View Full Version : valve clearance and Compression

26.05.2019, 01:44 AM
Thanks in advance for any advice on the following situation. I am rebuilding a ducati 900 air cooled engine. After a good deal of fiddling, I finally seemed to have the valve cllearances ideal. I used the info off of this website to adjust clearances. When I first tested compression, it was barely 120 psi in either cylinder. This seemed low so I fiddled some more, focusing on the getting the closers tighter (especially exhaust) and the openersrs more loose. This was due to my lack of. shims options. In the process of fiddliing, I found that very loose openers (greater than .009 in ) gave me the best compression (140-145 psi)..the openers now for both intake and exhaust are way looser than spec but gives:me much compression, which I always thought meant power.

Am I correct in that assumption? Should I go for max compression and not worry if clearances are over spec?

26.05.2019, 10:20 AM
I'm not sure that your procedure for adjusting and measuring the clearances is correct. If a tighter opener clearances are reducing the compression I would suggest that the valves aren't fully seating. You should be aiming for opener clearances of 3-4 thou inlet and 4-5 thou exhaust and all closers at 0-2 thou.

27.05.2019, 06:32 AM
One other thing! too tight on the closing shim will result in rapid cam and closing rocker wear, as Derek suggests try lapping in the valves if the seats do not need re-cutting.

29.06.2019, 07:23 AM
If looser opening clearances are giving more compression, then the closing clearance must be too loose, or possibly the valve timing is out, if say, the belts have been fitted with he timing out. Have you set the closers so that they can just be rotated by finger pressure? You will need a lot (or the exact) shims!