Topic: CB550f hesitation@3-4k rpm when warm

[thebeekeeper]

Good evening!

About a month ago I purchased a '75 CB550F. Since then, I have done some work on it in effort to get it running as it should be (for a 35 year old machine). This is my first bike, so I am learning a lot as I go and am hungry to know more about my wonderful bike!

That being said.... I have this annoying hesitation around 3000-4000 RPMs only when the engine is sufficiently warm. If ridden with slight choke, the bike accelerates extremely smoothly and gives me the impression that it also has a bit more power cold than when the engine is warm. Once I am past this troublesome RPM band, the engine has fantastic acceleration throughout! In neutral, the bike will accelerate through this trouble spot without hesitation.

I have gone through the carbs about 2-3 times to become acquainted with them. I am running the Keyster replacement kit for the 550k. I synced the carbs at about 1300 rpms and was unable to get the bike to idle smoothly at anything below this (the bike "sounds" happy @1300).  I checked the points, and they seem good. I adjusted the tappets and there is no longer much of a "ticking" noise. After I adjusted the tappets, I synced the carbs again.

I have quite a bit of backfire when the throttle is closed and the bike is slowing in gear at a higher RPM. The exhaust is the stock 4-1 and has the expected multiple holes (maybe 5-7) due to rust located all over the muffler. I replaced the old air filter (looked like it could have been 35 years old ) with the foam UNI filter and oiled it using their oil.

I have heard rumors of a "flat spot" in the carbs somewhere shortly after idle, is it possible that the UNI filter is giving me slightly too much air which only becomes a problem when the carbs are transitioning the fuel delivery between jets? Could the hole infested exhaust cause me issues with back pressure that could give me these (I am assuming) lean conditions?

These are my only two guesses and I would greatly appreciate any words of advice anyone is willing to extend to me.

Thank you, kindly, for your time!

Best,
Joshua

[JZEROE]

That's the flat spot. Learn to love it--either that, or learn to love head work.

[Gordon]

No head work needed.  Look in the FAQ in "The thoughts of Hondaman".  He goes over a few fairly easy things you can do to get rid of the flat spot, and they work. 

[chadbenson]

yeah I'm a new '76 550 owner myself and I've experienced and read up on the "flat spot". It's definitely a design thing and unless like everyone else has said about doing head work then you will have to live with it.

[Gordon]

It's definitely a design thing and unless like everyone else has said about doing head work then you will have to live with it.

Did you read my post?

[TwoTired]

About a month ago I purchased a '75 CB550F. ....
That being said.... I have this annoying hesitation around 3000-4000 RPMs ....
 I am running the Keyster replacement kit for the 550k.

There's one part of the reason.

I have quite a bit of backfire when the throttle is closed and the bike is slowing in gear at a higher RPM. The exhaust is the stock 4-1 and has the expected multiple holes (maybe 5-7) due to rust located all over the muffler.

Assuming backfire in the exhaust.  Here is another reason/clue.
Where is the idle mixture screw position?  And, did you change the stock solid tipped needles standard for the F to the hollow tipped ones standard for the K?

I have heard rumors of a "flat spot" in the carbs somewhere shortly after idle, is it possible that the UNI filter is giving me slightly too much air which only becomes a problem when the carbs are transitioning the fuel delivery between jets?

Don't blame the UNI . 

Could the hole infested exhaust cause me issues with back pressure that could give me these (I am assuming) lean conditions?
These are my only two guesses and I would greatly appreciate any words of advice anyone is willing to extend to me.

Holes in the muffler reduce exhaust pressure, which effects cylinder scavenging.  Therefore, available oxygen for the next combustion cycle.
Stock carbs are tuned for this eventuality and deliver A/F mixture accordingly.
Back pressure has changed (holes). And carb tuning has been changed (used K carb parts/settings instead of F carb parts/settings.)

You also haven't stated what slide needle position you selected,  F or K?  At this point (muffler holes), you may be better off with the K Slide needle position.
Otherwise, seal the muffler holes, and revert back to F carb internal parameters.

My Fs with stock carb configuration, stk muffler, and a UNI in stk box, do have a bit of a flat.  But, nothing that a slight throttle change doesn't overcome immediately.  I like to thing that is the "best economy range", or (spark plug cleaning range).   
If I am in traffic and at a speed where it is bothersome, I just drop a gear selection.  The engine likes it, and the stock muffler doesn't blare to cops that you are speeding.

[thebeekeeper]

Thank you for the response, gentlefolk!

Gordon: Thanks for the advice! I can't read enough about my bike and have spent most of my time trying to get know her rather than doing studying for class! Oh, well! I will definitely look at that FAQ.

TwoTired: I appreciate you taking the time to give me a detailed response, thank you. As for purchasing the 550k carb rebuild kit, I did not even know the difference between the K and F models at this point in my motorcycle education. I purchased, at the time, what I thought was a "one size fits all" sort of kit for the '75 550. Turns out that was my first lesson... and many more have come since then and will continue to come, or at least I hope so!
 
Interestingly enough, I believe the gentleman who owned the bike before me also used the 550K kit as I noticed that I pulled out size #100 mains. I replaced everything except the needles, which I am assuming is also from the 550K kit the previous owner had put in. That being said, I did not think to check the needle position and sort of hoped the bike would work with the needles as they were. Would it be worth me pulling the carbs off to check the slide needle position?

My idle mixture screws are all around 1 1/2 turns out, give or take 1/16 of a turn. Would you recommend me playing with these? Could that affect A/F mixture around 3-4k during normal riding conditions?

I suppose, in the end, I am just not exactly sure what the bike should feel like during acceleration. It could be that someone knowledgeable , such as yourself, could take my bike for a spin and tell me that the slight hesitation I am experiencing is normal. However, a bigger part of me wonders if you would ride it and possibly tell me that I am not even close to having it run like it should. Ultimately, the obsessive nature within me to know everything about my vehicles will nag at me until I have some sort of confirmation that my bike is in being ridden in a "happy" condition.

Either way, one would have a difficult time trying to wipe the smile off of my face whilst I am ridding the bike around! I do rather enjoy the ride!

Again, thank you for your time!

Best,
Joshua

[TwoTired]

Interestingly enough, I believe the gentleman who owned the bike before me also used the 550K kit as I noticed that I pulled out size #100 mains. I replaced everything except the needles, which I am assuming is also from the 550K kit the previous owner had put in. That being said, I did not think to check the needle position and sort of hoped the bike would work with the needles as they were. Would it be worth me pulling the carbs off to check the slide needle position?

Kinda depends on how much the the bike bugs you the way it runs now.  And, what kind of deposits show on the spark plugs.
FYI, the original F model carbs had #98 mains, primarily because of the stock muffler pressure.  The slide needle position was in the 2nd clip position from the top.  No need to look at them if they are in the proper position.  But, I don't know how else you will know unless you look.  Your call.  You will have to do a carb vacuum sync after taking out the slides to look at needle position.  But, you probably have the K needle profile instead of the F needle profile.  ...Which also effects where the needle is supposed to be clipped.  K needles were clipped in the 4th position from the top.  Honda made all these subtle carb tuning variations for a reason.  It would certainly have been cheaper to make one carb fits all models.  But, they didn't.

My idle mixture screws are all around 1 1/2 turns out, give or take 1/16 of a turn. Would you recommend me playing with these? Could that affect A/F mixture around 3-4k during normal riding conditions?

Could.  But, I'm not making any guarantees.
Those screws control the mixtures of the pilot circuit.  They are also called Idle air bleed screws.  The pilot circuit is a complete separate circuit all on its own inside the carbs.  They always deliver some fuel and any throttle position, but they are the most dominant at the idle throttle position.  The K pilot screws were made differently from the F pilot screws.  One thing you can do is take them out and look at them to see if they have been changed along with other parts in the carbs.  F screws are solid tipped, K screws are hollow tipped and cross drilled.
If you believe the hesitation is caused by a lean mixture, then tuning the screws in 1/8 or a 1/4 turn, should richen the mix a bit at idle and also a little at higher throttle settings.  Solid tipped screws have a more drastic effect than the hollow tipped screws.  So, you will have to feel this out on your own.  If you go too far in, then you can expect the spark plugs to more easily carbon foul and have increased sooting of the exhaust system.  If you go too far out, the engine will wheeze when you quickly twist the throttle (more than 1/2 a twist at low RPM) instead of accelerate under load.

I suppose, in the end, I am just not exactly sure what the bike should feel like during acceleration. It could be that someone knowledgeable , such as yourself, could take my bike for a spin and tell me that the slight hesitation I am experiencing is normal. However, a bigger part of me wonders if you would ride it and possibly tell me that I am not even close to having it run like it should. Ultimately, the obsessive nature within me to know everything about my vehicles will nag at me until I have some sort of confirmation that my bike is in being ridden in a "happy" condition.

Measure off 1/4 mile.
The 550 does the standing 1/4 in 14.02 secs (professional rider and track, 6-7 mph head wind, 383 ft above sea level, and 73 degrees)
0-30 in 3.0 sec
40 - 3.9 sec
50 - 4.5 sec
60 - 5.4 sec
70 - 7.5 sec
80 - 9.5 sec
90 - 13.9 sec
Weight as tested with rider 610 Lbs

Bigger than stock tires will likely slow you down, FYI.  (so will a dry chain).

Cheers,

[FunJimmy]

TwoTired

I have a stock '75 CB550F and am currently cleaning the carbs.
All things being stock except for HondaMan's trimmed advance springs and a Dyna S ignition, have you found raising the needle one notch
to be beneficial?

Thanks
FJ

[TwoTired]

TwoTired

I have a stock '75 CB550F and am currently cleaning the carbs.
All things being stock except for HondaMan's trimmed advance springs and a Dyna S ignition, have you found raising the needle one notch
to be beneficial?

Thanks
FJ

Not with the stock exhaust, air induction and a Uni NU-4055 Filter.

Cheers,

[FunJimmy]

Thanks for your reply.

Cheers