Topic: Cb400f Runs crummy when warmed up then overheats

[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.