Ducati.ms - The Ultimate Ducati Forum banner

1 - 11 of 11 Posts

·
Registered
Joined
·
367 Posts
Discussion Starter #1
Hi all,
I checked the valve clearances on my 1998 748 a few weeks ago. Basically they all stayed the same except for my exhaust closer clearances on both cylinders. They all got slightly tighter than my measurements two years prior. However, I can't figure out why this would be. Looking at the valve layout, I'd think the clearances on the closers would only get larger.

-Valve seats deteriorating would let the valve and closer shim ride up into the head
-Valves lengthening would let the closer ride higher into the head
-Wear on the top of the closing rocker arm would pivot the arm away from the cam
-Wear on the bottom of the closing shim would pivot the closing rocker arm away from the cam
-The half rings deforming would let the closing shim ride higher leading to the rocker arm pivoting away from the cam

And yet my closing clearances got tighter. The only other thing I can think of is that I have carbon buildup on the valves which is not letting them seat into the head all the way. I can see I have some crap on the intake valves, but I didn't pull the exhaust headers so I couldn't see those (I checked clearances with the heads on the engine in the frame).

Anyway, any thoughts on this would be appreciated. Am I all backwards on how these wear? Do I need to do anything - clean anything out?

John
http://ldsg.snippets.org/HORNS/ducati.html
 

·
Banned
Joined
·
11,256 Posts
Well, there are a few easy ways to test some of your theories. If you can perform a leak-down test, that will help prove or disprove the valves closing. Also, you can pull the shim stack out and move the valve with your fingers to insure its moving properly with no friction. Valve guides are notorious for failing, so that might be something to check out as well.

Usually the closers get looser, as the closers bed into the 1/2 rings.



100% free webcam site! | Awesome chicks and it is absolutely free! | Watch free live sex cam - easy as 1-2-3
 

·
Registered
Joined
·
367 Posts
Discussion Starter #3
Thanks for the reply. I'll work on a leakdown test. I redid the closing shims on all the exhaust valves and they did all slide smoothly. Also, the half rings had not been touched on these for a long time - ie, they did not need adjustment during the 4500 miles or so I've put on it. So I think they were bedded in. All the movements were small - it's just that the clearances were at the small end two years ago and so the slight tightening I measured this time was enough to make me adjust them to a slightly looser fit.

John
http://ldsg.snippets.org/HORNS/ducati.html
 

·
Registered
Joined
·
87 Posts
valve clearances

Assuming that you had the rocker on the base of the cam lobe both times, the likely reason the closers got slightly tighter would be carbon and fuel/oil additive build up on the valves and seats. It's not that uncommon.

A more serious cause would be bent valves which is uncommon and normally caused by broken or incorrectly installed belts or someone rotating the cams with the belts off and the piston near TDC. Let's hope it's the former.
 

·
Registered
Joined
·
367 Posts
Discussion Starter #5
I ordered parts for a leakdown tester today so I should be able to play around with it by this weekend. I've done all the service on my bike since I bought it, so I know I've never turned over the cams while the piston was there, etc. So if anything was messed up by someone else, it was done years and thousands of miles ago. What I'm looking at now seems to be gradual wear / build up.

If I've got carbon build up on the valves, is there a chemical or other easy method to remove that? I'd rather not disassemble heads at the moment... I can see some gunk on the intake valves, but I can't see the exhaust valves. I do think I've been running rich though. My plugs are a bit black and my exhaust exits are pretty sooty.

Thanks,
John
http://ldsg.snippets.org/HORNS/ducati.html
 

·
Registered
Joined
·
367 Posts
Discussion Starter #7
I did a leak down test today - I had about 7% leakage from both cylinders. However neither cylinder seemed to be leaking out the exhaust. The horizontal was leaking air into the crankcase and a bit out of the intake valves. Also, at locations other than TDC, the vertical would leak air into the crankcase as well.

Here are some pics of the leakdown tester I built in case anyone's interested. Not shown, there's a sparkplug grafted onto the end of the flexible hose. I did get some leakage from the sparkplug due to not being able to torque it into the head since the hose was attached, but not too much.



There's a plug in the connection between the pressure regulator and the second pressure gage with a 0.040" diameter hole in it which is the FAA standard for engines under 16L displacement (from what I understand from doing some reading).

 

·
Banned
Joined
·
11,256 Posts
Yea, you're results tell me, its time to pull off those heads and take a gander. You could be dealing with some carbon built-up or perhaps seat material thats flaked off. There is a whole host of issues you could be dealing with, so I'd focus on taking the heads off and doing a through cleaning of everything and then going from there.

Whilst your in there, might as well replace the piston rings and check for crank bearing slop...



100% free webcam site! | Awesome chicks and it is absolutely free! | Watch free live sex cam - easy as 1-2-3
 

·
Registered
Joined
·
367 Posts
Discussion Starter #9
Thanks. Would you think if I've already reshimmed the valves to be in spec it would hurt to ride for a while? I've just finished doing belts, valves, etc. and would rather wait until fall to really tear into it again. It sounds like a good winter project, maybe dropping in some new pistons at the same time.

John
 

·
Registered
Joined
·
367 Posts
Discussion Starter #11
Thanks for the help - rebuild scheduled for this fall. I also figure I need to check on the oil plug in the crank. I don't know if that's been replaced or not, so for the moment I just watch the filter screen when I do oil changes. I'd rather catch it before it backs out though...

Thanks again,
John
 
1 - 11 of 11 Posts
Top