Suck Squeeze Bang Blow - Big Jimmy's Motorcycle Forum
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post #1 of 1 (permalink) Old 04-04-2011, 07:07 PM Thread Starter
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Suck Squeeze Bang Blow

Suck Squeeze Bang Blow
or for the politically correct and exacting but humorless engineers '
Intake Compression Power Exhaust

There are two major pressure changes in a four stroke internal combustion engine , when you light the fire which is easy for you youngsters with your electronic ignition and when you let go of it . You go to people like Falicon with your cranks and A.P.E. or Branch with your heads . Pistons are weight matched within 1/8 gm or less .Even intake and exhaust ports are cc'd .Combustion chambers are volume matched within 1/4 cc or less . Some spend thousands on dyno time getting the perfect jetting or fuel map .
Then you adjust valves with a feeler gage ?
The valve clearances suggested by the manufacturer are just that a suggestion to keep you from holding a valve open or beating your valve train to death . Until Yamaha recently decided to experiment with 90 degree cranks all the inline fours used a flat (180 degree) crank with #1 and #4 at TDC and #2 and #3 at BDC . each pair firing alternately . Until the CBRXX no one thought to balance out this rocking couple but that's a story for another day . The point is why would you want to introduce any further variations in balance by scattering major pressure pulses over as much as 10 degrees of crank rotation ?
Use a positive piston stop to establish TDC and adjust the degree wheel to match a pointer . Insert the correct feeler gage between one exhaust valve and the rocker and adjust till there is a slight drag . So far this is basic a valve adjust . Now place a dial indicator on the spring retainer of this exhaust valve and preload it at least 0.060" . Pick some arbitrary lift distance like 0.030" and mark it on the dial . Rotate the crank until that valve has opened 0.030" and mark the degree wheel . If you started with #1 now go over to #4 and adjust the exhaust valve so there is that slight drag on the feeler gage and move the dial indicator to that retainer and preload it . When it has moved 0.030" look at the degree wheel . It should be 360 degrees out from #1 AND ON THE SAME MARK . Unless lady luck intervenes it won't be . Variations in cam indexing and tolerance as well as deformation of the top of the valve make it unusual in the extreme for all this to work with simply a feeler gage .
In the end all exhaust valves should be opening at the same place in degrees plus or minus 180 degrees at 0.030" lift and be within specified lash limits . If you have gone this far then repeat for the intakes even though the effect on the engine is not nearly as dramatic and closing more critical than opening .
Now of course when confronted selective shim valve adjustment you are limited to just how close you can get to ideal . Or are you ? If you have enough experience to have adjusted valves on an 8 valve kawasaki , Suzuki or other then you remember they came in half millimeter sizes ie 2.30 , 2.35 etc . I have sizes in 2.325 , 2.375 . If you think this is overkill imagine what you find in a factory race team's box of shims . I don't have to imagine it , I've seen it .
I do have a curious and humorous way of doing this that I have used to demonstrate the principal in a classroom and as a garage joke with friends . I'll set up the degree wheel ahead of time and expose the valve adjustment . I found a hose that just screws into the spark plug holes rather than continually removing the valve from my compression tester .
To the end of the hose I attach a rubber , yes a prophylactic , un-lubed , with a rubber band . The compression stroke inflates the "balloon" and the exhaust opening rapidly deflates it .If you really want to drive yourself crazy try the above procedure with the dial indicator and then repeat it with the balloon . I recommend a very little light oil on the seat . Now observe what a change of just 0.001" (insert a feeler guage between the rocker tip and valve or the cam lobe and "lifter" ) does in relation to opening in degrees . Then for those that obsess try just a 0.00025" or 1/4 thou change and record the difference in degrees at actual opening *. This "shade tree" device , with a little practice will also give a indication of low pressure cylinder sealing that a leak down tester will not unless equipped with gages costing several hundreds of dollars .

I'll attempt reason without anecdote or explanation .
If your cam specs are @ 0.040 lift and intake opens 19 degrees BTDC and closes 46 degrees ABDC and exhaust opens 34 degrees BBDC and closes 16 degrees ATDC ....
ALL exhaust valves are opening at 34 degrees before bottom dead center for their respective cylinder AND staying within acceptable valve clearances ( .007" to .010" ).
The lash or static clearance has far less impact on engine performance if kept within the manufacturer's specified limits than does adjusting the exhaust valves to open at a specific point in crank rotation .
I lifted representative specifications from an older megacycle 493-x1 cam

So it's up to you . Call it lazy to use just a feeler gage and get out simple tools and take a little extra time and get it right or use just a feeler gage and get close .


suck squeeze bang blow
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