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Re: 998 Cylinder Balancing
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.,,,
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I love how descriptive you are gerald... a cylinder head rattle... on WHAT??
Cylinder head rattles are most often caused by excessive valve clearance or loose cam chains. Check the manufacturer specs and measure the clearances and adjust if necessary.
There are a number of other things that could cause the rattle, if that doesn't solve it, but I'd suggest taking to to a mechanic to figure out what the problem is.
Yamaha YZ 426 F
Cross / motocross
71.6 out of 100
Engine and transmission
426.00 ccm (25.99 cubic inches)
Single cylinder, four-stroke
Bore x stroke:
95.0 x 60.1 mm (3.7 x 2.4 inches)
Valves per cylinder:
Chassis, suspension, brakes and wheels
300 mm (11.8 inches)
Rear suspension travel:
315 mm (12.4 inches)
Front tyre dimensions:
Rear tyre dimensions:
Front brakes diameter:
250 mm (9.8 inches)
Rear brakes diameter:
240 mm (9.4 inches)
Physical measures and capacities
105.0 kg (231.5 pounds)
998 mm (39.3 inches) If adjustable, lowest setting.
1,303 mm (51.3 inches)
2,171 mm (85.5 inches)
827 mm (32.6 inches)
375 mm (14.8 inches)
1,490 mm (58.7 inches)
I had a 4 stroke 500CC engine doing the same thing. It was sneezing fuel backwards out of the carburetor, and backfiring. I checked the valve clearances at Top Dead Center and found that I had no clearance on the intake valves. So, when compression would try to build, instead of staying inside the cylinder where it belonged, it shot backwards out of the intake (because the intake valves were ever so slightly open). So, check valve clearances and valve timing. Mine had to be set at .006 mm intake, and .010 mm exhaust. Runs perfectly now.
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.
If there is no clearance adjust the tappet clearance.
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.
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
wrench on the locknut on top of the valve, and turn it anti-clockwise.
be tight. Turn it about one quarter turn, and keep the wrench on it.
the crescent wrench turn the square headed screw either in or out
whether you need more or less clearance. We are talking about fractions
turn here. Then lightly lock the locknut, holding the square headed
the crescent wrench so it doesn't move, and check the clearance. Repeat
it's exactly right. When it's exactly right, tighten the locknut.
(7) Rotate the engine one revolution exactly:
Shine the flashlight into either sparkplug hole and rotate the engine
so that the piston goes down once and comes back up. Stop when it's at
(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
o-rings in place, the bike will have one or the other.
That's all there is to it!
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.
with the cylinder set at top dead center compression. using a feeler gauge between the tappets and the valves there should be .002-.003in clearance on the intake valves, and .006-.007in. clearance on the exhaust valves. thank you.
Valves are actuated only recently at Moto Guzzi by an overhead camshaft. Shim under bucket design precludes consumer adjustment due to shim kits price/availability. But it can be done! Not sure what the interval is, usually needs it between 12k miles, and 20k miles. If you hear noticeable tapping that doesn't quiet after warmup, it is time to check clearances. Good Luck, and nice bike, by the way. TT