Topic: Bike stumbles at idle to 1/4 throttle

[brett_bike]

You can't expect to have an engine run correctly with carbon soot shorting out the spark plug electrode.
As I said before, you cannot expect to snap the throttle full open and expect the engine to pick up.
However, you should be able the move the the throttle 1/2 of total travel and have the engine pick up briskly and reliably.

Lastly, if you want anymore help from me, you will have to answer the questions I asked you.

The original complaint was that the bike has a stumble on acceleration.
Did changing air screw position change this behavior?

For the separate problem of stalling...
Do you have you main idle knob set for correct idle speed when the engine is hot or cold?
Do you know the emulsion tube holes are clear?
Do all your spark plugs exhibit the same deposit color and pattern?

To answer your questions, I do not expect to "snap the throttle to full open and expect the engine to pick up".  As I stated, I give her a "flick at idle and it hesitates".  I give it maybe a quick quarter turn "snap" and it hesitates before the rpm's pick up.  However, it does not display this characteristic when cold.

On to your specific questions...

I set the air screws to 1 turn out and she blew out some gray smoke from the exhaust at a stoplight.  Still sputters when accelerating off idle and the plugs are still fouled.

Idle knob is set for idle when warm.
Emulsion tubes were proven clean/clear when I cleaned the carbs, I have a POR 15 lined tank.  Maybe something may have clogged it again.
All of the spark plugs do exhibit the same deposit color, slightly fouled with black soot but not as bad on the electrode itself.  That is a dark tan.

Thanks for all the time you people are taking to answer my questions.  I'm still new to this, especially considering all the combined years of experience you all have. 

[TwoTired]

The original complaint was that the bike has a stumble on acceleration.
Did changing air screw position change this behavior?

To answer your questions, I do not expect to "snap the throttle to full open and expect the engine to pick up".  As I stated, I give her a "flick at idle and it hesitates".  I give it maybe a quick quarter turn "snap" and it hesitates before the rpm's pick up.  However, it does not display this characteristic when cold.

mark your throttle position at idle and then mark you throttle position fully open.  Measure midway between idle and full travel.  That is 1/2 throttle travel.  Are you moving the throttle more than this when you ask for acceleration? Did changing air screw position change this behavior?

Idle knob is set for idle when warm.

What speed is that?  And, are you confident that the tach you have is correct for your bike?   
(I'm trying to rule out any possibility that you have set the idle too low.)

Emulsion tubes were proven clean/clear when I cleaned the carbs, I have a POR 15 lined tank.  Maybe something may have clogged it again.

Are all the internal parts of the carbs sourced from Honda?
Do you now what numbers are stamped on the slide needles?
Were all the orings and seals replaced when you cleaned the carbs?

Can you describe your choke procedure for start up?


P.S. You are the eyes, ears, and hands, for us who are trying to get your bike operating as you expect.  Details are important.  And, we can't diagnose what we can't visualize, or have test notes which we can scrutinize.


Cheers,

[brett_bike]

Here goes.  Changing the air screw from 2 1/8 turns out to 1 turn out made the problem worsen.

2 1/8 air screw from fully closed = acceleration at 2/3 throttle.  1 turn of air screw from filly closed = acceleration at just over 1/2 throttle. 

Idle speed at 1200 rpm's when bike is warm.  PO of bike is second owner and confident tach is stock.

PO says that all internal brass parts/jets are stock.  Number on slide needle is 272304, set on #4 clip position.  When I replaced the O rings in the carbs, I used Honda parts.  I did however have (what I thought were) "extra" O rings in the kits, I could not find where they belong so I figured they were extras. 

I do not have to choke the bike to start.

I think the carbs will be coming off again tomorrow or the next day for an inspection and see if I can find anything I may have missed.  Is it possible that the needle jet should be set at #3 because if the rich condition, or a smaller slow jet is the culprit.  Maybe there is something stuck that I missed in the emulsion tubes.  I love this.

Thanks again, especially you TT for taking the time to help.  Your wealth of knowledge boggles the mind.

[TwoTired]

Here goes.  Changing the air screw from 2 1/8 turns out to 1 turn out made the problem worsen.

2 1/8 air screw from fully closed = acceleration at 2/3 throttle.  1 turn of air screw from filly closed = acceleration at just over 1/2 throttle. 

PO says that all internal brass parts/jets are stock.  Number on slide needle is 272304, set on #4 clip position.  When I replaced the O rings in the carbs, I used Honda parts.  I did however have (what I thought were) "extra" O rings in the kits, I could not find where they belong so I figured they were extras. 

I do not have to choke the bike to start.

All this seems to confirm you bike is running too rich.  Question still is why?
Does behavior change when the air filter is removed for a quick test?

I think the carbs will be coming off again tomorrow or the next day for an inspection and see if I can find anything I may have missed.  Is it possible that the needle jet should be set at #3 because if the rich condition, or a smaller slow jet is the culprit.  Maybe there is something stuck that I missed in the emulsion tubes.  I love this.

Thanks again, especially you TT for taking the time to help.  Your wealth of knowledge boggles the mind.

Your welcome.
Do the air filter test before you get inside the carbs again.
Were it me, I'd prove all the air jet passageways were clear and fully open.
I'd also check the main, pilot, and needle jet orifices were unmodified from markings.
Next I would check the emulsion tubes (main and pilot), count the number of holes and measure hole sizes, too.

I've made some measurements to other carbs that can be compared.

Check the mains towers for corrosion that might interfere with oring seal.

Re-Check the float height.

[brett_bike]

Since last post I have pulled the air filter and tried 1 turn out from full close and 2 turns out from full close.  It ran the same as it did whether or not the air filter was in place.  However there was a difference between 1 and 2  turn of the air screw.  There was not as much stuttering when the air screw was at 2 turns out at this time.

Can I reach all the point you recommend I check with the carbs on?  Can I go in from the float bowls for all of these?  I'm worried I will damage the rubber boots between the air plenum and the carbs.  Then I have an entirely new problem.

I will recheck the float height.

The mains towers do have a little corrosion, but the jets still press and stay in.

[TwoTired]

Can I reach all the point you recommend I check with the carbs on? 

Technically, yes.  But, it won't be at all easy.  Depends on how determined, patient you are.
With the quirkyness your carbs seem to behaving, I would think close scrutiny is warranted.  They aren't responding well to the normal treatments.

Can I go in from the float bowls for all of these?  I'm worried I will damage the rubber boots between the air plenum and the carbs.  Then I have an entirely new problem.

I understand. 
There are techniques to minimize risk to the boots.  Have you read past posts that describe the carb removal insertion procedure?

The mains towers do have a little corrosion, but the jets still press and stay in.

The orings are not a retention device, they are sealing device.  Sealing is what matters.  Can you be confident they are sealing?

[brett_bike]

Well, I finally had a minute to pull the carbs off and go through them again.  I have included a pic of the guts of the 627b carbs just to show people who might be intimidated how basic it is.

The other photo shows what I missed in my prior "cleaning" of the carbs.  Could the fact that the holes in this emulsion tube were clogged lead to her running rich?  I assume this could be one of reasons she has been having the  stumbling issues. I went through the rest of the carbs and I will be installing them shortly.

[brett_bike]

I almost forgot about the other issue I came across.  I found this damage to the engine side of the "sliders", I must not have ever looked closely enough at them.  How much of a problem is this?

It looks as though the PO did a little "work" of his own to the bike.

[TwoTired]

Hey, wait a minute.  You told us the emuslion tubes were clear!!

Emulsion tubes were proven clean/clear when I cleaned the carbs... 

The emulsion tubes mix air with the fuel.  Block some of the holes and you can expect a richer mixture.

I don't like the looks of those slides with that damage.  Kinda the worst place to damage the slides.  But, I can't say for sure it will be a problem.  It won't be a problem above 1/2 throttle.  But since the carbs are vacuum synced at idle when the slides are barely open, changing the slide opening profile can make things weird (in technical terms) when the slides are opened a little more. (Some of the cylinders could go out of sync at different throttle openings, which may or may not be noticeable.)

If I didn't want to risk any more "weirdness" with the carbs on the bike, I'd replace them.

Be sure to send the PO a thank you note.

[brett_bike]

Hey, wait a minute.  You told us the emuslion tubes were clear!!

That was the only one that looked like that, I must have mixed it up when they were off the rack and did a different carb twice.  I never said I was a smart man.

At this point, do you think I should keep the stock setting for the needles (at #4)?  Considering that all the plugs were showing signs of running rich, I thought maybe I would set the needle to the #3 position to lean things up a bit.  Also, I never have to use the choke when I start the bike.

[TwoTired]

Where do you set the throttle when starting?

Did you check the float/fuel height yet?

[brett_bike]

Throttle is at about 1/8 - 1/4 twist.  Float level has been checked.

[TwoTired]

Throttle is at about 1/8 - 1/4 twist.  Float level has been checked.

And set to what?
Are you certain the floats aren't upside down?

Have you actually checked the fuel level in the bowls?

A too high fuel level can make the entire range too rich.

Think about when you drink from a straw.  The higher the liquid level in the container, the easier the easier fluid gets to your mouth.
The fuel jet pathway/circuit is just like that straw.  If the fuel level is high, it is much easier to get fuel into the carb bores.

Anyway, lowering the slide needles will make the fuel flow when the needle come into play leaner, and that would be form about 1/4 to 3/4 throttle position.
If you want to try that change, it's your call.  But, it bothers me that you don't have to use choke to start the bike, which, to me seems like your carb are running rich at all throttle positions.  Lowering the fuel level in the carbs would lean the entire operating range.

The fuel level is something you can test on the bench, btw.  You just have to make test plug at the drain hole and run a clear tube up the side.

[brett_bike]

Yep, float levels are set to 22mm best I can tell.  Maybe one'ish mm less.  I don't have the proper tool to check, I would just use a metric ruler.  Would that be enough to make it run too rich?

I made the changes I mentioned and the bike still suffers from the same stumbling issues.  However, it seems to pull better through the gears.  I think I will again pull the carbs, verify the float levels and see what happens.  I think I should order new air intake boots from DSS, I'm sure with all this removing of the carbs I will tear the boots sooner or later.

[HondaMan]

Order new O-rings for the intake casting tubes, while you're at it. 
30.8x3.2mm from Honda.

[brett_bike]

So, my next step will be to visit a local motorcycle salvage yard here in Denver and pick up a rack of used carbs and pull the sliders.  I'll do that before I go any farther, and the carb rack will be helpful to keep around for spare parts.  I think at the very least it will be a step in the right direction, if not remedy the issue.

[brett_bike]

Well, I visited my local salvage yard here in Denver and picked up a used 627b (same as mine) carb rack.  I pulled the sliders, installed them on my bike, did the carb sync, and guess what happened.

Anybody...

Nothing.  Still the same stumbling off idle and when I'm at cruising speed.  I'm either gonna have to pull the entire engine/bike apart and touch every nut, bolt and O-ring, live with it like it is, or burn this MF'er to the ground.

If I had any hair left I would be pulling it out.

[Spanner 1]

Page #1, post #6....anybody ?

[w1sa]

Page #1, post #6....anybody ?

+1.   Especially at this stage, will be time well spent.

[brett_bike]

Spanner, I completely forgot about that.  You're right, it is an easy job.  Since your last post I've been to the garage and did what you suggested, took me ten minutes.  I cracked the garage door and tried to give it a quick start, but she's feeling a little finicky I guess.  She wouldn't start, and she back fired on me.  WTF?  Thanks again, I will look into it more and post the results tomorrow.

[brett_bike]

Back firing was due to the asymmetrical ring(?) on the mechanical timing advance being but back together in the wrong direction.  My mistake, I put it back together the opposite of the way it came apart so things were not firing at the correct time.  Further update to come after I take her out for a ride.

[brett_bike]

So here's what's going on...

Seems to start off from a stop smoother, but she still stumbles a little at cruising speeds.  At this point I'm completely stumped, I've done everything involved in a tune up and more.  I guess now I will try lowering the fuel level in the carbs to work on the "rich " issue, then try replacing the O rings on the intake per Hondaman's suggestion.  I might as well have installed pods with all this work I've done to the carb's.

Also, she will still start without using the choke.

Anyone else have any ideas?

[TwoTired]

So, you are saying that the symptoms have not changed from your very first post?

Starts without choke when cold?

[brett_bike]

Mostly.  It is a little better at start from stop, but still stumbles at cruising speed.

And yes, she starts without choke when cold.  She will start with choke when cold also but, it stalls almost as quickly as it starts.

I think I'm going out for an evening ride, maybe I will take the air filter out and see how she responds.

[TwoTired]

And the spark plug deposits?