Topic: CB400F running woes - idle, carbs, timing, you name it?

[bggann]

 
Purchased a '75 CB400F from a freind (who I trust and who is a very capable mechanic).
Bike had been sitting for about 1 year, garaged.

Upon fueling and running- had traditional drippy carbs (needle valves) and didn't want to idle well. He said it was running well when put away (and I do trust this person to know and tell the truth).

Cleaned the carbs - Expecting that to solve the problem.  Has not - in fact, getting worse.

Has not (and we are both experience and both somewhat baffled - I've had several other SOHC 4's incluing a '73 350F.)
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Current state
- Bike idle is 'questionable' - may idle (rough and not nice), or roar (2000 rpm) or die.
- Seems rich (by smell).
- Running with idle screws way in - 1/4 turn out rather than the 2 +/- 1/2 recommended.
- Occasional carb overflows.
- Cruise is "burbble"
- Accel has some misfires.
- Stock air filter (have not run with it out - will do so).
- Close to stock exhaust (muffler is different).
=============
What have I done:
- Pulled the carbs off and cleaned the bottoms (main jets, needles, idle jets).  Really not that dirty.
- Run several tanks of clean premium fuel through it
- Checked:
   - Valves - all good
   - Cam Chain - tightened
   - Carbs syncd
   - Timing set (problematic - see below).
   - Adjusted carbs (running at 1/4 turn out - not 2.  2 - smells RICH).
   - May have vacuum lead intake #3 but not clear.  Very small change in running on spray of starting fluid.
- Done all this multiple times.

Plans:
- Have Carb kits ordered
- Have Dyna S ordered.
- Re-clean carbs.
- Pull tank valve filter and look for crap.
- Ask for help!

Timing Woes:  Timing this beasty is very hard as the base plate seems to move back and forth right and left.  When you set dwell (gap) then try to time it, if you let the baseplate move, then your dwell is all wrong.  I set dwell and timing by holding base plate right - but I can only do static timing, if I try a light, it bounce around.  Then to get 2-3 points timing right, I have to close the points up a bit from where they should be.

Hoping the Dyna S will solve this.

-----------------
Bob

[gregk]

i had a similiar problem and checked the search and found TT had suggested shimming the timing plate on the lower right had side.  Then you can get the dwell the same on both sets of points

Greg

[Hush]

If carbs leaking, replace float valves.
Even though they look good after sitting for a year they will have moulded to the area they are sitting in, either floats hanging or floats jammed hard up into carb body.
Either way replace them suckers, it will have a huge effect on all your other problems once the correct volume of gas is getting to the motor.

[svenD]

tiny impurities in the fuel will hang up the floatvalves causing rough running,try draining fuel from the float chambers and inspekt the result.

[TwoTired]

Timing Woes:  Timing this beasty is very hard as the base plate seems to move back and forth right and left.  When you set dwell (gap) then try to time it, if you let the baseplate move, then your dwell is all wrong.  I set dwell and timing by holding base plate right - but I can only do static timing, if I try a light, it bounce around.  Then to get 2-3 points timing right, I have to close the points up a bit from where they should be.

See:
http://forums.sohc4.net/index.php?topic=5752.msg47539#msg47539

The stock paper air filter is good for one year outside the plastic bag.  Fibers collapse and clog, leading to a choking effect.

Have you checked for air leaks at the carb couplers?

Cheers,

[bggann]

 
It's all in the timing - or so I thought.

The hints about shimming the base plate were great - I did this and re-timed (static - I sold my timing light about 2 weeks before I bought this bike because - hey - never use it any more...). 

Pulled the tank petcock and cleaned it out - some gunk there.  May need to flush the tank - thinking of stuff getting into needles.

BTW - have carb kits ordered, will replace needles and seats when they show up.

Got a pretty decent idle at 1200-1400 - way better than before, #4 is not quite right - pipe cold and I can pull that plug wire and not a ton of difference.  Plug looks nice, not rich, perhaps a bit lean, but not bad.

But - it idles!!! Whooee - take it for a ride, now it drops off on accel misfires at rpm's greater than 3-4K.  Thinking floats?  But, need to borrow Norms timing light and dynamically time it.  At least it is ridable - in that it seems to idle.

Did try w and w/o air filter.  Worse with NO filter.

BG

[bggann]

Well - It's not all timing....

The bike ran down the canyon pretty good this morning and idled away happily at the gas station (till I turned it off to fuel).  Then going down the road - progressively worse till I finally pulled over and - behold- pouring gas syndrome.

Ran the rest of the way in without a lot of trouble - using manual float control - as in turn the fuel on and off....

arrggghhh.  The problem is this is not a 'project bike' this is my daily driver....

[bggann]

   Floatin' down the road on the CB400F - will only a little ripple.
I'm getting it dialed in - each day I tackle one more thing.  Today - I put an in-line filter between tank and carbs, yesterday pulled the petcock and found nasties so I think that part of my reoccuring float problem is gunk.  So far - so good.

Then I popped the float bowl off #4 as that was the colder cylinders.  Floats way off (26mm or so, should be 21).  Running much better - still a high RPM burble, but I need to check the rest of the floats, I suspect they are all off.

Only died once at a stop sign today - idling, then just stops.  Getting pretty quick at finding neutral, flipping up the peg, down the kick and starting the baby up.  Wonder when that new starter switch will be in....

Bgann agaan