stevegbr
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Post by stevegbr on Jul 22, 2014 21:51:52 GMT
i think the compression should be around 185 +/-14 psi, bear in mind though its age.
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Post by mikef on Jul 22, 2014 22:00:59 GMT
Hi Scania. In answer to your question about compression pressure, I have not seen any figures published for the Sd400 but your readings seem much the same as mine. I have no running issues that I know of. So I would guess that 120 PSI is about right. I would suspect that your flat spot at low throttle opening could be due to a carb problem or an air leak around the inlet manifold rubbers. All the best. Mike.
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Post by scania on Jul 22, 2014 22:15:09 GMT
I just figured out something interesting. Atmospheric pressure in the UK and Ireland is around 14psi (except for high on mountains).
Now most petrol engines have a compression ratio of about 10:1, that means that they compress the air in the cylinder to ten times more than normal atmosphere pressure.
Therefore with this theory then a new SD would have a cylinder compression of 14 x 10 = 140 assuming no leaks.
In that light my 120psi does not sound too bad to me, but I wonder what is the minimum level where by it would be considered low compression?
Also what would the average 1980 SD400n read in Psi?
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Post by scania on Jul 22, 2014 22:19:18 GMT
Hi Scania. In answer to your question about compression pressure, I have not seen any figures published for the Sd400 but your readings seem much the same as mine. I have no running issues that I know of. So I would guess that 120 PSI is about right. I would suspect that your flat spot at low throttle opening could be due to a carb problem or an air leak around the inlet manifold rubbers. All the best. Mike.
I had not thought of checking the carb boots for air leaks, I will do that next. I am not too anxious to strip down the carbs, I am capable of cleaning the carbs if I good a good run at it but chances are it will end up in a biscuit box for months!
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Post by scania on Jul 22, 2014 22:20:42 GMT
i think the compression should be around 185 +/-14 psi, bear in mind though its age. If 185 is the average then I'm miles out !
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stevegbr
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Post by stevegbr on Jul 22, 2014 22:22:36 GMT
dont forget the age mate, that is a figure i got off the net, dont take it as gospel.
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terryc
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Post by terryc on Jul 23, 2014 5:46:02 GMT
I think the compression figure is around 170 but I've seen readings as low as 95 and they still run ok
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terryc
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Post by terryc on Jul 23, 2014 5:52:34 GMT
i think the compression should be around 185 +/-14 psi, bear in mind though its age. If 185 is the average then I'm miles out ! thats a maximum not an average it applies to a factory set-up in an unworn engine our bikes hav'nt seen a factory for thirty years .while not perfect 120 is ok
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Post by rich on Jul 23, 2014 9:47:30 GMT
they are a bit low but they are even...........cold dry test with no choke and ign killed, full throttle and both plugs out, whiz eng over say 5 compressions..........
some light oil ( 10cc ) for a wet test and figures much higher??
repeat with hot engine also.......
anyways is there any chuffing smoke or fumes from the exhaust and CAM COVER breather??
see cmsnl for exploded views and print the buggers off if you can.............400 has economy diaphragm or power diaphragm which may be holed........ you have a strobe timing light?? so how is the ignition advance..........f seen at idle then ign advances smoothly, jerk free till max lines are seen, relative to the cast in tdc pointer on upper c/case about noonish..........valve clearances ok ( and cam and balance chain adj ) as well as air filter...........
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Post by mikef on Jul 23, 2014 14:48:26 GMT
Hi All. As I understand it, the static compression ratio is the ratio of the cylinder volume when the piston is at TDC to the volume when the piston is at BDC. In theory this would then give a compression pressure of the compression ratio X atmospheric pressure as stated by Scania. However there is a thing called dynamic compression which takes into account valve timing ( valve overlap I think it's called), so the actual pressure is less than the theoretical. I hope that makes sense. Mike.
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Post by scania on Jul 25, 2014 0:48:30 GMT
I think the compression figure is around 170 but I've seen readings as low as 95 and they still run ok Interesting, It occurred to me since that I measured the engine compression with the engine cold. I might have been higher if I measured after worming up the engine.
Someday I will re-measure with the engine warmed up and repost the measurements here.
I was really surprised that both cylinders had exactly the same psi, 120psi on the dot !
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