Lobe position on valve clearance check '81 CB750C

baldjeff

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Hi all, I'm in the middle of adjusting the valves on my '81 CB750C. From what I understand the cylinder pairs are supposed to be in line with each other as you rotate the crank right? In other words, when the notch on cylinder 4, exhaust side, is at the 3 o'clock position, then cylinders 1 and 3 exhaust lobes should both be pointing towards the exhaust. That's not the case for me. When the notch is at 3 o'clock then cylinder 1's (exhaust side) lobes are pointing upwards and cylinder 3's lobes are pointing towards the exhaust. As I turn the camshaft 90 degrees, then cylinder 1 (intake side) are facing towards the intake and 3 are facing upwards. Same goes for cylinders 2 and 4 as I continue to turn the crank, they're not in line with each other. Is something way off with my camshafts? Is that going to effect my clearance readings? I just want to make sure I did this right before I order a ton of shims. The white lines on cylinder 1 are 180 degrees apart but not on cylinder 4.

I posted my compression readings in another post. If it gives you any additional info they are as follows: cylinder 1 - 145, 2 - 150, 3 - 167, 4 - 142.

Here were my clearance readings, all in inches: Exhaust side, cylinder 1 - .0025 and .002, 2 - .003 and .003, 3 - .0025 and .002, 4 - .003 and .0015
Intake side, cylinder 1 - .004 and .003, 2 - .004 and .004, 3 - .004 and .004, 4 - .004 and .004

Also, as I turn the crank there's a fair amount of resistance then it almost falls into 3, 6, 9, 12 o'clock positions. I'm only turning clockwise from the right side of the engine. Is that how it's supposed to feel to turn the crank? Thanks in advance for your help.
 

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amc49

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Yes, the engine changes resist6ance based on your running it up on compression, then the resistance goes down when you pass it. The valve springs change it too.

It's hard to set valves on these and one reason why nobody ever did them. They can change the clearance based on checking one and then messing up the setting by rotating engine and check it again, it can vary easy by one thou or more. The cam clearances are more than the valve clearances and it allows the valve springs to push the cams around in the cap looseness and then your check number appears to change. Anything you get at a firm .002" or less is effectively zero with engine running and why we set at .005" instead of the book .003" as .002" of any number you get is not real with the engine up and running. It disappears in the cams centering up once oil gets pumped in and the spring loads average out.

Valves set firmly at .002" CAN burn the valve in use, I've done it. Go to 5, then the least it can be is 3 realworld. You do not want anything under .004" and .003" for sure. By shooting for .005" then a range of .004"-.006" works well, the engine will make no tappet noise beyond normal.

I for one never use the marks as they can show the same problem, a lot of people back in the day used to set them at TDC after compression, at that point both valves are firmly closed and the tappets will both turn easily. The logic being that that point made the most sense to have them set properly at. Or open the valve to max lift then rotate 1/2 turn to cam lobe exactly opposite valve to be dead sure you are well off the clearance ramp and set then like the race guys do.

You should know that the end of cam valve setting marks were different on the early ones, they changed them thinking in '81 at some point. Honda was by then realizing they themselves were having trouble setting the clearances and fumbling around trying to find a better way to do it.

The easiest most reliable is to open the number up from 3 to 5, the engines go much longer with no valve issues that way.
 

amc49

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Anything you got there at around .002" or less is in valve burn territory. The valves due to the crap quality tend to erode in seat recession and the clearances then tend to close up rather than get looser like most other engines do. Why they burn valves so easy.

You can compare the compression readings with the tight valves to see if anything may be going on there, often it shows, the tight ones will be down on compression. Book spec on a new engine is 170 psi.
 

amc49

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Look at your 1 and 4, the exhausts may be leaking. Exhaust side gives the most trouble with valve recession, the valves are low quality steel with a simple heat treat and the recession removes the heat treat to then burn pretty quick. If lucky you may be just leaking because of the clearance being under zero, and not burned yet. Gotta fix that quick by going looser. Often the loose valve being held open carbons up to not seat well, you adjust it correctly and then take bike out and drive the crap out of it hard to beat the carbon back and then the valve may go back to sealing better. Then check it again to make sure it still has the bigger clearance in it, it can close closer if it beats enough carbon off.
 

baldjeff

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Thanks AMC! No clue what I would do without your wisdom. So it's not an issue that the lobes are facing different directions? It shows 30k miles on the odo. Should I just go ahead and rebuild the top end since I have the engine out already? Is it worth several hundred dollars? I would have to buy a valve spring compressor, piston ring compressor, piston rings, maybe some valve stems, gasket set, etc. I currently don't have anything. I'm calculating all OEM parts. I'm not sure if I trust after market stuff for the engine. At the same time I don't know if I'll ever take this insanely heavy thing out of the frame again. And I desperately need to clean around the spark plug holes, the PO didn't take care of this thing at all.
 

amc49

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I'd see if you can bring it back first. The rings are likely OK, the valves and mainly exhaust are the common issue and you can't get many engine parts any longer. The valves CANNOT be ground, seats can, replace valve only at like $25 each, you will discover this engine to be very expensive to rebuild.

Been so long since I used the OEM valve setting marks I can't say, I didn't like setting like that, probably OK what you are doing though. I did so much race stuff I was used to setting with the cam lobe for that valve opposite the valve.

Most of them go down due to exhaust valves leaking, then owners tear into the carbs thinking it's them. It CAN be though as the carbs are a handful just by themselves.
 

baldjeff

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Ok I know I’m an absolute idiot. I forgot the valve tool on cylinder 1 intake as I turned the crank 90 degrees. Are the valves definitely bent now? There wasn’t much resistance, I didn’t force it anymore than normal. 🤦*♂️

Also, can you leave the shims out as you turn the crank? I want to try to play musical shims but I’m afraid to turn the cam with no shims. Thanks again for your help!
 

amc49

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You may have bent a valve, you often don't really feel it. Roll engine back around and recheck your original clearance, if now it is wider/looser it is bent.

You absolutely cannot rotate the engine with no shims in it, the tappets damage and cams too.

The engines demand nothing less than the clearest thinking on how and what to do next and why so many problems. Drinking beer while doing it is a FU.
 

baldjeff

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I don't think its bent. Clearances are the same. I would've had to go a full 180 degrees for all of the valves to be in the same position right? Have you ever had a tough time getting the shims back in? The tool keeps slipping and will keep one bucket down and the other will pop back up. I can't seem to get both to stay down at the same time. It's only happening on one pair, all the rest are back in already.
 

amc49

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If the tool is worn or not wide enough to positively hold both tappets down then one will try to pop back up. I at first made my own tool back in '80 and it worked fine but was not hardened and later the wear from use let it do that. A later tool bought was hard enough it never did that.
 

baldjeff

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FFS, it was such a pain to get all the shims back in but they all look properly seated. I double and triple checked every one before turning the crank. After they were all in I rotated it a few times to check for anything hanging up and it all went smoothly. I went back and checked clearances again and now some of them are reading 0 as in no clearance at all. Any idea what I screwed up now?
 

amc49

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Either you put one in upside down (worn side down now) or they have not settled fully in place yet. The stamped on thickness numbers stay down to preserve them so you can identify them later. Oil will hold the shim up a half thou until pounded out dead flat with engine running.

Remember as well I pointed out you can check and get a number and then roll the engine around to then check and get a different number, the cams being pushed around by springs in the clearances thing I spoke of. Why you throw away the .002" to be sure of where you are at.
 

baldjeff

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Thanks for all the help so far! So I don't have to worry about any other damage? Should I just proceed to order the required shims as I previously measured them and disregard the new lower clearances? I did measure each shim with a caliper and they were the same size stated on the shim so I'm not sure if that rules out the worn side possibility. Once the new ones are in, theres no way to bench test them? I have to fire the engine to make sure they're fully settled first? Is there a test for bent valves without taking the head off? What are the chances thats the problem? If I have to replace the valves, will new ones even seat properly or will I have to change all of them for consistency? Thanks again!
 

amc49

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Even a slightly bent valve will leak compression, run a test on that one again. The shims really don't wear other than wearing the thickness number off and why you face them down. They can wear but only after LONG use.

You put the new shims in and CAREFULLY, you can really mess up not making sure they are all the way seated. They can tiddley-wink out of place to break cams and such. Roll engine around a few times and recheck the clearance with feeler gauge.
 
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