Topic: Cb400f big bore low rpm bogging issue. Help please

[ioginy]

Good day all.
I'm scratching my head on a low power issue off idle.  That being said, I'm no expert, so chances are good its an easy fix.  Here are the details though, and if anyone has some insight I would very grateful.

The bike:
1974 cb350f with a 1975 cb400f engine. Engine has a 466 big bore kit in it and a dyna-III ignition. This was all done before I got it.  It also has aftermarket 2 into 4 exhaust but they are routed with 1 and 4 into the right, and 2 and 3 into the left.  I've not seen this before on these bikes and if anyone knows anything about that, I would be curious.   I guess it sort of makes it run like a twin.  Also, air filter is stock. 
Carbs: float height is 22mm.  Was at 21 but was getting a little bit of overflow on one of my bowls when on the side stand. Main jet is 78 (stock was 75) and slow jet is 35 (original was 40).  Mixture screw is at 1 turn.
Spark plugs look great with this setup.  Tried putting the Jets back to stock and it was running extremely rich, and the problem seemed to be worse.  The floats were also at 24mm when I first got it.  Changing the height didn't effect the main issue at all.
I just put new Magna coils in, with new leads and spark caps. 
The bike starts, warms up, idles and rides great.  When the bike is still not totally warm, it isn't and issue, but when it warms up there is a significant bog between 1k rpm and 2500rpm.  If I blip the throttle before starting it seems to help, but with any load, in that room range it bogs for a second and then everything is fine. 
Fuel flow appears to be good, bit I am going to replace the float valves and seats as a precaution when they arrive.  Carbs are nice and clean as well.  There is a small bit of white smoke out the right pipe (cylinder 1and/or 4) when starting cold, and when idling. I have ordered a new head gasket (big bore ones are hard to find) and will be checking the valves when that arrives.  Haven't done a compression test yet, but will when I get a minute. 

I think that's all.  Sorry for the illiad but I figure more information is better than less. 

If anyone has any ideas, I would be grateful.

[Don R]

 part of the problem is these don't have accelerator pumps and the fuel mixture must crutch the roll on throttle issue.

[Mark1976]

   Gotta have some pic's of the plugs, if they're old ignore that. I gotta see the pipe as well, is it baffled very well. I take it the needles are in the stock position. Putting on an aftermarket exhaust that's not baffled well can make for a notoriously off idle flat spot as you described. Now throw it more displacement and a smaller idle (slow) jet, #35 vs. #40 and it'll get even worse. You only went up to 78's (you may need 80's if the exhaust is open or poorly baffled) on the mains and that's not much. Put the #40's back in, see what that does. It may still stumble a little once warm, but that'll tell you if your headed in the right direction, if it does, try richening up the idle mixture 1/2 a turn in. that may get you closer. That pipe you have will be the real variable. You really ought to have a set of gauges to sync as well. All this assumes that you are in fact running the stock airbox and filter and the brass that's currently in it is not the cheap after market stuff that is so pervasive these days. Welcome to the world of carburetion, one step at a time.
   Hope the coils are 5 ohm, check your battery voltage at idle since your running a dyna ignition, they can over tax what little charging system the bike has.














?

[ioginy]

part of the problem is these don't have accelerator pumps and the fuel mixture must crutch the roll on throttle issue.

That is definitely something I was pondering.  I really appreciated that feature on my 79 cb750. 

Gotta have some pic's of the plugs, if they're old ignore that. I gotta see the pipe as well, is it baffled very well.
I'll see about getting some pics of those soon.  The plugs are n
I take it the needles are in the stock position. Putting on an aftermarket exhaust that's not baffled well can make for a notoriously off idle flat spot as you described. Now throw it more displacement and a smaller idle (slow) jet, #35 vs. #40 and it'll get even worse. You only went up to 78's (you may need 80's if the exhaust is open or poorly baffled) on the mains and that's not much. Put the #40's back in, see what that does. It may still stumble a little once warm, but that'll tell you if your headed in the right direction, if it does, try richening up the idle mixture 1/2 a turn in. that may get you closer. That pipe you have will be the real variable. You really ought to have a set of gauges to sync as well. All this assumes that you are in fact running the stock airbox and filter and the brass that's currently in it is not the cheap after market stuff that is so pervasive these days. Welcome to the world of carburetion, one step at a time.
   Hope the coils are 5 ohm, check your battery voltage at idle since your running a dyna ignition, they can over tax what little charging system the bike has.

I'll see about grabbing some pics of the plugs and pipes with the baffles out soon. 
The plugs are new, and with the current setup, are reading very good.  I did try the original 40 jets already and the plugs came out black and the problem was worse.  I have a set of gauges and the carbs are perfectly synced.  The airbox and filter are stock...  The jets are branded Keihin, as I have had many issues in the past with variations in jet sizes on 4 cylinders that I do not wish to have again. 
The coils are 5 ohm, and the battery is new and fully charged.  Also every bulb is an LED and that has been great.  The issue definitely isn't coils or battery though, as it was the same before I installed the new coils and there was no change once they were in. 
I have been inclined to think it is the exhaust from the beginning, but I'll see what helpful tidbits folks have once I get some pictures of the baffles. 

[Mark1976]

   As long as all the o-rings on the brass are good, I can't imagine it being rich with the stock jetting and a 466 displacement bump. Stock cam? Make sure you have a fresh airfilter. If you leave the jetting the way it is now, try enriching the idle mixture  by 1/2 a turn maybe even 3/4 from the stock setting, to see if the 35 idles are your culprit. If its a lean off idle stumble it should improve. Don't go pulling everything apart until you have an idea what you problems maybe. Throwing in an aftermarket exhaust (and depending on how restrictive it is or isn't) will determine what you'll need to do with your jetting. I've 2 different pipes for my 400f (kerker, ontario MT) and each one gives a different plug reading. The kerker is fully baffled and gives the best overall readings, maybe even a smidge rich (quieter as well). The OMT pipe runs a bit leaner but it is a much less restrictive pipe. So your exhaust will impact your jetting. I run 85 mains and a 35 idle, stock needles in the stock positions, stock airbox and filter. But I've got some minor head work, and a web cam, I do run dyna coils and I run points. My personal experience,  is to dump the dyna, (I have one sitting in a box on the shelf for years) they have a tendency to drain the battery on these bikes, the points based ignition work best for all around riding. This bike really isn't that difficult to set up as long as you maintain the stock airbox. That's just my experience, yours may vary.

[david 750f]

Go for a ride and give it a little choke at the problem rpm, that should tell you if it’s rich or lean.

[Tim2005]

Just wondering, are you very familiar with riding a  400f?  They don't really have any power under 2500 rpm, they are a very revvy engine.

[ioginy]

Just wondering, are you very familiar with riding a  400f?  They don't really have any power under 2500 rpm, they are a very revvy engine.

I am indeed new to cb400f's  I just completed this rebuild and it is my first experience with small 4's.  I was concerned I was just riding it like a twin and that was making it feel wierd.  I definitely have zero issues if I bring the revs past 2500. 

I have a feeling you may have just sorted my issue out.