Topic: Idle Mixture for CB 750 F2

[pdeg]

Gurus 

Have a 750 F2 and wondering what the suggestion would be for what would be the number
of turns out  for the idle mixture.

I have just rebuilt the motor and finally put the air cleaner on.  Have set the mixture at about 2 turns
(or more ?) and the plugs are covered in soot..  I had to replace them.  The accel pump is working now too.

I have some vacuum gauges now.  Is is possible to adjust the idle mixture by aiming for max
vacuum ... plus slightly richer....  Hard to do with the other pots ??

The carbs are pretty well flogged out... they have lost the chrome plating and I have lost count
of the number of screws that I have had to tap out to the next size up,

I have a repair kit ?  Can I just replace the needles ?  The kit has seats too... but I am afraid
to start punching stuff out of the bodies.  Can I just drop the needles down a step ?

May be interested in replacing the carb if there is another fresher set out in the internet 

Tx guys
Peter

[hsas.69]

The stock F2 needles aren't adjustable without shims. Did the plugs foul from idling or riding? What setup are you running? What mods, jets? Have you set float heights with clear tube? Need more info to be able to help.

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[SOHC Digger]

Initial setting for your idle screws is backed out 1 3/4 turns.

[SOHC Digger]

And you're not aiming for the highest vacuum. When adjusting your idle screws, you're aiming for the highest RPM.

First, set all your screws to 1 3/4 out. Then hook up gauges to sync your throttle lifters. When doing this, #2 is not adjustable and is therefor your benchmark. You want to adjust the other carbs so that their vacuum is the same as #2.

Once you've done this, set your idle RPM to about 900 to 1000. Adjust your idle screws individually to achieve the highest RPM. AFter each carb is adjusted, return the idle to 900 to 1000 and adjust the next carburetor.

[bytio]

The mixture screws on the F2  control the amount of fuel not air like others, e.g. Unscrew means richer, and visa versa

[martin99]

The stock F2 needles aren't adjustable without shims. Did the plugs foul from idling or riding? What setup are you running? What mods, jets? Have you set float heights with clear tube? Need more info to be able to help.

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Depends on the carb. PD41s have  adjustable needles, PD42s do not.

If you are running a stock airbox, I would start with stock settings. Mains should be 105, slows 35. Clip position 2 from top (PD41), IMS screws 1 3/4 turns out from lightly seated.

As Bytio says, remember the IMS screws act differently to the earlier roundtop carbs. Turning out enrichens, turning in leans it out. Might explain your sooty plugs.

[hsas.69]

To my knowledge all the F2/3s had 42s

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[SOHC Digger]

http://www.hondachopper.com/garage/carb_specs/carb_specs.html

F2s and F3s had 35 slow jets.

[pdeg]

Tx for the good advice chaps.. appreciated.

Mmm .. am stock, and they are pretty tired..

Fairly sure the mixture is on the fuel side... out is rich.  Also suspect that I can adjust the needle height.
Is on the middle one at the moment.  Can I change that without stripping the whole lot down ?

Have checked float height with clear tube.  Abut 1mm down from the top. Will check again.

Will go back to 1 3/4 and stick the gauges on them.  This is the first time the plugs have been out
since new rings and valves went in..  so some of the soot may have been oil from the rebuild ?

Suspect put the stock air cleaner back on with its venturi tubes has probably richen it up a bit..

Hardest part is finding a screwdriver that will fit under the carbs to adjust the idle screws 

Am also casting an evil eye on the coils .. even though they are fairly new (stock) ... when I put
a clamp timing light on them... only 1 of the HT leads on the same coil is giving me a flash on the
light... could be the cheap light.....  but #1 has always has seemed to not work as hard as the rest.

[Bailgang]

To my knowledge all the F2/3s had 42s

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My 77 F2 has PD41's and the needles are adjustable.

[martin99]

Tx for the good advice chaps.. appreciated.

Mmm .. am stock, and they are pretty tired..

Fairly sure the mixture is on the fuel side... out is rich.  Also suspect that I can adjust the needle height.
Is on the middle one at the moment.  Can I change that without stripping the whole lot down ?

Have checked float height with clear tube.  Abut 1mm down from the top. Will check again.

Will go back to 1 3/4 and stick the gauges on them.  This is the first time the plugs have been out
since new rings and valves went in..  so some of the soot may have been oil from the rebuild ?

Suspect put the stock air cleaner back on with its venturi tubes has probably richen it up a bit..

Hardest part is finding a screwdriver that will fit under the carbs to adjust the idle screws 

Am also casting an evil eye on the coils .. even though they are fairly new (stock) ... when I put
a clamp timing light on them... only 1 of the HT leads on the same coil is giving me a flash on the
light... could be the cheap light.....  but #1 has always has seemed to not work as hard as the rest.

How do you reach the conclusion your carbs are 'tired'? The important bits are made of brass and rarely need anything other than a good clean.

Well done for verifying float height with the clear tube method. In my experience it's a PITA and I would say you need to revisit it several times to be sure the level you are seeing in the tube is consistent. If it stays at 1mm, it's a little too high IMO. I'd aim for nearer 3-5mm. Think of what is happening to the fuel in the bowls on the road - like four miniature washing machines so if floats are set too high it is quite likely overfuelling will occur intermittently. I take it you have checked each carb and not just one of them? Don't think that by setting the float heights all the same that the levels will be the same across the rack - maybe when they were new but not after forty years of use and abuse. I have had to set my float heights individually, ranging from 14.5mm to 17mm, to achieve a fuel level of 3-5mm across the rack.

You can change the clip position with the carbs on the bike, but you need the dexterity of a brain surgeon and little girl's hands. If you drop one of those tiny washers and can't retrieve it, they'll have to come off anyway.

Middle clip position is a mod usually done to accomodate pods - did it have these or stacks before? Second from top is stock, which if you change it back will lean the mixture back down.

Reading between the lines, I'm guessing you're only describing your plug condition at idle, in which case clip position is irrelevant for now. You absolutely must pull the slows, if you haven't done so already, and make sure they are clear. This can't be done in situ as they have tiny holes in the sides that you can only get to if you take them out. Doing this made a night and day difference to the idle on my F2, and others I've owned before.

My advice - bite the bullet and remove the rack. Pull the slows and clean them (you can also verify these are stock too), reset clip positions, verify float levels (much easier off bike and with an auxillary tank) and meticulously clean. Only after doing this, and with the bike still not running right, would I begin to troubleshoot electrical, and even then not necessarily the coils as they are known to be pretty bullet-proof too! If you have a good spark then chances are they are ok.

[SOHC Digger]

Get one of these tools. It's an angle drive screwdriver that makes adjusting idle screws on the PD carbs a breeze compared to any other tool. I have one and it's well worth it. Here's the link:  http://m.ebay.com/itm/201383767167

[pdeg]

Cause the float tangs have little indentations where the needles have worn a little dimple...  No idea how to get rid that.
Suspect they "hang" on there sometimes flood... 

And the slides have warn off the chrome and are down to the brass...

I think one problem is that the clear tubing I am using (silicon) is too small a diameter and the capillary action
is giving me a false reading....  will try some thicker stuff.

Have had a good go at the idles jets with the guitar string (E# ?) and everything.  Took a brave bill and
a pair of multi grips pulled the slow out of #1.  Even bought some new ones ... tx to a bloke on the forum
who gave me the link.  Have not used them yet..  Why on earth did they make them push in .. Grrrr.
Is it worth putting a bit of heat on the carby to pull them ?

But I take your point  and I suspect you are correct..  floats are too high.  Amongst other things . Is the slow speed that is
flooding it..  Motor is still new so I have not  opened it up much.

Have not really looked at the main needles.. just vaguely remember the E clip in the middle somewhere.
Not sure how many positions there are.....

Will pull the rack, get the carby cleaner and guitar string and have another go at them.  Have some
new float needles too...


Tx very much for the advice and moral support...    Appreciate it ....

BTW ... that tool looks the "duck guts"..  Sadly the Aussie is getting smacked at the moment so I got one out
of Asia..  $30 delivered in a month :-(

Tx again

[flybox1]

i made my fuel screw tool from stuff i had lying around.
I JB welded a power driver bit (with a small tip) into an old socket wrench socket that fit its base.

as far as the float tang dimple, grab some 000 steel wool and a cold beer.  it'll buff that right out.
just had to do it on this leaky carb...and you can see the wear-ring around the float valve.  together they made this carb tough to tune.

[enwri]

Are your upper vents clear? they can cause problems when clear tubing, flooding especially. On 41s at least. This bloke used his string.

 http://forums.sohc4.net/index.php/topic,149344.msg1703658.html#msg1703658 

And fill the carbs slowly.

I have a 77 k, pd 41s, and tried to put the needle back to stock, second from top, was on the top (leanest) position.
It started and ran but just stopped the instant any throttle was given under load.
I put it back, huge difference, all stock jets.

Old hard overflow tubes fit nicely on the mixture screws, stiff enough not to twist. cut the ends at an angle and you can count turns by braille.

Mine end up at about 2 1/4 turns out.

[pdeg]

Ahhh.... many tx for the link to an earlier thread.  These are exactly the symptoms I am getting too..

Did not know about the upper vents..

They are coming off again.

BTW ... any hints on how to hold the rack in a bench vise ?  Something interesting with G clamps possibly ??

[martin99]

I used some bits of wood I had laying around, and bent up some corner brackets to hang the carbs on-

[evanphi]

Get a hobby bench sort of thing that has a wide "vise" surface...

Like this:

[flybox1]

I have a vertical stand like martin99 made.
having a holder like this allows you to check many things while the carbs are off the bike.

[pdeg]

Ahh.... tx

Good idea... certainly easy to make... and I have a couple of bits of timber around the place..

Mate .. you making me sad with that beautiful set of carbs... Soda clean ?  And it looks like the
original screws too.  Half of mine had to be re-tapped..

I saw a listing on ebay for an old set missing the floats .. but the aluminium looked good.  $200....

Or perhaps some after market ones ?  Saw Cycle X had some....