Tag Archives: lash

Honda CM400 Valve Adjustment

Posted on by Motopsycho

Today I’m going to show you how to perform a Honda CM400 valve adjustment. This basic procedure covers 1978-81 CM & CB400T Honda twins. This engine is from a 1980 CM400. Please refer to a proper CB/CM400 service manual to verify the exact procedures & specifications for your motorcycle. I will give the valve lash & misc. other tune up specs at the bottom of the page.

<cm400 valve adjustment tools>

Gather up the tools you will need along with a copy of the appropriate service manual. Please note that it is not necessary to remove the engine from the motorcycle to perform this procedure, I already have this engine out so that I could do some some fabrication work & painting to the frame. This is the long delayed Project wAmmo bobber that I should have finished months ago, but now I am back on it with a vengeance. You will need to remove the fuel tank, gear shifter, and whatever other parts are necessary so that you can remove the valve cover & the left side crankcase cover. Once all of that is done then remove both sparkplugs.

<cm400 spark plug removal>

After you remove the spark plugs, switch sockets & turn the engine in the direction indicated by the arrow on the alternator rotor. The big rusty flywheel looking thingy you see in these pictures for those of you who have never seen one before. This one had to have some of the rust sanded off so that I could see the markings on it.

<honda cm400 alternator rotor>

Turn the engine and watch for the intake valve rocker arm on the side you are adjusting to move down and then back up. These little Honda twins have a 3 valve per cylinder layout with 2 intake valves & 1 exhaust valve per cylinder.

<cb400 rocker arms illustrated>

Once the intake rocker arm returns to the top continue to turn the engine slowly and line up the next “T” mark on the flywheel with the pointer on the engine case, as it comes around. If the exhaust rocker arm starts to move you have gone to far & must circle the engine all the way back around & start over. Do not turn the engine backwards to get to the timing mark if you miss it.

 

<Honda cb400 timing marks>

Then verify that the piston is indeed at top dead center. On this engine it is fairly easy to do just by looking into the spark plug hole.

<Honda cb400 piston tdc>

With the piston at top dead center for the cylinder you are adjusting both the intake & exhaust valves should a little bit of play in them unless the engine has severe wear or improper maintenance that has caused valve recession which will close up the gap. Too much lash is also detrimental to your engines performance and will cause your engine to tap very loudly. Too little lash will eventually lead to a burned valve if it doesn’t close completely.

Loosen up the lock nut for whichever adjuster you choose to start with, here I am starting on the exhaust side.

<cm400 exhaust valve locknut>

Then insert the proper size feeler gauge, loosening the adjuster with a flat screwdriver if needed.

<Honda cb400 exhaust valve lash>

Then carefully tighten the adjuster screw & lock nut until the feeler gauge is able to be removed & re-inserted with just a little bit of drag, but the next size larger feeler gauge should not fit. It will be necessary to hold the adjuster screw with the screwdriver as shown below while you are tightening the lock nut. Be sure to recheck your lash after you tighten down the lock nut for good, sometimes you may have to readjust to compensate for the adjustment screw moving when you torque the lock nuts.

<Honda cb400 intake valve lash>

Once you have all of the valves adjusted properly replace the engine covers being sure to inspect & replace all gaskets & seals as needed.

Valve lash and some miscellaneous tune up specs are below:

Intake valve clearance 0.10mm +/- 0.02mm    0.004″ +/- 0.0008″

Exhaust valve clearance 0.14mm +/- 0.02mm    0.006″ +/- 0.0008″

Idle speed 1200 rpm +/- 100 rpm

Spark plug   NGK-D8EA  or ND-X24ES-U

Spark plug gap  0.6~0.7mm    0.024~0.028″

Oil capacity  3 liters    3.2 U.S. qt.s

Hopefully this has helped someone out.

Happy Wrenching!

Recon Re-Assembled!

Posted on by Motopsycho

Actually this article is a little late, I actually finished this engine up months ago, rode the snot out of it, and sent it home with its happy owner. Here are some pictures of the process. This is not nor is it intended to be a step by step guide. It just your chance to see something the average person normally never gets to see.
Let’s start with a view of the transmission and shift drum sitting in the now very clean engine case.

Add the crankshaft,

To keep the gaskets in place I use Hondabond 4 gasket sealer. This and similar products do not allow the gasket to squeeze out the way silicon based sealants sometimes do.

Here it is with the front half in place.

Next pic shows the reverse lock out lever (red arrow) and the shift actuator rod (gray arrow) installed into the rear crankcase half. The shifter rod passes completely through the crankcase to the linkage that operates the shift drum.

Back to the front of the engine here is the oil pump drive chain.

Now insert the camshaft, chain, and tensioner, be sure to line up the timing marks as noted in the service manual.

After adding the rest of the small parts that were laid out in a carefully organized fashion on the workbench put the front and rear covers back on.

The cylinder wear was very light so I felt safe just honing it out and replacing the piston rings.

Here’s a shot of the cylinder installed to the crankcase with the piston at top dead center

After everything else was back together I was ready to line up the timing marks and set the valve lash.

Check your service manual for the specification for your engine.

Everything else was just a reversing of the diss-assembly. Just keep putting parts back on until you have a working four wheeler again.