Here I’ve already removed the tank and all of the necessary engine covers.
Since I have already put enough miles on this bike since I got it running to warrant an oil change I decided to re-check the valve adjustment for two reasons; one is that as long as the engine had been sitting without running I want to keep a close eye on it for a while & two so that I could show everyone how I do it. As you can see in the photo above I’ve already removed the gas tank, all four spark plugs and all of the appropriate engine covers. The first thing to do before you adjust the valve lash is to adjust the cam chain tensioner. On a 1980 Honda CB650 this is done by loosening the nut on the rear of the cylinder just a little bit. Do not remove it just loosen it some
This is the nut you loosen to adjust the cam chain tensioner.
Then put the correct size wrench on the hex spacer behind the nut at the ignition unit on the right side of the engine. Then rotate the wrench clockwise slowly 4 to 5 turns as you simultaneously tighten the lock nut that you loosened at the beginning of this step. This is also the first thing you should try if you own a motorcycle and you can hear the cam chain rattling, but if after making the correct adjustments you still have a rattle then it will be time to start replacing parts. Keep the wrench that you used to turn the crankshaft handy, you are gonna need it a few more times.
The Clymer manual describes this as a 15/16″ nut. That is wrong this is a 24mm. OOPS
Here in this picture taken on the left side of the engine you can see 2 of the three openings in which we will be working to check & set the valve lash. The adjusters for the intake valves are on the back of the head in front of the carburetors and the exhaust valve adjusters are on the front of the head behind where the exhaust pipes stick out. The first step is to get the number 1 cylinder to top dead center. Take your large wrench and rotate the crankshaft clockwise and watch for the intake rocker arm on the first cylinder to drop down into the head and start to rise back up. Then look at the timing marks on the ignition advance unit (photo is further down the page) and continue to slowly turn the engine until the 1.4 T mark is aligned with the pointer that is cast into the crankcase.
The camshaft & rocker arms are in the left hole & one of the adjusters is in the right hole.
At this point both the intake & exhaust valve on cylinder number one should be loose enough both rocker arms to be wiggled. If not you either have a valve that is way too tight or you did not stop turning the crankshaft at the right mark, either way you should verify which problem you have before moving on. a simple way to see if the cylinder is at top dead center is to take a long small diameter wooden or plastic dowel and insert it into the spark plug hole. If the piston is at the top of the cylinder the dowel will not go in very far at all.
With the number one piston at TDC on a 1980 CB650 you should be able to adjust both valves on the number one cylinder, The exhaust valve on number two, & the intake valve on number three. Once you have those done you need to repeat the step above but this time watch the intake rocker of cylinder 4 on the right side of the motor cycle. With that one at TDC you can adjust both valves on number four, the exhaust valve on number three, and the intake valve of number two.
Lets talk about tools for a little bit. All of the shop manuals show a special tool for adjusting the valves, but the simple truth of the matter is that for a number of engines you don’t need them and this CB650 is one such example. For the lock nut, I just clamped a pair of Vise Grips around an old cheap 10mm socket that I have on hand and then just used the proper size of flat-head screwdriver to turn the adjustment screw with. Works great for me on this motorcycle, your mileage may vary, if you break something I’m not responsible, etc.
Macgyver was an amateur!
Speaking of tools let’s get the feeler gauges to set the valves with. The ones that I use are from Snapon and are about a foot long. The also came with a nifty holder that is very handy for working with the really thinner sizes in hard to reach places. The intake valve lash setting for this generation of Honda CB650 is .05mm (.002 inches) and the exhaust setting is .076mm (.003). So to do this job I will get out three feeler gauges in sizes .002,.003 & .004 (.1mm). Why three sizes? I’ll explain in a minute.
The next picture is of the .002 feeler gauge slipped in between the rocker arm & camshaft. This is where you measure the lash on this engine. Basically what I do is turn the adjustment screw until I can just slip the feeler gauge into place with just a little bit of wiggling. You should tighten the lock nut each time you do this as it may affect the final adjustment. If you tighten the lock nut and find that your lash setting has changed tighten the adjustment screw a bit to compensate, re-tighten the lock nut and check it again. Usually after I do all of this, and I am satisfied with my setting, I then take the next larger feeler gauge (.003) and try to insert it into the gap. If it doesn’t fit great I move on to the next one but if it slips in I readjust the lash until the correct sizes slips in fairly easy but the next size up wont go in. You may wonder why not just set it a little tight and not worry about it? I like to set these things exactly as needed for the best performance. The other reason is the way that motorcycle engine valves wear. Very rarely is there any wear at the top of the valve, most of the wear occurs where the valve closes at the valve seat in the head, this causes the valve lash to get tighter as the engine wears & not looser. This is especially problematic when you are running old motorcycles on the toxic, corrosive, & environmentally unsound corn juice that passes for gasoline in this day & time. If you must err on the side of caution it would be a little tiny bit better for your valve lash to be just a hair too loose than to be to tight.
For really thin feeler gauges like this .002 I recommend a holder like this one from Snapon tools.
Next take the .003 feeler gauge & set the exhaust valves as shown here. Then use the .004 gauge to make sure your adjustment is just right. It is especially critical not to over tighten the exhaust valve lash. If the valve wears down and is not able to close all the way due to a lack of clearance you may get a burned valve & a big repair bill.
If you have a late 70s or early 80s Honda with the factory electronic ignition you definitely want to perform this next step. First get yourself a set of nonmagnetic feeler gauges. DO NOT not use steel feeler gauges to set the magnetic pickups aka pulse generators on these bikes.
Non-magnetic feeler gauges are a necessity to set air gap on the pulse generator of most OEM electronic ignitions of this time period.
The range of settings for the air gap between the pulse generators (black boxes in the picture below) and the trigger mounted on the end of the crankshaft is .012-.016 inches (0.3-0.4mm) You can also see the pointer & the timing marks that I mentioned above in this picture.
1980 CB650 timing marks, pulse generators & advance plate
Believe it or not I have never seen any motorcycles with this gap set correctly from the factory, but the system is still good enough so that most bikes run without any problems whatsoever. Still if you have one of these and it runs okay except for a little surging & hesitation the pulse generator air gap should be the first thing you check. To set mine I just rotate the engine until the trigger ( little square nib sticking out of the crankshaft ) is aligned with nib on the pickup, loosen up the adjustment screws, stick the feeler gauge in place, & hold it all together while tightening the screws back down. Then rotate & repeat to do the other one.
This makes a huge difference in engine performance if it is set correctly.
There you have it, put a little lube on the advance mechanism behind the plate, reinstall all of your covers, spark plugs & fuel tank. Now it’s time to fire it up & check it out. Once you are sure that you did everything correctly & your engine is sounding just the way it should take it out for a ride & enjoy the fruits of your labor!