CB550 - full trottle performance

[Erny]

After I finished assembly of my CB550 back to life (http://www.sohc.co.uk/index.php/topic,23823.msg234963.html#msg234963) I did test of newly set carbs and observed following:
When accelerating using full trottle, bike accelerates but slower than I would expect (roar noise is increased of course), almost having feeling that opening full trottle is somehow "braking" engine revving fast. But closing trottle a bit (maybe 1/8 or 1/4) significanly increasing acceleration speed.
Based on my understanding how these carbs works, in such full trottle opening only main jet is in chanrge of A/F ratio, thus logically I would assume mixture is lean so I should replace main jets by bigger ones? I did not do plug test (yet) waiting for good weather occation...

Current bike and carb setup:
- bike is practically 100% stock, air filter element is aftermarket from cmsnl, replaced as new 2 years ago (up to 3.5k miles on it)
- carbs are 022A type, 38/100 jets, genuine Keihin.
- carbs are clean, setup according to HSM, otherwise than high spped (full trottle) running very smooth

Strange is that I did not observe this behaviour, maybe I just did not realise or this is "new" issue.

Any ideas?
If conclusion is to test bigger jets, how big then? 105? 110?

Note: normally I would expect changing main jets on otherwise stock setup will not bring any improvement and all works as it shoud

[Bryanj]

Full throttle at what revs?

[Erny]

Approx 7k and more

[K2-K6]

Showing just a little lean (vacuum is down slightly when throttle is wide but revs not yet high enough) should clean up if you lift needles to bring in main jet earlier.

You could try two notches to see what you get as if it cleans up all ok. If there's still some effect then it would suggest one jet size larger.

[K2-K6]

With the above,  you may also have to fine tune the idle mixture a very small amount to make it leaner if it feels too rich in low to mid range. Emphasis on very small, just to move and refine it.

[Erny]

Thanks. Rising needles means carbs to pull and sync them again
Main jets change I can do with carbs on bike

But still wondering if this is not caused by something else as I have stock setup, I would expect default settings shall be fine (needle position, jets..)

[K2-K6]

"Thanks. Rising needles means carbs to pull and sync them again
Main jets change I can do with carbs on bike"
it's certainly a pain if you have to change them. Suggested two steps as if you made one step and found improvement but not quite enough, then you'd have to do it all over again. Whereas jumping two steps answers the question of if it needs larger jets, if it's not successful then the next step is to consider a jet change.

They do different things,  needle height versus main jet size. The larger jet is focused on fuel demand at full revs and full throttle, the needle position changes the points at which the jetting changes in volume on the way there.

Needle position and idle air screw are never absolute fixed position.  They are there to compensate, in fine adjustment, for the jet sizes which are fixed. They need to be set for the environment and conditions in which you are running the engine and can quite happily be different from the manual which is always given as baseline setting from which to optimise.

[deltarider]

All the aftermarket exhausts systems back in the day, advertised with: 'no rejetting required'. All, but one, which was a special Sebring 4-1 racing exhaust. I don't see why your bike, having such close to the original exhausts, would need bigger main jets. Air intake is stock, isn't it? Generally speaking I advise to leave the jet needles alone. My experience is limited to OEM parts however. Before you do anything, I'd have a look at the plug noses. If you have a dealer nearby, you may consider a run on a Dyno. Expect to need a 6% CO for a good driveability for a bike of that era.

[K2-K6]

"Before you do anything, I'd have a look at the plug noses."

You'd have to run it consistently in the "error"range for the plug colour to mean anything reliable for that error,  which I doubt you could do.

[Laverda Dave]

I would be tempted to go on a dyno. Might cost a bit more but at least you will have a full read out of what the engine is doing across the entire rev range and this will give you a chance to make small adjustments whilst you are there to potentially fix the problem.

[deltarider]

I would be tempted to go on a dyno. Might cost a bit more but at least you will have a full read out of what the engine is doing across the entire rev range and this will give you a chance to make small adjustments whilst you are there to potentially fix the problem.

Yep, bike as it is now, definitely deserves it IMO. Oh and if you do, please make recordings.

[Erny]

Short update after today's test ride (~50 miles)

Issue is present in wide range of RPMs, I confim it from ~3500 revs ->
Simply, when at whatever revs I'll open trottle fully, I feel engine does not accelate as good as if I close trottle a bit (approx 1/8-1/4).

After ride I pulled plugs (I installed them new, 55 miles before), all cyls are lean. See attached pics. Good news are are equally lean (I have never achieved that before!)

So, rising needle 1-2 notch looks to be next step..

[Erny]

Yes, I know but I cannot avoid resync, right?

[philward]

Short update after today's test ride (~50 miles)

Issue is present in wide range of RPMs, I confim it from ~3500 revs ->
Simply, when at whatever revs I'll open trottle fully, I feel engine does not accelate as good as if I close trottle a bit (approx 1/8-1/4).

After ride I pulled plugs (I installed them new, 55 miles before), all cyls are lean. See attached pics. Good news are are equally lean (I have never achieved that before!)

So, rising needle 1-2 notch looks to be next step..

Don't ride it too hard for too long with it running that lean Erny

[K2-K6]

Ah ha
"Issue is present in wide range of RPMs, I confim it from ~3500 revs ->
Simply, when at whatever revs I'll open trottle fully, I feel engine does not accelate as good as if I close trottle a bit (approx 1/8-1/4)."

That makes it a different problem to initially described,  in as much as it looks like it could take a one size up main jet if it's right across the rev range from about 3,500 upward, and worthwhile trying that first given this information and the plug colours.

You may find that if a jet change was too far,  then in future you could either check with the larger jet plus needle drop, or it would persuade you to use original main jet plus lifted needles as they would be slightly different solutions.

At least you could assess it now with less work if you have any main jets to try just one size larger.