cam sprockets spun out of place



10 replies to this topic
  • Rich_Clarke

Posted January 10, 2001 - 11:06 PM

#1

I have been noticed a lack of power for some time now so i took my bike in and found this.

I have low compression due to my bilitanium hydrolic clutch decompression (add on) lever pulling on the exauste valve enough to hit the intake slightly bending both.

and my exauste cam sprocket spun out of place by about a tooth and a half.

no wonder i could not get this thing up hills.

Was just wondering if any of you have had your cam sprockets spin on you.

  • Boit

Posted January 11, 2001 - 12:11 AM

#2

Whoa Nellie! Great info here! This illustrates the need to have a little bit of slack in that compression release lever. This makes sense to me. In this case, every time the exhaust valve opens and closes, it has pushed against the decompression shaft which in turn causes a snatching...or jerking effect with the exhaust cam. Since the cam sprockets are pressed fitted, over time this jerking or snatching effect causes the exhaust sprocket to begin migrating. Once it starts moving, its stiction to the cam is compromised and it moves easier and faster. I bet that once you noticed a decrease in power, it decreased almost exponentially. That is, it lost power at an accelerated rate. Anyone who has the stock decompression setup, try this....with the engine idling, put a small amount of pressure with one finger on the decompression(compression release) lever. You will feel instantly a violent pulsating sensation in your finger. Just think what happens if that compression release cable or hydraulic actuation is too tight. This is an excellent example of why it's very important to set up all your controls properly...especially when it comes to the cylinder head. On page 4-18 and step 3 of the camshaft removal instructions it states..."loosen: the camshaft sprocket bolts"...Obviously, Yamaha decided at the last minute to change the camshaft/sprocket design. They must think that we are idiots. Looks like Yamaha is succumbing to the "Lead Rock Singer" syndrome. Translated, that means, "I can do no wrong".

  • ande749

Posted January 11, 2001 - 01:46 AM

#3

I thought that the slack was required to prevent the valve from burning. The compression shaft presses the valve from the right hand side of the valve lifter. How does that snatching/jerking effect occur? Does the shaft and the cam lobe contact each other or what happens? I thought that if there is no slack and the valve is slightly open continuously, the result would be lower compression and eventually burnt valve, just like with too small or no valve clearance at all.

I'll check that spinning thing during the next valve clearance check, though. I think that this kind of information is where these owners forums are at best. Official importers and dealers don't often announce about things like this, at least I've never heard. They propably rather have the bike for a repair after the damage is done and then give you a nice bill.

Ande

  • Boit

Posted January 11, 2001 - 02:32 AM

#4

Ande: you are exactly correct in that too tight of an exhaust valve will "burn" the valve and seat. In multi-valve heads such as the Yamaha, the compression release shaft acts only on one valve...the far right one. Now, imagine that this one valve is not allowed to close properly while the other one is(3 intakes: 2 exhausts cylinder head design). An out-of-time exhaust valve throws everything out of whack. In this case, one exhaust valve is not allowed to close completely, as you correctly describe, and allows burning fuel to seep past it during combustion causing the "burnt valve syndrome". Think of what the violent combustion explosion does to a valve/spring/cam lobe if that valve is not seated during the combustion process. This explosion( for lack of a better term) is meant to drive the piston down and create power. If the valve is not allowed to seat and seal off the combustion chamber, all kinds of terrible things begin to happen. Combustion pressures are enough to overcome any valve spring pressure and cause the valve to do all kinds of destructive things. As the cam rotates and the lobe comes around to close the exhaust valve for fuel intake, that valve is once again not allowed to seat. The vicious cycle continues. In this design, valves are acted upon directly by the cams so any mistiming can be transmitted to the cams. Loose lash is better than tight lash in this design. Am I mistaken?

Ande: No, the compression release shaft puts downward pressure on the shim bucket only. It does not come in contact with the cam lobe or camshaft at all. It's postioned in such a way that it is beside the cam lobe. That bucket is pretty big in diameter...relatively speaking.

[This message has been edited by Boit (edited 01-11-2001).]

  • ande749

Posted January 11, 2001 - 03:31 AM

#5

At the time that ignition/explosion happens, exhaust cam lobe shouldn't be in contact with the shim bucket. So even if the pressure in combustion chamber would force the valve against its seat, it should have no effect on the cam. Damaging part would be the compression release shaft. But in order to keep the engine running, the valve can be only very slightly open. Thus there could be only very little movement caused by pressure. I'm not sure, if these are the destructive things you ment.

I believe that only serious mistiming affects could be transmitted to the cam, that is when the piston and the valve have a contact. I experienced that with FZR1000 (similar 5-valve head) resulting two bent valves, no other damage, rather tender touch, I think... Before I opened the head I just set the timing right. The engine did run but there was a considerable loss of power and it sounded as if it was running only on three cylinders. That taught me something about circumspection... One should never hurry while assembling engine!

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  • Hick

Posted January 11, 2001 - 08:23 AM

#6

Geez, The Applied Physics of Valvetrain Theory 101…

I think I may actually UNDERSTAND what Boit is trying to say. Let me say it differently though (not that it will help, I flunked Physics. Twice.):

The cam, via the lobes, encounters resistance every revolution as it opens the valves. The steeper the ramp of the lobe the greater the acceleration of resistance. So if we hold one valve slightly open it misses the initial ramp up of resistance. By the time the lobe contacts the valve bucket it is at a much steeper angle, producing a much more abrupt force for the cam and gear to deal with.

I think what Boit is saying is that this abrupt resistance on the second exhaust valve (i.e. it offers zero resistance then suddenly applies a lot of resistance when the steep angle of the lobe finally whacks it) finally caused the cam gear to rotate out of position on the cam (WHACK-WHACK-WHACK-WHACK, 5k times/minute). It makes sense to me that the two are related (compression release stuck and cam gear slipped).

But the main reason to periodically check for play in your comp release lever is that the valve buckets, like lifters in my Chevy, rotate to distribute the wear and friction. If the comp. release is contacting the bucket at all it will not rotate and wear unevenly and fail prematurely. This happened to someone here when they got a small rock wedged in their lever and it even damaged the head a little in addition to ruining the cam and bucket.

But what do I know?

  • Boit

Posted January 11, 2001 - 04:58 PM

#7

Hick: Thanks for you additional comment and info. You articulate it very well. Another example of destructive forces at work in an engine is the bad habit a few people have in lugging an engine. Have you ever ridden in someone's standard shift automobile when he/she is tooling along about 20mph in high gear? The damn thing will vibrate violently! I saw a guy bend all the pushrods in his 383 Superbee by repeatedly doing this. His timing chain stretched so much it jumped a tooth on the gear. It also cracked several of the connecting rod big end bearing surfaces. The moral to all this is that an engine requires operation within the design parameters. Valves must open and close at their proper time and must seat completely. The ignition must fire correctly...etc. Once demands are placed on the engine that exceeds its design capability, it will most likely self-destruct...sometimes quickly and without warning...other times, it will be gradual.
A lady I dated a few years ago kept driving her V6 Mustang at 60mph on the interstate long after the hot light came on. She drove it til it seized. I asked her why she didn't stop when the warning light first came on. She replied, "Well....it was still going..." Women's logic! God, help me!

  • Boit

Posted January 13, 2001 - 04:01 AM

#8

Ande: Valve lash it set properly when the engine is cold due to contraction of metals when cold compared to hot. As the engine warms up, the distance bewteen the cam lobe at TDC of the compression stroke should be such that the lobe puts no downward force on the bucket/shim/valve stem. The valve spring is supposed to hold the valve closed during cumbustion. If the valve lash is set too tight with the engine cool, then, as the engine reaches operating temperature and the metals expand, the valve may not close completely and not seal off for combustion. Some of the combustion will take place past the valve(we've been speaking about the exhaust valves here) and even into the exhuast pipe. This is accentuated as RPM's increase and it will misfire violently. It's these errant combustion processes that will wreak havoc on the cylinder head components.

  • ande749

Posted January 14, 2001 - 03:06 AM

#9

Boit and Hick: All you said make a lot of sense.

Because it has been some two or three months since I checked the clearances, I don't quite remember the exact composition of cam and the sproket. I've been wondering that should one have a cam bearing failure, changing that ball bearing wouldn't propably be easy. In that sense bolts would have been better choice. They would also prevent the possible spinning.

Ande

  • Taffy

Posted January 14, 2001 - 06:02 AM

#10

lads this is absolute bunkum!

rich wouldn't have been able to ride the bike without it stalling if the decomp was always open.

secondly, the cams first contact with the bucket would still be at the most acute of angles.

thirdly, we can sort all this tosh out when they go to refit rich's cams. they will have to re-time the wheel. are we all agreed? then we'll find out.

knowing most workshop mech's the shop will just order a new cam inc. the wheel costing rich a packet. all because they can't do the valve timing themselves.

rich, your problem is somewhere else.

if your workshop want the valve timing figures i will send them to you.

if you want to fit a falicon timing wheel & do it you can get my figures & have a power boost. i know you won't take the second option because when in doubt you will go back to standard. i can't blame you.

rich, couldn't you hear a loud 'ticking noise' while all this was happening?

didn't your bike stall on left handers?

sorry lads but i think you're wrong.

something to dt with the snarled up valves, probably. how did that happen?

get that & you've got your answer.


Taffy

[This message has been edited by Taffy (edited 01-14-2001).]

  • Boit

Posted January 14, 2001 - 06:02 PM

#11

An engine WILL run with an exhaust valve that is not allowed to seat completely...say for a degree or two of cam rotation. Compression would not be as high as design, but since the YZ is 12.5:1, it would still be plenty high enough to combust. So, it's not bumkum.

Let me add that one of the RPM limiting factors of a 4-stroke is "valve float". This is when the valve springs can't close the valves fast enough to follow the cam profile. The rev limiter will protect this from happening as long as the valve springs haven't weakened beyond design. In a high performance 4-stroke such as the YZF, it's imperative that the engine is maintained in a high state of tune. With an engine that's capable of 11,200 RPM's(rev limited), any mistiming is magnified exponentially. I'm not saying that because the decompression lever was too tight and slightly held open one exhaust valve, that this was the definitive cause of the cam sprocket spinning. I'm simply suggesting it as a possibility if the engine was operated over time with this occuring. If may have been as simple as the sprocket fit wasn't tight enough to begin with. Quality control isn't 100% on anything. Maybe the engine assembler had a hangover and didn't feel like rejecting what he/she knew to be a loose fitting sprocket. Nobody can say for certain.

[This message has been edited by Boit (edited 01-14-2001).]




 
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