Author Topic: How do you Shim the Needles on the Later Model PD carbs? Here, I'll show you...  (Read 6736 times)

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Offline KB02

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Working on the carbs on my 750 K8 and I knew that I needed to raise the needles a bit, but on the SOHC4 750, in 1978, the needles are not adjustable. By doing some searching, I found that many people just said to shim the needles up a bit. BUT, despite the amount of searching I did, I could not find an example anywhere of how to actually do this. Not being scared, I figured I would dive in and document so that others might share in this.

First of all, can this be done with the carbs still on the bike? Yes. It's far more of a pain in the butt than taking them off, so I took them off.


First thing to do is to take off the tops of the carbs:


With the 78 PD carbs, you will need to remove the set screw that holds the upper arm to the throttle arm (the actual part that twists).

As you can see on the other three carbs, the adjustment portions (for carb syncing) are not effected by this, and the set screw screws right into the twisting rod. In theory, the sync of the carbs will not be effected.

Doing each carb one by one is the best way to do this. Less parts flying all over the place. First, lets take a look inside and see what we're dealing with:

All kinds of fun little pieces. What we need to concern ourselves with most is the big arm in the middle, and circlip and the little plastic washer. You will want to remove the circlip and the washer so that the middle arm can be slid off to the side and be removed.

With the parts off and this arm out of the way, we are free to pull the whole slide assembly out.


Make sure you still have all the pieces.


There are two screws that hold the next piece in place This part holds the needles in place with a small spring. Simply remove both screws (don't loose them, they are small) and pull the part out of the slide. The needle can then be removed.


The washers I used, I simply bought at the hardware store, in the metric section. M2.5 Flat Washers.


Each washer is about the space that would sit in between each section on an adjustable needle. This leads to even greater fine tuning if you really wanted to go that far. I only wanted to raise the needle "one position," so I placed on two washers (one for a spacer, the other for where the new clip position should be.

One Washer:


Two washers


Reassembly, as they say in most manuals, is reverse of removal.

I hope this helps someone down the road. You can also search around and find needles from a '77 750, but since they are no longer available from Honda, sometimes it is best to work with what you have.

-Bob
KB02
1978 CB750K Project
2000 Ducati ST2
...and a pedal bike

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Offline 754

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A few things,
 Never trust stamped parts to be flat, check them. Preferrably with a micrometer, and record thickness. Ant burrs or rags should be removed so they are actualy flat & parallel.

  Different  sources will give different thicknesses. And the stock spacing increments should be noted, so you know how much you have changed your settings.

 I should have a set of slides and  clip needles somewhere for these, but I need enough to make it worth the while.. Slides are different between tose 2 years.. one uses plated Brass, the other is anodized Aluminum, IIRC
Maker of the WELDLESS 750 Frame Kit
dodogas99@gmail.com
Kelowna B.C.       Canada

My next bike will be a ..ANFOB.....

It's All part of the ADVENTURE...

73 836cc.. Green, had it for 3 decades!!
Lost quite a few CB 750's along the way

Offline eurban

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Yes washers are rarely of exact uniform thickness so you will get bit of variation when using them as shims.  Probably not a huge effect but it might be wise to bring a micrometer with you to the hardware store.  I have seen a Suzuki? carb needle shim recommended by MRieck on this site; perhaps someone knows the part number.  I went the route of replacing my 78ks main jet needles with a new set of 77k adjustable needles from David Silver Spares.  These came with new needle jets (the "bushing" that the main jet needle rides in an out of) which seemed to have a positive effect on the precision (and the effect of adjusting the clip position on the needle) of the mixture produced. . . .One thing that I have found is that it is very easy to roach out the screws that hold the linkage piece to the slide, especially since you can't press too hard against them with the slide held in your hand.  If you drill a hole in your work bench you can set the needle through the hole and then press down hard enough to remove the screws without issue.

Offline KB02

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. . .One thing that I have found is that it is very easy to roach out the screws that hold the linkage piece to the slide, especially since you can't press too hard against them with the slide held in your hand.  If you drill a hole in your work bench you can set the needle through the hole and then press down hard enough to remove the screws without issue.

Good tip. I was able to hold them in my hand and get the screws out. I only had one that I had to "redesign" to accept a flat head instead of a Phillips head once I got it out. But that one was pretty bad before I got to it.

And you are both right, washers are not consistent and perfect. I just wasn't able to find another alternative.
1978 CB750K Project
2000 Ducati ST2
...and a pedal bike

Join the AMA today!!

My project thread Part I: K8 Project "Parts Bike"
My project thread Part II: Finishing (yeah, right) touches on Project "Parts Bike"

Offline MRieck

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Yes washers are rarely of exact uniform thickness so you will get bit of variation when using them as shims.  Probably not a huge effect but it might be wise to bring a micrometer with you to the hardware store.  I have seen a Suzuki? carb needle shim recommended by MRieck on this site; perhaps someone knows the part number.  I went the route of replacing my 78ks main jet needles with a new set of 77k adjustable needles from David Silver Spares.  These came with new needle jets (the "bushing" that the main jet needle rides in an out of) which seemed to have a positive effect on the precision (and the effect of adjusting the clip position on the needle) of the mixture produced. . . .One thing that I have found is that it is very easy to roach out the screws that hold the linkage piece to the slide, especially since you can't press too hard against them with the slide held in your hand.  If you drill a hole in your work bench you can set the needle through the hole and then press down hard enough to remove the screws without issue.
I'll get the #. I'm real busy doing other things so stuff like this gets put on the back burner. The Suzuki washers are .020.....all the time.
Owner of the "Million Dollar CB"

Offline 754

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I am pretty sure that a hundred of these off the shelf washers would be around 5 bux. If they are inconsistent, its usually burrs you can work off with a stone.

 The 20 thous sounds good...1/2 mm.

 If someone has a surface grinder, they could easily grind a handfull to a custom thickness.
Maker of the WELDLESS 750 Frame Kit
dodogas99@gmail.com
Kelowna B.C.       Canada

My next bike will be a ..ANFOB.....

It's All part of the ADVENTURE...

73 836cc.. Green, had it for 3 decades!!
Lost quite a few CB 750's along the way

Offline eurban

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My experience with "off the shelf" washers is that thickness is inconsistent, sometimes significantly.  However if you buy a good supply cheap and use your micrometer you should easily be able to make a good number of matched sets. . .

Offline MRieck

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 Suzuki part # for .020 shims is 13382-44030. As I said...they are always .020 and flat. They are also nice because they allow 1/2 clip adjustment.
Owner of the "Million Dollar CB"

Offline Clyde

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I used 3mm stainless steel washers for a set of Mikuni 28m carbs and found them to be within .001" of each other at .020" (which as stated above is 1/2 clip adjustent)
SOHC4 #1909
Honda CB750 K0(original and unrestored), K1(in pieces), K2(restored), F1(restored), 76 750a (awaiting restoration), 1966 Honda CB72
Suzuki GT750 1972 (restored), Kawasaki Z1 1973 (restored)

Offline KB02

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I bought 16 and used 8. I can measure once I get new batteries for my calipers.  ::)  Right now the bike is running much better than it was, so until I can get a permanent fix (Suzuki spacers - Thanks for the part number, Mike), I'll run with it for now. Won't be the only Suzuki parts on the bike.  ;D
1978 CB750K Project
2000 Ducati ST2
...and a pedal bike

Join the AMA today!!

My project thread Part I: K8 Project "Parts Bike"
My project thread Part II: Finishing (yeah, right) touches on Project "Parts Bike"

Offline fastbroshi

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  Also a good part # to know for those doing jetting exercises.  I'd been to Fastenal and their selection of washers was crap, would have to order some.   Not to mention I don't need a bag of 5000.
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Offline MarkCR750

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What a terrific "how to", I just wet to shim my carbs, took the top off one of my carbs and thought "right......... what the bloody hell do I do now?!", found this post and it made it a very easy job, thanks Bob top bloke.

Offline MiGhost

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Great work!
This should go into the FAQs should that it does not get lost.
~ Ghost

Grey Ghost '80 CB650C: Updated Stock Restomod. Period Custom Cruiser (OEM harbags & trunk, Wixom Ranger fairing, Jardine turnouts)
Bad Moon '83 XJ750MK Midnight Maxim: Full Dress Tour Deluxe w/ X-1 Fairing