Topic: carb troubles, can't get it to run rich

[JHansen]

Howdy folks...

Trying my hand at tuning carbs, here is my current situation on my CB400

Came home tonight and checked timing. Running advanced >5 degrees, all cylinders.

Hooked up my carb syncs, which brings me to my first question. Manual calls for press. between 16-24 cm-hg. What is this converted into Standard? My gauges have no reading for cm-hg. I got all the needles in the identical spots. Sounded better. Snapped throttle to WOT quickly. At about 1/2 throttle, motor stumbles and bogs, if I snap to wot and leave it there, motor dies. Note all adjustments were made at operating temp.

Road tested and decided it was time to play with air screws. Started with screws 2 1/2 turns, as called for in manual. Had same stumble problem. Turned screws in, to 1 1/2 (richer, right!?) Stumble was not quite so bad, seemed a little better, motor slightly more responsive, but still with the stumbles. Went in on all screws 1/4 turn more. Road tested. Motor does not like that last 1/4 turn in. Sounds like is misses almost, like it's not getting enough air, duh. Came home, pulled plugs. The do not appear to be carboned up at all, and I can't get the bike to blow black smoke as if it were running lean. Plugs were slightly brownish on tips, and ceramic electrodes were bright white, but with no blisters. Plugs don't look like it's running rich. Valves were adjusted during engine reassemble, but have not been re-adjusted since, motor now has 60 break-in miles on it. Additionally, feel like the motor very slightly surges and hiccups at 40-50 mph @ 4000 rpm. Again, this is very, very slight.

Got home after 5 road test miles and to my dismay realized I hadn't tightened down my sync screws! S*^@! I tightened them down and tomorrow night will re-check my carb synch. Hoping vibration didn't throw it out, but if that were not an issue, there probably wouldn't be locknuts for the screws, right!

Back to my <1/2 throttle stumbling problems. What might be the cause? Here are my thoughts:

1.) Valve adjustment; only I can't believe they'd be that far off after only 60 miles
2.) fuel/air mix not right. I am running pods on it, and therefore I believe I need less air than specified in my manual dialed in on the carbs.
3.) Possible I have debris clogging some part of my carbs? Would this debris need to clog all 4 carbs to cause such trouble? Or would one clogged up carb do the trick? If fuel supply was clogged, would that explain why I can make the motor run rich, even on purpose?
4.) Some other thing I can't think of... This is the first time I have dialed in carbs, am fairly inexperienced with them

I know lots of the issues posted on these pages are carb related, but I'm at a slight loss here. Thanks so much to all who have taken the time to answer my previous posts, they do help so much.

So what do you guys think? Thanks! John

[TwoTired]

Snapped throttle to WOT quickly. At about 1/2 throttle, motor stumbles and bogs, if I snap to wot and leave it there, motor dies. Note all adjustments were made at operating temp.

You will never be able to snap from idle to WOT without the motor dying.  The slides are mechanically linked to the twist grip.  And, there is no accelerator pump to enrich the mixture when the carb throat looses vaccum.
When you snap open the throttle, the carb bore vacuum drops to near zero or to atmospheric levels.  All the fuel metering devices in the carbs require a differential pressure between atmosphere and the carb bore to flow fuel.  The change from stock filtration intensifies the vacuum loss.

You should, however, be able to input 1/2 throttle range changes with good engine response.  At least with the stock air filter, anyway.  Pods are likely to need changes to achieve this.

Throttle response on these machines relies on running over-rich at idle, and good fuel levels in bowls of the carbs to keep the emulsion tubes as full as practical.  This is the closest jump off point for the fuel to enter the carb throat.

Plugs were slightly brownish on tips, and ceramic electrodes were bright white, but with no blisters.

Classic "too Lean" symptoms.  At least you won't have to clean them much while you are richening things up.

Back to my <1/2 throttle stumbling problems. What might be the cause? Here are my thoughts:
4.) Some other thing I can't think of... This is the first time I have dialed in carbs, am fairly inexperienced with them

There are many facets to tuning carbs.  Throttle response is last on the list.
1. The main jet is selected to provide the correct mixture for WOT operation.  It's kind of a maximum supply device.  The slide needles further restrict it's flow at lower throttle settings.
2. The slide needles are adjusted to provide the correct mixture for midrange operation 1/4-3/4 throttle.
3.  If the idle air bleed screw doesn't have enough range of adjustment, the slow, pilot or idle jets must be selected.
4. Throttle transition from idle or slow jet system to the slide needles begins to suck from the emulsion tubes.  Emulsion tube levels (derived from the float height) and the throttle valve cutaway determine the mixture during this transition.

It is best to put temporary markings on your throttle, so you have a clear indication about what part of the carb you are operating on when you do your plug chops.