Topic: hesitation off idle

[KeithB]

I did read some other posts on this subject and tried the suggestions but still having problems.

1972 CB550 with stock air intake. 4 into 1 Mac exhaust.(don't have stock exhaust available) Valves adjusted and timing dead on with strobe.
D8E plugs with 5k caps and Honda Man ignition. All carb boots tight.
I tried the HondaMan suggestion of raising the needle and installing lower size main jet(110 down to 100) as well as tensioning the advance springs.
1 tooth smaller front sprocket.

This is just in the low end of the throttle range just after clutch engages and the engine stumbles a bit before "catching" at around 3000 rpm.
After that it pulls to redline like a rocket! Starts and idles just fine.
As I turned the idle screws out and it gets better but at 3 turns out, It's still not right.
At 1/2 turn out, it's hard to start and the hesitation is much worse and won't idle.

Any suggestions would be appreciated.

[flybox1]

Stock plug is a D7EA.
You did not mention the plug color, or the condition of the emulsion tubes or the pilot jets. These can have a huge effect on off idle for any bike.
Typically with a less restrictive exhaust, one would increase the mIn jet size, so your decision to go down one size is confusing.
I'm not super familiar with the 5xx's, so hopefully youll hear from dave500, Duanob, and TwoTired....

[scunny]

are you cracking open the throttle or gently accelerating

[KeithB]

Gently. I always roll on smooooooothly with these bikes.

[lucky]

Hondaman told you to go to the 100 main jet because that is the stock size.

I would go to the #40 idle jet instead of the #38.

I would put the needle in the stock position as long as you do not change the stock intake filter or air box.

You said Honda Man said to raise the needle but you did not say FROM what?
The stock position of the needle in the slide would be clip in forth groove down from the top.

Then turn mixture screw back to about one turn .


A stock bike with that exhaust should work with just a slight increase of the mixture screw. Just get it so that at idle in the shop, and a blip of the throttle, it does not backfire.

[KeithB]

Thanks for taking the time to reply.
A couple of corrections...
Main jet from 100 to 90 and clip is now in the 5th groove down from top.
I may have to change that back as the MAC exhaust is probably a bit more free flowing than stock.
I have not had a chance to do a proper "plug chop" for colour.
The Hondaman adjustments were made as per his post about "dealing with the 500/550 flat spot".
Seems to work as it's all good above 3000 rpm.
Your suggestion of going to a larger idle jet makes sense and, as I was going to replace them with new ones anyway, I'll go to #40

[TwoTired]

The CB550 wasn't introduced until 1974.   How can you have a 72 CB550?  A hybrid of parts?  What carbs do you have?

The MAC 4-1 with silencer requires no jetting change from stock on a CB550K engine expecting a 4 to 4 exhaust.

#38 pilot, Needle in the 4th clip from the top, and #100 mains.  Float height 22mm.  For the earlier non-PD carbs.

Don't know why you are using D8 plugs, I always hated them in favor of the D7.

For the early style carbs, turning the air screws outward will make the throttle twist response wheezy and stumbly as it leans the idle mixture.  Opening the slide then raises the throat pressure, reducing fuel jet flow along with an inrush of air.  The idle mix has to be over rich to gain RPM reliably.
I don't believe you can properly compensate for this by raising the slide needle, and reducing main jet size.

Of course, I also don't believe the stock bike had a "flat spot" either.  At least, not until ethanol was added to the gas.


Do check the plugs for soot deposits.  That can cause a "flat spot" all on it's own, until temps come up to clean off the soot from an over rich mid range setting.

Does the "Hesitation" recovery burble back to life, or resume cleanly, as though suddenly "switched on"?

[KeithB]

1974 NOT '72 (bad fingers!) Had a '72 CB750 before....

"Does the "Hesitation" recovery burble back to life, or resume cleanly, as though suddenly "switched on"?"
Switched on. At about 3000 rpm it just lights up.
What does that indicate? Rich or lean?

Will go back to D7  as the plugs are a bit sooty and try to do a highway speed plug chop.

[TwoTired]

If it burbles back it was rich before recovery, missing firing cycles until their was finally enough oxygen to burn the hydrocarbons reliably.
If it "switches on" it was too lean to fire before recovery.

But, I'm confused.  The carbs don't know much about RPM (Just air velocities and throat pressures), they do know about slide position, which governs which fuel metering device is dominant.
Best if you mark the throttle to know at what throttle position your hesitation occurs.

I'm also wondering how it achieved 3000 RPM if it was hesitating. 
 I'll have to think about that some more.

[KeithB]

This hesitation is just a slight drop in power just as I come off the clutch a go to roll on some throttle.
Perhaps it "switches on" a bit above 3000 but certainly runs very well after 3500.
New plugs tomorrow and I should probably restore jets and needles to stock.
I will mark throttle position but I'm estimating it happens at about 1/4 throttle.

This was a bike from a wrecker and ran like complete crap when I got it.
I suppose making any mods before testing it with stock settings was an error but having read HMs post on the "flat spot" I thought I would get ahead of the game.
Just trying to reduce how many times I had to take those %$#@$ carbs off 

[lucky]

The CB550 wasn't introduced until 1974.   How can you have a 72 CB550?  A hybrid of parts?  What carbs do you have?

The MAC 4-1 with silencer requires no jetting change from stock on a CB550K engine expecting a 4 to 4 exhaust.

#38 Main, Needle in the 4th clip from the top, and #100 mains.  Float height 22mm.  For the earlier non-PD carbs.

Don't know why you are using D8 plugs, I always hated them in favor of the D7.

For the early style carbs, turning the air screws outward will make the throttle twist response wheezy and stumbly as it leans the idle mixture.  Opening the slide then raises the throat pressure, reducing fuel jet flow along with an inrush of air.  The idle mix has to be over rich to gain RPM reliably.
I don't believe you can properly compensate for this by raising the slide needle, and reducing main jet size.

Of course, I also don't believe the stock bike had a "flat spot" either.  At least, not until ethanol was added to the gas.


Do check the plugs for soot deposits.  That can cause a "flat spot" all on it's own, until temps come up to clean off the soot from an over rich mid range setting.

Does the "Hesitation" recovery burble back to life, or resume cleanly, as though suddenly "switched on"?

"Slight error.: "#38 Main, Needle in the 4th clip from the top, and #100 mains.  Float height 22mm.  For the earlier non-PD carbs."


should be:
#38 IDLE jet, Needle in the 4th clip from the top, and #100 mains.  Float height 22mm.  For the earlier non-PD carbs.

[TwoTired]

should be:
#38 IDLE jet, Needle in the 4th clip from the top, and #100 mains.  Float height 22mm.  For the earlier non-PD carbs.

Quite right.  Correction made.

[KeithB]

OK. So i restored to 100 mains and replaced the O rings on the jets, clip 4th from top, checked the #38 idle jets to make sure they are clean, installed D7EA plugs.
Will check timing and advance again.
It started but after dealing with those F#$#@! carbs and intake, I'm taking a break to restore sanity before going out for a test drive.

What a catastrophic intake design!!!!!!

Thanks for the help

[KeithB]

The story so far...
Went back to stock 100 mains with new O rings
checked #38 idle jets...clean
needle at 4th clip from top
floats at 22mm
idle screws 1.5 turns out
new D7EA plugs
checked points gap and timing with strobe....OK

At 1/8 throttle it still hesitates and stumbles and if I hold at 1/8, it clears up around 3000 rpm
If I roll on to 1/2 throttle it cleans up almost right away.
Tried 1/4 turn in and 1/4 turn out on idle screws....no difference.

[flybox1]

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]

Forget about spark plug color and forget about plug chops right now.

Just get it to run right.

Many times owners look at plugs that have black on them and think it is running rich, but in fact all it has to do is idle for a minute and it can blacken the plugs.
Remember these engines are 40 years old and back then nice tan colored plugs
were not happening. Now we have fuel injected production cars and the plugs are "tan" all the time. New direct injection engines are running 65:1 fuel air ratios.

In some places like Tucson the plugs are always "tan" because it is so hot outside.

[Duanob]

The story so far...
Went back to stock 100 mains with new O rings
checked #38 idle jets...clean
needle at 4th clip from top
floats at 22mm
idle screws 1.5 turns out
new D7EA plugs
checked points gap and timing with strobe....OK

At 1/8 throttle it still hesitates and stumbles and if I hold at 1/8, it clears up around 3000 rpm
If I roll on to 1/2 throttle it cleans up almost right away.
Tried 1/4 turn in and 1/4 turn out on idle screws....no difference.

Although TT is correct on the settings for the 550K model with 4X4 exhaust I think I would try going with the 550F settings that come with a 4X1 exhaust. Pretty much teh same carb with all jets the same size but the needle position is 2nd clip from the top. All other settings are the same. Also make sure when you pull the needles take some 0000 steel or brass wool and clean the needles. I didn't see in your post where you vacuum synced the carbs, either. Anytime you mess with the needles even just taking them out and checking them you need to resync the carbs. That will help a lot as well.  I think you're almost there. I went through the same thing on my first bike but perserverance and making sure everything was correct my 550 runs like a japanese watch.

I know you say the pilots are clean but they really need to be clean! With a K&L carb cleaning wire kit you should be able to get the smallest wire through the 38 pilot and that's it. If you can get anything bigger someone drilled them out and fcuked them up. And measure them as well they should be 28mm long, Keihin makes other jets in different lengths that won't work. You just never know when buying these bikes what a PO did to them. Starting from base zero stock parts is the best place to start.

[KeithB]

I did vacuum synch the carbs and I use guitar strings to check the idle jets.
As for moving the needle, it was my understanding that the needle didn't really come into play at low throttle/rpm settings.
The engine pulls like a champ to redline once the stumble clears up.
I'm willing to try it but want to be sure as those carbs are the WORST PITA to remove and replace,with the stock air box, on any bike I have worked on!!!
If there is a "trick" I have not found it yet.
I'm keen to use the stock air intake but pods are looking better every time I have to pull these SOBs 

[Duanob]

Yes but your hesistation is off idle when the needle starts to open. If you start dumping too much fuel all of sudden you will get a hesitation, especially with a more restricted exhaust. Check the chart below.

Not trying to be a smartass but I can remove and replace the carbs in my sleep having done it more times than I can count. Remove the filterbox assembly, remove the intake bolts from the block, and all cables and clamps and fuel hoses. Push the airbox rubbers into the venturi box (hopefully they're still flexible). Pull the intakes/carb assembly as one away from the head, twist it down to clear the studs and pull out the left side to avoid the clutch cable. Takes 20 minutes. replace the opposite sequence. Don't forget the o-rings that go between the intakes and the head. In fact if you haven't checked them I would replace them, that's an area of possible vacuum leaks.

[KeithB]

Nice chart!
Well it certainly looks like needle position would have an impact at 1/8-1/4 throttle.
Looks like carbs come off...again...(sigh)

[flybox1]

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

[KeithB]

I did check the new plugs just after getting the idle set and checking timing and before going out on the street for a test.
They were quite dark.
That was with the idle screws out at 3 turns and probably too rich.
Before I throw another $18 out on new plugs, I will try to re-clip the needle.
It didn't really occur to me that a 4 into 1 MAC is probably a bit more restrictive than the stock 4 into 4 (DOH!)

[Duanob]

WOW! plugs are expensive in Canada. I get them for $2.30 USD each at my local autoparts store. NGK D7ea.

[flybox1]

WOW! plugs are expensive in Canada. I get them for $2.30 USD each at my local autoparts store. NGK D7ea.

+1...my last shipment of denso's were $1.40ea

[goldarrow]

Amazon denso x24es-u $2.50 each