Topic: Bike stumbles at idle to 1/4 throttle

[brett_bike]

'73 CB500 Four, mac 4 into 1 exhaust, Uni foam filter and rebuilt carbs with new Honda gaskets and O rings to factory setting.  New points, coils, plug wires and boots.  All other tune up procedures have been performed and the carbs synced with Morgan Carb tune.

The problem is, when I take off from a stop until I reach about 1/4 throttle (or aprox. 3000 rpm's), she stumbles and blubbers.  After that, she pulls strong until I get to cruising speed, then she stumbles a bit more.  As long as I'm accelerating, there is no stumbling.

I have done a plug chop and the plugs do look a bit fouled.

Please tell me if this is the correct thing to do.

I think I need to install a smaller idle jet because it is showing signs of running rich.  One jet is #40 and another jet is #100.  I'm not sure which one is the slow (idle) jet.

[fmctm1sw]

I have the same kind iof issues.  The 40 would be your slow jet.  I'm working a foam filter mod among other things to work on mine...

[brett_bike]

I did try removing the stock air box and filter, then replacing it with an oval K & N filter and mounting bracket that I got from the PO.  It seem to run a little less rich, but still rich.  I think I am getting closer.  I've considered installing pods to see if that allows even more air flow to reduce the fouling, but I'm afraid that it might cause issues at highway speeds.  Also, I would prefer to keep the stock air box and Uni filter because it seems to produce more of a vacuum(?) effect.

Example: When I run it with no air filter and give it gas, it seems to just get louder without the pull I usually get with the air box and filter attached.

Is it possible that the Mac exhaust has something to do with it?  I assume it has more flow than a stock 4 into 4.  is the lack of back pressure from this exhaust an issue?

[TwoTired]

What setting are you using for the pilot screws?

[brett_bike]

The screw on the air plenum side of the carb?  2 1/8 turns out seemed to work the best.

[Spanner 1]

Have you ever checked the operation of your mechanical timing advance ? Now would be a good time to undo it and lube/check for free operation....just a hunch.

[brett_bike]

I have not.  I wouldn't even know where to start.  Time to dust off my shop manual.

[Spanner 1]

Easy enough to do.....entails removing the points plate, dont need to un-do anything on the plate, points etc, 3 screws and then the 10mm nut on the end of the points cam. First mark the points plate position with a Sharpie so it holds the timing when put back together, then remove the 10mm and slide the cam/advance mech off . Lube the cam to shaft surface , weight pivots, springs and put back together with the 'tab' located in it's cut-out. check for free op. of the 'fly weights' while off the bike...put back and yer done ! A sticky advance will mess up your bikes running for sure.....

[brett_bike]

Sounds easy enough, I'll try that and whatever other hints I might get.  Thanks Spanner1

[HondaMan]

The screw on the air plenum side of the carb?  2 1/8 turns out seemed to work the best.

The air screws should never be more than 2 turns out. Turning them in leans it out: try 1.5 turns to clean up the plugs.

When an engine transitions from one A/F mixture to another, there is an interruption in the power. If it transitions from lean to rich, there is a burst of power: if it transitions from rich to lean, there is a stumble, like a partial miss. This is the same for all engines.

so, the general idea is to try to even out the mixture from one range to the next, to make the power flow steady across the RPM range. The overall mixture is set by the jet sizes, and that is determined by the volumeric efficiencies involved (at least, on these pre-EPA-restricted engines, that is...).

So, if your plugs are dark, lean it out first, until they clean up to a light gray or tan. Only then should other changes be introduced, and then in the form of overall balance via jet changes.

The #40 pilot is correct for the CB500: with the stock air screw range of 1.5 to 2.0 turns, this provides mixtures from approximately 14.0:1 (1.5 turns) to 11.0:1 (2 turns). Normal idle mixture on these bikes is in the range of 12.5:1 to 13.0:1, so the #40 pilot is fully capable of providing that range.

Installing a K&N filter on the 500 usually calls for adjusting the air screw in about 1/8 turn from its previous setting, as the increased airflow will cause more lift over the idle jet (and thus more fuel at a given slide opening). In my experience, installing the foam air filters goes the other way, requiring about 1/8 to 1/6 turn out on the air screws.

My 'gut' feeling at this point: check the O-rings in the carb throat castings where they bolt to the head: they may be very hard, and leaking. Contrary to popular belief, vacuum leaks at this location will make low-speed circuits run rich on pulse-type carbs. This O-ring, BTW, is the same one as is used on the little valve caps, 30.8x3.5mm (I have a whole BAG of them, since I've been fixing 550s with this problem all summer!). 

[TwoTired]

The screw on the air plenum side of the carb?  2 1/8 turns out seemed to work the best.

That's part of the stumble problem.  It's at least, one turn out too far, making the idle too lean.  Since there is no accelerator pump, the carbs have to run rich at idle for sudden throttle opening not to starve.  It should accept up to 1/2 throttle travel changes under load and still accelerate briskly.
Don't ever expect the engine to pick up under load from snapping the throttle open from idle.  The sudden loss of a carb throat vacuum all but stops the flow from the fuel jets.

[Deltarider]

Turning them in leans it out

Hate to contradict you. Standard setting for a 500 is 1 turn out ⁺/_- ¹/₈ of a turn. ⁷/₈ turn out is richest, 1¹/8 out is leanest within Honda specs.
I know in FAQ http://www.motorcycleproject.com/motorcycle/text/specs.html
mentions a 1¹/₂- 2 turn out, but since I've not found a source where this derives from, I believe this information is corrupt. The other http://forums.sohc4.net/index.php?action=dlattach;topic=5410.0;attach=3002 is incomplete as it doesn't mention the 649A Keihin.
Only place that I know where Honda contradicts Honda is on page 9 of the Honda Shop Manual 500-550 where in the chapter Maintenance Operations under 2.Carburetor Adjustment a standard adjustment of 1 ⁺/_- ³/₈, which is odd.
By the way, it's high time we all try to clarify the mist around the 500's air screw. Mine has a hollow tip and has a crosshole drilled in. I've learned from a mechanic and by experience that it's position also affects mixture at higher revs, although I can't explain this...
Maybe there are models around with airscrews that have a solid tip that, maybe, require different adjustment, but I have no data that suggest this. In all parts lists (500, 500K1 and 500K2) the airscrew has exactly the same partsnumber, so no modification.
At idle my 500 stinks an awful lot. Leaning out brings a beautifull idle (it's really purring then) but at the cost of less driveability under acceleration. All who know more on this subject, please chime in.

[brett_bike]

The screw on the air plenum side of the carb?  2 1/8 turns out seemed to work the best.

That's part of the stumble problem.  It's at least, one turn out too far, making the idle too lean.  Since there is no accelerator pump, the carbs have to run rich at idle for sudden throttle opening not to starve.  It should accept up to 1/2 throttle travel changes under load and still accelerate briskly.
Don't ever expect the engine to pick up under load from snapping the throttle open from idle.  The sudden loss of a carb throat vacuum all but stops the flow from the fuel jets.

I have had it on that setting (1 turn out +/- 1/8), but I dies when I get off the freeway and come to a stop.

Also, if she already runs a little rich, would that not make her more rich?  

By the way, thanks to everyone who has taken the time to help me with this.  I am determined to figure this out.

[HondaMan]

Turning them in leans it out

Hate to contradict you. Standard setting for a 500 is 1 turn out ⁺/_- ¹/₈ of a turn. ⁷/₈ turn out is richest, 1¹/8 out is leanest within Honda specs.
I know in FAQ http://www.motorcycleproject.com/motorcycle/text/specs.html
mentions a 1¹/₂- 2 turn out, but since I've not found a source where this derives from, I believe this information is corrupt. The other http://forums.sohc4.net/index.php?action=dlattach;topic=5410.0;attach=3002 is incomplete as it doesn't mention the 649A Keihin.
Only place that I know where Honda contradicts Honda is on page 9 of the Honda Shop Manual 500-550 where in the chapter Maintenance Operations under 2.Carburetor Adjustment a standard adjustment of 1 ⁺/_- ³/₈, which is odd.
By the way, it's high time we all try to clarify the mist around the 500's air screw. Mine has a hollow tip and has a crosshole drilled in. I've learned from a mechanic and by experience that it's position also affects mixture at higher revs, although I can't explain this...
Maybe there are models around with airscrews that have a solid tip that, maybe, require different adjustment, but I have no data that suggest this. In all parts lists (500, 500K1 and 500K2) the airscrew has exactly the same partsnumber, so no modification.
At idle my 500 stinks an awful lot. Leaning out brings a beautifull idle (it's really purring then) but at the cost of less driveability under acceleration. All who know more on this subject, please chime in.

Well...not to contradict your mechanic, but...it's physically impossible for the idle circuit in these carbs to function past 30% of slide opening. The venturi simply moves upward and backward at that point, so the airflow pattern across the idle jet becomes laminar at that area, and the lift stops...that's how these carbs are designed, all of them, by Keihin. On none of them does the vacuum lift the fuel: that is too difficult to control at low speeds, and leads to poor low-throttle engine speed control. The only place I have ever seen the idle screw controlling fuel flow on motorcycle carbs is on 2-stroke engines, which are, of course, a whole different critter.

Regarding the air screws settings: the 1.5 to 2.0 turns is for the 022/022A/022B series of carbs, which are the only ones I've seen on a U.S. CB500, as far as I can remember. My '71 CB500 ran at 1.5 turns, and the last year of the CB500 (U.S.), with the offset gas cap on the tank, ran at 2.0 turns, stock settings ("B" series carbs). Could it be that Euro CB500 bikes are different? I know they were made far longer over there - in the U.S. they were only here from '71-'73, and were superceded with the 550.

If you have an air screw with a hollow tip, and not the kind that has a long, thin, tapered needle, the screw is regulating airflow to the idle jet's emulsion chamber. This is the small chamber where the tiny cross holes on the pilot jet break up the solid fuel from the bowl as it is pushed up the jet (lower pressure in idle venturi, plus higher still pressure in bowl below, pushes the fuel up the jet: it is not sucked up by vacuum, as popularly misunderstood). The small open chamber where the tip of the idle jet sits strains the incoming air in tiny bubble streams to aerate the fuel as it rises to the idle port in the carb. The lifting action on this fuel is controlled by the air flow that comes in via the air adjusting screw, so opening it further lifts more fuel, while closing it down lets less fuel be lifted. Closing it all the way cuts off the lift force, and makes it dead lean. In a nutshell, that's how these pulse-style carbs work...

If you have a needle screw, as is found in some older motorcycles and many cars, the needle regulates fuel flow and there is no idle fuel jet. There often is, instead, an AIR jet in those carbs, which are notorious for clogging by dirt. These carbs can go dead rich, because the fuel is usually fed by suction, which pulls both air and fuel into the throat. These are often found on lawn mowers, old Harleys (Linkert carbs) and some other machinery. They often have only one jet, and the fuel screw.

About the stinking idle and loss of driveability at more than 1.125 turns out: you might try enlarging the emulsifier holes in the 'mainjet holder', aka 'emulsifier tube'. These are usually too small for today's fuels. They were OK in the 1960s, but the oxygenated and slower-burn fuels the world uses today (for less emissions) need more air: opening these holes is something I do a lot around here. It smoothes out the transition from idle circuit to low midrange, quite a bit. The 500 was always subject to this "flat spot" because of the long length of the intake runners (they are almost 10 times the diameter of the carbs, for ram tuning performance), so they are also overly sensitive to the smallest of vacuum leaks at the head, where those O-rings live. Those O-rings typically leak after 3-4 years because of the heat and the gas hardening them: this O-ring is the same one as is used on the valve caps, so you have a bunch on hand that you can try out. 

[brett_bike]

My carb is a 627b. Would that have anything to do with the issues I'm having. Since the specs I have them set to are for the 022b (?) series.

I set the air screw to 1 1/8 turns out for the ride to work (15 miles) and it stalled when I got off the freeway and came to stop signs.

[TwoTired]

I set the air screw to 1 1/8 turns out for the ride to work (15 miles) and it stalled when I got off the freeway and came to stop signs.

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?


Separately, and for the debate about the air screws... 
The pilot circuit operates in parallel with the mains and throttle valve.  Whenever the exit port in the carb throat is below atmospheric pressure, it will draw some fuel from the pilot circuit.  This exit port is separate from the mains/throttle valve exit position.

The more restriction that is in the inlet tract such as air filter, ducting impedance or AIR HORN RESTRICTOR, the deeper the effects of pressure differential all along the intake tract and the more the pilot circuit will deliver at all throttle positions.

How it works.
The source of the pressure differential is the piston falling during the intake stroke while the intake valve is open.  This begins a negative pressure wave that travels toward the source of the equalizing pressure (the surrounding atmosphere).

If you stopped the piston at BDTC, and all the seals were perfect, and you blocked the inlet completely.  The negative pressure would equalize throughout the intake tract. (It would take some amount of time for all the molecules within the tract to spread out evenly.)
In this case, both the pilot delivery port and the throttle valve port would experience the exact same pressure differential, assuming the carbs air vents are NOT sealed.
When the engine is running, the intact tract is pulsing with air flow from inlet to intake valve, but pressures along the tract are always trying to equalize during and after each intake event.  The carb ports and venturi exist within this tunnel, with each end of the tract fighting for dominance the pressure within that tract.
The intake tunnel walls and obstructions impede the equalization flow of air within the tunnel, and effect how fast the equalization process occurs, as does the size of the inlet presented to the equalization source.  The inlet, in effect, becomes an orifice which then controls how fast the inlet tract can equalize with atmospheric.  Smaller orifices delay equalization allowing the entire tract to remain in a deeper offset from atmospheric, as long as a negative pressure exists at the other end of the intake tract tunnel, this includes the carb's fuel delivery exit ports, both the pilot and throttle valves.
The greater the pressure differential, the more fuel delivery will occur.  A restrictive air filter or an AIR HORN ahead of the filter, will increase all the differentials in the intake tract, making the pilot circuit deliver more throughout the operating range at whatever slide position, and is a major reason why the mains are restricted to #78 for the 649 carbs (which may also have some other differences to the rest of the world where 627b, 022A, 087a, and 069A carbs prevail without the air restrictor snorkel ahead of the air filter).

In summary, if you have what is pictured below on your bike, then you can expect to have more pilot circuit delivery at all throttle positions and engine speeds.

[Deltarider]

Well...not to contradict your mechanic, but...it's physically impossible for the idle circuit in these carbs to function past 30% of slide opening. The venturi simply moves upward and backward at that point... etc

Let's see if we can establish any differences between US and Euro carbs for the CB500. I've attached a picture in which I encircled the small orifices for the pilot jets. I believe you have the same. Since I have found no other partnumber in the Honda parts lists than 16016-323-004 for the airscrews - and I believe I've seen them all (parts lists) - I conclude there has been no modification. Therefore if ours have a hollow tip and cross hole I would think all have them.

If you maintain your view that by turning the air screws in the mixture will lean out, we have a problem, because ours definitely enrichen.

If I turn the air screws from let's say 1¹/₈ out to just 1 turn out and take my bike immediately to the Autobahn to maintain only speeds of say 130 km/h constantly until I have to refuel, the difference in fuelconsumption is already significant, indicating mixture at higher rpms has become richer too. I know this from experience. Most mechanics deny the influence of the air screw for higher rpms, all but one. He has had a CB 500 himself for years and all his advices have proved right (not that that means much). Just to make sure: our carbs have in the mouth (engine side) and just in front of the slide a tiny little hole that's fed by the pilot system. I take it your carbs have that too.

Regarding the air screws settings: the 1.5 to 2.0 turns is for the 022/022A/022B series of carbs.

It's just that I have not seen an official Honda source for this setting.

[Deltarider]

In summary, if you have what is pictured below on your bike, then you can expect to have more pilot circuit delivery at all throttle positions and engine speeds.

Aha! That could explain why the euro's have #78 main jets in stead of #100.

[fmctm1sw]

The lifting action on this fuel is controlled by the air flow that comes in via the air adjusting screw, so opening it further lifts more fuel, while closing it down lets less fuel be lifted. Closing it all the way cuts off the lift force, and makes it dead lean. In a nutshell, that's how these pulse-style carbs work...

   Man, I've had this wrong all along.  I always thought that turning the screws out would mean more air meaning a leaner mixture.  Those 500 carbs are the same type as my 350 I assume...

[Deltarider]

Man, I've had this wrong all along.  I always thought that turning the screws out would mean more air meaning a leaner mixture.  Those 500 carbs are the same type as my 350 I assume...

No, no, you were right.

[fmctm1sw]

Well, I'm back from my ride.  I put my air screws at 1/2 turn out and rode to a gas station.  In practice, for me anyway, my flat spot is much less pronounced when I turn the screws out.  I stopped at the gas station and went from that 1/2 turn to 1-1/2 turns.  Much better (but still not perfect) on the way back.  Hopefully that foam filter I'm working on will help it more.  It is also much colder here now (like 65 instead of 85) so that may affect it as well.  It makes me wonder if you could enlarge or even add holes to the slow jet like Hondaman has done to the main jet emulsifier tubes?  Still very hard to start (I noticed the battery cranks a lot slower than my '72) but runs like a bat out of hell anywhere other than that spot right above idle...

[brett_bike]

In summary, if you have what is pictured below on your bike, then you can expect to have more pilot circuit delivery at all throttle positions and engine speeds.

Below is what I have on my bike.  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.   I need to put on some highway miles tomorrow, maybe that will clean up the plugs a little.  Also, when it idles and I give it a flick of the throttle, it hesitates.  I know not to do that when riding because it has no accelerator pump like a 750, but I thought a little flick at idle would be OK.

[TwoTired]

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?

[Spanner 1]

15 mins. can check the advance mech....a good 15 mins spent on any bike with mech. timing IMO.and most folks have never looked at it.

[TwoTired]

If you have an air screw with a hollow tip, and not the kind that has a long, thin, tapered needle, the screw is regulating airflow to the idle jet's emulsion chamber. This is the small chamber where the tiny cross holes on the pilot jet break up the solid fuel from the bowl as it is pushed up the jet (lower pressure in idle venturi, plus higher still pressure in bowl below, pushes the fuel up the jet: it is not sucked up by vacuum, as popularly misunderstood). The small open chamber where the tip of the idle jet sits strains the incoming air in tiny bubble streams to aerate the fuel as it rises to the idle port in the carb. The lifting action on this fuel is controlled by the air flow that comes in via the air adjusting screw, so opening it further lifts more fuel, while closing it down lets less fuel be lifted. Closing it all the way cuts off the lift force, and makes it dead lean. In a nutshell, that's how these pulse-style carbs work...

I must respectfully disagree with the last part of your analysis/description.  The air screw does control the amount of air premixed with fuel in the pilot circuit.  As such, turning it out adds more air, making the mix leaner,  turning it in makes the pilot mix richer.  For this carb body casting, there are two different pilot screw designs from the factory.  Most are hollow tipped with the cross hole.  This does not allow an adjustment that completely starves all air being delivered to the idle jet emulsion tube.  The exception is the 069a set up that uses solid tipped air screw tips where you can block all air going to the pilot jet emulsion tube.  However, even with the air screw passage blocked, there still exists a passageway from the exit hole in the carb throat where low pressure exists (running engine assumed) to the pilot jet entry hole, which sees atmospheric pressure from the carb vents.  Flow exists whenever there is a pressure differential.  Therefore, you cannot shut off pilot jet fuel delivery with the air screw adjustment.  The air screw adjustment only determines how much air is available to be premixed with the fuel at the pilot jet emulsion tube.

Many years ago, on a CB550 with 022A carbs, I decided to peak the idle air screws with the use of an exhaust gas analyzer.  In the garage, I ran the engine and opened the air screws to get lower hydrocarbons on the scale reading.  The idle increased with each adjustment, reading each cylinder's exhaust pipe.  So, I kept lowering the main idle.  It took about a half hour before I could turn the screws out no more and get a lower hydrocarbon reading on the analyzer.  I believe I ended up with 5 or 6 turns out from seated (likely the end of effective adjustment range designed into the air screw circuit.)  The engine was purring smoothly, as this was the last thing I did for the routine 3000 mile tune up, full service.
I was excited about the test drive.  I didn't even get to the end of my short 35 ft driveway before I knew there was a problem.  With the bike under load, nearly any throttle opening movement caused the engine to wheeze and threaten to die, with a MAJOR stumble.  Acceleration was only possible if I gave it extremely meager throttle opening change, like 1/16 travel.  Knowing it didn't have this behavior before, and that the stock air screw setting was about 1 1/2 turns out from seated (From Honda Shop manual Supplements chapter 16), I then brought a screwdriver along and began turning the air screws in about 1/4 turn at a time and test driving the bike.  With each adjustment, the throttle response behavior improved with less stumble/wheeze when adding throttle under load.  I kept doing these air screw adjustments until the engine would accept up to one half total throttle travel snap changes under load in any gear with the engine responding reliably, predictably, and briskly.  The bike was once again joyously rideable.
Curious, I checked each pilot air screw for final setting, to write down in my manual.  But, I didn't write down anything because the screws were all finally set to 1 and 1/2 turns, which was already printed in the shop manual!
Yes, the bike idled with a higher Hydrocarbon reading on the Exhaust gas analyzer.  But, because there is no accelerator pump the idle must be on the rich side for engine pickup.  Anyone who has vacuum synched the carbs knows that when you open the slides, the intake tract (and carb throat) pressure rises dramatically toward atmospheric pressure.  The lower the differential pressure across any fuel delivery circuit in the carbs, the less fuel flows in that circuit.  Opening the slides suddenly, allows a great deal of oxygen to enter the cylinders, but vacuum loss stops, or severely reduces, fuel delivery.  This air fuel mixture change won't support combustion and the engine stumbles/wheezes in response, until the air velocity increases enough to create an pressure drop due to venturi action (which restores pressure differential across the fuel delivery orifices.  So, for the engine to pick up you need to start with an over rich mixture that still supports combustion.  Or, the slide opening action must have some kind of fuel enrichment (like an accelerator pump).

Well, that's how I believe it all works.  Readers will have to decide on their own which is right, I suppose.

Hope this helps.