Topic: Cb400f Runs crummy when warmed up then overheats

[Icarus]

Hey all,
     So I finished building up the 400f a couple weeks ago, finally got it plated and on the road for some test rides.  New rings, lapped valves, new gaskets and all that.  Once I got it all together, I wasn't sure which break in method to use (short and rough or long and gentle) so I opted for doing a few good pulls up a steep hill then switched to around town riding, variable speed/rpm and no highways for now.

Anyways, went for a ride over the weekend and it was running good until about 40 minutes in when I started to come to some lights/stop signs and it really wanted to die at idle and I had to rev it to keep it from dying.  Also, it had very little power when accelerating from stop, had to really rev it which wasn't the case on shorter rides. 

The next day I checked the timing with a light and adjusted the timing for 2/3.  It was off and I think ,maybe they weren't firing consistently and that caused the previous days problems at idle.

So the next day I go for a ride and it seems to do a similar thing after it gets real warm.  Wants to die at idle, but then after letting it cool it runs good again.  My GF had to take it to work today because the car is getting repaired.  On her way home she is cruising fine until she hits traffic and has to sit...and sit revving because its wanting to die at idle.  This makes it get even hotter and it just gives up.  She said the kickstarter was very hard to move at that point (AWWW CRAP SEIZED?!?) but by the time I come to the rescue 30 min later, it kicks over easy and starts right up and idles easy.

What is going on?  Why does it run good then crap out?  I was thinking too rich ( I have idle screws 1.75 turns out though)  Stock airbox and 4-1 exhaust.  Seems like its overheating but shouldn't be so easy to get to that point.  When I was cruising and had problems the first day I spent very little time idling, plenty of wind over the engine.


Oh, the battery is unhealthy but I just got the new one so I can pop that in to get rid of one pesky symptom (after riding a while, and then dying at idle I would have to kick start because the battery didn't have the juice for the starter).  I tested the field coil and stator and they appear to be within the normal limits and the have a new solid state regulator/rectifier in there.

[flybox1]

good, properly charged battery, first, then let us know if the symptoms persist.

look here
http://forums.sohc4.net/index.php?topic=87086.0

[Icarus]

Reading through that thread now.  I should also mention the carbs are only bench synced right now.  I have a synchronizer on its way, hoping to have it by Friday.

I will report back once the fresh battery has been tried out.

[MoMo]

What color are the plugs?  Carbs out of sync would not cause the problem you're describing.  Battery first...Larry

[Icarus]

Momo, the plugs are sooty black.  I am guessing this is because it was sitting at idle in traffic before dying and doesn't excessive idling usually make for a rich looking plug?

[Icarus]

OKay i have the new battery charged up and in the bike.  I was re testing all the charging stuff and I got these numbers:
green/white 4.5ohms
yellow/yellow/yellow .6-.7 ohms (my multimeter doesn't show the next decimal)
neither have continuity with the frame, the battery at idle reads 12.08V but when I increase the rpm to 3k, 4k, 5k the voltage only goes up to 12.16V.

According to the manual it should get up to 14.5v, does this mean I have a regulator problem?  Its a new unit.

[Matt Chapter]

Yes, you should being seeing a higher voltage across the battery than 12.xx when charging.  You might have a corroded or loose connection if there are new parts involved.

Also, no, I don't think that in a stock setup the plugs should be very sooty after idling. Sounds like to me you really need to have the carbs synched.  Have you cleaned and rebuilt them?

[Icarus]

Matt, yeah I rebuilt the carbs going to vac sync them as soon as I have the meter. 

I am charging the battery again because it sounds like it should be reading higher than 12.09 at full charge.  Also took apart the battery and reg/rect connections and they're nice n tight with dielectric grease.

[Tim2005]

Could it be that your new solid state reg/rect is faulty? Do you still have the original parts to swap back and try?

I'd certainly fix this problem before investigating the running rough thing, as that may well just be caused by low volts & weak sparks.

[Icarus]

Yes, I am wanting to get the whole charging system sorted to see if that fixes things.  I am in contact with the regulator/rectifier maker and doing more tests once the battery is ready.  I have the OEM regulator and rectifier but since the bike was sitting for 20 years before I got it, I do not know if they are in working order.

this thing sure is fun when its running nice.  Its my first 4 cylinder and its so dang smooth compared to my old triumph. 

[vames]

Also, make sure it's running well before you send your girlfriend out into dangerous traffic on it!

[Icarus]

haha she was warned!  She rides a lot and new what kinda guff it was having but wanted to ride it anyways.

[TwoTired]

When you installed the rings, what end gap did you set?
Rings too tight will bore seize when hot and expanded.

Check the voltage across the Green and white when the engine is at idle and again revved.  Battery voltage level there make the alternator work the hardest.   But, you should be checking the charging system with a known good and fully charged battery.


Charging issues won't make the kick start lever tight or cause the engine to seize.  Engine seizure, o approach to that will make the starter motor draw a lot more power from the battery.

[Icarus]

Okay further battery report:

Charged to 13.03V, let it sit for 2 hours and it measured 12.74V.

At idle the battery voltage read 12.26V and upon revving it only went up to 12.31V.  This is with headlight off.

TwoTired, for the measuring of the green/white at idle and revved should I disconnect them from the regulator/rect or keep them hooked up and just expose a small part to measure?

My ring gap for the top two rings was .010" which was right in the middle of what the manual said it should be.  I am blanking on what the oil rings were.

I agree that the charging situation wouldn't lead everything tightening up.  Trying to figure out I need to take the engine all apart again.  guhhhh

[TwoTired]

TwoTired, for the measuring of the green/white at idle and revved should I disconnect them from the regulator/rect or keep them hooked up and just expose a small part to measure?

This an "in operation" test.  You want to see what the regulator is feeding the alternator field when the battery is demanding power.

At idle, the alternator can't make much power even if the field is given full battery voltage.  However, when revved, there should be plenty of power, provided the voltage regulator drives it as it should.  The vreg really should only trim back voltage to the field when the battery gets to 14.5v, (or at least 13.8V).

[Icarus]

Solved one of the problems.  Thanks to your white/green wire running check.  I found that the green wire from the regulator had a crappy ground.  I extended the wire and bolted it straight to the (-) battery terminal and then did another volate test and at higher RPMs it was definitely getting more juice.

Next test will be going on a bit of a ride to see how the charge holds up and if there is still an overheating issue.  I am hoping that the low battery charge that forced revving to keep it going at idle just caused too much heat while sitting in traffic.  Of course, the other possibility is that the bike gets hot, and the rings get tight making idling difficult so it takes more power to keep things moving.

I will let you know what I find.  I worry that the thing is seizing up but I don't know why it would.  I took out the worn rings and measured the cylinders only to find they were still well within standard bore limits, so I put new standard sized rings in after a quick hone.

[lucky]

It does sound like it was seizing up.
That is standard in that situation to not want to turn over and then when it has cooled off to be easy to turn over.

You did not tell us if the it has the stock air box and stock air filter or the exhaust configuration.

Good thing your GF did not get killed or hurt by being hit from behind with a stalled bike. Just think if that did happen.

[Icarus]

Stock airbox with new filter (intake system is old so might have some leaks around the ports.  The headers are the stock 4-1, not sure if the muffler is OEM or not.

Lucky dun worry, my GF knows how to ride a bike.  She isn't out there in high heels and shorts.  Her bike stalled while sitting stopped in traffic, surrounded by others cars also sitting still in traffic.

[Icarus]

So after further test riding, the charging system is now working great!  Unfortunately the bike still doesn't want to idle once i've been on the road for ~15minutes.

I have tested for fuel flow, one by one I drained each carb bowl and then opened the petcock to see how readily the fuel came from each one.  Seemed good.

The bike idles good while cold.  I use a bit of choke when I first start it and it takes just a minute or so before I can un-choke and get on my way.

Does anyone have ideas for things to test to maybe try and zero in on the issue?  Oh, I installed new condensers and coils but that had no effect.

[Icarus]

Also, when its wanting to die at idle I can rev the throttle to keep it alive and then the RPMs slowly settle back down to 1200-1400 and then it stumbles to nothing (if I don't keep revving).  When the light turns green I have to give it more throttle than usual and ease of the clutch slower.  Seems like maybe not all cylinders are firing? Then they all kick in and it runs good again once the RPMs increase.

I would think timing but then why would it run good for a while then get crummy?  Possible AAU sticking open?  I have timed it with a strobe but I guess I can always do it again.

[MoMo]

I forget if you said the carbs have been cleaned, what you are describing sounds as though the pilot  and midrange emulsifier jets are blocked...Larry

[Icarus]

I did rebuild them but its possible one of those didn't get perfectly done.  I have sprayed some carb cleaner through the idle circuits.

I must admit I don't exactly know how to clean the mid range emulsifier jets.  Is that the tube that the needle is in?

[MoMo]

I did rebuild them but its possible one of those didn't get perfectly done.  I have sprayed some carb cleaner through the idle circuits.

I must admit I don't exactly know how to clean the mid range emulsifier jets.  Is that the tube that the needle is in?

Yes, and jets are most always clogged.  I remove the slides to service them but I have seen people remove the tubes without removing the slides...Larry

[Icarus]

Sounds like I need to take my carbs off again!  Well at least I can do it quicker each time.  Now I just need to decide if I should mess with my needle heights while I have the slides out.

Does the idle becoming crummy once heated make sense with dirty emulsifiers?  I had thought that if the idle circuits or something were getting clogged, it would always have trouble idling until they were cleaned.

[MoMo]

If everything is stock, needle jet heights should be ok.  Make sure you clean the chamber that the emulsifier goes in...Larry

[AJK]

Momo, the plugs are sooty black.  I am guessing this is because it was sitting at idle in traffic before dying and doesn't excessive idling usually make for a rich looking plug?

At a superficial level, it sounds like your air mixture screws are set too rich. I'm not sure what the recommended starting position is for the cb400. If you have the manual, set them out according to the manual. I.e. It might say 1 turn instead of what you have them set at.

If you simply turn in the screws and the idle speed picks up, then you are heading in the right direction. Just make all screws an even amount of turns from lightly touching the base, and set them to the manuals default settings to start with.

Also check that your timing is firing at the || marks at say 4000 rpms, and let it sit where it will when idling. Check that both 1.4 & 2.3 are firing at the || mark at those revs.

There are a myriad of other things that could be checked, but just check this simple tuning first.

[Icarus]

This weekend I took the carbs apart to clean the emulsion tubes, and boy were they funky! A bunch of sediment, so cleaning those is definitely going to be helpful I think.  Also, I noticed that the carb cutaways that you bench sync with seemed very uneven and almost completely closed on a couple of the carbs.  This was after I had vacuum synced them.  So When I reassemble, I will bench sync again, then double check with gauges.

I also soaked the airbox boots in the xylene/wintergreen solution to revitalize them a bit, hopefully providing a better seal.

AJK, I think I have the idle screws set at 1.5 turns out.  I will double check them and try setting them a bit leaner when I get things back together. 

I will report beck when I can take it for a long enough ride to test for the weirdness.

[mikejorg]

Have checked your ignition coils?

I had a similar issue where my CB750 would run fine for about 20 minutes then die out. After cooling for a bit it would run fine again, but then die after riding for a bit. I went through my carbs thinking it was that but the same thing would happen. It ended up being the ignition coils. They would over heat and I would get no spark.

[Jasondo]

I also have a 400F and had this very same problem over the summer. I would ride 10-15 minutes til it got nice and warm and die at a stop sign . wait 20 minutes and bam!  fires right up and of i go for another 15 minutes.  After pulling my hair out much like you are I discovered it was a case of vapor lock. The vacuum builds and simply stops the flow of gas. after 20 minutes the pressure stabilized and starts again only to shut off further down the road. Try riding with you gas cap popped open. Its worth a try....

Jason

[Icarus]

@Mikejorg, yeah I suspected the coils since my GFs just had them crap out with similar symptoms.  SO I replaced them on the 400f but it didn't solve it.

@Jasondo, I can give it a try.  My current gas cap rubber is all dry and cracked so I am guessing its not sealing well but who knows.

[AJK]

Icarus, in this order, check:
 - Check valve clearances and adjust cam chain.
 - points dwell (points gap)
 - ignition timing (as i mentioned in my previous post)
 - float levels, carb sync (can be benched synced fairly closely using a small drill bit)
 - stock air screw settings from base, then set idle setting.

Go according to the manual. If these superficial checks dont fix it, then you need to go deeper. Make sure you have no air leaks around your carb boots & that your fuel cap is venting properly.

[Icarus]

@AJK Yes, I have done all those things  (well I am in the process of renewing the rubber boots, waiting to put them back in).  I have timed it statically and checked with a light. I just checked all the float levels again after putting the emulsion tubes back in.  I bench synced it then vacuum sync adjusted.  I did all the easy stuff before asking  .  Now I just have to see if cleaning the emulsion tubes, doing the boots and getting everything synced up again will have it fixed.

My cam chain only makes a little bit of noise.  I have loosened and pushed through the downward pointing opening (the adjuster is stick and doesn't tighten too well on its own spring).

Oh, I am also changing the oil.  Its only had ~200mi since the new rings/valves and such but I hear thats a good interval to get some fresh in there after engine work. 

[sfront]

Hey! New member here. I've recently acquired a 1975 CB400F that has the EXACT same issue Icarus's bike had. I know this is an ancient post, but after searching for more hours than I care to admit, this is the only one I've found with the same issue. But much to my dismay, no solution was ever given. Was this problem eventually solved? Or do I now have this bike?
Thanks
Steve

[Flyin900]

There was so little real info given by the original poster that this thread is useless for you at this stage. He never answered what the rebuild ring gap was and was all over the place with attempted solutions.

I would suggest you start a new thread being a new member and detail all the work you have (had) done and the symptoms. There are usually never any quick fixes to most complex issues. These small bore fours are different than their bigger cousins with special attention required to bore specs, if you or someone else rebuilt the engine.

[newday777]

Hey! New member here. I've recently acquired a 1975 CB400F that has the EXACT same issue Icarus's bike had. I know this is an ancient post, but after searching for more hours than I care to admit, this is the only one I've found with the same issue. But much to my dismay, no solution was ever given. Was this problem eventually solved? Or do I now have this bike?
Thanks
Steve

Welcome aboard the forum sfront.
Saying you have the exact same issue doesn't tell us much. You'll have to be more specific to exactly what you have for symptoms.