Volume control screw.. ed me!*

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cascade.king
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Volume control screw.. ed me!*

Post by cascade.king »

(*I posted this over at thehaflinger.com, but I value the opinions of the members of this board as well.)

So I dedicated a few weekend hours to rebuilding my Zenith 32NDIX today. When I got my Hafi, the carb was loading up badly when the throttle was pressed. You could see fuel pour into the right side throat, and none in the left side.
So..
Disassembled, soaked all metal parts in a parts cleaner bath (20 mins per dip) and brushed and reassembled.
Used the Hafi NDIX rebuild kit sold by SAV, for what it's worth.
Things came apart OK, and went back as before. Bowls were a mess.
It's all clean now.

Before;
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After;
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So here is my problem;
The carb had a mystery left volume control screw installed. Upon review, it looks like a Solex screw.
I have 2 new and correct volume control screws now.. and they were the last pieces installed.
Right side went in just fine, but the left side was apparently damaged by the incorrect threads on the bodge-job Solex screw. The new unit will NOT screw in.

Do you folks think a tap with the correct thread size (if you know this, please tell me) will do it, or do i need to Helicoil the thing to do it properly?
Mike
1971 North American 700AP Haflinger Pathfinder
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Re: Volume control screw.. ed me!*

Post by TechMOGogy »

Looks great
I am not a carb guy but I would start with rethreading it and go from there - parts manual seems to be useless but I would just measure one of the new idle adjusting screws
Sorry that's all I got :)
72 Pathfinder | 75 710M 2.7i | 96 350GDT Worker
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Jimm391730
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Re: Volume control screw.. ed me!*

Post by Jimm391730 »

Do you folks think a tap with the correct thread size (if you know this, please tell me) will do it, or do i need to Helicoil the thing to do it properly?
If the threads are just bunged up, chase 'em with a tap. Not knowing exactly how deep the hole goes or what is at the bottom, I'd use a "bottoming" tap which does NOT have much thread lead in and it also has a flat end; using this type of tap, you will run out of threads before you likely touch the bottom of the hole. Use a micrometer or calipers to measure the screw diameter (in mm) then count out five or ten threads and measure that length; divide by the number of threads you measured and you should be able to know exactly what size and pitch tap to get.

Because you are only chasing the bad threads, just gently work the tap through the threads by hand; repeat if necessary. Check for smooth fit of the screw.

Helicoils are good when the threads are missing, i.e., stripped out. You shouldn't have to go that route, especially since this thread doesn't have any real force applied to it (there is nothing trying to "rip the screw out").

Best of luck.
Jim M.
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Re: Volume control screw.. ed me!*

Post by Jim LaGuardia »

If you don't have a tap, one of the top screws can be used to carefully correct the treads as they are the same thread.
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edzz
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Re: Volume control screw.. ed me!*

Post by edzz »

:idea: remember a little oil can make it easier to run the screw in when used as a tap.
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cascade.king
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Re: Volume control screw.. ed me!*

Post by cascade.king »

Jim, I noticed the same thing. I made a mild attempt with no luck on reassembly.
The threads are just too bunged up. I need to bring the base into my shop and tap it. I'm nervous, though, since the end tapers. I figure I just need a few threads to hold the screw, since the spring tension will keep is stable.

Also, the angle is key too, I don't want the screw to bottom out at a bad angle and let to much/little volume pass.

Ug.
Mike
1971 North American 700AP Haflinger Pathfinder
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Re: Volume control screw.. ed me!*

Post by Heinkeljb »

The taper and the hole at the bottom need to be in good condition for the screw to work correctly, so as suggested use a "bottoming" / flat ended tap as the last chasing tap to try and get the thread correct all the way down.

If I understand taps correctly three are 3 sorts to do the job. The number 1 is smooth a long way up the shaft with threads only starting to be cut once it is quite deep into the hole. this is likely to hit the bottom before actually re-cutting any thread for you. A number 2 has a shorter smooth section so would be the one to use to start re-cutting your thread. A number 3 would be used last as it has thread cutting edges right near the tip. It could be used as the only tap but getting it started at the right angle and trying to correct a cross threaded section could be difficult.

Good luck with the repair. Might be worth while try to sort a replacement in case it all goes pear shaped.

John
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cascade.king
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Re: Volume control screw.. ed me!*

Post by cascade.king »

Well.. After a frustrating trip to my local hardware emporium, I have to ask..

Does anyone know the size of the volume control screw on the carb?

I tried to measure it, and it appears to be an M5, but the threads are a mystery..

They do NOT appear to be 0.80. They appear to be a coarser thread/less thread per cm.


Does anyone know the thread gauge on these things?

I need to get this exact, since I'm going to threadchase the hole in the base (if the correct tap is even available).

Any help would be SOOOO appreciated.
Mike
1971 North American 700AP Haflinger Pathfinder
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edzz
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Re: Volume control screw.. ed me!*

Post by edzz »

Jim L has indicated that the top screws on the Zenith 32NDIX are the same thread as your air screw. I don’t know if the Zenith 32NDIX and the Zenith 36 use the same size top screws however the screen shots are from the Pinz spares book and provide the screw call out and also the thread info for the air screw on the Zenith 36.
Also is a photo of a screw I modified for use as a tap. You could modify your good old / spare air screw for use as a tap by, cut off the tapered tip, blunt the first 2-3 threads and cut a slot using a dremel with a cut-off wheel or use an xacto saw blade to cut the slot. Not the best tap and you may need to remove it often to clear the chaff however with patience and a little oil it should work if you can’t locate the correct tap.
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cascade.king
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Re: Volume control screw.. ed me!*

Post by cascade.king »

So, am I reading this right.. its an M5 x 0.90 ?
Weird (but not surprising..)

Thanks edzz!
Mike
1971 North American 700AP Haflinger Pathfinder
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edzz
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Re: Volume control screw.. ed me!*

Post by edzz »

M5x0,9

I'm not good at interpreting metric speak however the screw shop should be able to make sense of it.

Also like I wrote this is info from the Pinz books and I’m only guessing that the 32 and the 36 have parts commonality.
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cascade.king
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Re: Volume control screw.. ed me!*

Post by cascade.king »

Success!

M5 x .9 is correct for the screw.
Got a cheap (but not TOO cheap) tap and chased the threads. The hole was really cross threaded, so I had to plunge the tap in a far as I could without bottoming out.

Please keep in mind that I did this with the base removed from the rest of the carb, so I could blow out the shavings.

Thanks to ALL who added info/input on this one.
I did NOT want to source a new part.
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Mike
1971 North American 700AP Haflinger Pathfinder
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