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compressions

i have not been on the forum for ages gave bikes up but i am addicted so brought a Triunph TRW it will not start i have electronic igntion good spark double checked timing and a new amal carb i checked the commpressions they are 75 lbs oiled one bore throught the timing hole it only came up an extra pound so i dont think its rings is 75 lbs good it has a 6 to 1 compression ratio i have converted it to 12 volts

email (option): roger.beck@node6.com

Re: compressions

75 lbs/sq. inch is low....Did you do the test with the throttle fully open?....There won't be much pressure if the air isn't drawn in to be compressed.....Ian

email (option): ian@wright52.plus.com

Re: compressions

roger beck
i have not been on the forum for ages gave bikes up but i am addicted so brought a Triunph TRW it will not start i have electronic igntion good spark double checked timing and a new amal carb i checked the commpressions they are 75 lbs oiled one bore throught the timing hole it only came up an extra pound so i dont think its rings is 75 lbs good it has a 6 to 1 compression ratio i have converted it to 12 volts
Atmospheric pressure is close enough to 14.5 PSI on average .
14.5 x 6 = 87 psi
So if you had absolutely no losses and the compression ratio is really 6:1 the best you could expect is 90 rounded up
SO 75 is reasonable particularly if you took the measurement on a low pressure day.
This is why atmospheric pressure is always on dyno tuning data sheets, it does make a difference.

Then there is the accuracy of your test gear which is generally poor unless you paid a healthy 3 figure sum for it .
The fact that there was 5/8 of SFA difference wet & dry tends towards good ring sealing .
So that leaves you with valve to get ulcers over.
Leak down tests are a lot more useful than strait compression numbers .
And if you do one you should be doing it at both the theoretical PSI ( 90 ) and a low pressure , around 20 PSI done first
You can hear air rushing past a valve at 20 PSI just as well as when 90 PSI is pushing it along and it is a lot less dangerous.

email (option): bsansw1@tpg.com.au

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