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[Rcb1000]

Hi all,
Just a quick note after registering. Great forum, learned a lot. I am now retired and needed some winter projects. Picked up a 1982 cb900f a few months ago that has turned into a challenge (carbs and valves). Just recently picked up a 1982 CB750SC. Runs good, 24k miles but needs complete tuneup. Looking forward to advice.Thanks.

 

[amc49]

Run the valves if they have not been done, what causes them to run bad and burns the valves, nobody ever adjusts the valves. Go for .005" instead of the service manual number of .003", the engines will love you for it. Any valve at .002" WILL burn yet manual says it's OK.

 

[Rcb1000]

Understand. I assume you are referring to the cb750sc. I think I read your previous post concerning setting valves at .005 inches. Is that your recommendation on the 900 as well? I have gone thru about $60 in shims just trying to get them to stay in spec. Cam looks good, supposedly 20k miles on bike. Thanks.

 

[amc49]

There is no difference there in '79-'83 750-1100 as the heads are the same and so are the problems. The bikes were designed in the era of low lead fuel but the difference in even very low lead and none at all (now) makes the valves tend to recede very quick as they are ordinary soft steel hardened on the outside only and very thin at that. Ethanol if used in your local fuel will make that worse.

The cams in the head are actually looser than the valve clearances meaning the fairly strong valve springs will push the cams all over the place to make any numbers you get checking the valves not real to a point. I have come up with .002" of anything you get for a clearance as not being real at all and why you go to .005". Meaning, if you measure a valve at .003" you are really looking at only .001" real world clearance. I have burned more than one valve that showed having .002" (true .000) believe me. The extra space makes the setting of them last whoppingly longer too, instead of setting every 4K miles you can go to like 15K setting periods if they were done right.

The performance then slowly drops not setting the valves to where most guys blame the carbs (they DO have their problems) and then why the carbs are commonly off bike when you buy one. I have no idea why all guys think everything is always the carbs as an issue, maybe because they are the least understood part of bikes.

 

[Rcb1000]

Very, very helpful. Understand. Thank you for taking the time to respond. I will heed your advice.

 

[amc49]

Prove it to yourself, I don't know everything. Check a same valve over and over by rerotating the engine to again find the place to set them exactly and then if you measure very carefully like needed you will often find the number you got the time before is no longer the number you get on the repeat reading, it changes. Why Honda changed the marks on cam end at some point in production trying to find a better place to set valves, they knew something was whacky but never figured out exactly what. A lot of people ignore the marks to set each lobe opposite and set them on the base circle that way, or set them on compression stroke and TDC since that is where it matters most.

For some odd reason Honda made the cams in the caps looser in the middle than the ends, no one knows why. Maybe fearful of the chains pulling too hard on cams to seize those journals in the middle? Who knows? Or maybe for more engine expansion in middle due to heat?, the outsides cool on head ends too so they run cooler.

 

[Rcb1000]

Ain't that the truth! I get different readings with each cycle. I am using the indent on the right side front cam as shown in the Clymer manual to position cam at 12, 3, 6, and 9 oclock to check valves although the cam with the indent is supposed to be only 80 and 81 models (mine is an 82). Anyway, just trying to shim them on the high end .005 spec as I can.