testy

Author Topic: CB550F-Slight stumbling around 2-3k rpm in low gear when throttle barely cracked  (Read 161 times)

0 Members and 1 Guest are viewing this topic.

Offline Winters

  • Full Member
  • *
  • Posts: 4
  • 1976 CB550F
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: February 11, 2019, 10:19:10 pm by Winters »

Offline toma

  • Full Member
  • *
  • Posts: 16
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

  • Full Member
  • *
  • Posts: 4
  • 1976 CB550F
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.

Offline TwoTired

  • Really Old Timer ...
  • *******
  • Posts: 20,716
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

  • Full Member
  • *
  • Posts: 4
  • 1976 CB550F
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

  • Really Old Timer ...
  • *******
  • Posts: 20,716
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

  • Full Member
  • *
  • Posts: 4
  • 1976 CB550F
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 »

 

;
Honda