Super Jet superjet avgas rehabilitation =)

[ISeeWetPeople]

Hi i have R&D head and domes in my jetski.. i want to run pump gas instead of avgas 100ll and pump gas mix (40% avgas and 60% pump). my domes are now 37cc and compression was about 150each.. pistons are wiseco 85.00 mm so its a 781cc engine. i didnt build it so i dont know why it has so big domes and so low compression. can anyone tell me what size domes i should buy. i dont want to make my 37cc domes bigger because i maybe use them later again.

but if you have time you can tell me how to make domes bigger.. previous owner says that take 0,5mm out of tha squisband area.. but what does it meen in cc:s is it 40cc or??

 

[OCD Solutions]

What octane are you calling "pump gas" and what fuel rating system is used in Finland?

I would not consider 150psi high compression and would have no problems running 91 octane pump gas in that motor, provided that your measurements are accurate.

A stock yamaha engine comes with 150-160 psi right from the factory. I just sold a bone stock 701 that had 155psi in each cylinder.

 

[ISeeWetPeople]

Hi.. In finland we have 95 and 98 pump gas. and 100ll AVGAS. I dont know avgas in usa
but. our 95 is 90 in usa and our 98 is 93 in usa.. and our 95 has 10% ethanol and 98 has 5% etanol persentage by volume

 

[OCD Solutions]

Unless I am missing something, i would run it on the 93 with the domes you already have in there. No need to change anything. In fact, i would even run the lower octane if it was significantly cheaper.

I have a ported 760 with 34cc domes that gives me 185psi compression. I run 91 octane out of the pump with no issues.

 

[ISeeWetPeople]

Is there something else that you should know of my superjet i am kind a new in 2 stroke engines and modifying them. So i dont know all factories that can chance the gasoline that i use

 

[Freakrider]

150 psi will run on 87 octane, been there done that a long time...what OCD is telling you is accurate...is your compression tester a good one because that's about stock #'s ?

also a quick google search shows highest elavation in Finland as 660 feet in the southern region, so that doesn't seem to be a factor....

 

[Penken]

measure squish. if squish is super tight you can still get deto in the squish band area, even if compression is low.

 

[ISeeWetPeople]

Last year my jet had bad luck.. i forgot a t-shirt in my jetski that i use for drying extra gasoline i spild.. i went to drive and that shirt twist around my driveline shaft.. so shirt brake my trim cable some electric and half cooling system went down so the result was engine overheat. as you can see my pistons

I have now new pressure tester and new pistons but as you can see my engine



is not in ready yet.. but i will put it together by weekend so then i can measure pressure (hope its higher than 150..) and squish..but i have never measured squish.. i maybe know how to do it but my english is so bad that it would take me 4 ours to write it for you guys to read.. so if any one have time to rehearse it i would appreciate it ..
ill let you know the readings soon as i get it

 

[ISeeWetPeople]

funniest part was that when the shirt brake my cooling lines.. there was nothing to stop water to come in. so my ski sink

this story lesson was that YES A FU....G T-SHIRT CAN SINK YOUR JETSKI:bigok: so be careful out there you never know what a hat can do

 

[ISeeWetPeople]

and our 98 (usa 93 ) cost 1,7euro ($2,25) and 100ll cost 3,30euro ($4,37) so its too expencive to ride 100ll and hard to find other countries

 

[Cross]

A lot of factors other than compression determine if you will need higher octane gas or not. I see you are running a dry pipe as well as MSD total loss. You will need to get some more information once you get your motor together. You will need to check the compression, sqiush and timing curve on the MSD. These all factor into your need to run av gas. If you are running a lot of timing even though your compression is low you will probably still need to run av gas.

 

[ISeeWetPeople]

thank for reply.. what about the timing curve im little lost in squish and curve i havent really figure it out how to calculate squish and how to use that info.. is there somewhere somekind of chart or? and timing curve that too i have to learn but its hard is there thread for timing for stupid people anywhere?=) and sguish band area and the size of it for retarded vol1.1

 

[ISeeWetPeople]

ok now i have more info

rev2 drypipe
novi maxflow 48mm carbs
M16 monster reeds by R&D
squish is about 1.4mm
pressure 180-185psi
static timing is 25
max speed retard is 17
Retard begin speed 4000rpm
rev limit 9000
max timing is on max
domes are 37cc
now i run 60% 98ron (93pon) and 40% 100ll i think its 94-95pon totally

and i need to run on pump gas 93pon
what size domes i must run??

 

[MileHighJSC]

ok now i have more info

rev2 drypipe
novi maxflow 48mm carbs
M16 monster reeds by R&D
squish is about 1.4mm
pressure 180-185psi
static timing is 25
max speed retard is 17
Retard begin speed 4000rpm
rev limit 9000
max timing is on max
domes are 37cc
now i run 60% 98ron (93pon) and 40% 100ll i think its 94-95pon totally

and i need to run on pump gas 93pon
what size domes i must run??

Compression seen on gauge and running compression in the cylinder are not the same.
http://www.x-h2o.com/threads/82584-can-i-run-93-octane&p=1267482#post1267482 Read post #20
The bigger the bore the more prone the motor is to detonation. The flame front has to move faster as it has further to go in the same amount of time, ie same rpm yet bigger bore. A faster flame front is hotter so it can induce hotspots which create detonation. Basically.
That is why big bores usually have bigger domes, and lower compression. Your head stock was 37cc Then it was cut. you need to CC your domes to get a good reading. You also have a dry pipe so your EGT is a little higher than a wet pipe. I think 42cc on dry pipe BB760 is safe to run 91 us octane.

http://www.adaracing.com/personal-watercraft/head-kit-yamaha-760-superjet-girdled-billet/ya760mhdkit

Check this link but keep in mind you are bigger than 760 you are 781