Author Topic: CB550F - Stumbling at 1/8 throttle  (Read 341 times)

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Offline Winters

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CB550F - Stumbling at 1/8 throttle
« on: February 10, 2019, 05:43:40 pm »
EDIT: title has been adjusted accordingly; read newest post. You can skip this post. Thanks


(OLD) Hi friends,

I have searched and searched but have not seem to come up with an answer for this, to put some info in place:

3k maintenance was done (however air cleaner element is not brand new); cleaned and sync'd carbs (with floats set to 22mm, when using clear tube method, 1-4 shows 1mm below gasket surface and carbs 2-3 show at the surface - but I don't think that should be an issue? I've seen 1-3mm below gasket surface suggested?) idles perfect at 1k. Runs up to redline smoothly. When throttle cracked 1/16 (or a slight hair lower than 1/16) and more the issue goes away. IMS set to 1.5 but just moved it to 1 5/8 and it seemed to be smoother there. All pipes within the same temperature. I can start it in the cold (today 20 degrees F) without choke (with a hair of throttle to sustain rpm after it's on); however it with choke I don't need to hold the throttle as long as it has been running for at least 30 seconds (I was told these bikes idle at 8:1 but originally I thought that if it can turn on without choke cold it was too rich?). Advance seems to be operating properly and full by ~2500 (give or take 100-200rpm). Carbs are 069A, running Keihins, 38 pilot, 98 main, needle at 2nd clip. Stock CB550F except a yoshimura replica exhaust with baffle (not sure of brand, could be motogp). Compression was 155-160 across the board as I last recall (did that about three months ago).

To reiterate my issue:

When I'm riding, coming off idle, as it reaches around 2k-3k (throttle probably 1/32 to slowly increase revs, e.g. neighborhood riding) it will stumble (I'm guessing this is the 8-stroking you call it?) when the throttle is cracked a hair more it smoothens out. When riding greater than ~1/16 throttle it will never see that hesitation (not really noticeable to me, but with the pipes I can hear the stuttering, feels pretty smooth of a ride though, pardon me as I'm new to these). No hesitation coming off idle to ~1700rpm at the same throttle positions described above. Also with the throttle closed in any gear and revving down, there are no pops, and it downrevs smoothly through all the rpm.

Before I tell myself this is the nature of the beast (I'm new to carbs too, and from what I hear, without an accelerator pump a change in fuel circuit will be noticed due to inherent design? higher rpm demands more air/fuel and that little of throttle won't suffice?). I'm wondering if anyone has any ideas for me.

My apologies if the details were either too far or too little for some of you reading this.

Thanks.
« Last Edit: March 31, 2019, 09:55:16 am by Winters »

Offline toma

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Sounds like some crud in your low speed jets. They have tiny holes in them. Take them out and put them in muriatic acid for a few minutes. If that doesn't fix it and you have modified the engine some way to reduce back pressure you may have to go bigger.
Also sometimes what appears to be a jetting problem is a timing problem . Good luck
Acid  = caution!

Offline Winters

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Edit:

(OLD) Hi thanks for the recommendation! But to go on that,

When I took my carbs apart I checked the orifices, used solvents and compressed air on all the parts, and checked them by shining a light through them, also ran an E string through them for good measure. Bowls themselves were surprisingly clean. Draining them the gasoline had no sediment in it and the bottoms of the bowls were the same. Would not a bad slow jet show symptoms from closed up to technically 1/4 (probably more felt up to 1/8 only) throttle regardless the rpm range?

For example if I'm riding above that ~2-3k range and I decrease throttle to that position it's fine but when I'm in that range I get that stuttering sound (remember below the ~2k range its also smooth).

Also, regarding the pipes, decently long, end past the rear pegs, heard the factory 4-1 is more restrictive than the 4-4's used on the K's so I took a gamble that this exhaust would be ok (read mac 4-1's also didn't need rejetting); haven't done a plug chop but any 1/16 throttle to WOT feels great. She screams and sounds smooth. Also idles smooth.
« Last Edit: March 31, 2019, 09:55:36 am by Winters »

Offline TwoTired

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Clean or put in brand new spark plugs.  Ride the bike and put it in your stuttering mode for as long as you can.
Hit the kill switch and pull in the clutch, coast to stop and remove all the spark plugs to examine the deposit patterns.

This will tell you if you have a rich or lean condition for the stumble.  Really ought to have a brand new air filter for the test, too, as well as performing a fresh 3k mile tune up.  Not smart tuning carbs to an ill tuned engine, imo.

However, I suspect that the exhaust change now requires a different slide needle profile.  One that more closely resembles the K model profile.  Find some 022A, carbs and rob the slide needles from them for use in your 069a.  (And scratch off the set up numbers, cause they won't be 069a anymore.)

I'm also surprised you are still using 98 mains.  You will probably be happier with 100s...which are also found in the 022a carbs.  A WOT plug chop should confirm that suspicion.

I don't think any aftermarket parts were made specifically for the F model, as there is a much greater population of K models in existence, than F.  Would you target parts you make for a larger or smaller marketplace?

Cheers,

Lloyd... (SOHC4 #11 Original Mail List)
72 500, 74 550, 75 550K, 75 550F, 76 550F, 77 550F X2, 78 550K, 77 750F X2, 78 750F, 79CX500, 85 700SC, GL1100

Those that learn from history are doomed to repeat it by those that don't learn from history.

Offline Winters

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I was thinking 100 mains would be better for the exhaust; however I was planning to do that after I sorted this range out. (Plug chop at WOT to see where I'm at later)

I'll see what I can sort out if the weather is OK this weekend. 3K tune up was done about 200 feet ago lol, about two weeks ago. Set everything to factory spec just for starting measure.

Thanks for the information. Would have never guessed to go with a different taper/needle dia; curious as to why/how it would alter/help this case? I'm sure you're right but I would've felt like adjusting an F model for another 4-1 would require less than a K model for a 4-1.

Honestly glad to hear from you, did not know these forums were that active. I've seen plenty of posts from between you and Hondaman.


Offline TwoTired

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Be aware; 4 into 1 is a style, not a specification.  Each exhaust design has a flow and back pressure profile over the RPM range.  A true, race designed, 4 into 1 will be tuned to boost scavenge effect in an upper RPM range.  At lower RPMs, it will actually foul the flow of gasses due to neighboring cylinder firings entering the common pipe and reflecting back to other exhaust ports.  It's all about timing and frequency.  Racers only care about that sweet RPM spot for racing.   Street bike drivers care about streetability.  Many owners care about emulating the style of race bikes.  Honda tried to placate them with the F model, and still made a streetable bike.

Honda well tuned the carbs for the exhaust delivered with the bike.  It's really up to aftermarket manufacturers to either match the engineering specs of the original, or provide tuning data for the carbs to have them deliver air/fuel to the new exhaust requirements.
I think most mfgs. just make an exhaust that functions for the widest number of vehicles they fit.  Or, they just punt the engineering off to the style buyers to figure out.  It just doesn't matter to them that some may not have the skills to get the bike working as well as Honda parts do, once they have your money.

That's why I say that, if they did the engineering at all, they made them work with the highest popularity of bike model in existence, and that was the K model.

The stock F pipe was a high pressure pipe, for evening the back pressure spikes from neighboring cylinders at lower RPMs, quietness, and emissions reduction.  This allows leaner tuning of the carbs, pretty much throughout the entire operating range.   Aftermarket manufacturers sell to buyers who want a louder pipe and the "race" look, where desired HP gains can occur when you rev and flog the hell out of the motor.  They aren't much looking for all RPM street performance like Honda did.

Anyway, the jetting setup for the K model is far more likely to work better for an aftermarket 4 into 1.  They had the same engine, and the 4 into 1 exhaust is freer flowing with less back pressure than the Honda 4 into 1 system.  Hence the change in carb tuning parameters.

IMO, the main jet is set for the maximum demands of the motor.  As it has a bleed effect for other throttle positions, it's "leakage" effects tuning the needle and pilot jet.  So, I set that first.  Then move on to the slide needle selection, and pilot jet air bleed, for throttle response off idle (a 38 pilot jet pretty much works for all the 550s, PD carbs excepted).   But, the pilot system does provide some bleed for other throttle settings, too.  So, there may be some juggling in order to get the same ideal tuning that Honda originally provided for operation anywhere in the world.

It is all far more easy to reach end goal with an exhaust sniffing dyno that will provide you a fuel map for whatever assemblage of parts you wish to test and tune.  Then you can address the specific carb section/RPM based on test results.  Of course, stock parts don't need this extra attention.  Just build it the way Honda did, and go ride. ;D

Otherwise, a test and try can eventually get results, with a variable time in test and try mode.  A test track and plug chops is a 2nd best alternative.

Cheers,
Lloyd... (SOHC4 #11 Original Mail List)
72 500, 74 550, 75 550K, 75 550F, 76 550F, 77 550F X2, 78 550K, 77 750F X2, 78 750F, 79CX500, 85 700SC, GL1100

Those that learn from history are doomed to repeat it by those that don't learn from history.

Offline Winters

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Whew, loads of great information! I see everything you mean. I just originally looked at it as a diameter/collector style basis without taking in aspect the minor details of the revrange/scavenging affect/turbulence et cetera. The PO did provide me with a slightly dented factory 4-1 (muffler date stamped replacement item in the early 2000s) though, which is why in the meanwhile I'm probably going to look to properly tune for it. I did read somewhere that dents don't really affect much flow in pipes though, that is if it is not pinching the tubes shut.
« Last Edit: February 12, 2019, 07:20:39 pm by Winters »

Offline Winters

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Re: CB550F - Stumbling at 1/8 throttle
« Reply #7 on: March 30, 2019, 05:11:36 pm »
Hello friends, going back on this post to update - new air filter. Fuel level 2mm below gasket surface for all bowls except #2 is 3mm. New 38 pilots on all of them as well, though the originals looked good.

When I was doing a carb sync it would still have that stumble and the vacuum was dead even across the board. At one point I left the fuel valve off and as the bowls seemingly started to get lower on fuel I'd assume - it stopped having that stumble at 1/8 throttle.

Plugs show ok color, sooty on #4. Plugs 1-2 show a dark brown and 3 shows less dark of brown; however when leaving the fuel valve off it'll clear up at 1/8 after some time.
I then turned all IMS to 2 turns out with cyl4 at 2.5 turns out and turned the valve back on and it had the stumble again at 1/8; however, when leaving the fuel valve off it'll clear up again after some time.

Do you think I should set fuel level to 4mm below the surface? or should I try 35 pilots? Or what else? I thought 2-3mm was the norm from reading what I could find on the forums and elsewhere on the internet. I usually hear people going up in size because of more airflow with exhaust/intake setups but my plugs are already pretty dark with 38's at 1.5

Specs is factory everything except motogpwerks 4-1 exhaust and emgo air filter. I threw some keyster 110 main jets in there (the PO had them laying in a bag... however plug chops show ok results) and clipped the needles down to 3rd notch (though I haven't had any ridability issues past 1/8 throttle prior anyways, but I didn't ride it enough to where 98 jets would run it too lean for too long so I probably didn't notice the mismatch with much less anything under 105 jets for this exhaust)

Oh also feeling the exhaust from idle to 1/8 to past 1/8 goes from hot to warm to hot; looking like as you have said the slides opening and induction changes requires different demands. Great knowledge! Carbs are definitely interesting.
« Last Edit: March 30, 2019, 05:16:44 pm by Winters »

Offline BRG-BIRD

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Re: CB550F - Stumbling at 1/8 throttle
« Reply #8 on: March 30, 2019, 07:21:45 pm »
Have you checked the emulsion tubes for wear? Mine were worn out and the bike would stumble off idle. It may not be your issue but it wouldn’t hurt to eliminate them as an issue.
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Offline BomberMann650

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Re: CB550F - Stumbling at 1/8 throttle
« Reply #9 on: March 30, 2019, 10:56:39 pm »
Why so much tuning for initial throttle opening?

Understanding that smooth running motors are nice and all.  But plug chops at 1/32nd throttle?  Seems obsessive in a bad way.

Give the bike a chance to be a bike.  Twist the #$%*in throttle!
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Offline TwoTired

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Re: CB550F - Stumbling at 1/8 throttle
« Reply #10 on: March 31, 2019, 08:45:12 am »
Without knowing the differences in carb throat pressure caused by an EMGO filter, or the back pressure profile of the changed exhaust.  I would start the carb adjust odyssey, with 100 mains, and slide needles from a 022a, or 087a, or 627b carb, clip it in the fourth position from the top, and use 38 pilot jets.  The pilot screw setting would remain in stock setting, until I had verified the main jet by doing a full power plug chop and reading the deposits for proper color.  Then I would do full load plug chops and read the plugs for correct deposits. Adjusting the needle position according demands.  Then check the needle taper by doing 3/4 throttle position plug chop, again looking for correct plug deposit color, and then 1/4 to 1/3 throttle position full load tests.

Finally, the air screws are set to minimum open position that allows, rolling forward slowly in top gear, and snapping the throttle to 1/2 position and having the bike accelerate smoothly without hesitation.  Won't be fast, but reliable acceleration.   If the screws are too far out, the bike will wheeze and fall flat, like you snapped the throttle full open.  If it reliably accelerates with more than 1/2 throttle,then the screws need turning inward.   There is no valid reason for any of the air screws to be in different positions at the end of tuning, unless there is something different about cylinder mechanicals, or carb dissimilar issues, which should be corrected before any fine tuning is performed.

Also note that carb tuning is last on the tune-up schedule checklist.  Compression, valves, and ignition timing should be spot on on BEFORE any carb fine tuning.  And the spark plug must be either repeatedly replace or kept immaculately cleaned before each plug color test is performed.

It is all a lot faster and easier with a dyno exhaust gas readout.  But, a convenient test track of some sort will suffice that allows full power, full load operation of the bike.

Others may do it a different way.

Cheers,
Lloyd... (SOHC4 #11 Original Mail List)
72 500, 74 550, 75 550K, 75 550F, 76 550F, 77 550F X2, 78 550K, 77 750F X2, 78 750F, 79CX500, 85 700SC, GL1100

Those that learn from history are doomed to repeat it by those that don't learn from history.

Offline Winters

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Re: CB550F - Stumbling at 1/8 throttle
« Reply #11 on: March 31, 2019, 09:54:49 am »
Hi BRG-BIRD. The emulsion tubes are clear of deposits. There was a little powder in the body/housing but I cleaned that as well.

BomberMann650 - I'm new to these bikes/bikes/carbs in general I apologize; Learning about them more and more about reading it allows me to correctly describe symptoms! It's more of a neighborhood riding is at 1/8 to coast around 25 and it makes all sorts of 8-cycling there so I'm trying to cure it! It idles well and it revs well but I guess it is that initial opening of the carb throat that is operating too rich.

TwoTired - I'll go back into it again, thanks.
How much throttle; or leak range, would you say the needles/mainjet supplies at 1/8? Everything else is spot on - 155 psi across the board of compression, valves set to 0.002/.003 respectively, and ignition timing reads dead on from F to full advance at 2500rpm. I pulled plugs before every chop and cleaned it to where the ceramic was whiteish so I could read it - no use of steel wire wheel here to prevent ends getting on the plugs. When I threw the new pilots on I went again and blasted air through the pilot orifaces. Earlier I was finding some binding on the linkages but I fixed that and did the idle plug chop with a new sync - I thought that would have been my issue for showing cylinder mismatches.

I can't get a dyno but I could get a wideband - what AFRs would I be chasing?

Offline TwoTired

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Re: CB550F - Stumbling at 1/8 throttle
« Reply #12 on: March 31, 2019, 12:00:50 pm »
Eight -cycling, to me infers burbling or mixture too rich.  But, I don't know how much your 4 to 1 is reacting to back flow at low RPM.

According to a Honda carb technical series, the transition from pilot jet to jet needles is dominated by the slide cutaway design. FYI

But, your carbs aren't in stock configuration.

In top gear at low speed, how much throttle can you give it before the engine falls flat?  When you back off throttle, does it resume cleanly or burble back in to clean running?

Are you saying that you are happy with main jet plug deposits, as well as mid range?

Cheers,
Lloyd... (SOHC4 #11 Original Mail List)
72 500, 74 550, 75 550K, 75 550F, 76 550F, 77 550F X2, 78 550K, 77 750F X2, 78 750F, 79CX500, 85 700SC, GL1100

Those that learn from history are doomed to repeat it by those that don't learn from history.

Offline jimbojangles

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Re: CB550F - Stumbling at 1/8 throttle
« Reply #13 on: April 02, 2019, 08:41:50 am »
I'm not sure about the CB550F, but I had the same issue with my 750. I don't think your being obsessive. This drove me nuts, because you ride at part throttle more than you realize, until a problem develops there. I experienced this most when trying to ride a constant 25-30 mph through a neighborhood or in a parking lot. The bike just burbled and stuttered. Any other throttle position was perfect.

On my 750 this was completely cured by replacing the needle jets. Not the needles or idle or primary jets. Just the small brass part that the needle slides up and down into. On the 750 this was held in the carb by the emulsion tubes and could be pressed out from the other side with a chopstick. I'm not familiar with the 550 carbs, but this might work for you.   
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