Topic: hesitation off idle

[TwoTired]

Tried 1/4 turn in and 1/4 turn out on idle screws....no difference.

Try one turn +/- 1/2 turn.  If it recovers from hesitation cleanly and without burbling back to life, the hesitation was caused by a too lean mixture condition.  Turn the pilot screws inward to enrich the mixture.

Bears repeating.   Are you sure the main jet's emulsion tube holes are clean and not enlarged?

Are you sure the #38 idle jets are stock Keihin manufacture, rather than aftermarket?

Are the carbs still vacuum synced?


Unless you put the high pressure stock muffler back on, the stock F setting won't be appropriate.  The MAC 4-1 muffler, even with the baffles is still not a high pressure muffler.

[KeithB]

Try one turn +/- 1/2 turn.  If it recovers from hesitation cleanly and without burbling back to life, the hesitation was caused by a too lean mixture condition.  Turn the pilot screws inward to enrich the mixture.
 tried that...very little difference. Certainly not a "clean recovery"

Bears repeating.   Are you sure the main jet's emulsion tube holes are clean and not enlarged?
They are clean and all look like they have not been drilled

Are you sure the #38 idle jets are stock Keihin manufacture, rather than aftermarket?
No...will check.

Are the carbs still vacuum synced?
WIll check again but I have done rough "bench" syncs on my 750s and had better results than this.

So you are not convinced that changing to the leaner clip position is not worth a try?

[KeithB]

I knew it was a mistake to post how much I pay for plugs.
It certainly didn't take long to get the inevitable responses of how much cheaper I can get them elsewhere.
After having an alarming number of motorcycle shops close in Toronto, I'm keen to support a guy who is a 10 minute drive from me, has stuff in stock and can get me parts I find on the internet at competitive prices.
I don't have to wait and I don't have to pay the inflated shipping charges of internet companies.
30 minutes round trip and back to my project is worth a little extra $$

Many thanks to the people that took time to point out the better pricing as I'm sure that info will be very useful in helping to get my tuning issue corrected.

[flybox1]

you're welcome!  just trying to help you throw out less money   

Before I throw another $18 out on new plugs....

nothing wrong with supporting the local guy when he has what you need.
you can also burn off the carbon and reuse your plugs 

[goldarrow]

Sorry, wasn't aware that you were in Toronto.  Price listed are US prices

[KeithB]

US always seems to get lower pricing and we have higher taxes too.
Helps with heathcare

[Duanob]

Amazon denso x24es-u $2.50 each

Don't do it you will regret it. Your better off paying a bunch more for the NGKs.

[Duanob]

Unless you put the high pressure stock muffler back on, the stock F setting won't be appropriate.  The MAC 4-1 muffler, even with the baffles is still not a high pressure muffler.

Good to know. I have a MAC and it's pretty quiet, i figured it had decent amount of backpressure and baffling.

[flybox1]

Amazon denso x24es-u $2.50 each

Don't do it you will regret it. Your better off paying a bunch more for the NGKs.

of course he will.  those are the wrong plugs for a 550 

[goldarrow]

Amazon denso x24es-u $2.50 each

Don't do it you will regret it. Your better off paying a bunch more for the NGKs.

of course he will.  those are the wrong plugs for a 550 

Hmm....explanations?

I've actually been running that x24es-u.  I do lots of commute.  50miles one way.   Basically run between 60-75mph for an hour.

[flybox1]

D8EA's & x24es-u's are best 750's
D7EA's & x22es-u's are best for 500/550's

[goldarrow]

D8EA's & x24es-u's are best 750's
D7EA's & x22es-u's are best for 500/550's

Yes

I also read that x24es-u for 550 is good for longer, extended ride. 

[TwoTired]

Try one turn +/- 1/2 turn.  If it recovers from hesitation cleanly and without burbling back to life, the hesitation was caused by a too lean mixture condition.  Turn the pilot screws inward to enrich the mixture.

tried that...very little difference. Certainly not a "clean recovery"

Earlier in reply #7, you said it recovered as if switched on.  That IS a clean recovery to me ,and indicates it was too lean during the hesitation event.
Are you now saying it burbles after the hesitation missing firing cycles randomly until it settles down to smooth power?

Are the carbs still vacuum synced?

WIll check again but I have done rough "bench" syncs on my 750s and had better results than this.

Your are chasing "nits" and vacuum sync can be one of those.

So you are not convinced that changing to the leaner clip position is not worth a try?

Not if it is already too lean to pick up without hesitation.

Have you tried "cleaning" the plugs with some sustained high power operation?  D7 ought to clean it's own plugs at WOT if the main is proper size.

[KeithB]

Sorry...misunderstood your question.
The clean recovery happens the same with screws adjusted from 2.5 turns out to .5 turns out.
It burbles a bit then catches and pulls quite nicely.
If held at 1/8 throttle it cleans up at around 3000 rpm
If I roll on the throttle to about 1/2 it cleans up quicker.
The screws seem to have no effect on when the clean up happens.
Changing plug temp seems to have made no diff.

I did a full vacuum synch with the install after cleaning and re-building.
Have not done one since changing needle height.

Plugs were brand new and I did some WOT runs through 3 gears.
Still stumbled the same. Plugs still a bit black.
When I put the new plugs in and was setting idle and checking timing, they went quite black.

I have the carbs out again and willing to try to lean out needle.
Otherwise , I put them back with no changes and check sync.


Here is a pic of the jet.

[TwoTired]

Sorry...misunderstood your question.
The clean recovery happens the same with screws adjusted from 2.5 turns out to .5 turns out.
If held at 1/8 throttle it cleans up at around 3000 rpm

I don't get it.  If this were a mixture issue, to lean or too rich to make fire in the cylinders, how can the rpm advance to 3000, particularly under load?

If I roll on the throttle to about 1/2 it cleans up quicker.
The screws seem to have no effect on when the clean up happens.

The pilot screws set base A/F mixture, which must be on the rich side to compensate for no accelerator pump.  Changing the pilot mix doesn't effect how it runs or hesitation traits?

Two things happen when the throttle is advanced more (slides open).  The jet exit ports see less pressure differential leading the less fuel delivery and more air is presented to the engine simultaneously.  It must go leaner until the air velocity regains the pressure differential though venturi effect.  To keep the engine running during the slide opening transition the mixture begins over rich, so the leaning doesn't cause engine hesitation.

I will note that the transition off idle can be effected by slide cutaway angle (changes the venturi velocity).  But, you're the first in this forum that seems to have no other options than to change that cutaway.

Plugs were brand new and I did some WOT runs through 3 gears.

Still stumbled the same.
Were the plugs still sooty?
We need better clues as to what the engine is actually doing during the complaint hesitation.

Here is a pic of the jet.

Looks like proper Keihin, to me.


Starting to wonder about the spark advance curve/operation smoothness.

Is the clutch slipping?

[KeithB]

Have to put this aside until Sunday.
Will address your questions in detail then
Thanks TT

[jcarthel]

Have you considered the timing is advancing at the wrong curve? That would explain when she comes on all at once,  the timing is advancing too late. I think you should put in fresh plugs then take it the redline in at least 3 gears. Kill the motor at wide open throttle just before redline in 3rd gear and clutch it. Coast to a stop and pull your plugs. All the car magazines recommend this method to getting the jets just right in your vintage musclecar. Maybe she is just going to be a little rich at idle, that why I think you should check your plugs and consider your mixture at wide open throttle. You definitely don't want her too lean under wide open throttle.

[iron_worker]

Definitely check your timing advance is functioning correct. I believe you should have full advance by 2500RPM ... hopefully someone can confirm that. Should probably be in your manual too.

If that is all working fine then the symptoms you describe almost sounds like too rich off of idle. The fact that it responds better if you crack the throttle more makes me think this.

TT, I'm not sure why you believe that a bike can't accelerate at all if the mixture is too rich or too lean. It is possible that it will misfire, then hit, misfire, misfire, hit, etc. The engine will still accelerate just not smoothly like it should. It's not like every bike to ever have a mixture issue wouldn't accelerate past idle???

IW

[Deltarider]

Definitely check your timing advance is functioning correct.

+1
A strobelight can show if the advance mechanism operates smoothly. Full advance is at 2500 rpm.
If it's stuck somewhat, report back.

[Duanob]

Sorry...misunderstood your question.
The clean recovery happens the same with screws adjusted from 2.5 turns out to .5 turns out.
It burbles a bit then catches and pulls quite nicely.
If held at 1/8 throttle it cleans up at around 3000 rpm
If I roll on the throttle to about 1/2 it cleans up quicker.
The screws seem to have no effect on when the clean up happens.
Changing plug temp seems to have made no diff.

I did a full vacuum synch with the install after cleaning and re-building.
Have not done one since changing needle height.

Plugs were brand new and I did some WOT runs through 3 gears.
Still stumbled the same. Plugs still a bit black.
When I put the new plugs in and was setting idle and checking timing, they went quite black.

I have the carbs out again and willing to try to lean out needle.
Otherwise , I put them back with no changes and check sync.


Here is a pic of the jet.

Do they measure 28mm long? I found a couple of shorter ones in my carbs once.

[Duanob]

D8EA's & x24es-u's are best 750's
D7EA's & x22es-u's are best for 500/550's

Yes

I also read that x24es-u for 550 is good for longer, extended ride.

the last set of Nippon Densos I used lasted a very short time before they were fouled and not firing. So short of time I kept ruling them out as a problem but they were the problem. I probably just stick with NGKs from now on.

[Deltarider]

Here is a pic of the jet.

It's the right one allright.

[TwoTired]

D8EA's & x24es-u's are best 750's
D7EA's & x22es-u's are best for 500/550's

Yes

I also read that x24es-u for 550 is good for longer, extended ride.

the last set of Nippon Densos I used lasted a very short time before they were fouled and not firing. So short of time I kept ruling them out as a problem but they were the problem. I probably just stick with NGKs from now on.

I had a similar experience with the ND X22s.  Once they fouled, cleaning was ineffective and would randomly fire/not fire annoyingly.  I went back to NGK D7 and never had a repeat foul event.

I hate both the ND and the NGK in the colder heat ranges in the CB550.  Perhaps if I was operating the engine at 90% or more power setting through the hot desert, I'd want the colder heat range.  But then, I just don't ride that way anymore.

[TwoTired]

TT, I'm not sure why you believe that a bike can't accelerate at all if the mixture is too rich or too lean.

Because fuel only burns in the cylinder when within an A/F ratio range.  To me, hesitation is the lack of combustion cycles.

It is possible that it will misfire, then hit, misfire, misfire, hit, etc.

I observe this descriptor when there is too much fuel for available oxygen IE. too rich.  The engine misses cycles because the extra fuel quenches any flame before it can light off, if the over rich is a borderline condition.

The engine will still accelerate just not smoothly like it should. It's not like every bike to ever have a mixture issue wouldn't accelerate past idle???

Could be a description difference, or meaning.  Hesitation, wheeze, or no response to throttle advancement is what I expected the complaint was all about.

Anyway, with a slightly lean condition, it always burns what ever it has in the chamber, as there is plenty of oxygen to do so.  It just doesn't burn very long during the power stroke, making poor power.

I learned this from 2 cycle engine adjusting by ear.  When very rich, they will 4 cycle, 6 cycle, or even 8 cycle.  Very lean and they, make poor power (and run hot) but they do fire on each cycle of the power strokes.  If you go so lean as to go outside combustion range, they quit, until some more fuel is available, then resume as if nothing went wrong.

The phenomena carries to 4 cycle engines too, 8 cycling and more, as the exhaust cycles don't completely evacuate the cylinder for the new fresh charge.  I call this burbling as the engine clears away excess fuel for proper and reliable combustion.
The recovery from a throttle transition can be diagnosed in this way.  Clean recovery and the mix went too lean, burbling through recovery means the mixture was too rich to fire on each combustion stroke.

For combustion to occur it must fit into an air/fuel ratio range to combust at all.  I describe hesitation as when the engine is firing on any combustion strokes.   If it is too rich during the hesitation, the engine will burble as it gains RPM.  If too lean it won't advance at all.  And this is typical for mechanical slide carbs set for idle that is not rich enough when the slide is raised, raising carb throat pressure and making more oxygen available for combustion.

If the OP and you are using different meanings of the words I'm accustomed to, then just ignore my input.

Cheers,

[lucky]

put in a new set of plugs
let it idle 3-4 min.
pull the plugs and post pictures of the tips so we can see color...

Lucky, how would the OP know how to get it to run right when they dont know whats going on in the first place.
you like exploratory brain surgery for s simple head cold?

plugs will show you the rich condition going on at idle - 1/4 throttle turn. 
dropping the needles like duanob says, will lean you out

I can tell how it is running by how it sounds.
No popping on decel in the shop.

Plug readings are not reliable in the shop.