Author Topic: Hanging idle when hot CB400F (Read 4818 times)

3pintrace · « **on:** May 20, 2018, 06:45:00 AM »

Hi everyone,

I just bought a 1975 CB400F and I'm having an issue that I cant seem to figure out. I'm no ace mechanic but would like to do this myself.
The issue: It starts up easily, idles great, warms up with the choke gradually being turned off. Runs and rides well, good power but... when the engine gets up to full temp the idle will hang up at around 2200. If I set the idle at this point, it will eventually drop back down and die. If I set the idle when it drops, it will hang up again. If when its hung up, I very gently press down on the top of one of the vacuum adjust screws it will immediately drop to a good idle. If I blip the throttle it will sometimes drop down but as it gets hotter, it doesn't seem to work.
Here are some specifics.
Stock air box, exhaust, jets. (I am at 5k elevation)
Cant find any vac leaks using the carb cleaner method.
New throttle cables, not sticking. Lots of spring tension bringing linkage back down, hits stop with a "clunk" when not running.
Vacuum sync carbs.
air screws about 1 3/4 turns out
Electronic ignition, all cylinders are firing.
No fuel delivery issues.
When the choke is open the stop screw is in spec.
Let me know if I forgot any needed information.
My next step was going to be to check float levels, check the pilot jets are clear (the PO said they were just cleaned).
Any suggestions would be great, I'm just trying to avoid going down the wrong road or making things worse because I'm a hack. Could the slides be sticking? but that wouldn't be temp specific?
Thanks for any help!

Don R · « **Reply #1 on:** May 20, 2018, 09:13:46 AM »

It sounds like you're been thorough, I'd suggest float levels or timing advancer, if it's still in use. Good luck and watching with interest. My 836 started doing this also, it has 4 cable carbs so I'll begin there.

3pintrace · « **Reply #2 on:** May 20, 2018, 02:30:56 PM »

Update.
Checked the floats and they are all good. Checked the pilot jets, nice and clean #40's. Rechecked for air leaks, nothing.
The hotter the bike, the worse it gets. Since I'm at elevation, should I try to go down a size of the pilot jet? Would a too big pilot cause this?

Thanks for any help. Sooo frustrated!

robvangulik · « **Reply #3 on:** May 20, 2018, 02:48:04 PM »

Start simple, try turning in the IdleMixtureScrew to 1 turn out.

mrbreeze · « **Reply #4 on:** May 20, 2018, 08:06:51 PM »

Sounds like the carbs aren't synched. We had a guy on here the other day that said his carbs had been synched but came to find out it wasn't done properly. Make sure all other adjustments are on before synching (ignition,valves,etc.). Also you need to check your fuel in bowl levels (not float adjustment)

strynboen · « **Reply #5 on:** May 21, 2018, 01:03:41 PM »

yes do a vaccum syncro..one cylender gets a lot more gas then the rest..and hold the rpm up..vhen you press that one dovn..the engine dies..

Patrick · « **Reply #6 on:** May 21, 2018, 01:13:36 PM »

If you synched your motor the first time with the bike sitting in place without at least a fan on it, you synched an overheated engine and probably are running lean on one or more cylinders. I would redo that first. Too rich ruins your plugs, too lean melts your motor.

Patrick

3pintrace · « **Reply #7 on:** May 21, 2018, 03:10:52 PM »

I did have a fan running when I vacuum synced. It was idling perfectly (it was hot but not as hot as it gets) rpm's would drop right back down. Then I go for a ride, get after it a bit and come to a stop and the rpm's hang up again. if I adjust the idle down at this point, the bike will die when the rpm's decide to finally drop. Only variable seems to be the engine temp. I have checked the temp's of the the pipes and they are all the same. Since I'm 100% stock and at 5280 feet, I'm thinking of trying #38 pilot jets. Not that hopeful that it will help but all I can think is that I'm running a rich idle circuit. If I was asked what I thought, I'd say air leak for sure but I cant find one.
One other think I found was that the junction between the headers and muffler was leaking a bit of water on a cold damp morning when I was warming the bike up. Can't imagine that would have that dramatic affect.
I'm gonna try to run it and pull the plugs to get a better idea of rich/lean condition. I'll also try to do the clear tube test.

Thanks for the suggestions guys.

knowsnothing · « **Reply #8 on:** May 23, 2018, 07:11:59 AM »

Check your boots for any cracks/tears/pinches etc. Really sounds like an air leak to me too even though i know you checked it. Also double check the throttle cables and their routing just to be safe. The fact it only happen as the engine warms would suggest its from the thermal expansion of something or the effect of the expansion on something.

I know very well how tricky it can be to get the carbs in on these little beauties so maybe take everything off, inspect rubbers (before and after carbs), check the clamps to make sure they haven't stretched too far and are giving and nice tight seal, and try again. Sometimes that is all it takes. Good luck

Deltarider · « **Reply #9 on:** May 23, 2018, 08:08:58 AM »

Use a strobe to see if advancer functions as it should. Plug chops at idle are next to useless IMO (you may read what I posted elsewhere). If bike sat a long period of time, there's a chance slow jets are (partially) blocked again.
Here's a simple test. Have the engine run at a bit above idle. Fully close airscrew #1 and then open 3 turns. RPM should change. Repeat for #2, #3 and #4. If there's no change in running that idle circuit has a problem.

TwoTired · « **Reply #10 on:** May 23, 2018, 09:09:28 AM »

If you are confident your vacuum sync guages are reporting accurately...
Do check for mechanicl advance sticking both cold and hot.

Higher altitudes make mixtures richer. And runaway idle speeds indicate one or more cylinders are getting too lean.

Perhaps you are using oxygenated fuel which would in effect lean your mixtures. Hot engines more efficiently atomize and utilize available mixture and increase leaning effect.

A simple experiment would be to turn yourmixture screws inward to enrich the idle mixture somewhat.

Cheers,

Tim2005 · « **Reply #11 on:** May 23, 2018, 11:07:15 AM »

A couple of thoughts. Firstly those linkages between 1&2 and 3&4,the ones with the adjust screws, can occasionally deteriorate like that (admittedly I've only seen that once & at very high miles) - could be worth giving those a real good dose of contact cleaner then wd40 to see if it makes any difference. Secondly, 5000 does seem pretty high for stock jetting, but admittedly I've little experience of those heights on a 400f... so 38s would seem well worth a try

3pintrace · « **Reply #12 on:** May 25, 2018, 07:57:39 PM »

Thanks for all the suggestions. So I figured out that the air screw on the #3 carb has no effect on the idle but turning in any of the others stalls the motor. So what’s my next move? Got to be an air leak but using carb cleaner or starter fluid sprayed around the boots, linkage etc the idle is not effected?!
Thanks for all the guidance!

jgger · « **Reply #13 on:** May 25, 2018, 09:45:10 PM »

If turning the screw has no effect and starter fluid doesn't change anything, I would think that the passage is blocked. Might be ultrasonic bath time.

Deltarider · « **Reply #14 on:** May 25, 2018, 11:32:55 PM »

Ultrasonic bath is no guarantee though. Fortunately your type of carb is not difficult to clean. Here's what I'd do:
a) Verify that fuel reaches the #3 floatbowl. Open or even remove the drainscrew and open and close the petcock a few times. If no fuel, gently tap the floatbowl with the stub end of a screwdriver and hopefully the needle will be freed. If not remove floatbowl and inspect floatvalve and -needle.
b) If fuel reaches floatbowl with drainscrew open, check that overflow tube (and pipe) vent.
c) Slow jets easily block after some time of inactivity. After removal of floatbowl, unscrew the slowjet and see if it is blocked. Clean it. Caution: don't use anything harder than brass. I prefer copper wire as shown in pic. With the slowjet removed, squirt carburetor- or brakecleaner (using a straw) in the opening where slow jet sat. It should come out of the tiny hole that's right in front of the gasslide (engineside).
d) The little opening in the side of the carbs mouth (airfilter side) should also be free (in pic: pilot system air jet). It usually is. Only when bike was run without an airfilter, there's a chance it isn't.
e) Unscrew the airscrew in the side of the carb and clean it. Squirt cleaner in the opening where the airscrew sat. It should come out of the opening where the slowjet sat.
f) Repeat e) when the cleaned slowjet is back in its place and verify if cleaner exits from the slowjet.
It's a bit of a chore. See it as an adventure and you'll have learned a lot about your carb.

3pintrace · « **Reply #15 on:** May 26, 2018, 06:04:11 AM »

Cool. I’ll report back once I get it cleaned up.
Thanks,
Dave

3pintrace · « **Reply #16 on:** May 26, 2018, 08:14:17 PM »

So....
Took off the carbs, cleaned all the brass. Made sure the orifices were clean by spraying carb cleaner through them. Replaced the pilot jets with 38’s. Replaced all the air screws and springs. Vacuum synced them. Replaced the spark plugs. Set screws at 1 1/2 out (checked that they were set for best idle). Starts up perfectly, idles like a champ...until it gets hot. Same frickin thing! Thought I had it. Went for s short ride, coming back to idle like it should, going great. Then it stalls, I turn up the idle so it won’t and then it hangs at 2300 rpm. Getting diiscouraged. Sigh!

Deltarider · « **Reply #17 on:** May 26, 2018, 11:31:44 PM »

Does turning the big central idle speed knob help after engine has warmed up? Then there would be no problem at all, for it's very normal practice for other CB owners to create an idle at say 1200 rpm after engine has warmed up.

Quote

Set screws at 1 1/2 out (checked that they were set for best idle).

If by 'set for best idle' you mean to achieve a highest possible rpm, then it's the wrong procedure, I'm afraid. I suggest - if there's no airleak and the central idle speed screw offers no solution - to play the airscrews a bit more. In your case: enriching by turning them in.
If all this does not help, please note that p.16 of the CB400F Parts List suggests that a #40 slow jet is standard and even a #42 is a possibility... http://www.honda4fun.com/pdf_parts_list/pdf_spac400/CB400F-F1-F2/CB400F-F1-F2.pdf

robvangulik · « **Reply #18 on:** May 27, 2018, 01:32:56 AM »

I suggested a week ago to turn the ims in, now you've even put in a smaller pilot jet, you are making it run lean even more!
You can be at altitude, but the bike is telling you it needs more fuel.....

3pintrace · « **Reply #19 on:** May 27, 2018, 05:53:21 AM »

My plugs had black soot on them so figured it was running rich. I tried to dial in the air screws when the 40’s were in but it didn’t help. If I got past 1 turn out the motor would want stall. I could dial all of them 2+ turns out with no effect past 1.5 out. To me, this said smaller jet but clearly I don’t know squat! Should I try 42’s? Switch to a bicycle?!

Patrick · « **Reply #20 on:** May 27, 2018, 06:35:00 AM »

The air jets are just there to smooth the idle by slightly balancing the the air flow at idle. They are not going to help performance at all.

I found in some old sets of carbs that the needle jets wear out. Replacements can be hard to find, but if they are worn they are letting fuel through the mains even at idle, making it really difficult to fine tune the bike. You also can have issues tuning if you used the carb needle from most aftermarket carb rebuild kits. Aftermarket kits generally do not have needles with exactly the right angles, so they leak fuel and run rich. I'm betting either you, or a previous owner, used a carb kit and switched the needle, but not the jet.

As for solutions, if this is the case, the last time found some stock needle jets and installed them my air screws went from two and a half to three turns out (with a big different between carbs) to slightly tighter than stock and all screws turned within one-quarter revolution. Otherwise, just keeping turning the air screws and hope you find a balance.

But again, this only affects your idle. Not once you twist the throttle.

I understand carbs, but I really hate them.

Patrick

Deltarider · « **Reply #21 on:** May 27, 2018, 07:27:22 AM »

Quote from: 3pintrace on May 27, 2018, 05:53:21 AM

My plugs had black soot on them so figured it was running rich.

Where was the soot, on the noses?

Quote

Switch to a bicycle?!

Isn't that's what we all should do and make our environment livable again? I ride my bicycles more and more. No dressing up, no irritating standing at traffic lights. Most destinations I go to are within 20 km and all in all I arrive sooner on a bicycle. No cage around my head. No time lost at gasstations. No costs. We have an excellent and ever improving infrastructure for bicycles. The centre of Amsterdam is not the cheapest place to live and people that live there can easily afford cars. Nevertheless car ownership keeps on falling and is now around 40% of the households. I Was in Sevilla, Spain recently and was happy to see another city converted.

3pintrace · « **Reply #22 on:** May 27, 2018, 12:49:46 PM »

The soot was all over, see pic. Would the needle jet hypothesis work with the only happening when very hot fact? Under that same theory, could I raise the clip on the needles to stop that extra fuel?
Thanks

Deltarider · « **Reply #23 on:** May 27, 2018, 02:27:58 PM »

I have no idea. BTW, are you sure the little O-rings around the main jets are still OK and do they seal well?

3pintrace · « **Reply #24 on:** May 27, 2018, 03:16:16 PM »

Would old o-rings around the main jets cause this condition? If so, I’ll rip the carbs apart and check. Was thinking maybe I should just buy all new main jets, needle jets and needles from 4 into 1. But only if people here think it could help. Trying not to through money and time away.

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Author Topic: Hanging idle when hot CB400F (Read 4818 times)