[asnyder]

Hello,

I have very little mechanical savvy FYI. I just got this CB400A last month and it's having trouble idling, driving seems fine, but it is barely staying running and stalls at idle. I have to rev it up a little at idle and it's difficult with learning how to ride to have to worry about this too.
I have:

1. Changed oil and filter
2. Refilled battery with distilled water to proper levels
3. Removed air filter, cleaned with air cleaner solvent, dried, then soaked foam filter in 80/90 gear oil and squeezed out excess and reinstalled, as per label and manual
4. Put proper amount of Seafoam in gas tank, swirled around and ran it around for a while
5. Filled with 87 octane (in USA so pump octane), per manual bike only needs 86
6. I unplugged the two crankcase breather drains as per the manual, it says that water and deposits are supposed to come out, one nothing came out, the other (connected to under what I think is the carb) had gas coming out, not water. I plugged it back up again as it didn't seem to stop trickling after a minute...

None of this has changed the rough idle and stalling.
Any easy things to check? I've seen an idle speed adjustment in service manual and heard people talk about it but I cannot find it.
Is there a fuel filter somewhere that might be gunked up? I've seen aftermarket ones that are inline but I don't see anything.

Thank you for your help. Pictures of where to check would help as I don't really know my way around an engine.

 

[Mayhem]

Hello,

I have very little mechanical savvy FYI. I just got this CB400A last month and it's having trouble idling, driving seems fine, but it is barely staying running and stalls at idle. I have to rev it up a little at idle and it's difficult with learning how to ride to have to worry about this too.
Thank you for your help. Pictures of where to check would help as I don't really know my way around an engine.

The fuel filter is going to be best gotten at after you run it near dry, or are okay with draining the gas back out. You filter is nothing more than a micro perforated plastic tube
accompanied by a rubber O-ring that slide over the fuel delivery tube, which is inside the tank.

Until you can get a proper shop manual Go HERE to CMS, and find your bike.
there is a few things they don't have info on, but I have yet to find better for getting the proper size specs for missing or broken bolt replacements.
It don't seem like even the HONDA Shop manuals include each Bolt by length x diameter x thread pitch, all equally important when replacing from a hardware store stainless steel bolt bin, so not to bottom out prematurely, or end up being too short. the right diameter for obvious reasons, and thread pitch (the difference between extra fine, fine, common, and coarse threads basically) So not to cross thread by using the wrong type.

this is a hard to see "strainer" or fuel filter.
and the other "image" is the whole tank assembly
(CMS link is direct to 78 CB400A Hondamatic listing)
*CLICK TO ENLARGE*

 

[asnyder]

Thanks I have the clymer and honda service manuals.

Your attachments both say invalid link.

 

[J-T]

The idle speed adjustment is a black knob below the carburetor bowls. It's most easily accessed from the left side. Turn it clockwise to increase idle speed.

There are 2 drain hoses that come from the air box. These are about 3/4" outside diameter and should have plastic plugs in them which, when removed, drain condensation from crankcase ventilation. There is a vent hose from the battery and one from the bottom of each carburetor. These are about 1/4" O.D. and are not plugged. If one of the carburetor drain hoses has a plug and leaks fuel when unplugged you may have a malfunctioning float valve, cracked overflow standpipe, or leaking bowl drain screw.

 

[Mayhem]

Thanks I have the clymer and honda service manuals.

Your attachments both say invalid link.

I have updated the and they should display actual images now.
The Go HERE CMS link also should lead directly to the 78 CB400A Hondamatic page.

Aside from that Clymer is okay-ish for most Modern Cars, but Do ̶y̶o̶u̶r̶s̶e̶l̶f̶ , Do Your Honda a favor...Disregard the Clymer and stay away from Haynes Manuals as well, Stick to using the OE Honda Manual.
They are the Holy Grail of making proper adjustments,or repairs to these old bikes.

 

[doode]

If the idle screw doesn't help, you may have an intake leak. Check around for cracks...

 

[asnyder]

The idle screw did help, it doesn't stall at idle, when in gears 1 or 2 it idles low at stops but I can handle it.

Big problem this morning as I walked into garage to fuel leak on floor. I traced it to those two crankcase breather tubes that come from bottom of carbs. The one that contained fuel cracked, apparently from me inspecting it. Bike Bandit has breakout diagram and parts for that particular section of the engine but unfortunately doesn't have the tube, I think it has the caps. Should any clear vinyl tubing work or is there special automotive tubing, what size?

1. It shouldn't contain fuel at all, just water according to manual.

 

[EazyDR]

Those are overflow tubes and should never have fuel in them. They are overflowing as either the overflow tube in the carb is cracked or the float needles are not sealing. The needles could be worn out or your floats themselves may be leaking and not raising high enough to close. If fuel is pouring from one side, try tapping gently on the carb bowl and see if it stops.

Be sure you are turning of your fuel petcock after each ride. Failing to do so can lead to fuel in the oil = lower viscosity.

Welcome to the forum!

 

[J-T]

It could be helpful if you posted pictures of the hoses but I think EasyDR has it pegged. Sounds like someone (PO) plugged the overflow rather than fixing the problem. Can't overemphasize the importance of turning the petcock off whenever you park the bike. Since this condition has likely existed for some time I'd just change the oil and filter to be sure.

If the engine is being slowed at idle significantly when in gear your clutch may need an adjustment.

 

[asnyder]

Yes these are the two carb overflow tubes, they are next to a larger diameter single hose that seems to come from airbox area. They are bundled together and each come from the bottom of a carb. Caps are gone so PO screwed screws into them to plug them. I need to get the caps (which bikebandit has) and the tube (BB doesn't have) (to fix the hole in it), does regular vinyl tubing from hardware store work or will they melt? The left carb overflow is the one that is having problem. I will tap to see if it stabilizes but I think it will need taken apart and fixed/cleaned. Hopefully there are detailed videos or pictures, along with service manuals, I have little mechanical knowledge.

 

[J-T]

The drain hoses coming from the carbs are not supposed to be plugged, only the hose from the airbox.

Auto parts store or motorcycle shop would be a better source for petroleum compatible hoses.

 

[EazyDR]

The drain hoses coming from the carbs are not supposed to be plugged, only the hose from the airbox.

Auto parts store or motorcycle shop would be a better source for petroleum compatible hoses.

+1. I used vacuum hose that was fuel rated. The line does not have to be clear, you will know if fuel is coming out.

 

[asnyder]

Got it, bad that they were sealed. The larger hose from the airbox is not sealed so I'll seal that one.

 

[asnyder]

So for taking out and cleaning, rebuilding the carbs, am I going to need to get the pressure gauges too for the carb sync once finished? Also, what exact parts do I need to replace the seals, o-rings, etc for the carbs. All I've seen is people post to the Honda OEM parts pages but it looks like I need three different items on most. There are carb rebuild kits (3rd party) that look like they include everything but people here warn against using them. Thanks!

 

[jetcash]

After you do the carbs, you should check the valve clearance, it's easy. My 'matic did the same thing when I bought it; stalling at stops, pissing fuel out the carb drains. After the carb rebuild it was significantly better, but after adjusting the valves, I can sit through a whole stoplight cycle with out taking it out of gear.

 

[peejay]

J-T,

asnyder said in his opening thread it's a HondaMatic, therefore no clutch

 

[WoundedCyborg]

I see youre starting on a new direction to solve your issue but I figured you'd want to know I have a 1980 hondamatic cm400a and it has a very similar issue. Weak idle, and will randomly stall. Except mine started making a terrible hiss and rattle sound from the bottom of it closer to the rear. I just paid my local dealer to diagnose the issue for me. In my year of having it its always needed the carbs cleaned often and always had fueling issues. But anyways, when I get a call from them in the next few days I'll let you know and maybe you may get you closer to a solution yourself!

 

[asnyder]

I have the carbs half torn apart now... just needing another small tool to get in a tight spot. Lots of varnish, dirt, rust, goo (from gaskets completely liquefied), etc. Some of the jets pins were plugged up.
Carb kits will be here monday as well as my Simple Green Aircraft cleaner for my ultrasonic cleaner. Should make the cleaning go a lot easier. Also got a 2 cylinder carb synchronizer and a tachomoter to do the idle adjust.
I'm using pictures and knowledge from here and Mr. Polecat's 23 part video series on cleaning his.

My removal of the carbs took about 90 minutes because I couldn't find directions other than what was in service manual that said to loosen clamp screws around boots on both sides of carbs and wiggle out. Would have been 10000x easier to remove one set of the boots but I don't have the small wrench needed. Will do it for sure to put them back on.

Is there a list somewhere of the number and sizes of allen screw head style replacement screws to replace all of the screws on the 2 carbs (VB24a)?

FOUND IT: Stainless Steel Allen Head Screws: 8: 4x12mm, 8: 4x16mm, 4: 5x12mm, 5: 5x16mm, 4: 6x14mm
Thanks: http://www.hondatwins.net/forums/63-fuel-supply-carburation/24191-rebuilding-vbulletin-carbs.html

EDIT: I've gone to three different hardware stores and no one has stainless, metric, allen head screws in all the sizes I need. Are these the ones with a cap or the ones without, eg. a straight shaft and no head

 

[longdistancerider]

I get the screws at Ace Hardware Ace Hardware Store Locator | Click Here To Find Your Local Ace Hardware Store and Services ohio|30
These are socket head type Allen screws

 

[asnyder]

I get the screws at Ace Hardware Ace Hardware Store Locator | Click Here To Find Your Local Ace Hardware Store and Services ohio|30
These are socket head type Allen screws

Socket button head or socket cap?