Topic: CB400F Hanging Idle & rough running - out of ideas

[telerick]

Hi Everyone,

I have been chasing this problem with my '75 400F for months now and it's driving me crazy, I have found a few posts in this forum and others with CB400 and 550's with similar issue but none of them seem to have found a definitive solution - was hoping someone might be able to point me in the right direction.

Basically, the bike won't idle properly and the issue appears when the bike is warm, and gets worse the hotter it is. The most annoying this is the bike used to run perfectly. I didn't use it for about 2 years, and as part of re commissioning I gave the carbs a clean out as they had got a bit gummed up over that time. And ever since then, the issue has been there.

It starts happily without choke even on a cold day and idles ok cold, but to me it seems increasingly rich as it warms up.
Once warm, the idle is rough and it tends to cut out. If I turn the speed idle screw enough to get it to idle without cutting out, it will sit there then suddenly rev up to about 3000RPM. Turn the idle screw down again, it has little effect until the revs suddenly drop, it stumbles and cuts out.

I have done and checked the following to try and resolve this with no success, with one exception I will describe below:

Bike is standard, standard air cleaner and intake setup, standard exhaust (even with the original Honda branded muffler!), 5K NGK plug caps

Fresh premium E5 fuel (With new inline fuel filter)
New condensers
New points & points gap set
Confirmed throttle cables are free and not sticking (I installed them new when I first got the bike)
Automatic advance unit checked and lubricated
Ignition timing set using strobe light
Automatic advance operation checked with strobe light, advance start RPM is in spec and its not sticking
New spark plugs (Denso X24ES-U) changed from NGK DR8ES-L
New battery
Carbs cleaned thoroughly 5 times now, idle circuit confirmed clear between idle circuit inlet, slow jet, idle air adjuster screw and idle circuit outlet just under the throttle slide
New No.40 slow jets from David Silver
New No.75 main jets from David Silver (o-rings fit nice and tight in the jet tower)
Checked and cleaned emulsion tubes
Checked choke operation, all 4 opening and closing evenly
Float height set to 21mm
Cleaned fuel tank and new strainer - confirmed good flow from tank and through filter
Valve clearances set to 0.05mm
Valve timing checked - timing mark perfectly aligned
Checked for air leaks around intake boots and carbs
New intake boots (engine side)
Ran without air filter - issue still there
Adjusted idle air screws from 0 to 4 turns out in 1/2 turn increments - almost no effect, but seemed slightly better the further out they were screwed.
Vacuum synced the carbs (in the short interval where it was actually running properly, see below)
Checked coil impedance primary and secondary

As well as the idle issue, I am also getting the following symptoms:

Heavy soot deposits on spark plugs even after a fully warmed up ride (on all plugs)
Slight midrange stumble and lack of torque lower in the rev range (much less pull than when its running right)- this seems to get worse every time I take the bike out
A mist of fuel can be seen spitting from the carbs into the air cleaner side, particularly as you open the throttle slightly and the engine gets a bit stumbly. Very similar to the CB750 post from a few days ago. Intake manifold is very fuely

Now I did think I had fixed it - after installing the new idle jets (I dont think the jets actually made any difference, it seems whatever the issue was happened to be resolved temporarily). Once fully warmed up it was running perfectly. I took this opportunity to vacuum sync the carbs, after which it was idling smooth and nice and responsive.
Took it out for a ride, it was almost like it was back to its old self. Powerful, responsive, idling perfect. But about 10 minutes in, the engine started to stumble and feel sluggish. I stopped and found fuel flowing from the carb overflow tubes. Drained the bowls and opened the fuel tap again, overflow was resolved. Started it up and behold, it wont idle anymore. Exactly the same symptoms as before, with the added bonus of backfiring and stumbling on anything over 1/3rd throttle. Full throttle felt like you slammed the brakes on. I managed to nurse it home, at that point I was inclined to cast it into the bin.

No idea where to look from here. Since then I have cleaned the carbs again and fitted new intake boots, and still the idle problem and the midrange stumble / full throttle engine fully cutting out is still there (actually worse if anything). It pops and bangs and throws me around like a ragdoll at anything above 1/3rd throttle

Thanks everyone and sorry for the long post, wanted to convey as much detail as possible.

[newday777]

Welcome to the forum telerick
How did you clean the carbs exactly? Sure sounds like plugged idle circuits. Plugged emulsion tubes can do it too.
How did you clean the tank?
I suspect that you might have some rust still(flash rust even a little can separate from the tank and get in the carbs causing overflows and extra gas levels and rich running.

Have you used a bore scope to closely inspect for any rust in the tank?

[Ozzybud]

Sounds like the plugs are fouled. Once fouled unless they are cleaned properly the spark will travel down the insulators intermittently and give you all kind of issues..I always run my plugs thru my spark plug cleaner every time I make a change when troubleshooting. Yes it could be this simple! I didn't see that you had replaced  or cleaned the plugs

[Tom R]

I had the same exact issue….only cured by complete disassembly and rebuild of the carbs.
My guess is air passage or emulsion tube blocked up.I would turn the petcock off to get it to idle right. Certainly no way to go down the road.
I found a list of oring sizes (on this site I believe) and bought enough for several carb racks, and just bought the top cap gaskets. Use original brass. I was lucky and was able to reuse the float needle valves
No choke cold start is unfortunately not the norm anymore, but she idles perfectly now.

[scottly]

Check the actual fuel level in the float bowls using the "clear tube" method.

[telerick]

Hi, thanks for the replies!

I cleaned the carbs with carb cleaner and compressed air, and some little little cylindrical brushes for the larger passageways. I payed extra attention to the idle circuit, sprayed the carb cleaner through the idle circuit and made sure it was flowing out everywhere it should be.

I flushed the tank with a detergent, it looks quite good condition inside from what can be seen through the cap, looks too good to be the original tank, but there are some spots of rust visible and quite a bit of rust came out.

Would enough debris get through both the fuel strainer and in-line filter to cause an issue?

With regards to plugs, I put a new set of plugs in and it made no difference unfortunately.

Looks like maybe another carb clean is on the cards, but a full tear down as you say Tom replacing all the seals. I want to avoid replacing as much brass as possible as I have read people having issues with aftermarket brass

[Nurse Julie]

Where abouts in UK are you? , we have plenty of 400/4 owners on the UK site that may be able to offer advice or even hands on assistance. Join us on www.sohc.co.uk if you haven't already done so.
I also sell full Viton O ring kits for the carbs.
Strange that your bikes starts with no choke on a cold day, that's not normal at all 🤔🤔🤔

[newday777]

Did you get pictures of the tank and carbs before you began?
Unfortunately your spray cleaning of the carbs lacks the ability to get rid of the hardened gas in the idle circuits and detergent is good for removing old gas to an extent but alone will not remove rust.
You need to either do an electrolysis cleaning method or get a full tank of Evaporust if only minor rust.
If it looked like the tank I'm attaching then you need to go the electrolysis method.
Yes even minute rust will clog strainers and filters. The tank needs to be spotless.

[Mark1976]

Check the actual fuel level in the float bowls using the "clear tube" method.

I think this is the most reasonable place to start, post pics and lets go from there.

[HondaMan]

The bike is running rich: that's what causes the problem you are describing with the 400F (and the 350F).
But...before going after that problem, I'd strongly suggest installing the original brass jets. If the new jets you have are not genuine Keihin brass, they are NOT calibrated the way they should be. This will cause a host of troubles.

After that:
You removed the carbs, which disturbed the seal they once had between the hoses and the carbs: did you also remove the rubber spigots from the head? If so, the first thing I'd suggest would be replacing the rubber O-rings that seal those flanges to the head. When these leak, they make the carbs run rich, and they are now hard as plastic and will not seal. The O-rings are available at CMSNL.com, #16173253004 as 27x2.4 size. I've found the 27x2.5 size more commonly available, and it fits. If the rubber spigots are hard and/or the clamps stretched to the point where you can close the clamps fully face-to-face with the screw mounts, they also may not be sealing - from being hard, now. Try reshaping the bracket that backs the head of the screws to make them flatter and help close down harder on the hoses. I have also made flat-sided washers for those screws to give them more purchase on the clamp closure faces.

You mentioned changing the points and condensors: if the condensors are the Daiichi brand (often have no markings whatsoever) then they don't work, right from the box. Install your old ones instead: these parts do not 'wear out', but often get changed anyway. Currently only TEC (Tokyo Electric Corporation) of Japan is making good ones, and they are expensive parts at up to $18 USD each. They don't wear out (I reiterate), so try the old ones. Those alone can cause the problems you are having: it has happened that way to me several times since 2002 or so, when China started making counterfeit Daiichi condensors and selling them - sometimes with Daiichi stamp markings, sometimes not. A very weak spark will cause the problem you're having as it lets unburned fuel build up until a warmed-up engine cannot ingest it anymore. This is not coil-related, nor plug caps, as some may mention, since your bike ran just a couple of years ago. The bad condensors WILL cause weak spark, hence the symptoms.

Along the way, setting the carb's idle mix screws too far out will cause buildup of unburned fuel, too, which will make this bike 'stick' at high idle because of the timing of the intake cam, which makes this engine slow to scavenge excess fuel. They should not be more than 2-1/2 turns out, as they run too rich when turned out further than that. Most of the time I set them at 2.0 turns, +/-1/4 turn.

Running high-octane fuel will cause the engine to 'overrun' the throttle closure when hot. This, too, is caused by the cam's timing. Modern fuels burn far, far slower than the 1970s versions because today the fuel must help to light off catalytic convertors - so, it burns slowly and long, like premium gas did in the 1970s. Use lower octane gas to help prevent the tendency to 'hang' at high RPM when the throttle is let off. High octane will also cause darker sparkplugs.

[Trevor from Warragul]

I've owned my CB350F for over 30 years. It ran perfectly on leaded fuel "back in the day". When Australia phased out leaded fuel, it started to run rich and foul the plugs. Going to one size lower on the main jet solved that problem.

[telerick]

Thanks for taking the time to reply, it is much appreciated

I have not done clear tube method, I will certainly give this a try!

Annoyingly with regards to fuel, it seems you can only get E5 (5% ethanol) in 'premium' high octane fuel guise in the UK. 95 octane is almost always E10 which I want to avoid

On the next removal of the carbs i'll pop the original brass back in - the pilot jets that were in it before don't look like original Keihn as I cant see the logo on them and they are quite pitted - but worth a try

The new points are Japanese Futaba brand, not sure about the branding of the condensers but they do have a 'Made in Japan' marking and are from a reputable Honda specialist. The old condensers appeared like were on their way out as the points were arcing quite badly with the old ones - but they are actually 'ND' branded condensers - worth a try.
The problem was, however, there before I changed the ignition components and brass. The list of things I have done is things to try and rectify the issue, so the problem was there before doing any of those things and still remains after - but I am cautious that I can be causing new issues by replacing things that didn't need to be replaced.

I have indeed removed and replaced the rubber spigots and accompanying o-rings that connect the carbs to the head

The tank is nowhere near as bad as that Newday, just a few spots of rust visible in an otherwise good condition silver finish inside. Didn't get a picture but might give it another wash out with evaporust or similar

I am in Essex Juile, I will take a look at the UK forum, thanks for the info!

It runs almost perfect when its cold. Smooth idle and responsive throttle. You can actually hear the engine getting progressively rougher as it warms up

[HondaMan]

I've owned my CB350F for over 30 years. It ran perfectly on leaded fuel "back in the day". When Australia phased out leaded fuel, it started to run rich and foul the plugs. Going to one size lower on the main jet solved that problem.

This is similar with the 750 and 500/550, too. That's why I so often recommend lower-octane (USA-type) fuels: it helps. I dropped a 5 size in my own 750 about 3-4 years ago when the gas got 'readjusted' here for 'less pollution', but was really about protecting and aiding the startup of platinum-catalyst convertors in cars.

[Tim2005]

You've done most of the things I would, however the way it was the same without the airfilter fitted is rather odd & points away from a carb issue. It won't be fuel either, I'm UK (Norfolk) too and I find 400Fs run great on E10. The stopping at full throttle is rather odd as well. As per Hondaman's advice, what's the spark like? I'd look into that further and do a compression check too, just for peace of mind. Check what voltage you are getting at your coils too.  Also, what condition is the silencer in? Any chance of a collapsed baffle or other blockage in there?

[jaytee-nz]

The fact that it starts without the choke when cold is a sure sign of your needles / needle jets being worn out. It makes the engine run rich and that's why it will start without the choke and run fine, but as it warms up it's getting too much fuel.
Have you replaced these (with genuine Keihin parts)?. I had a CB750 with the same symptoms and worn needles was the problem.

[telerick]

I am suspicious of the spark. With the plugs out the spark looks nice and strong but it might not reflect what the spark is actually like in a pressurised fuel air mixture. Although I have gone through the ignition system and I cant see anything wrong. Coil primaries are exactly 5 ohms on both, i'm only getting about 10.5v in the primary circuit, i'm guessing this is because of the resistance of the coil? I cleaned up the contacts for the coils too. Secondary side was around 24K ohm on both, with 5K plug caps installed.

The needles are a little pitted and the jets worn slightly but nothing major. Worn needles / jets is definitely a route to go down but the fact it ran fine before it was laid up would say to me it wouldn't be that. I think after the next round of actions if its still not resolved i'll look at replacing needles and jets.

I have the carbs out again, going to give another clean and put the original brass back in. Going to invest in a compression tester to see if that identifies any issues

I plan to do a clear tube method to verify float levels, re-install carbs and give it a try. Then swap out the condensers to the old ones and see what we get

[telerick]

Hey everyone!

Quick update: Carbs removed, cleaned, sprayed carb cleaner down the idle jet fitting through a tube and observed a good spray coming out of both idle circuit inlet and outlet on all carbs

Put original brass back in (main jets and idle jets) with new o-rings

Found a bent lifter linkage bracket on one of the throttle slides - straightened it out and now all the sync adjusters are even with a bench sync - but I don't think this has anything to do with the problem as it must have always been like it back when the bike ran fine and was just compensated for at the sync adjuster

Bench synced the carbs using the calibrated dill bit method

Double checked float height @21mm - Have not done clear tube as of yet as I need to work out how to connect a tube to the drain holes with the stuff I have

Compression Test: all cylinders 130PSI +/- 5PSI (Less than the 170PSI in the manual, but this seems quite normal reading other posts)

Installed carbs and ran bike - started ok but sounded uneven. As it started to warm up, it needed more and more throttle to keep it going and ended up with the exact same symptoms as before (rough / dying idle and suddenly revving and getting stuck at 3000RPM). It also develops an occasional puffing / popping sound as one of the carbs is spitting back into the intake.

Swapped condensers back to the old ones - no difference.

So unfortunately no progress as of yet

Did some tests whilst bike was hot to see if I could catch something that was changing with a hot engine:

Compression test: all cylinders 135PSI
Resistance primary coil: Both 5.3 ohm
Resistance secondary (with 5K caps) 26.2K ohm & 25.6 K ohm

Plugs all heavily sooted (I cleaned them before running the bike) - all look equally so

I did find that a baffle in the silencer (the tube the leads from the inlet of the silencer) has rusted and broken off inside so its floating around in there. I am going to order a new silencer and see how it goes! and in the meantime see if I can find some screws that fit the carb drains to make some adapters for clear tube method.

[newday777]

The only thing that will clean out the idle circuits is heated water/cleaner mix. Ultrasonic or old school boiling. Spray cleaner doesn't do it.

[Tim2005]

130 compression is really low in my thinking, I checked my commuter 400F last year, it was last rebuilt in 2008 and must have done 50000 miles easily since then and it is on 150-160 psi still. I'd try checking with another gauge.

It'll be interesting to hear what the exhaust  and clear tube fixes come up with too

[telerick]

So clear tube (modified pen) shows all carbs to be filling a couple of mm (around 4mm) below the bowl flange - which I believe is correct? - my modified pen just about held up to test all 4 before disintegrating and leaving a chunk of plastic inside the drain hole so they are going to have to come apart again anyway.

What is strange though, before I pulled the carbs I was having a play around and found if I turned the fuel off, after 30s or so the bike ran absolutely perfect. It smoothed out, ticked over at 1200 and not even a stumble on tip in, nice and responsive throttle.

As soon as I turn the fuel back on, bam, it splutters, gets lumpy and cuts out. Like the instant I turn the fuel tap.

So this pretty much confirms the rich idle condition - but the question is, do I increase the float height or would I be masking the real issue by doing that? Or maybe that is the issue? as it must be something that is common between all the carbs, as they are all running rich (float valve o-rings were replaced on last re-build and I am certain there isnt fuel going by as theres no overflowing even with the fuel on for 10 minutes)

With regards to compression, I have read a number of threads with various opinions, some saying it should be close to the Honda spec and anything less is suspect, others saying you will never get that number. It seems to depend on the type of gauge and volume of the tubing - most posts I have found people report around the 120-130PSI mark even with recently refurbished engines after honing and fresh rings. The main thing I was looking for was consistency, and they were all almost identical which I am quite happy about.

[Nurse Julie]

No point in increasing float height, they are set at that for a reason. Have you removed the fuel tap, and taken it apart to see what's happening. It sounds like the rubber gasket may have desintigrated or something similar. Also, are you turning the fuel tap the correct way?

[telerick]

Yes fuel tap is cleaned and has has a brand new OEM strainer and seals, and verified good flow both from the fuel tap and through inline filter. Also replaced all the fuel lines.

I do get the same effect both on reserve and on positions. Its like once fuel the fuel is off (with engine running), as fuel is used out of the bowls it leans out to the perfect mixture. As soon as fuel is turned on, the bowls fill to a higher level and make it too rich again.

[Nurse Julie]

Does it smell very petrolly when the bike is running?

[Trevor from Warragul]

if I turned the fuel off, after 30s or so the bike ran absolutely perfect. It smoothed out, ticked over at 1200 and not even a stumble on tip in, nice and responsive throttle.

To me, this points to the float height being the problem. When the fuel is turned off, the float height gradually lowers as the fuel in the bowls is used up. Try lowering the float heights.

Also, on my 350 Four I replaced the float bowl screws with cap heads. This means the bowls can be removed without taking the carburetors off.  Saves a shedload of time!

[jonda500]

If winding out the air screws helps it you could try swapping out the pilots for #35's - you can do this without removing the carbs if you have some good extra stubby screwdrivers (I shortened mine by cutting half the handle off). - but I like to get the main jet size dialed in first..
My 500 has 100% oem setup yet mysteriously only runs well with the main jets increased from #100 to #105 and the pilots decreased from #40 to #35 (and the air screws only a 1/2 turn out from seated)
John