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TKRIV

Rear 2 x 4 Carb 1965GS

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I need the rear carb (3925S) for 1965GS. If not the whole carb just the body will do as it's been damaged in the idle circuit.

I know this will not be easy to find as they are normally sold in pairs.

Cheers

Tom Kunek

InkedGS Rear Carb Marked.jpg

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Might be easier, and definitely less expensive, to fix the one you have.

 

Jon.

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Yes Jon agree 100% will be cheaper to repair but it would take a specialist who has the skills to work in such a restrictive location.

I would imagine that person would need fine welding stills also.

Do you do this type of repair work?

Tom K

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I would be really lucky to find a sad carb but with a good body assembly.

Fingers crossed.

Tom K

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13 hours ago, TKRIV said:

Yes Jon agree 100% will be cheaper to repair but it would take a specialist who has the skills to work in such a restrictive location.

I would imagine that person would need fine welding stills also.

Do you do this type of repair work?

Tom K

Tom, I no longer rebuild any carbs, due to demand on my time for the manufacture of rebuilding kits.

 

Welding skills??? My welding skills consisted of a checkbook, and a trip to a welding shop. My own welding appeared like the underside of the desk I inherited in third grade!

 

But the welding is not that difficult. Weld the hole completely full, and then re-machine the casting. Have had to do that to more than one AFB, where some "mechanic" drilled out a main metering jet.

 

Jon.

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On January 9, 2020 at 4:32 PM, TKRIV said:

I need the rear carb (3925S) for 1965GS. If not the whole carb just the body will do as it's been damaged in the idle circuit.

I know this will not be easy to find as they are normally sold in pairs.

Cheers

Tom Kunek

InkedGS Rear Carb Marked.jpg

How did that happen ?

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Looks like PO or "Mechanic" thought that the idle/air metering port needed to be cleaned out with a screwdriver.

Damage is consistent with flat screwdriver.

It would not take a lot of pressure to damage casting, it's very thin at this location. 

Tom K

 

GS Rear Carb 2.jpg

 

Edited by TKRIV
Removed duplicate photo (see edit history)
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I know that this is the primary side of the rear carb.  I don't think that will have anything to do with the idle circuit.  Are you having a problem with idle or other driving parameters??   Does it appear to cause a problem??? And IF it does what are the symptoms???

 

Tom T.

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The idle adjustment on the rear carb is controlled by the large brass screw in the front of the rear carb.

This is the idle air bypass adjustment.

Normally the primary butterflies are slightly held open by a idle adjustment screw on the throttle linkage.

This is not the case with 2 x 4 Carters.

The big brass screw bypasses just enough air to give correct idle speed when properly adjusted along with the 2 mixture screws

located on each side of the big brass Air Bypass Screw.

There is a past post where Tom Mooney gives a great explanation on how to tune 2 x 4 setups. 

 

We should remember that whenever a past owner wanted more HP they would typically start

to modify the carby. People who have no idea about the operation of a carb yet play with the accelerator pump, metering rods, jets...

 

Nailhead does not have a smooth idle because of the damage in the air idle circuit as per damage identified.

It's a bit like "Happy Wife, Happy Life"

 

Still looking for a old 3925S Carter Carb. If not the complete carb, then just the Body Assembly.

Cheers

Tom K

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Tom K,

 

   I rebuild these carbs. ALL the time & well versed on them. Right now I have 4 dual quad assemblies to rebuild for people around the world so I'm very versed on these carbs. as well as the 1x4's.

   I still don't believe that defect has anything to do with idle speed.   Remove the big brass screw & blow in compressed air to feel IF ANY compressed air is coming out of that area.  I looked at one of the quads I already have apart & that it appears to have no correlation.

   ALSO I'm very versed on NailHeads in general as I have three I'm in the process of rebuilding so I basically know about ALL there idiosyncrasies.

 

Tom T.

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Hi Tom T,

Thanks for getting back to me. I must admit that I am now confused which is easy to do!

Yes, If I blow compressed air into the hole (with the brass idle screw removed) air does come out via 2 small ports just above the butterfly plate.

You can see in the photo how I have pulled a length of string.

My understanding of the 3925S carb's idle is set via how much air is allowed to pass through this circuit and is not set 

by the idle screw which is located on the linkage.

What am I missing?

Tom K

409323185_GSRearCarbS.thumb.jpg.9719a0721b92d9c6cc6136c3da71b0eb.jpg

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Tom K,

  When you blow air through the hole where the large brass screw is threaded into the carb body you should also have air coming out UNDER the butterflies.

Tom Mooney

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Thank You Tom M & Tom T,  The penny has dropped. I get it.

Yes the air does flow to a small chamber and then comes out UNDER the butterflies via slots on the base of the carb shown with arrows.

I now understand why the damage that is present would have little effect on this flow of air. 

I know that my problem is with the carb because my friend has a GS which runs nice and when I fitted his carb

on my intake my nailhead also runs sweet.

The carb technician who rebuilt my carbs is back from holidays next week so will ask him to review his rebuild.

Cheers

Tom Kunek

Inked2x4XX.jpg

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Tom K,

 

   Tell the re-builder to make absolutely sure the front & rear throttle blades are CLOSED allowing NO light to be seen from the throttle valves when they are closed.. You can do this yourself very easily by tuning the carb. upside down with a light under it to check for light & make sure the throttle is closed all the way. 

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