Topic: CM400T Newbie - Valve Adjustment & Carb Leak

[a1980cm400t]

Hello Forum.  I am new here and need some advice please.

The story begins after I changed the oil and filter on my 1980 CM400T, I thought I noticed a strange ticking, tapping, knocking sound that I never noticed before.  I posted my question over on the Honda Twins Forum along with a YouTube video demonstrating the noise I thought I was hearing.  Some said to check the valves and so I did this afternoon and made a bigger mess and should have left it alone instead (hindsight is 20/20).

Here is the YouTube video:

https://www.youtube.com/watch?v=S8kxEu3Rnvw

In an attempt to check the valves, I ended up breaking the right side top cover bolt inside the top, managed to get leaking left carburetor, and rpms went from a steady 1200 to now upwards of over 3000 at idle before I turned the engine off.

Using the information on page 3-3 of the Honda Shop Manual:

** I took the bike seat off
** I turned the fuel petcock to off and disconnected the fuel line from the gas tank
** I removed the gas tank from the bike
** I removed the fat tube on the right side of the bike that connects to the top part of the head cover (I don't know what that tube is called)
** I removed the clips that are attached to the frame that guide the spark plug wires to give me enough room to pass the top end cover through the frame
** I removed both top end cover bolts - gasket was in amazing condition so I saved it/re-used it
** I removed the top cover exposing the valves
** I removed the left crankcase cover and shifter
** I rotated the flywheel counterclockwise until the arrow on the flywheel matched up with the T in order to get TDC
** I then worked on the left side exhaust valve adjusting the gap to .006 inches using a feeler gauge
** Next I worked on both the left side intake valves adjusting the gap to .004 inches using a feeler gauge
** I tightened everything down and rotated the flywheel another counterclockwise revolution making sure the arrow lined up with the T and repeated the steps on the right side
** I put everything back together and that's when I over-tightened the right side bolt on the top cover and broke it
** I put the tank back on and reconnected the fuel line to the petcock and turned it on
** I put the broken right side bolt back into its hole and firmly held it in place by hand so I could try running the engine
** Engine started and ticking sound is still there but also now the rpms are not constant at 1200 but jumped up to 3000 before the I cut the engine
** That's when I noticed the right carb is leaking gas under the float bowl by the brass screw above the tiny black hose - I never touched the carbs so not sure why it's leaking???

Everything was working before I decided to play motorcycle mechanic and now I've created a mess for myself all because I wanted to see if I could get rid of that ticking sound. I also read that it's good practice to check the valves every season and since I bought the bike used I thought to myself, well, it's worth shot to check it out.

Should I just have my bike towed to the Honda dealership? Or is this a fix a simpleton like myself can do? Totally bummed out!

PS - I also posted on Honda Twins Forums - I need all the help I can get please - Thanks.!

[calj737]

Here's the parts fiche for your bike's valve cover. The bolt you broke off is part #11
http://www.cmsnl.com/honda-cm400t-1980-usa_model7286/partslist/E++01.html#results

The carb screw that is leaking is probably the bowl drain screw, and there should be a small seal behind it from the looks of the parts fiche. Probably corrosion has it eaten away and it's dripping.

There's a member here, Harisuluv, that focuses entirely on carbs and may have some of the small parts for your carbs if you need them. You've posted in the wrong section, should've been under "Other Bikes" as you're a twin, not a SOHC4, but leave that to a moderator to correct.

The bolt you broke off was probably over-tightened by a previous owner. Happens quite a bit (ham-fisting bolts in an alloy case). You'll likely need to pop the cover off, then grab the threads and it'll likely turn out pretty easily. Just replace the cover carefully to avoid any leaks.

When you do install those types of bolts, don't go nuts on cranking them down; just a good firm "tight" by hand is all that's required.

The "ticking" you hear/heard may be from the carbs not being vacuum synch'ed, may be an exhaust leak. This would be true if the valves are now properly adjusted and the cam chain is also. Carbs are a prominent source of noise when not synchronized, and only a vacuum gauge will accurately get them "timed". Search YouTube for demo video on how and what that is. You can build your own or buy a set of gauges.

The link I posted has your entire motor assembly and part numbers if you back up one page. It can be very helpful where the manual may be to brief or not illustrative enough to explain your assemblies.

Hope this helps-

[a1980cm400t]

Thanks calj737

Can setting the valves incorrectly cause the RPMs to idle too high?

Can setting the valves incorrectly cause the Carb to leak?

Can setting the valves incorrectly cause oil to get into the carb?

So much to learn!

[calj737]

Thanks calj737

Can setting the valves incorrectly cause the RPMs to idle too high? Not really. If you're idle is too high, more times than not, it's an adjustment on the carbs themselves, the idle screw. If the idle "jumps around" it can be related to vacuum leaks between the carbs, the intake manifolds, and the carb boots.

Can setting the valves incorrectly cause the Carb to leak? Leak fuel? No.

Can setting the valves incorrectly cause oil to get into the carb? Im a bit confused by "into the carbs". Generally, if the valves are improperly set, it causes running issues, rough, lack of power, noisy engine. In extreme cases, you could damage a piston or cause some other engine problems, but from what you describe, it doesn't seem like a valve adjustment issue.

So much to learn! Welcome to the crowd!

If I had to speculate, I'd suspect many of your issues are carb related. And these need certainly to be perfectly tuned, to insure other variables can be identified properly. Spend some in depth time reading the manual, the a Carbs FAQ, and read some threads on the "twins" site about adjusting your carbs, then carefully undertake doing that. Take lots of pictures as you go, and keep all the components from each carb separated and together in small plastic bins.

[a1980cm400t]

Thanks for the input calj737!!!

[faux fiddy]

Welcome to the forum.

There is another place you might go to if you haven't yet:

hondatwins.net