63 CSR compression reading.

[bigjohn]

Hi all. I have asked this question before but have had no response, I wanted to know if anyone knew the correct compression reading (using a compression tester) for a 650 CSR twin with 8.5-1 compression pistons. If not, would anybody who has a 650 twin with the same compression ratio as mine and the engine runs well, who also has a compression tester, spare a few minutes to test theirs to see what the reading is? My reading is 115 psi both cylinders which I think is a little low and the bike is a bit "flat".

Thanks

John.

[Stuoyb]

Can't help with the specifics of your engine, but on a car with an electric cranked engine I was always advised that anything over 100psi was a healthy engine with a top reading in the range of 125psi to 130psi for high performance engines.

[clive]

"Flat" engine may be the timing or faulty auto advance if it is fitted with one.

[petert120r]

115 psi sounds pretty healthy to me. I get roughly the same on my Bonnie with 9:1 CR

[moley]

jUST REBUILT MY 61 CSR WITH ORIGINAL HEPOLITE'S AND GET 110 BEFORE I HAVE RUN THE ENGINE, SO SOUNDS OK TO ME !! aNDY

[bigjohn]

Hi - many thanks for all your prompt replies. Yes I always thought that sort of figure was healthy enough but a mechanic I was speaking to the other day said about 150 psi was more like it with 8.5-1 pistons. Plus in this months Practical Classics there is an article on compression testing which gives a formula to arrive at the correct figure - compression ratio x 1.4 to give the figure in bar. So 8.5 x 1.4 = 11.9 bar, which is about 170psi! Also in my BSA A65 manual it gives a figure of 140 psi for the basic engine.
I have set the timing till I am blue in the face it has 2 brand new carbs set up perfectly, I have tried it on one carb, checked the gearing and yet at 60mph it sounds as if it is nearly on its limit. It has a manual advance magneto and always starts with no more than 2 kicks, hot or cold. Have been trying to get to the bottom of this for about 2 yrs It feels as if the engine is tight and needs to free off but I don't think the engine has been done recently and I have done 500 miles on it since I had it. It really feels as if it is straining all the time.
Thanks again

John.

[dodger]

Compression test should be conducted with the engine hot.
A screw in type gauge should be used, the type with a rubber bung you hold on by hand are in my experience useless.
The other cylinder plug should be removed and the engine kicked over as rapidly as possible at least a half dozen times.
My 1961 CSR with 8.5:1 +40 Hepolite pistons, using the above method, registers 140 - 150psi, both cylinders should be within 10psi of each other.

Laurie

[1608]

When I tested the comp. on my G12 ( tester with the rubber fitting ) engine hot both plugs removed and throttle wide open I achieved 120psi. 7.5-1 pistons