Question about 2003 Ducati 998

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998 Cylinder Balancing

I recently noticed (as I was checking valve clearances) on my 998

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That when opening the throttle the front butterfly seems to open fully but the rear butterfly stop short by a few degrees. In other words, front and rear butterflies do not open exactly the same amount. The bike has done 9k miles and apart from maintenance and valve checks, have never bveen worked on. It is in pristine condition, runs great and pulls strong well beyond the 10k redline. I have not had 4V desmo's before and therefore cannot compare power wise, but she feels rather quick to me. For what its worth, it has the 50mm Termi half system and matching ECU. Is this normal and just a result of the factory balancing exercise?,The length of the throttle linkage rod is set in the factory on a test bench. It is a brave person that messes with that linkage rod. What is more accurate is to do a cylinder balance using carb sticks to check if cylinders are balanced on idle. If they are out by one or two degrees at full throttle the difference is minmal and not something that an average street rider should worry about. (When is the last tiem you were banging off the limiter for extended periods of time?) At the same time check your throttle stop set screws to see if they are in the correct place. If the cylinders are balanced at idle I would not worry about it.,,,

Posted on Nov 10, 2008

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Exhaust blows and sucks when turned over on trx300 Honda Fourtrax atv. What's likely the problem? I suspect something to do with the valves, perhaps a top end rebuild or just new top end gasket and o-ring...

Hi and welcome to FixYa. I am Kelly
The exhaust valve is remaining open. This can be caused by carbon stuck under the valve tapered head area, a cracked valve seat or..... a broken valve spring. NOT head gasket. Before you take the head off.. open the valve cover and check for a spring problem. If there does not appear the be a spring problem watch the valve travel and then make sure with the piston at the top of cylinder that the rocker arm has 0.003 - 0.005 clearance between the tappet and the end of the valve.
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After you have the proper valve clearance check for the previous symptom. If you still have that symptom then your going to hae to remove the head and inspect the valve tapered area and valve seat for carbon, cracks and abnormal wear.

Thanks for choosing FixYa.


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Inlet .0024 (two and a half thou.)
Exhaust .006" (six thou.)
(6) Set the valves:
Set the valves on the cylinder on the COMPRESSION stroke. Put the wring wrench on the locknut on top of the valve, and turn it anti-clockwise. It will be tight. Turn it about one quarter turn, and keep the wrench on it. Then, using the crescent wrench turn the square headed screw either in or out depending whether you need more or less clearance. We are talking about fractions of a turn here. Then lightly lock the locknut, holding the square headed screw with the crescent wrench so it doesn't move, and check the clearance. Repeat until it's exactly right. When it's exactly right, tighten the locknut. Recheck.
(7) Rotate the engine one revolution exactly:
Shine the flashlight into either sparkplug hole and rotate the engine slowly so that the piston goes down once and comes back up. Stop when it's at Top Dead Centre.
(8) Now do the other cylinder:
Check the gap and set if necessary the other cylinder's valves
Repeat steps (5) and (6) for the other cylinder.
(9) Button it up:
Replace the valve adjustment covers, making sure to keep the gaskets or o-rings in place, the bike will have one or the other.
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If the valves were bent, the engine wouldn't run worth a darn, if at all. What I suspect is that the exhaust valves have sank their way into their seats and are now positioned too far upward in the cylinder head to attain adequate tappet clearance. Exhaust valves have more of a tendency to do this because of their high thermal loading during operation. Then, as the engine continues to be operated - and the tappet clearance diminishes, the valves run even hotter, causing damage to the valve and seat ares.
So, to answer your question; yes, the previous owner did neglect it - by failing to keep the valve tappet clearances set correctly.
The only cure for this, sadly, would be to remove the head and replace the exhaust valves and recondition the cylinder head's valve seats. Of course it may turn out that the cylinder head valve seats are worn beyond repair, requiring a new cylinder head.

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