Topic: CB400f won't idle

[Loudo]

I bought this completely stock bike as a roller with boxes full of parts.  New rings, gaskets, took carb rack completely apart and gave it a thorough cleaning, new o-rings at the carb manifolds.  Float levels set to factory spec, air screws likewise.  Everything put back together, bike fires up and runs but won't idle unless the choke is left almost fully closed even when warm.  So, yes...it will eventually idle when warm but only with the choke about 7/8 closed.  Needs full choke and full gas to start, need to keep the revs up or it dies.  If I try to tease the choke open it will die.  All four cylinders are firing, there's no misfiring so I think the timing is good.  The revs are not staying high and slowly returning to normal when the throttle is reduced so I don't think it's an air leak situation (been there, done that with a CB175.)  Spark plugs check reveals some soot, so it's running rich.  Makes sense since I'm choking it even when warm, but it won't run otherwise.  I can ride it and it shifts fine but of course in the upper revs it rides like a bike where the rider forgot to open the choke.    I'm preparing to tear the carb rack out and clean everything again and stare at it really really hard until it behaves, but thought it would be a good idea to throw this out at the collective wisdom of y'all and get your thoughts.  It's probably something simple, but my experience is with two cylinder bikes so this four cylinder thing has me back on the learning curve I suppose.

Lemme hear it, folks...I hafta get this thing out of the garage and on the road.

[jessezm]

You need to make sure the throttle valves are opening enough to get an idle.  My guess is that when you choke it, it engages the throttle cam a bit and opens the valves enough to run.  But when you put the carbs back on you may not have set the idle speed screw high enough and the slides are just bottomed out all the way.  

If that isn't it, you need to check your idle circuits.  Did you bench sync the carbs before installing?

[jessezm]

Also, this may sound really stupid but it just happened to me yesterday....  Make sure your fuel tap is open!!!  if you're low on gas, put it on reserve, or fill 'er up!  Mine had the exact same symptoms for a few minutes before I figured it out...

[Loudo]

Thanks for the reply....if by bench sync you mean ensure the slides are all at the same height and move in concert, yes.  Also yes on the fuel tap...I have it on "R" just to be safe.  I'll look into the throttle cam issue, thanks for that.

[Bodi]

Check the emulsifier tubes on the pilot jets. The tiny cross holes allow air to mix with the fuel and form an emulsion, we might call it foam. When these are blocked the fuel doesn't foam. The result is rich mixture. I've noticed that the pilot jet emulsion tube holes clog more than the main jet emulsion tube (which a separate piece "above" the main jet).
Clean the tiny holes with a strand of copper wire, one pulled from a harness wire is about right if you have any spare harness wire. Once unplugged spray carb cleaner through them..

[MoMo]

Are the little black tubes between the carb bodies? Are you positive the idle circuits are clear, both the jets and the carb body?  good luck with your bike,  Larry

[Loudo]

Thanks for all the replies so far.  Fuel tap is open, black tubes between carb bodies are attached.  I had all the jets out and thoroughly cleaned when I stripped the carbs for overhaul, so I'm pretty sure it's not the emulsion tubes.  There's a chance that despite the screen in the fuel tank, some crap has gotten through and is plugging my fuel line, a possibility because even though very little surface rust is present in the tank, it has been in storage for a long time.

By the way, has anyone else tried to attach the main fuel line to the main fuel rail barb AFTER installation of the carbs?  Gosh, maybe instructions are ordered in a certain way for a reason. 

It'll start if I choke it all the way AND give it full throttle.  Some popping in the muffler during warmup.  Once started I can keep it going by modulating the throttle but I can't take my hand off the throttle or it'll die.  After it warms up, I try easing the choke off ver-r-r-ry slowly and the revs climb, something I associate with leaning out based on prior experience.  If I continue to ease off the choke, it dies as if I just cut the switch.

Given these symptoms, I'm thinking that for whatever reason it's a fuel delivery problem.  Any other advice or wild guesses would be appreciated before I rip the carb rack out and stare at it all mean-like.

[plug1]

I had the same problem w/ my 400f. I just rode it and kept on the throttle some when I came to a stop. Eventually she would stay running, but it took forever.  I contributed mine to a lean condition; my air box was in pretty bad shape. I've since put pods on and re-jetted and now she idles very nice and steady.

[Loudo]

Okay, before I remove the carb rack I realized I forgot to ask a question that's bugging me, which is:  Is it possible for one carb with a problem to drag the whole operation down with the symptoms I've described?  For example, if my idle circuit on one carb was plugged but working fine on the other three, would that create a bad-running situation in which it's difficult to tell if the problem is one carb or two or three and if so, how do you tell which one?  (Or two?)

It's pretty clear which side is ailing on a two cylinder bike, but this four cylinder business is more challenging.  I'm inclined to think that since all four headers on the bike are getting equally warm and it revs fine with no cutting out, the problem is in the fuel supply for all four carbs.  Opinions?

[jessezm]

Yes, it definitely sounds like a fuel delivery problem, and yes, one carb being way off can throw off the others.  The reason I suggested checking your fuel tap/level was because those are the exact symptoms I get when I forget to turn on the tap.  It runs fine on choke, then as I close the choke it revs up (you can actually watch your throttle tube twisting as it is being moved by the cam on the choke mechanism I think), and then totally dies when the choke is turned all the way off. 

Question:  if you can run it by twisting the throttle to keep the revs up without the choke on, can you hold the revs stead at 1500 or so? 

Another question: how did you do your bench sync, and what position was your idle speed screw set at when you did it?  I set my slides using a 1/8" drill bit, and made sure the idle speed screw was partially engaged so that after setting the slides, I could back off the screw and close them all the way, but also turn it in and open them up more. 

My advice would be to ensure you did your bench sync right, and while your carbs are off check all the pilot circuits again for crud, as well as all the air passageways.  I followed a thread here recently about doing this carefully with copper wire or a guitar string if I recall...

[Loudo]

I've been following the directions described in this document:  www.cb400f.btinternet.co.uk/carbv02.doc

To answer your question, with the choke off, it needs to stay way revved to keep running, like 4-5k.

I note that 2.8 and 2.27 call for a 2-3mm gap between the fast idle screw and the fast idle plate, but after using a manometer to balance the carbs the required gap called out for in 3.3 is 0-0.3mm.  I never got to the balancing part so closing the choke had no effect on the carb slides which presumably should inch up a tad when the choke is fully closed, which I assume is the purpose of this contraption, yes?  So I went ahead and adjusted it to a 0.1mm gap and it now DOES open the slides slightly but it still doesn't idle right.

I don't understand the purpose of setting that gap loose and then tightening it up later.

I also note that these instructions address setting the max open for the carb slides, but not a max closed.  This is probably an important component of bench synching that I didn't do.  Thought I did, but didn't.

Question on your fuel tap...if you never opened it, wouldn't your engine eventually stop even with it fully choked?  I assume what you've described is the engine running on the initial fuel available in the carb bowls until it's "vacuumed" out, but is there something else I'm missing?

[jessezm]

Question on your fuel tap...if you never opened it, wouldn't your engine eventually stop even with it fully choked?  I assume what you've described is the engine running on the initial fuel available in the carb bowls until it's "vacuumed" out, but is there something else I'm missing?

That's correct.

[jessezm]

If you can't hold the idle under 4-5k and you've adjusted the slides mechanically as close as you can, then I think you can rule out my first theory that the slides are just bottomed out.  It really sounds like your idle circuit is clogged somewhere.  I would go back and check all those tiny air passageways in the idle circuit.  Sorry, this sounds really frustrating!

[Loudo]

I haven't taken it on the road or even reassembled but discovered that my pilot jets were all plugged...the tiny cross holes were clear but the tiny orifice through the body of the jet was occluded.  I didn't bother to study the workings of the low speed circuit enough to know that a hole THROUGH the jet existed and needed to be clear.  Of course now as I look closely at a carb diagram I can see that the fuel in this circuit had to come from somewhere, but I guess I thought it was just an air function when I was cleaning them.  My initial puzzlement caused me to grab a spare pilot jet and look through it, seeing the hole (through a magnifying glass!) and experiencing an "aha" moment.

Other than that and a little debris in the float bowls I didn't see anything strange.  I'm gonna dump my gas and rinse again, reassemble and fire it up.  Now that I can see how tiny that pilot jet hole is (1/64") it's easy to see how easy it can be plugged.

Thanks again for all the suggestions, they helped...and stay tuned.

[Loudo]

OK....I put it all back together and fired it up.  It WILL idle now and act normal as it pertains to a cold start.  I used jessezm's advice and positioned the idle speed set screw so that I could use the screw to bottom the throttle slides.  It's running a lot better, but is exhibiting behavior I associate with an air leak which is that if it's at normal idle (~1,400 rpm) and I blip the throttle it climbs up to 3,000+ and slo-o-o-owly wants to come down.  Before I start throwing wrenches around the garage and saying bad words (well, WORSE words) I suppose I should allow for the fact I haven't balanced the carbs or messed with the air screws.

On a positive note, it seems to shift fine and the clutch is very nice.  I can't seem to get my chain guard attached in a way that does not conflict with the rear sprocket.  It doesn't look damaged, so I'm puzzled what's going on here.

I don't know how new piston rings affect what I'm doing here, but I imagine there's a break in period I need to be aware of.

[jessezm]

First, I'd check for an air leak at the intakes--use an aerosol can of WD-40 or some other lubricant/penetrant and when the bike is idling at 1400rpm, spray around the rubber intake boots.  If the idle climbs, there's your problem.  you can "soften" the intake boots with hot water and wintergreen oil (do a search for the thread).   It could well be that the carbs aren't well-synchronized, but you've done it twice now on the bench, and I doubt that's the issue.  A good bench-sync should be close enough for decent running.

Next thought is one or more of the slides are getting hung up, either by a sticky, old, non-lubricated or mis-routed throttle cable or assembly, or the slides themselves are sticking inside their bores.

When the bike is turned off and you twist the throttle and let go, do the slides spring back to the closed position freely?  Were they stuck at all before you took the carbs apart?  did you thoroughly clean and polish them?  Have you lubed the cables, and are you using both the push and pull cables?

Another possible culprit is the advance mechanism--have you checked to see that it moves freely, springs back to position from advanced, and that the springs aren't worn out?

Finally, a note about your new rings....   Do not sit and do all these tests with the engine idling in your driveway!  Idling your engine with a new set of rings is pretty much a no-no as far as break-in is concerned (at least that's what I was told over and over).  Did you have your cylinders honed when you took apart your top end?  Letting the bike idle can cause them to glaze over before the new rings are properly seated.

You can determine pretty quickly if it's a vacuum leak.  But if not, I'd try to put some break-in miles on those rings before digging to deeply.

[Loudo]

Finally had a chance to do some wrenchin' today.   Got it started, raced and died and died and raced but it finally warmed up enough to idle decently except the idle kept wanting to cli-i-i-mb up to 3k.  Sprayed some WD-40 around the rubber carb boots, didn't notice any change in idle.  I decided to get some miles in under load to break in the new rings if nothing else. Geared up and drove around for 12 miles in the 'hood taking it easy, on the highway for a couple miles just to go through all six gears.  No problems.  Still wanting to idle too high...I can drop the revs using the clutch but it wants to climb again.  Overall though it ran surprisingly good especially at speed...no stumbling or uneven running.  Next I'll check the advance mechanism.

The thing I noted mostly though was the weird handling.  The bike's tires are 100/90 front and 120/90 rear and these are oversize for the bike, perhaps significantly so.  I felt like the rolling inertia was heavy and twitchy when turning in sharply.  Not real confidence-inspiring.  My frame of reference by comparison is my CB77, a slightly lighter bike with correct tires and it handles SO much better as does my CB175.  My plan is to put that 100/90 on the rear and put a spare 90/90 that I have on the front.

Comments on the motor stuff and tire stuff and any other stuff are welcome.

[MoMo]

I had oversize tires on one of my 400 fours, took them off and put closer to stock size.  If a CB77 handles better than your bike you need to change.  Are the tires old?  Tires older than 5 or so years need to be replaced if you are going to do any type of aggressive riding.

Where are the air screws set?  If set too lean, the symptoms will be similar to an air leak.  Each cylinder should be set individually by ear, by turning air screw out till idle on that cylinder falters and then back in a half turn.  You can actually hear the cylinder reach its smoothest idle.  My 400s usually wind up between one and a quarter to one and three quarters out....Larry

[Bodi]

The 400 is very sensitive to tire wear and pressure. With low tire pressure it gets twitchy and falls into turns. With flat bottom worn tires it gets very weird on turns and develops a tank slap oscillation around 45mph. If you have old tires they're probably hard and slippery and due for replacement, getting closer to stock sizes would be a good idea. Both your tires are oversize for the stock rim widths and the tire profiles will be distorted, many people use them anyway without complaint though. Stock rims are 1.60" front and 1.85" rear: take a look here http://www.dropbears.com/u/utilities/tyrerim.htm for tire size recommendations.
Check the rear swingarm side play. The mount bushings are known to wear especially when lubrication is neglected (as is normal it seems) and the steel collar the bushings run on also wears oval, this part is long discontinued and unavailable though. The stock "plastic" bushings are available but very nasty to install. Bronze bushings are also available but I have yet to find a set that fit properly and an oval bushing continues to be an issue. "Hondaman" here has been refurbishing the collars and fitting bronze bushings for a decent price, contact him about it if you want this done... I am not sure he is still doing them.
Also check the steering stem bearings for slop. Just get the front wheel off the ground (a heavy friend sitting on the back of the seat while on the centre stand should do it) and see if there's any play in the stem bearings. There's a tapered roller bearing conversion available but unless the races are damaged the ball system Honda used is pretty good, it will require cleaning and regreasing after 30 years though. If they need adjustment you should do the clean and grease as well. Actually you should do it anyway if you plan to ride the bike much.

[Loudo]

Here's a photo of the bike currently along with my proposed front tire (Avon 90/90) next to the current front tire (CS Hi-Max 100/90)  The 120/90 rear tire is also a Hi-Max but of a different tread.  The current tires look fine and are not old/cracked, I'm inclined to think my handling problems are due to the tires being too large.  I will also be looking at the bushings and bearings.  I have the front fender, it's just not on until I get this tire thing squared away.  The only non-stock parts I know of are the shocks and mirrors.

Bodi, I note what you've said about stock wheel sizes.  My understanding is that the measurement is from the inside of the beads.  I can't measure the distance between beads with the tires on but the outside measurement of my front and rear wheels are 2-3/8" and 2-13/16" respectively.

[plug1]

The closest to stock sizes would be 100/90 rear and 90/90 front. I recently replaced the original tires with 110/90 rear and 90/90 front Avon Roadriders on my 400/4 and love the way it handles. Those tires on your 400 look rather big for those rims. Also, you may want to add a fork brace since the front fender is removed. That might improve your handling a bit.

[Loudo]

plug1, thanks for the reply.  I didn't think about the bracing qualities of the front fender, but it's only off 'til I change the tires.  100/90 rear and 90/90 front is what I'll end up with.  I'm still trying to figure out if the wheels I have are the stock size.  Anybody out there have some outside calipers they can measure their rims with and tell me what they find?

[plug1]

  I didn't think about the bracing qualities of the front fender, but it's only off 'til I change the tires. 

Sorry, I missed that in your last post. I removed my fender and added a brace after reading recommendations on this forum. Anyway, I'd say you have stock rims. I took a half-assed measurement with a tape measure on the inside of the rim and got 2 7/16" front and 2 3/4" rear. That was before I read your measurements in your last post. I'd say within 1/16" is sufficient considering the methods used. My rims are definitely stock, btw. Hope this helps.

[jessezm]

Where did you get a brace for you forks?  I can't find one anywhere for a 400f that's a bolt-on.  Do you have a picture?  Thanks!

[plug1]

I just used the brace from the stock front fender. I drilled the rivets out to remove it, cleaned it up and painted it. Not exactly a Telefix, but it does the job.






This one's just for $hits and giggles.