest to do with the top of the (exhaust port) sleeve on a 64x cylinder? it sits down approx 1 mm from the aluminium cylinder, but if removed flush with the cylinder surley that would change the port timing?

 

[Nuklear6]

nuclear what do you sugest to do with the top of the (exhaust port) sleeve on a 64x cylinder? it sits down approx 1 mm from the aluminium cylinder, but if removed flush with the cylinder surley that would change the port timing?

The miss match between top of exhaust sleeve opening and the top of the port in the casting should be left as it is. The minimal effect on the Ports max velocity and flow isn't worth the extra work and piston pop up you would have if you grind up to fix that. I've run both ways and with equal port timing there was no difference. Only reason I even ran with out the mismatch was I got a cylinder that had already been "fixed" by SBT. Had to drop that cylinder almost .100" to get the exhaust port where I wanted it. Then grind the intake Ports back up some cuz at that point they are way too low.

 

[ryan]

ben down all the bull bb this stroker that. all that stock port,std bore 701. i like to use my boat not have to put gas in it every hour. i went back to my rag tag b.s. 701 and love every min of not working on it. still role and flip it . mind you i dont do flat water. ant no body got time for that.

 

[imperialgtr]

The miss match between top of exhaust sleeve opening and the top of the port in the casting should be left as it is. The minimal effect on the Ports max velocity and flow isn't worth the extra work and piston pop up you would have if you grind up to fix that. I've run both ways and with equal port timing there was no difference. Only reason I even ran with out the mismatch was I got a cylinder that had already been "fixed" by SBT. Had to drop that cylinder almost .100" to get the exhaust port where I wanted it. Then grind the intake Ports back up some cuz at that point they are way too low.

thanks for the reply, just to clarify i am gonna be decking my cases by 2mm and ordering custom cut domes do i still just leave the sleve untouched at the top of the exhaust port or do i match them up? thanks

 

[Nuklear6]

2mm is a bit too much in my opinion. If you lower stock 760 port timing by 2mm you will be lowing the port opening by over 5 degrees. If you lower it that much it will still have good bottom end but will start to sacrifice mid range. 1.5 mm will be plenty to give you good bottom end and not sacrifice mid range. I would leave the port roof on the sleeve just how it is. Dont want to go too far into what i spent alot of time perfecting but dont forget to widen the exhaust port and grind the transfers back up a little bit. also dont forget when reducing the volume in the combustion chamber by .060" (cutting the domes for new tdc height) that you will increase your compression ratio. Depending on what manufacturers head/dome your using you prob wanna order your domes with 2 cc's larger combustion chamber, before you cut them for the squish relief.

 

[imperialgtr]

10 4 nuclear

 

[MikeyB]

Good info

 

[lamar311]

This is really interesting to me as I am running a 61x case with 760 cylinder. I plan on boring 1.0 over to 85 mm and have heard about decking the case. I have to deck my head anyway as I am blowing head gasket. Has anyone cut a stock head to meke this set up work and bump compression?