Hilko Posted May 3 Posted May 3 Can anyone tell me what readings I should get on a compression test? I only have the Haynes manual which tells me each cylinder should read between 126 and 171psi with a service limit of 100psi. Are these correct and how much variance is acceptable between cylinders. I am getting about 150 psi on all but number 2 which is reading about 138. Are these OK or will it be noticeably sluggish? Quote
imago Posted May 3 Posted May 3 The important thing is that the pressures are roughly even across all four. 150 is decent and 138 is 8% down on that which is acceptable for a road bike. A general rule of thumb is that you don't want more than 15% difference between the highest and lowest on an inline four. Anything more than that and it starts to get noticeable when you try to balance the carbs, once you get to a 20% difference you'll start to notice it on idle and running. Check the plugs to see if the low cyl is running lean, if it is then it's likely to be a valve and get worse. You can also do a leak down test to see where the pressure's escaping. Quote
Captain Chaos Posted May 3 Posted May 3 47 minutes ago, imago said: 15% difference between the highest and lowest 15% of the highest, or 15% of the lowest reading? Quote
imago Posted May 3 Posted May 3 (edited) 22 minutes ago, Captain Chaos said: 15% of the highest, or 15% of the lowest reading? The lowest being 15% below the highest. If you have 150, 136, 142 and 127 for example, then 127 is 15.5% so you'd start to notice it when you tried to balance the carbs. For a customer's bike I'd suggest a leak down test, head off, valves and seats ground etc. But, if it was mine and a well used road bike I wouldn't bother. I'd just carry on using it until it warranted a full refresh. Edited May 3 by imago Quote
Gixer1460 Posted May 3 Posted May 3 Maybe i'm a pessimist but i've always worked on 10% variance across cylinders high to low (maybe due to racing / turbo use?). I'm quite surprised at the factory allowing near 30% variance! Quote
Hilko Posted May 5 Author Posted May 5 So it looks like my readings are within spec then. I think I will check the valve clearance just for peace of mind. Hopefully I won’t need to go any further. Quote
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