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Wide valve clearance, correct cam positions?

c3powil

'82 Nighthawk 750sc
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So I'm measuring my valve clearance and I'm getting some wayyyyy wide readings. It looks like whoever did it last confused the .05mm spec with .005 inches. I'm getting readings ranging from .100mm to .150mm. This is safe for the valves, but surely would decrease performance, right?

I may not be checking the clearance at the right cam positions, though. The manual is extremely vague. It says to align the mark on the exhaust cam shaft with the front valve cover mating surface. I get that part. However, there are two markings on my exhaust cam....

It seems to me that as long as the cam lobe is not depressing the shims/bucket I should get an accurate reading.

Any help?
 
Better look closer at that manual, the spec is .08 mm. or .003". The range is .002"-.005" and I can tell you for a fact that the OEM setting of .003" can and will burn the crap out of valves. Use .005" as a MINIMUM and the engine won't see that at all, rather it will last MUCH longer in the long run as the valves on these tend to close up with wear rather than loosen up. Why? They are surface hardened only and once fuel went to true unleaded the seat wears faster than the tip. The bikes were built in an era of low lead fuel and that small amount of lead still performed wonders saving the valves. We are not there any longer.

Here's why they burn and why you have fits getting the same readings over and over. Simply put, look at the cam clearances in the head. They are .005" and even more. There is flat NO WAY you can hold a closer number than that with the loose cams being pushed around in the holes by the springs to alter the valve clearances to not be what you think they are. You can set as carefully as you want and say .002" and engine running will then not be closing that valve to get a compression leak and engine then runs like pure crap. Why you go .005", to make sure while running you got hopefully a truer .003" or so.

My first one of these I set close and perfect, just exactly as the OEM spec said. The bike promptly burned like 6 valves in less than 5000 miles. I'm no rookie setting valves either, been doing it for many years on all types of engines. The next head I loosened up to .005" after discussing the issues with others having same trouble and voila, engine then lasts forever.

The problem is so bad that there are those who look for the bikes at super discount as so many owners have set valves too close as well as the shops. Often you can redo the valves to looser and a motor that was missing before from valves not closing then goes to running like a bat out of hell. The carbs often get blamed for it and then carbs will be in a box to get bike even cheaper. You can make big money knowing that.

Honda knew there was a problem with setting them and attempted to change the marks in '82 I'm thinking, but the problem still there. Basically you can check the clearance and roll cam a bit and while still on the cam base circle the clearance will change one or two by simply moving it. Because the springs next to the one you're setting are pushing cams around in the head. It'll drive you crazy.
 
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Two marks on exhaust cam says it's an '82 model. The earlier ones only had one mark that you moved 90 degrees at a time. The later one you actually move it about 100-105 degrees and why the second mark is there, to not get confused. You need the 1982 service manual addendum to explain fully how to set the clearances, or simply ignore it all. Some set each cylinder at TDC since that is the most logical place to make sure the valve is set right what with what I just told you in the post above this one.

I'm telling you brother, set at .005" if you want that sucker to rev all the way up and last. Too close will often kill revability up around 8000+, they then don't pull all the way out. Later as valves get worse the not pulling out gets lower and lower until engine often won't run over 6000 right. My stuff set at .005" revved all day long hard to 9500-10,000 rpm. Does that sound like decreased performance to you???

You can get that addendum for free from www.cb1100F.net but you gotta join up and free. Look around there and you will find what I've posted here all over the place, I'm not nearly the first guy to go there loosening up those valves. It's a given there if you want that sucker to run right.
 
Thanks amc! I read what you said in the other thread I posted, and it seems I was the one who got it mixed up. That's great news. So my target is .005", but what is a good upper limit spec? I know one of my valves was closer to .03mm so I'll have to loosen it up. Checking clearances at tdc should be simple. I can pull the plug and check for tdc on compression stroke and check valve clearances from there, and also do spark plug gaps while I'm at it.

Your story seems to be supported by other things I have read out there. It seems people are changing shims and turning over the engine, only to discover the clearance they thought they had was not what they really had. Loose cams might help explain that, as well as inaccurate shim sizes.
 
http://www.cb750c.com/publicdocs/CB...83CB750ServiceManualChapter261982Addendum.pdf

and page 11 of that pdf document (26-11 by the actual pagination there) will show you how to use those double marks. Of course you'll probably have to join that site as well, a sister site if you will to the DOHC 1100 site. Free joinup and they do not hassle you at all.

You got exact TDC marks on the ignition left side rotor if you yank the cover.

Some use .004"-.006", I've seen it listed by like the Andrews cam people back when they used to make cams for these. I use .005"-.007". They don't begin to make noise until around 8 or 9.
 
Got em all set up right! My lowest clearance is .0045", on just on intake valve, and my highest is .007". I figure the intake valve running a little tight is not as big of a deal since it's the exhausts that tend to run hotter.

Now on to carbs...
 
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