Author Topic: '79 CM400T questions: CV Carb Conundrum (Read 20269 times)

Pinhead · « **on:** August 21, 2009, 11:27:09 PM »

I just picked up a 1979 CM400T that I was originally going to give to my brother. My dad said h's not ready for a motorcycle yet, though. I had already bought the bike so I guess I have a bike to ride when I tear down my CB650.

Just a few questions about the CM though.

The bike runs good down the road; has good power and torque throughout the RPM band. However, the engine doesn't like to go back to an idle. In other words, if I'm sitting in neutral, and rev the bike up a bit and quickly release the throttle, it'll hang out around 3000 rpm for about a minute or so before then it finally drops back down to idle speed. Also, how do I adjust the idle speed? It idles at about 1500 rpm which seems kind of high for a twin.

What's the "crossover box" for on the exhaust? Does it act like an H-pipe to aid in exhaust scavenging?

I'll probably have more questions later on but I haven't had a lot of time to really take a super close look at it.

KB02 · « **Reply #1 on:** August 22, 2009, 05:08:16 AM »

Sync your carbs. There's only two, so in that respect, it's easy. The painful part is getting to the adjuster. It's buried in between the two carbs. That should help the engine revving thing. The idle is adjusted but a simple thumb screw. Should be pretty easy to find.

As for the cross over pipe, I have no idea what that is for.

Johnie · « **Reply #2 on:** August 22, 2009, 05:37:56 AM »

The CM400 is an easy bike to work on. I restored a 1980 pictured below which probably looks like yours. Still have it too. The carbs on there are real easy to work on. You do not have to remove them to get at the jets. Drop the bowls and take the top off and clean away. I am able to adjust my idle by that large idle screw mentioned. Pretty easy to get at. Just look under the tank by the carbs and you can not miss it. Do you think your throttle cables are or hanging up or dirty? You should have 2 of them. These CM400's are pretty reliable bikes. I bought, fixed and sold 4 of them the past 2 years. The Honda service manual is real helpful. Click the pic to enlarge...

mystic_1 · « **Reply #3 on:** August 22, 2009, 06:12:20 AM »

Quote from: Pinhead on August 21, 2009, 11:27:09 PM

if I'm sitting in neutral, and rev the bike up a bit and quickly release the throttle, it'll hang out around 3000 rpm for about a minute or so before then it finally drops back down to idle speed

Change your air filter and check for intake leaks while you're down there. Both are common causes of this on CV carbs.

mystic_1

Pinhead · « **Reply #4 on:** August 24, 2009, 03:47:53 PM »

I've isolated a problem with the throttle response... The left cylinder doesn't fire at idle. Just above idle and every throttle position thereafter it runs great.

Without taking the carbs completely apart, I did a little troubleshooting on the stuff that I could get to. Here's what I found.

The first pic shows the outside of the carb. I believe this is the air cutoff valve?

Looking at the third pic, I was able to get the left cylinder to fire by covering the bottom hole. However, if I covered that hole and the manifold vacuum port, the engine would continue to miss (run like before). What does this indicate?

Pinhead · « **Reply #5 on:** August 24, 2009, 04:42:51 PM »

Got another piece to add to the puzzle... The engine idles on both cylinders if the offending carb's idle air mixture screw is all the way out (just barely pressing against the spring). Idles even better with the screw removed completely and the hole blocked with a finger...

Johnie · « **Reply #6 on:** August 24, 2009, 05:56:33 PM »

Quote from: Pinhead on August 24, 2009, 04:42:51 PM

Got another piece to add to the puzzle... The engine idles on both cylinders if the offending carb's idle air mixture screw is all the way out (just barely pressing against the spring). Idles even better with the screw removed completely and the hole blocked with a finger...

I have restored and sold 6 CM400's in the last 4 years. Everyone needed the carbs cleaned. I would start there. This is a 30 year old bike. Carbs can use a bath. Once that is done give it a try and move on from there if necessary. Good luck with it. Those are easy carbs to work on.

KB02 · « **Reply #7 on:** August 25, 2009, 04:39:26 AM »

I agree. Pop the carbs and give them a good ole cleaning.

kirkn · « **Reply #8 on:** August 25, 2009, 06:34:57 AM »

Quote from: Johnie on August 22, 2009, 05:37:56 AM

The CM400 is an easy bike to work on. I restored a 1980 pictured below which probably looks like yours. Still have it too. The carbs on there are real easy to work on. You do not have to remove them to get at the jets. Drop the bowls and take the top off and clean away. I am able to adjust my idle by that large idle screw mentioned. Pretty easy to get at. Just look under the tank by the carbs and you can not miss it. Do you think your throttle cables are or hanging up or dirty? You should have 2 of them. These CM400's are pretty reliable bikes. I bought, fixed and sold 4 of them the past 2 years. The Honda service manual is real helpful. Click the pic to enlarge...

Slightly off topic, but that is a pretty example of the breed... I must be showing my age, but I always thought those were very nice looking bikes. Nothing flashy or too "cartoon-y" in the overblown cruiser department. Good job on that one!

Kirk

Johnie · « **Reply #9 on:** August 25, 2009, 09:42:26 AM »

Quote from: kirkn on August 25, 2009, 06:34:57 AM

Quote from: Johnie on August 22, 2009, 05:37:56 AM
The CM400 is an easy bike to work on. I restored a 1980 pictured below which probably looks like yours. Still have it too. The carbs on there are real easy to work on. You do not have to remove them to get at the jets. Drop the bowls and take the top off and clean away. I am able to adjust my idle by that large idle screw mentioned. Pretty easy to get at. Just look under the tank by the carbs and you can not miss it. Do you think your throttle cables are or hanging up or dirty? You should have 2 of them. These CM400's are pretty reliable bikes. I bought, fixed and sold 4 of them the past 2 years. The Honda service manual is real helpful. Click the pic to enlarge...

Slightly off topic, but that is a pretty example of the breed... I must be showing my age, but I always thought those were very nice looking bikes. Nothing flashy or too "cartoon-y" in the overblown cruiser department. Good job on that one!

Kirk

Thanks Kirk. I appreciate the comments. That 1980 is the first bike I restored with my son. He learned to ride on that bike and never dumped it...thank heavens. It is fun to take that 400 out and ride the city and country roads. Why did I restore a CM400...it only cost me $50 from a friend. Only needed throttle cables and a carb clean to get it running perfect. Thanks again for the kind comments.

Pinhead · « **Reply #10 on:** August 25, 2009, 01:37:08 PM »

I agree, that is a really good-lookin bike.

Is it just me, or is the frame of these bikes kind of weak? It seems really easy to induce a wobble at anything above 55 mph. Just twitch the handlebars ever-so slightly and it feels like the entire bike is flexing...

Johnie · « **Reply #11 on:** August 25, 2009, 02:02:12 PM »

Not sure how strong the frame is on these 400's, but I suspect Honda took care of that department. Never had a wobble. Maybe the fact these were built low has something to do with it. I heard the 400 series was built as a low rider to try to get more ladies and short crotch men riding.

Pinhead · « **Reply #12 on:** August 25, 2009, 02:45:46 PM »

The design of the frame looks extremely weak in comparison to my '79 CB650; lots of curved pipe and very few triangular braces. Of course it's not designed to be a race bike, but I would think it'd be easier to make a smaller bike stiffer...

I was riding around without the rear grab bar because it was interfering with removal of the seat. I put it back on today and it seems to have stiffened the bike up a bit, just by grabbing the rear-end and shaking it around. Hopefully tonight on my way home from work I'll be able to notice a difference.

Pinhead · « **Reply #13 on:** August 25, 2009, 05:04:03 PM »

More questions about the CM400: How the heck do you change front fork oil? There's no drain plug! I don't want to have to take the forks off and flip 'em upside-down....

Johnie · « **Reply #14 on:** August 25, 2009, 06:06:33 PM »

Quote from: Pinhead on August 25, 2009, 05:04:03 PM

More questions about the CM400: How the heck do you change front fork oil? There's no drain plug! I don't want to have to take the forks off and flip 'em upside-down....

You got it...there is not a drain on the 400 I have. But on the CM400 custom there is a drain just like on the 750 forks. Not sure what the 400T has but it sounds like the same as my 400E. But yes, you have to remove the forks and take the top fork bolt out and dump it out. I took mine off to polish and completely clean the inside tubes. Then put in new seals.

Pinhead · « **Reply #15 on:** August 25, 2009, 10:56:10 PM »

Well on the way home from work tonight I drove with the grab bar installed. That one change made a huge difference in the bike's handling. No more wobble and it doesn't feel like I'm driving a slinky down the road.

The stock CM400 frame could definitely use some stiffening.

Johnie · « **Reply #16 on:** August 26, 2009, 05:19:13 AM »

Quote from: Pinhead on August 25, 2009, 10:56:10 PM

Well on the way home from work tonight I drove with the grab bar installed. That one change made a huge difference in the bike's handling. No more wobble and it doesn't feel like I'm driving a slinky down the road. The stock CM400 frame could definitely use some stiffening.

Good point...I never rode mine without the rail. Are you sure all your frame bolts, engine mounts and joints are tight?

Pinhead · « **Reply #17 on:** August 26, 2009, 09:22:23 AM »

That's the first thing I checked when I started searching for the cause of the wobble. Everything seems secure. The design of the frame is simply weak; during my search I grabbed the rear of the frame and could twist it ever-so-slightly by hand. The grab bar all but eliminated that particular flex.

tbpmusic · « **Reply #18 on:** August 26, 2009, 10:04:17 AM »

Quote from: Johnie on August 25, 2009, 06:06:33 PM

Quote from: Pinhead on August 25, 2009, 05:04:03 PM
More questions about the CM400: How the heck do you change front fork oil? There's no drain plug! I don't want to have to take the forks off and flip 'em upside-down....
You got it...there is not a drain on the 400 I have. But on the CM400 custom there is a drain just like on the 750 forks. Not sure what the 400T has but it sounds like the same as my 400E. But yes, you have to remove the forks and take the top fork bolt out and dump it out. I took mine off to polish and completely clean the inside tubes. Then put in new seals.

400T is the same thing - gotta remove them from the bike and turn them upside down.

Nice touch, huh??

Pinhead · « **Reply #19 on:** August 26, 2009, 10:10:59 AM »

How about using a long tube attached to the end of a large syringe? If the forks were bottomed out, would the tube be able to reach the very bottom?

crazypj · « **Reply #20 on:** August 26, 2009, 05:42:04 PM »

Quote from: Pinhead on August 26, 2009, 10:10:59 AM

How about using a long tube attached to the end of a large syringe? If the forks were bottomed out, would the tube be able to reach the very bottom?

Ghetto
do it right, your supposed to strip forks to clean them and change oil

PJ

Johnie · « **Reply #21 on:** August 26, 2009, 06:15:30 PM »

Quote from: Pinhead on August 26, 2009, 10:10:59 AM

How about using a long tube attached to the end of a large syringe? If the forks were bottomed out, would the tube be able to reach the very bottom?

No it won't reach the bottom. You may as well either do it the correct way or else I would not bother with it. Why cut corners. There is probably a bunch of crapola in the bottom that needs cleaning anyway.

Pinhead · « **Reply #22 on:** August 26, 2009, 09:55:53 PM »

I found a PDF of a Clymer manual if anyone needs one. It covers "Cb400 Cb450 Cm400 Cm450 Twins Nighthawk (1978-1987) - Service Manual 400 Page (Eng)" in PDF format.

I found this interesting image when I was searching for the manual for this bike, though... Is it a four-cylinder or a twin? If it's a twin, I WANT the tank, seat, side covers, rear cowl, etc.! I LOVE the style.

EDIT: Here's another pic; it looks like the Nighthawk WAS a twin!

kirkn · « **Reply #23 on:** August 27, 2009, 07:37:54 AM »

Well....

The Nighthawk name was applied to both the 400/450 twin and the 550/650/750 fours in the early- to mid-80s.

The "Hawk" name was applied to the CB400T/CB450T twin models.

Recall that the twins were available in many styling flavors - economy (spokes & drums), cruiser, standard / sport (Hawk), and a slightly different cruiser (Nighthawk). Even the ultra-cruiser Rebel.

I think your edit-added picture is of the Hawk. It had the larger, flatter-topped tank. The overall style of the Hawk mimicked the larger CB750F/CB900F styling.

The 400/450 Nighthawk styling was more cruiser. Smaller, more sloped-topped tank, different seat - more stepped (but not quite so stepped as the full-cruiser CM400/450C.

The Hawk name was also applied to the "first-gen" CB400 of '77 & '78 but it was a much more conventional-styled look. It was not nearly so "family-related" to the CB750. The earlier versions had the silver ComStar wheels. That picture you've attached has the later black ComStar wheels. Eventually, Honda abandoned the "built-up" ComStar wheels and went with a conventional cast wheel like everybody else had by then which they called ComCast.

*EDIT* Ah, at first I couldn't see the first picture you attached. Now I see it. It's labeled a CD400N, if I see the side covers correctly. I'm pretty certain you've found a picture of a non-US model. Those wheels were never available here in the US. And, it looks like that model had dual front disks. THAT was never an option in the US. They look very similar to the '82 CX500 Turbo wheels...

crazypj · « **Reply #24 on:** August 27, 2009, 12:14:33 PM »

That styling was originally for Europe only, (Eurosport)
Slimmer bikes than the US versions.
400 should be good for at least 108mph
Its a later model with that reversed Comstar style wheel, don't remember the 'holey' spoke wheels making it to Britain?

PJ

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Author Topic: '79 CM400T questions: CV Carb Conundrum (Read 20269 times)