Topic: Stumbles off idle: FIxed - Now a new problem

[KB02]

'76 750-F running pods (yeah, I know) and stock 4into1 exhaust.
When pulling from an idle or just low throttle, my bike has a bt of a stumble. If I just pour the throttle right on, it works fine, but a moderate twist shows the stumble in it's true fashion. Should I try increasing the idle jet size to the next size up?

[jdamman]

I have the same problem (see in post "missing at cruising") and am also hoping to stumble on the answer. (prob. 2 different problems-stumble and missing).  It seems to be kind of a transition problem, like there's a gap between the slow jet and the needle jet- am I thinking about this right, because mine seems to "jump the gap" with no problem when I use the throttle more agressively.
Thanks,
Joel

[eldar]

if you can whack it open and it goes great but a slow roll causes it to stumble, I would not jet up, sounds like there is enough fuel. For a short test, remove the air filter and try it and see what happens.

[TwoTired]

Should I try increasing the idle jet size to the next size up?

What is your Idle Air Bleed Screw setting?
Closing that down also adds fuel to the idle mix (by starving the air for premix), to a point.  It's adjustment range is limited.  I'm guessing That if you go less the 1/2 turn out from seated, you'll need bigger slow jet orifices.  Then you'll have to find a different happy place for the Idle screws.

The other possibility is the throttle valve cutaway.  Honda says this determines fuel mix during the transition between the slow system and slide needle fuel delivery.  However, changing the slide cutaway height is not a very easy adjustment to make, particularly if you want it lower.

[KB02]

I have the bleed screws set at the reommendation from my clymer's manual (I can't emember what exactly that is right off the top of my head). I'll have to play with that a little bit and see what going on there.

[TwoTired]

All the manuals assume you are working on a bike in stock configuration.  The settings and adjustment positions, come from Honda and they don't include a bike modified with pods.  You will have to readjust the carbs (possibly all the settings, idle screw, slow jet, needle position and taper, and main jet) to operate with your modifications.  In theory, even the throttle valve cutaway might require changing if the transition can't be compensated with other adjustments.

Good luck!

[Zeke]

Also, the stock idle screw settings are only defaults.  They should be fine-tuned on all bikes, preferably they would each have individual settings.

Though I gave up on that -- the "idle - drop" thing to get them individually adjusted should come with a voodoo shaman -- I found it impossible, so I just turned the screws out the same amount.  Little rich, turn them all just an 1/8th turn...

Now, correct me if I'm wrong, because I'm about to rejet my bike for new pipes:

If you need to be just a little richer at the transition from pilot jet to main (like around 1/4 throttle) Can't you remedy that by raising the needle a notch?

Raising the needle will let just a bit more fuel out at throttle position where the needle is still in the jet.

 

Zeke

[TwoTired]

Also, the stock idle screw settings are only defaults.  They should be fine-tuned on all bikes, preferably they would each have individual settings.

I mostly agree.  Though behavior is a little different between Idle Air Bleed Screws (IABS, or air screw slow jets) and Idle Mixture Screws (IMS).
Carbs with IABS don't have accelerator pumps.  The IABS are set for a compromise between as-lean-as-you-can-go and rich enough to still have good throttle response.  IMS is generally set to as-lean-as-you-can-go.

Though I gave up on that -- the "idle - drop" thing to get them individually adjusted should come with a voodoo shaman -- I found it impossible, so I just turned the screws out the same amount.  Little rich, turn them all just an 1/8th turn...

That should be fine, as long as the EPA isn't looking over your shoulder.

Now, correct me if I'm wrong, because I'm about to rejet my bike for new pipes:
If you need to be just a little richer at the transition from pilot jet to main (like around 1/4 throttle) Can't you remedy that by raising the needle a notch?

Raising the needle will let just a bit more fuel out at throttle position where the needle is still in the jet.

Yes, but the needle controls mixture from 1/8 to 3/4 throttle setting. Raising the needle effects whole range.  It may be more appropriate to change the taper of the needle.

Also, if you cruise at say 1/3 throttle, you may need a compound taper needle profile to compensate for Throttle Valve Cutaway design taylored for stock induction and exhaust characteristics.

This is why re-jetting can be problematic for a bike intended to be street driven and to run well whatever you do to the twist grip.

Cheers,

[KB02]

Okay, I am going to explore my ignorance and your inteligence here, TwoTired: Could you please explain to me in a little more depth what the "Thottle Valve Cut Away" is? And how do I adjust it?

Also, on that chart you posted (thanks by the way. You had posted that before and I love it, but I couldn't find it anywhere), would raising or lowering the clip on the needle move the posision of the needle in relitive to it's posision on the chart? I.E. - would rasing it a clip slide it to the left on that chart? Or right? Or not at all?

[Bob Wessner]

This might help on the cut-a-way question. It's the small beveled area at the bottom of the slide valve.

http://www.motocross.com/motoprof/moto/mcycle/carb101/carb101.html

Normally it is not adjustable since it would require some knowledge (which I don't have  ) and some precise machining.

[TwoTired]

It's a two axis chart, which depicts the effectiveness of a factor vs. the throttle position.
To use it, read along the bottom to find the throttle position of interest, then follow the line up wards to find the shaded shape area that crosses it.  The width of the shaded area along the vertical line indicates how much changing of that factor is in effecting mixture at that throttle position.

For example at WOT you can see that the Main jet dominates the mixture, but as the jet needle never fully withdraws from the needle orifice, the width of the needle at that point along its taper, also has a lesser effect on the mixture ratio.  In other words, you will get a more noticeable change in mixture by changing the main jet size at WOT than by changing the slide needle taper.  However, if you change the needle taper or clip position to improve the mixture at 1/2 throttle, you may have to revist main jet selection, as the needle has a lesser effect on WOT mixture settings, too.

Briefly, the throttle valve cutaway refers to the shape of the bottom of the slides.  You'll note the intlet side is angled higher than the flat bottomed side facing the intake valve.  That's the cutaway.  And the magnitude, or angle of that cutaway effects the mixture from idle to almost 1/2 throttle, with it largest effect at about 1/10 to 1/5th throttle position on the chart.

You may begin to see how a Dyno can be such an invaluable tool.  As with it's instrumentation you can chart it's mixture vs throttle position and know what's best to adjust with minimal impact on what is already properly adjusted.

You can also get this information on the test track over several runs and readings of spark plug deposits.  But, it takes longer.

Cheers,

[KB02]

Thanks Llyod, and Bob!!!!

[jdamman]

I got mine to move up from idle better by backing the bleed screws out (leaner right?) and the transition is better, I still have more fine tuning to do, and it also reduced the missing at cruising I was having.  Apparently my mixture was off at low load because of the bleed screws, causing a miss.  unfortunatly, I haven't figured out why it was only one side, yet I think my carbs have all had pretty similar bleed screw adjustments.  They are out 2-3 turns now I think, but their taper is different than stock, I replaced the orrg. ones when I put the Keyster carb kit in.  Has anyone else noticed a major difference between the behavior of the Keyster screws vs. the Honda ones?  I'm thinking of putting the honda ones back in.
Oh yeah, the only mod to the bike (CB750 K1) is a 4-2 exhaust, FYI.
Thanks,
Joel

[Ceej]

I think i understand most of the things in this thread after reading it a few times.  But one thing i dont know is....where are these bleed screws and how should i go about adjusting them.  Does backing them off make it run richer? or vice versa?

[TwoTired]

IMS and air screws are adjusted oppositely for a richening effect.  The topic bike was a 76 CB750.  It has Air bleed screws; out (CCW) for leaner.

If we knew what bike you have, we could advise.

Cheers,

[Ceej]

74 cb750

[TwoTired]

You have idle air bleed screws. Turning them in restricts the air premix and richens the mixture.  Turning them out is, of course, the opposite.  They have a limited range though.  Less than 1/2 turn out from seated is about as rich as you should go, unless you have hollow needles with a crossdrill.  These latter are self limiting, and prevent total closure of the air for premix.

Cheers,

[Ceej]

please excuse my ignorance.  I dont know where or what these idle air bleed screw look like.  Ive looked through my manual and found an "air screw" but this wasnt even looked at in detail.  So if you could give me a quick run down of how to adjust these i would greatly apreciate it.

[TwoTired]

Which manual do you have?

Honda shop manual shows it on page 79, fig. 6-15.
Clymer; page 64, fig. 75. 
And page 123 fig 7.  However, in typical Clymer fashion, they labeled the parts wrong.  24 is the idle screw and 25 is the throttle stop screw.
(It galls me to know they still make money spreading disinformation.)

Anyway, there is a screw slot in the side of the carbs near the air filter box.  Left carbs have it on the left side.  Right carbs have it on the right side.

Best to know where they are currently set.  Turn them clockwise, counting the turns, until lightly seated.  Excessive force can damage the needle, or worse, the needle seat in the carb body.

Now that you know the current setting, you can add or subtract turn increments to change the idle mixture.  Use the same setting for each of the four carbs.

Cheers,

[KB02]

Okay, I pulled the carbs, reset the neddles to the center clip and reset all the air screws (1 1/2 turns out) and the bike runs fine (YAY!!  ). Stumble is gone.

NOW, since I have done this, I have a new problem. For some reason gas stops flowing to two of my carbs. #1 and #3 carbs slowly drain out of fuel while riding and the bike (obviously) stumbles and dies. Since it was working perfectly before, and the only thing I did to the fuel line was take it off and set it to the side while I adjusted the needles, and the only thing I did to the carbs was the needles, I am at a loss as to why this is doing this.

If I just reach down, wiggle the fuel line and wait 5 seconds, the bowls fill back up and I'm off.

Any suggestions on this one?

[Bob Wessner]

Did you route them the same way when you reinstalled the lines after working on the carbs?

[KB02]

Yes, I did. Which is one of e reasons that I am confused. If I had done anything different, I would have a clue, but since everything is the same, I'm at a loss.

[Bob Wessner]

I guess I would check any and all things related to fuel flow starting with the tank/petcock and all vents. It does seem like an odd combination though. I would have expected something like 1&2 or 3&4 rather than what you are seeing.

One other question, do you have in-line fuel filters?

[Klark Kent]

Hey
Have you tried just waiting 5 seconds as opposed to wiggling and then waiting.  It may be the waiting and not the wiggling that is refilling your float bowls.  Did you replace the fuel line with modern fuel line, which is too thick most of the time to route the same way as stock line?  I have a pretty good kink in my fuel line right now that i will get around to alleviating, but it doesn't seem to restrict flow enough to effect running, so I am reluctant to blame the line- so try just waiting without wiggleing the line at all.
Hope this helps- if not just keep being deductive with the effects and you will find the cause.
-KK

[TwoTired]

NOW, since I have done this, I have a new problem. For some reason gas stops flowing to two of my carbs. #1 and #3 carbs slowly drain out of fuel while riding and the bike (obviously) stumbles and dies. Since it was working perfectly before, and the only thing I did to the fuel line was take it off and set it to the side while I adjusted the needles, and the only thing I did to the carbs was the needles, I am at a loss as to why this is doing this.

Any suggestions on this one?

You did introduce air into the fuel lines while they were removed.  And, the fuel feed is done by gravity.  Loops or areas of the line may have trapped air in them now, which would be exacerbated by a low fuel tank level.  If you have added in-line fuel filters, fuel line loops that have high points where air can collect, can be difficult to avoid.
 I'm not sure about the 750, but many carbs do not have a mechanical link for dropped floats to positively open the float valve.  Float valve opening relies on gravity and the head pressure in the fuel line and tank to actually push open the float valve.  If the float valves are sticking just a tiny bit and the air in the line cannot escape or equalizes the head pressure, no or low fuel flow may result.

Reroute the fuel line or bleed the air out of the fuel lines.  That's my suggestion.

Cheers,