Author Topic: '79 CM400T questions: CV Carb Conundrum (Read 20263 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

Pinhead · « **Reply #25 on:** August 27, 2009, 04:38:50 PM »

Here's another pic of the 400 with a likeness to the "F" bikes.

Pinhead · « **Reply #26 on:** September 06, 2009, 07:58:21 PM »

I did a little work to the 400 today to make it ride a bit better. The squishy ride had to be remedied so I added front fork oil (they were completely empty) and replaced the rear shocks with the stock shocks from my '79 CB650. This added a bit of stiffness and height to the rear-end which made the bike handle and ride much better.

Pinhead · « **Reply #27 on:** October 03, 2009, 06:00:59 PM »

I made a strange discovery today... The engine will idle with the tops of the carbs, springs, and slides completely removed.

crazypj · « **Reply #28 on:** October 04, 2009, 06:02:14 AM »

I've posted that several times, the butterflies control air flow and fuel goes through bypass port.
You could remove slides and it will still start and idle OK
It will probably cut out if you try and rev it because it goes too lean (never actually tried that

)

PJ

cheseburger · « **Reply #29 on:** October 04, 2009, 07:32:45 PM »

Got a 1980 cm400t. have cone air filters , cleaned carb pretty good and used air to smoke out rust from tank. Have 2 into 1 exhaust welding up one side of the crossover. 20 inch baffled cheap muffler. It is still loud. Now I need to have a good idea where to start with main and piolet jets. Any help would be wonderful. Stock I believe is 72mains.

Pinhead · « **Reply #30 on:** October 04, 2009, 07:53:51 PM »

Believe me, it's not loud. Loud is if you remove the crossover and run straight pipes. I'm not even driving my bike until I get new mufflers installed. My straight-piped CB650 sounds stock compared to the straight'd 400T. The 400 makes Harleys sound soft.

Alan F. · « **Reply #31 on:** October 04, 2009, 09:08:52 PM »

Quote from: cheseburger on October 04, 2009, 07:32:45 PM

Got a 1980 cm400t. have cone air filters , cleaned carb pretty good and used air to smoke out rust from tank. Have 2 into 1 exhaust welding up one side of the crossover. 20 inch baffled cheap muffler. It is still loud. Now I need to have a good idea where to start with main and piolet jets. Any help would be wonderful. Stock I believe is 72mains.

I looked up some stock jetting info, it's posted here:
http://forums.sohc4.net/index.php?topic=59429.0
-Alan

cheseburger · « **Reply #32 on:** October 12, 2009, 07:14:00 AM »

[quot[/quote] i started out with a 120 main and 72 piolet jet. this should let me know at least if im to lean or to rich. stock was 112 to 118. it has a 118 stock main jet. at least i can drill out jets. how many thousands is a jet from one size to another? thanks for the help

tbpmusic · « **Reply #33 on:** October 12, 2009, 07:45:00 AM »

Quote from: cheseburger on October 12, 2009, 07:14:00 AM

[quot

i started out with a 120 main and 72 piolet jet. this should let me know at least if im to lean or to rich. stock was 112 to 118. it has a 118 stock main jet. at least i can drill out jets. how many thousands is a jet from one size to another? thanks for the help
[/quote]

The jet sizes are in millimeters.
EG: A 150 jet has an orifice that's 1.5 mm in diameter. A 100 jet is 1.0mm, a 72 is 0.72 mm, etc.

I don't know what that is in inches, sorry.........

Crash007 · « **Reply #34 on:** November 08, 2009, 02:15:27 PM »

I got a CM400T recently and the carbs are in bad shape.

I have the repair manual so I will try to take apart and clean up. What do you recomend for a cleaning solution?

Also, what other bike model's can I pull the carbs off as a replacement?

Thanks

Crash

Alan F. · « **Reply #35 on:** November 08, 2009, 02:58:57 PM »

Berryman's is good stuff, simple green or purple power will work, boil the carb outdoors in 1/2 lemon juice and 1/2 water will work, there are many good ways to clean your carbs.
Become proficient with the search function on these forums and you'll learn everything you'll ever need.
-Alan

cb650 · « **Reply #36 on:** November 08, 2009, 03:01:09 PM »

Nieghbor while growing up had one with just "HAWK" on the side covers and was a auto.

Crash007 · « **Reply #37 on:** November 12, 2009, 05:50:49 AM »

Question on carb sync. I have the carbs off the CM400T, got the bowls off needles out etc for cleaning.

Looking at the carbs, when I pull on the throttle, the 2nd carb is not in sync with the first. Like at idle the first is closed the 2nd is about 15% open. I found the screw between the carbs to adjust and now when i open the throttle, both will open up to the same level.

Then problem is, when it goes to idle, the 2nd carb wont close on its own. It now has the free play to shut, but I have to push it shut with my finger to see.

The rods and springs connecting the 2 are very gunked up, I have not cleaned them yet. Do you think they are just rusty or loose or something?

THanks

C.

Pinhead · « **Reply #38 on:** November 12, 2009, 02:34:25 PM »

I had the same exact problem with my bike. I sprayed the hell out of everything with WD-40 and worked it back-and-forth to get it to work right. I haven't taken them apart to clean them yet, though, so that's probably what needs to be done.

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Author Topic: '79 CM400T questions: CV Carb Conundrum (Read 20263 times)

cheseburger

cheseburger

Crash007

Crash007