Compression ?

J

Jp116

X
New here and i know this is general question but i just need to know if i am in the ballpark.
760 cylinder from a WB2
riva head 34cc domes.
recently puked MSD enhancer replaced with stock ignition for now.
indicated compression of 148 and 150 Mag/PTO
are my PSI's lower then you would expect?
the deck has never been machined.
Also I run my bpipe top screw 3/4 turn open mid shut lower shut.
is there any benift of restricting the stinger line or is this done more for
closed course racing?
just interested in some input.
thanks and great site.
 
J

Jp116

X
Thanks for the replys
engine was warm
throttle was held open
and I said "indicated" compression
so not to be confused with compression ratio.
guess i'll take the head off and see whats in there.
I hate taking things apart when they seem to operating properly.
Maybe i'll double check my numbers first.
Cheers
 
crammit442

crammit442

makin' legs
Location
here
Riva domes are notorious for being cut or marked incorrectly. Might want to cc the domes to see what the actual volume. Tricky and I had a bunch of "same size" riva domes and no two were alike. ADA heads have much better quality control.

Charles
 
E

egbrig

Thank you! You are correct, bad termenology. I meant when you were doing a pressure test, and yes it does make a difference on a two stroke. Crammit442 is right, you can get very screwed if you don't check the domes yourself!
 
J

Jp116

X
Thanks for the quick replys and suggestions.
i'll check the cc's of the domes.
what is the best method? Titration rod?
 
Matt_E

Matt_E

steals hub caps from cars
Site Supporter
Location
at peace
Put a plug in, get a syringe with cc measurement, and fill the domes with measured volume of water.
 
J

Jp116

X
Well i took the head off and havn't pulled the cylinder but if i bring the pistons to TDC and put my thumb in the middle i can wiggle the piston back and forth and all around like .4mm or so. Def. time for a rebuild. going to run it for victoria day wkend. then pull it apart.
hopefully have enough meat to go to 85mm. i see on wiseco's site that anything larger needs new sleeves and i am already running 84.5 's
thanks for all the help.
 
Y

yamaslut

Guest
Matt_E said:
Put a plug in, get a syringe with cc measurement, and fill the domes with measured volume of water.
Click to expand...

stupid question maybe, but where do you fill them to??? right before the oring????
 
J

Jp116

X
if you use a piece of clear plexi with a small hole to shoot the water thru and seat it on the oring then fill the dome till all the air bubbles are out you would get the most accurate reading i would think.
my squish band looked good and no signs of any deto. i think its just well past the service limit and needs a new topend. cylinder walls were visable glazed but no scoring or marks. i am hoping my statment of "one more ride" dosen't jinx me and i end up imploding a skirt but i just need one more weekend out this Biatch!! knock on wood.
 
Proformance1

Proformance1

Liquid Insanity
Site Supporter
Vendor Account
Location
New York Crew
Ok, I have cc'ed and checked compression on a few 4 strokes. I do not understand how to check compression on a 2 stroke. On a 4, you can check the head the same way you do on a 2. Then you can check your piston height to deck at TDC. Then it's a simple calculation, especially for flat top pistons. Here is an example site for calculating 4 strokes:

http://www.angelfire.com/fl/procrastination/motor.html

On a two stroke, do you measure from the port height to the deck as the stroke length?
 
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