Topic: ATDC ? cam chain adjustment

[greenjeans]

I need to adjust the cam chain on my '72 CB750k before I adjust the tappets.

Is ATDC 15 degrees BEFORE TDC or AFTER ?
Another silly question:  when looking at the crankshft bolt from the RT side (points side) do the engine rotate clockwise or counter clock wise ?

Then manual I have really doesn't say more than 2 words about it:
1  Position 1 & 4 cylinders to TDC
2  Push in the push bar for the cam chain tensioner.  Install tensioner on the cylinder, loosen the tensioner adjusting bolts and retighten the bolts and lock with the lock nuts

I've checked the FAQ for a more elaborate (plain english for relative newbie) One of the posts states to properly set it you need to set the 1 & 4 to TDC and then advance it 15 degrees before tightening.  Another said the bike should be running to adjust properly...yet another 15 degrees BEFORE TDC.... Anybody care to clarify this ?

After reading many of the posts, I figure it may be a good idea for me to take off the tensioner and make sure it's working properly and not gummed up or anything. 
(It's been sitting since 2002)  I got it running the other day - only for a minute or so, but it was noisy.   

So......Can anyone point me to a link or explain in plain "newbie style" english the way to adjust the cam chain (after I clean the tensioner) with the bike NOT running. (waiting on a carb body, new points and condensers)    TDC or 15 degrees advanced....running or not running ?
*My carbs and tank are out of the way, engine is in frame.


Next....I'm on to the tappets which seems to be explained a bit clearer than the tensioner.   But, I do have some questions regarding that as well.

I start by aligning the timing and index mark to TDC. (This can't be hard to screw up right?)  Question:   At this point the valves at the #1 piston should be fully closed.  How can you determine that they are indeed "fully closed" ?  Is that just a given ?  Then I can loosen the lock nut and tighten/loosen and check the clearance of the tappets on the intake side 1 & 3 to .05mm and the exhaust side of 1 & 2 to .08mm     Then I rotate the crankshaft 360 degrees with the kickstarter and check the intake side of 2 & 4  .05mm  then  exhaust side 3 & 4 .08mm.   Sound right ?
This can all be done WITHOUT taking off the valve cover correct? 

Sorry for the long post, but it usually helps me just typing these things out.  Some time the rocks in the head need a little jumbling.

Thanks fellas

[greenjeans]

I know I'll get some "Look in the FAQ section!"   believe me, I have been for the
last 2 hours.... and I know I'm making this harder than it it....just have never done it.

I'm finding more than one way to do this...all are pretty much the same, the main differences being whether the engine has to be at TDC or 15 degrees before TDC or 15 degress after....which is it ?    And how do you know when it is 15 degrees before or after ?  Is there another mark that I don't know about ?

You guys set me straight on this, and I promise not to post anymore questions for at least the weekend.

[Bob Wessner]

Did you see this one?

http://forums.sohc4.net/index.php?topic=369.msg3328#msg3328

[paulages]

that link explains it pretty well, but to answer your other critical question, the crank spins clockwise as you are looking at the points assembly.

[mystic_1]

Is ATDC 15 degrees BEFORE TDC or AFTER ?

ATDC = After Top Dead Center.

Another silly question:  when looking at the crankshft bolt from the RT side (points side) do the engine rotate clockwise or counter clock wise ?

Clockwise.  Think of it this way: the crank shaft spins in the same direction as the wheels when moving forward.

So......Can anyone point me to a link or explain in plain "newbie style" english the way to adjust the cam chain (after I clean the tensioner) with the bike NOT running. (waiting on a carb body, new points and condensers)    TDC or 15 degrees advanced....running or not running ?
*My carbs and tank are out of the way, engine is in frame.

OK, here's a breif explaination:  The camchain tensioner consists of a spring-loaded roller that presses against the cam chain in order to take up slack.  The locking bolt on the cam chain adjuster locks the roller into whatever position it's in.

The cam chain runs over the cam, down around the crankshaft, through the tensioner, and back up to the cam.  IF you rotate the crank shaft, this pulls the front (descending) run of the chain tight, which moves the cam shaft.  The slack is then in the rear run of the chain.  Think of it like the final drive chain - the top run of the chain is being pulled on by the engine sprocket, so it is tight.  All slack will be in the lower run at this point.

So to set the tensioner, the idea is that, with the engine off,  you roll the engine forward to 15deg ATDC, which pulls the cam chain tight on the front run and moves all the slack to the rear run of the chain.  Roll the crank clockwise until you are right at TDC, then continue to rotate forward until the advancer's spring peg is just past the timing mark.  Does not have to be EXACTLY 15 degrees, you just want to make sure you've taken up all the slack in the chain.

At this point you loosen the cam chain adjuster locknut, and the built-in spring pushes the roller into the chain to take up slack.  You then re-tighten the locknut to lock the roller in place.

Do not rotate the crankshaft while the locknut is loose, as this will throw off the amount of tension that the roller is placing on the cam chain.

Do not rotate the engine backwards if you go past the timing mark. as this will allow the slack to move to the front run of the chain.

Do not adjust the cam chain while the bike is running - other bikes are adjusted this way but the CB750s are done with the engine off.

There's a much more succinct description of this procedure in the FAQ:

http://forums.sohc4.net/index.php?topic=369.0

I start by aligning the timing and index mark to TDC. (This can't be hard to screw up right?)  Question:   At this point the valves at the #1 piston should be fully closed.  How can you determine that they are indeed "fully closed" ? 

When a cylinder's valves are closed, you'll be able to wiggle the rocker arms back and forth a bit.  Check both exhaust and intake valves on Cyl #1 and make sure there's a little play there - feel the other rocker arms to feel the differnce between open and closed ones.

Then I can loosen the lock nut and tighten/loosen and check the clearance of the tappets on the intake side 1 & 3 to .05mm and the exhaust side of 1 & 2 to .08mm     Then I rotate the crankshaft 360 degrees with the kickstarter and check the intake side of 2 & 4  .05mm  then  exhaust side 3 & 4 .08mm.   Sound right ?

Not quite, set cylinder 1 to TDC of compression stroke, check both rockers are loose.  Adjust both valves on cylinder 1.  Now rotate crankshaft 360 degrees forward - this puts Cylinder 4 at TDC of compression stroke.  Check both rockers are loose, then adjust both valves of Cyl 4.  Not repeat the above, but using the 2-3 timing marks, for cylinders 2 and three.

On other words, position each cylinder at TDC of it's compression stroke and then adjust both valves for that cylinder.  Do each cylinder one-at-a-time.


Hope this helps

mystic_1

[greenjeans]

Thanks guys, we got snow here in North Texas and they are sending us home for the day....I'll start on it as soon as I get home.

Mystic, I have to adjust the valves as well.....not just the tappets ?
Guess I have some more reading to do....  adjust valves then tappets..
All this can happen with the engine in the bike and the valve covers on - right ?

ANy more links and advice off the top of your head ?

[Cvillechopper]

You adjust the tappets, which are what compress the valve spring.  Most people call it adjusting the valves but you are only adjusting the thing that moves the valve technically. 

This is all stuff that you'll become very familiar with and is done with the engine in the bike.  Make sure your engine is dead cold (like overnight sitting) to adjust the valves.

Good luck.

[mystic_1]

Right sorry.  Cvillechopper is quite right regarding what I meant.

This procedure is also called adjusting the valve clearances, which is performed by turning the tappet screws.

Yes, this can all be done without removing the valve cover, although if you can do so it makes it all about twice as easy.  Otherwise you have to try to fit your tools through the round tappet adjusting ports, which is doable but a bit tricky (see below).

Other tips:  Use a crescent wrench on the engine nut under the timing cover to rotate the engine, not the kick starter.  Using the engine nut is much easier and much more precise.

As Cvillechopper says, do this with a bone-cold motor, as any heat will expand the parts and throw off the measurements.

remember, when I say that the tappet will be loose, keep in mind that the gap is only a few hundredths of an inch.  they wont move much but once you get your hands on things the difference between a closed and open valve will become obvious.

Use a large flat tip screwdriver, or an actual tappet adjusting tool, to turn the tappets and also to hold them when you snug down the locknuts.

Double check the clearance after snugging the locknut.  You will have to loosen, adjust, tighten several times probably before you get it just right.

The feeler gauge should slid through the gap with just a little bit of resistance.  In other words, it should just barely fit.  To verify, try the next larger feeler gauge you have and it should not fit into the gap.

You'll have to curve the blade of your feeler gauge in order to slide it into the gap between the tappet screw and the valve stem.  use your other hand to steady and apply a bit of upward pressure to the tappet arm to ease things along.

Once you've done both valves on two of the cylinders, have a seat and drink a beer, you're working hard!

Since you're into it this far, this is a convenient time to check your points and timing, if applicable.

Make sure you do your maintenance in the right order, as spelled out in your shop manual usually.  Failing to adjust the cam chain, for example, will throw things off when you go to set the valve clearances and timing.  those things being off will make it hard to sync the carbs, etc.

best of luck to you. 

mystic_1

[Cvillechopper]

One suggestion I'd like to make is that you pull out the spark plugs before using the large nut on the crank to turn the engine.  Yes, you can turn it with the plugs in but there is a risk of stripping the nut off.  Without the plugs there is no back pressure so the engine turns much more easily.  I would also suggest buying the correct size offset box end wrench for this.  A large crescent will work but a box end wrench with an offset makes it easier IMHO.

Don't worry about trying to get the valve cover off unless the frame has been cut out above the engine.  You can't get the valve cover off with the engine in the bike on a 750. 

[moham]

Other tips:  Use a crescent wrench on the engine nut under the timing cover to rotate the engine, not the kick starter.  Using the engine nut is much easier and much more precise.

great tip. I wish I had heard that a couple weeks ago when I did mine...trying to advance to those marks using the kick shaft was a bit tedious, to say the least...

[paulages]

I would also suggest buying the correct size offset box end wrench for this.  A large crescent will work but a box end wrench with an offset makes it easier IMHO.

it's 23mm

[mystic_1]

I would also suggest buying the correct size offset box end wrench for this.  A large crescent will work but a box end wrench with an offset makes it easier IMHO.

Yeah normally I use crescent wrenches as little as possible, it's always easier and better to use a properly sized wrench.  Crescent wrenches tend to be harder to get on and off the fastener, and have a habit of rounding off corners.  I just figured more people would have a large crescent than a 23mm wrench   Good call on the offset box end, thats what I use.  In fact, a good set of offset combination wrenches is a great investment for anyone working on metric motorcycles in general.

Especially avoid using crescent wrenches on the tappet covers.  You can round those off or crack them just by looking at them too hard.


mystic_1