Checking valve clearance


Checking valve clearance

Postby bobber01 » Wed Dec 21, 2011 11:47 pm

How many follow the instructions on page 3-10 of the Honda Service Manual when checking valve clearance? I'll get different measurements when I adjust valves with its opposite valves fully open, thus the lobes of the valves I'm checking are 180 degrees away from the shims than if I follow the service manual sequence where some lobes aren't 180 away from shims. I find that when I follow the sequence in the service manual and then re-check individually (with lobes 180 from shim) I have to re-shim 90-95% of the time to get back within spec.
Want to do the valves in Dannys bike tomorrow, lower engine to get carbs off then crank to check compression. Want to keep as much carb 'crud' out of the engine as possible and disturb carbs internals as little as possible until ready to take them apart.
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Re: Checking valve clearance

Postby preston » Thu Dec 22, 2011 12:46 am

Follow the procedure in the Factory Shop Manual and you won't go wrong, Done hundereds and it's the only way to do it with the least crank rotations and have never had a problem with any since the '78 models. When in doubu always follow the FSM, it's 99% right.
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Re: Checking valve clearance

Postby Larry Zimmer » Thu Dec 22, 2011 12:42 pm

Agree with Preston regarding the valve clearance check. Relative to compression check, it sounds as though you plan to do that with the engine tilted and carbs removed. Keep in mind that your compression readings will be lower doing it in this manner. As the manual says, warm the engine first. The reason for the lower values is that the cold engine will not seal as well as warm; and, the cold engine will absorb some heat of compression resulting in slightly lower compression pressure. Neither will affect the uniformity of pressure cylinder-to-cylinder, though.
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Re: Checking valve clearance

Postby bobber01 » Thu Dec 22, 2011 1:04 pm

About the valve clearances, curious as to why there would be a difference when cam lobe 180 degrees away from valve rather than when between 90-120 degrees as most are when following the Service Manual. Enough of a difference that clearances are out of spec.
You're right about the engine being warm to get a 'true' compression reading. Just that when I was first rotating the engine by hand to check valve/cam movement (remember this bike has been sitting for a decade or more with no prep) I noticed a couple of cylinders did't seem to be building any compression when I'd put my finger over the spark plug hole while turning. Others would give a little 'poof' of air. Ideally I'd like to do a leak down test but all my tooling for that procedure is 450kms away at home. Looking for some idea as to if I have pull the cylinder due to stuck rings or damaged/burnt valves. Used my borescope and cylinder walls/piston tops looked good. I cann't fire up the engine the way the carbs are now, may have to go get a 1/2 mask if they stink as much as the fuel tank did when I took it off! LOL
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Re: Checking valve clearance

Postby EMS » Thu Dec 22, 2011 2:21 pm

bobber01 wrote:About the valve clearances, curious as to why there would be a difference when cam lobe 180 degrees away from valve rather than when between 90-120 degrees as most are when following the Service Manual. Enough of a difference that clearances are out of spec.


Very much depends on the actual cam contour. You may already be in the cam rise area at 90-120 degrees. Only way to be sure to really be on the cam ground is at 180°.
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Re: Checking valve clearance

Postby daves79x » Thu Dec 22, 2011 4:31 pm

Preston is right as far as I'm concerned, if the procedure in the manual is good enough for Honda, it's good enough for me. You'll go nuts doing it any other way. The base circles of the cams are quite consistent enough that if you set your valves on the loose side (.004-.005 in), you'll not have to worry just what the clearance may be at exactly 180 degrees from the shim. Save your time for other, more important issues.

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Re: Checking valve clearance

Postby Larry Zimmer » Thu Dec 22, 2011 8:48 pm

The reason the cam clearance check doesn't 'work' sometimes at the 90 deg point is that many cams,especially with mechanical lifters, are ground with a so-called take-up lift prior to the true profile. This will be on the order of 'only' 25 to 50 microns lift. (.001" - .002") But, it occurs over a relatively large cam travel. It's purpose is to gently take up the clearance before the high acceleration rate of the true profile without 'slamming' the cam into the lifter (bucket). For this reason, you may find, then, that your measurements differ between the 90 deg and the 180 degree by that amount. Note that not every manufacturer does or has done this. So, best bet is stay with the 180 point for true clearance.
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Re: Checking valve clearance

Postby preston » Thu Dec 22, 2011 11:02 pm

If you're only getting a "poooffff" out of some you're waisting your time on a motor that has been setting for years. If all you want to do is get it running and walk away and never see it again there are ways but not one I could sleep with. Otherwise it's time for a top end job at the minimum. What's the sludge in the oil pan look like??
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Re: Checking valve clearance

Postby bobber01 » Fri Dec 23, 2011 2:20 pm

I'm thinking that a re-ring and valve seal replacement may be in order for the engine however not my bike and not sure how much effort the owner wants to put into the machine or even what his long terms plan is for it. Sitting as long as it has MANY parts has seized up (brakes/cables/drive chain/etc).
It keeps my mind occupied while going thru treatments so I'm puttering along bit by bit. Getting the engine running properly is first on list and thats how I came about checking the valve clearances. Once get that sorted want to move on to the carbs and clutch before attempting to start it up.
Haven't dropped the oil yet so not sure whats in the base pan but will be done prior to starting.
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Re: Checking valve clearance

Postby preston » Sat Dec 24, 2011 12:06 am

Bobber, This CBX engine will keep you mind occupied for sure. If you need any help feel free to call, I'm more than willing to share my knowledge if these engines. What you do with my thoughts is up to you. Always remember to pay attention to detail, think ahead.

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Re: Checking valve clearance

Postby bobber01 » Sat Dec 24, 2011 11:25 pm

preston wrote:Bobber, This CBX engine will keep you mind occupied for sure. If you need any help feel free to call, I'm more than willing to share my knowledge if these engines. What you do with my thoughts is up to you. Always remember to pay attention to detail, think ahead.

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Thank you very much for the offer, may just take you up on it.
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While I'm at it hope all are having an enjoyable Season.
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