Super Jet 760 and 35cc domes compression???

[1eyedjim]

does anyone know what sort of compression i should get out of a 760 61x cylinder and a set of 35cc domes?
cheers
Jim

 

[chriselmore_1]

are they domes from a 701 ? or did you get 760 domes?

 

[1eyedjim]

they are supposed to be for a 760 i quoted this when i got them but i tried my compression tester on it a few weeks back but was only about 150psi. the ski runs great and has loads of power but just wondered if its down to the domes?? both cylinders are the same reading

 

[chriselmore_1]

35cc domes 760 at sea level should put you at about 200-205psi. my 38cc domes on a 61x cylinder bored to 760 give me 180psi

 

[1eyedjim]

i ring up tomorrow and check with who i got the head from then, cheers for the help

 

[QJS]

Two things....If the cylinder has been ported it will give you a lower cranking pressure if the exhaust ports have been raised, the other thing is to make sure you don't have a Mickey Mouse compression gauge, always use a calibrated one ie snap on.
Chris.

 

[1eyedjim]

it might be the porting then from what i was told the exhaust ports have been raised nut i dunno how much, will this be a down fall in the engine if the compression is that low? the guage shows 150psi on my mates standard 701 and 140 on my dads engine, its not the best guage but sure that it shows these psi readings it cant be to bad. i got the head from yourself chris so this saves me calling you. thanks for the info and help

 

[Matt_E]

Yes, raising ports will cause that.
Also, a weak battery or starter will cause lower compression readings.

 

[1eyedjim]

is the lower reading anything to worry about?

 

[chriselmore_1]

whats your elevation?

 

[chriselmore_1]

ok so your elevation is about 110 meters wich is about 400 feet. so if you are using a good compression guage 150 seems a little low to me?

 

[Matt_E]

so if you are using a good compression guage 150 seems a little low to me?

See post #8.

is the lower reading anything to worry about?

Not if the bores look good and the ring gap is acceptable.

 

[QJS]

If the engine is good and strong and in good condition I wouldn't worry too much, if you prefer we have smaller domes in stock.
Chris.

 

[RyanO]

85mm chamber 760 no porting @ sea level
32cc=225psi*
34cc=215psi*
36cc=195psi*
38cc=185psi*
40cc=170psi
*= must use 100+ octane fuel

 

[Matt_E]

Those numbers - if they are from ADA - assume a true 760 cylinder, which has higher exhaust ports than a 61X of the same bore.
In other words, compression on an unported 61X 760 will be higher.

 

[Crab]

at 84mm each 1mm you raise ports should change or reduce the compression by about 2.6389cc (If I did the math right)

 

[chriselmore_1]

well I don't think he raised the ports 20+mm! Lol. 150psi seems extremely low for 35cc domes

 

[fast superjet chase]

ya i would say its low i have 35cc in mine im at 5000 feet and im at 180psi in each cylinder.