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Author Topic: valve clearance and compression  (Read 979 times)
PoormansDucati
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« on: May 26, 2019, 09:56:13 AM »

This evening I thought I was ready to turn the corner on my Ducati  900 rebuild project when I ran across some info that now has me second guessing some assumptions I had about valve clearances and compression. In an earlier post, I described one of my setbacks when a damaged belt slightly bent two valves in the horizontal cylinder. It also dinged up the head on the vertical cylinder as well. I had been hoping to not have to deal with vertical cylinder, because as I know most of you know, there is not good access for valve work and most likely you will end up either taking the motor out of the frame or some variation on theme if you need to remove  the head or do a  ring  job.

No such luck here. Someone on another forum recommended a leak down test, which I really appreciate, as you don’t need to start the engine nor have anything like the starting system active. Unfortunately, my compressor decided  to give up the ghost, so I am back to trying to make sure the vertical cylinder is functioning properly, using a dry compression test, before I spend all the time in reassembly, only to find out I need to take it all apart again.

I had to totally redo the valves, one cylinder because of new valves and the other for new head. I  checked around about clearance recommendations after realizing there is quite a bit of variance in what is recommended by individuals and  the factory.  I settled on the ones from the Ducati suite web site. I got the clearances well within  recommended values, though when I checked compression. It ranged from 90 to 120 in a cold crank test . Checked
 a few times and confirmed.   

This value seemed a bit low so I fiddled around a bit and  found looser clearances produced more compression. I  got  both cylinders showing compression at 135 to 145psi. The only thing was is the opener clearances were very very loose and definitely out of spec (greater than .009 in).  I still was feeling relatively satisfied but then read  that starter rpms don’t produce the compression value needed to evaluate the engine. I also read that there is  significant potential to make errors when measuring clearances.  The last thing I learned was that the compression curve   (pressure vs rpm) implied that there  was more factors at play than  I had thought.

I  had assumed that more compression is always better, regardless if  the clearances aren’t to spec. This led me to try to maximize compression through valve adjustment. My questions are:

1.Was my assumption correct that more compression = more power = better performance? Or incorrect and...

2. Are there factors at play that limit the usefulness of this approach? For example, have I messed up the timing sufficiently  that the bike may have more pure power but not run as well as  before or as well as it could?
3. Given that my shim supply at the moment is rather limited (4 unused closers, 6 unused openers), would it be best to focus on the following strategy: ?
A. Use tallest opener shims for intake  (tighter).
B. Use shortest opener shims for exhaust (looser).
C. Try to run tallest closers on exhaust (tighter).
D. Try to run next tallest closers on intake (less tight).

Or…..

How would you prioritize which clearances are most important to have adjusted correctly?
What is the highest reasonable compression to shoot for in PSI if working with most stock parts?

Do people knowingly run bikes with valves out of spec  because  the net  end result is a faster, better  running bike?  Or is this an oxymoron?

 Thank you!

 Tony
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Howie
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« Reply #1 on: May 26, 2019, 10:49:04 AM »

If you decide to run "old school specs" after replacing any valve parts you want to re-check sooner than later.  Later specs are to accommodate for longer service intervals.  Back in the older days services were more frequent, every 3K miles.  Also the valve and valve seat metallurgy wasn't what it is today.  Then again, just to confuse things, we were using gasoline with lead wich helped protect valve seats.  IMO, old school on the loose side would work just fine.

Now, back to your compression test.  Small changes in valve clearance will not show a meaningful difference in a compression test, unless, of course, a valve is not completely closing. Temperature will though.  You will have less compression when cold.   Also how much oil is on the cylinder walls as this helps the rings seal.  You are doing the compression test with the throttle open I assume.

My experience is our bikes run better with old school specs.  My experience is the valve clearances hold up pretty well after break in unless the split rings are replaced with new ones.

I think you might have started an interesting thread, lots of different opinions out there from some very good people, some who are on this board.
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koko64
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« Reply #2 on: May 26, 2019, 02:41:40 PM »

+1 Will be watching this one[popcorn] Get that compressor going! waytogo

I will also be interested to see your hot engine, WOT, cranking compression test results when you are confident to run the motor enough to get it hot. I'm assuming you're running oem pistons.
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« Reply #3 on: May 26, 2019, 09:42:24 PM »

I'm a little unclear on what damage was done to which head, and what repairs have been made on each.

Can you detail that out a bit?
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