YZ exhaust cam timing = bent valves!?


30 replies to this topic
  • Rich_Rohrich

Posted June 09, 2004 - 01:40 PM

#21

YZ timing the exhaust cam therefore makes the cam timing safer!!!!




Taffy - Maybe you can explain to us poor poor slow witted sods how having the valve open farther at TDC with YZF timing is safer? Here on planet earth any delay in exhaust valve closing, like retarding the cam timing 20+ crank degrees with YZF cam timing reduces the exhaust piston to valve clearance. :thumbsup:

Miraculous as it might seem we do agree on one thing, the YZF timing is perfectly safe and wasn't the root cause here.

Do carry on and please enlighten us ohh Duke of Piffle. :awww: I'll make sure and take notes. :lol:

  • RichBaker

Posted June 09, 2004 - 01:56 PM

#22

So who makes good replacement springs for a YZ or WR? And what is the recommended replacement interval?

The problem I've seen with the YZF/WR engines is the springs lose tension over time so the rpm that the valves float at gets lower over time. Valves springs have a finite life, but they don't seem to get changed regularly by the weekend racer. This can lead to other issues as we have seen. The current crop of engines like the YZF450 and the CRFs barely have enough spring force when new to adequately control the valve at high rpm, so it's worthwhile to consider that as you service these engines, especially with the 250Fs. :thumbsup:





  • Rich_Rohrich

Posted June 09, 2004 - 02:05 PM

#23

So who makes good replacement springs for a YZ or WR? And what is the recommended replacement interval?





Kibblewhite is who we use for most cylinder head builds, but you can also get good quality pieces from Ron Hamp. The OEM springs on the 4xx are really quite reliable and cost effective if you keep track of them.

As for service intervals, that's the million dollar question. I've never seen guidelines for average racers that were very reliable. I always suggest that riders pull at least one intake and one exhaust spring every time they do a top end to do a quick check of the free length. If you keep good notes and start seeing the free length change it's worth investigating further. I realize that is sort of vague but it's kind of a gray area based on the usage the engine sees.

  • TallBoysWRF

Posted June 09, 2004 - 03:55 PM

#24

if you delay the cam timing you make the exhaust valve open after the piston has come and gone and it won't hit the piston on the overlap stroke either as the timing is too mild. at the very most the piston can only chase the valves home.

YZ timing the exhaust cam therefore makes the cam timing safer!!!!


Taffy


AH, Taffy,
You are thinking backwards. The YZ cam timing ADVANCES the exhaust cam, it doesn't delay it. :thumbsup:

  • Hamish

Posted June 09, 2004 - 10:11 PM

#25

How is the spring tension tested? Will they measure out to be a tiny bit shorter, or can they still be the proper length but too weak?


This is the procedure:
With the normal springs removed and a set of very soft test springs installed measure the length of the spring when the valve is closed. This measurement is called the 'installed height' Then you get yourself (borrow) a valve spring tester. Basically, it's a small set of scales that measures up to a few hundred pounds. Place the valve spring tester on the table of a drill press, and place the valve spring on top of it. Get on old valve and place it in the chuck of the drill press. Now turn the handle on the drill press and squash the valve spring. Using a vernier to measure the spring length, gently squash the spring until the height of the spring is the same as the installed height of the spring, then read the pressure on the gueage. This is your 'seat pressure'
To work out the length of the valve spring when the valve is open subtract the valve lift (less the valve clearance) from the installed height. Now continue to compress the spring until the spring is this length. Read the guage, this reading is the 'over the nose pressure'

I can say that Kibblewhite valve springs are good. Had a set in a modified ZX7 ninja for years, they replaced the APE items which didnt even last a year.

Believe it or not, but if you run a 2 valve Kawasaki Pro Stock Bike, valve spring need replaceing every meeting !!! (thats less than 3 miles riding)

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  • Rich_Rohrich

Posted June 10, 2004 - 03:48 AM

#26

Believe it or not, but if you run a 2 valve Kawasaki Pro Stock Bike, valve spring need replaceing every meeting !!! (thats less than 3 miles riding)


In the same vain, I was chatting with a cam guy who works with an NHRA Pro Stock car team about how short the life of CRF valves have become. He laughed and told me about the titanium valve springs in their engines only being good for 8 runs. Between racing and testing he figured their yearly valve spring bill was in the $80,000 range. 1000 pounds of open pressure doesn't come cheap. :thumbsup:

Suddenly I feel a lot better about the service intervals we have to follow. :awww:

The old rule still seems to apply:
higher performance translates to higher costs.

  • sabin

Posted June 10, 2004 - 08:00 PM

#27

Great info fellas!!! :thumbsup: :awww: :lol:

However you got me worried a little. As I did not have the correct pads some of my valves were set a little lose.

Intake must be 0.10-0.15 and mine is 0.17
Exhaust must be 0.20-0.25 and mine is 0.27

Do I risk demaging my valves?

  • Hamish

Posted June 10, 2004 - 09:52 PM

#28

Sabin, in my opinion that is fine what you have. My mentor (25years of race engine experience) always advises to run the clearance on the higher side. I wouldnt like them to be more that 0.05mm over the recomended maximun.

Hamish

  • Rich_Rohrich

Posted June 11, 2004 - 06:56 AM

#29

However you got me worried a little. As I did not have the correct pads some of my valves were set a little lose.


There is no reason to worry as long as you don't stray too far from the Yamaha spec. There is plenty of wiggle room in the spec on either the loose or tight side. :thumbsup:

  • TallBoysWRF

Posted June 11, 2004 - 08:05 AM

#30


if you delay the cam timing you make the exhaust valve open after the piston has come and gone and it won't hit the piston on the overlap stroke either as the timing is too mild. at the very most the piston can only chase the valves home.

YZ timing the exhaust cam therefore makes the cam timing safer!!!!


Taffy


AH, Taffy,
You are thinking backwards. The YZ cam timing ADVANCES the exhaust cam, it doesn't delay it. :thumbsup:


Taffy,
Turns out I was thinking backwards. The YZ spec timing does retard the exhaust valves as you said, but the piston has not come and gone when the valves open. The valves open as the piston moves up in its stroke to force the gases out. :awww:
Thanks Rich :lol:

  • SFO

Posted June 13, 2004 - 07:46 AM

#31

Welcome back, Taffy!
Erudite as always.
There are many great valve spring tools available, although the drillpress valve spring checkers tend to be on the lower end of the performance/accuracy scale if they are calibrated or double checked with a higher quality spring tester I might trust them.
The Yamaha valve springs are less than 10$ dealer which is a no brainer in regards to replacement when rebuilding occurs.
It is cheap insurance to R&R these springs every time the head is off.
In this instance of V to P contact I might also double check the degreeing of the cams to assure that the camshafts have not reindexed in relationship to their sprockets as they have been known to do on rare occasions.




 
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