Advice needed - stripped mixture screw threads!

Stevehose

Stevehose

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Unfortunate thing happened - my Zenith mixture screw turned fine up until today when i was making some post-pertronix adjustments. The screw would not turn easily and felt like it was binding in the hole. Turning it in or out did not improve it. I managed to get the screw out and the bottom half of the threads are gone. I installed the other screw to see if it was the screw or the seat that was bad - it's the seat because the other screw started to bind as it got about half way in so i backed it out.

Looks like I am in deep doo doo here - is it possible to replace the brass fitting that is pressed into the carb base? Any advice on this?

If not does anyone know of someone i can send the carb to for repair? Thanks.
 
Stripper?

Steve,

How much threads were good from the first needle? If 1/2 or more, it's possible that you can clean the threads in the bushing with a dental pick.The idle screw's most important feature is the needle and the O-ring seal. Otherwise, the second approach to try is a tap of the blind variety (squared end) taking extra care regarding the depth with a collar.

A machine shop might be able to help. The brass appears to be threaded into the throttle plate according to the parts diagram. But since the part's old, it's doughtful it could be that easy but perhaps a machine shop could. You might want to try a boat machinist/ repair shop as well.

My offer of a spare part holds if you're not successful.
 
Here is a pic of the needle, the shiny part of the threads is what is ruined. Whatever lurks in the brass seat is nasty - the pick is a good idea and will give it a try, I hope it can get far enough into the threads. If that doesnt work then the machine shop - do you think it is threaded or perhaps pressed in? Where can I find a replacement if I get it out? As usual, thanks for your ideas.
 

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Stevehose said:
Here is a pic of the needle, the shiny part of the threads is what is ruined. Whatever lurks in the brass seat is nasty - the pick is a good idea and will give it a try, I hope it can get far enough into the threads. If that doesnt work then the machine shop - do you think it is threaded or perhaps pressed in? Where can I find a replacement if I get it out? As usual, thanks for your ideas.
Click to expand...

Sorry thought that the threads on the carb itself were warped.
 
I didnt know Royze rebulit carbs - I've heard mixed reviews on their kits and their website is sketchy - but will investigate, thanks for your input
 
They only sell kits, no rebuilds, and the owner is a great guy and auto enthusiast. He owns several very original Chrysler 'letter' cars.
 
What about using a steel screw with same thread pattern to realign the threads? Problem is finding such a screw - the thread pitch on this mixture screw looks like .25 mm or so - very difficult to find I imagine - anyone know where to source such a screw to use to chase the threads?
 
screwed!

The idle mixture valve isn't metric! I just miked it. Try for 3/16 32 tap for machine screw. Sears Craftsman should have that size. Try for one with a square end vs. pointed. If the Home depot is closer- there's a Klien electrical multi tap tool might want to look at that. Looks like a multi tip screwdriver , but taps.

Possibly on the metal machine screw- file it down at the tip and cut grooves in it like a tap. Go slow.

One obscure thought- gunsmith if you know one close.

Loctite has a thread repair compound with a release agent if all else fails.
 
Does the screw you measured look like mine pictured below? It seems like the threads are finer than that? I showed the screw to a couple of industrial screw suppliers and even the local porsche/mercedes racing shop and they were befuddled with the thread count. I will go take a look at Sears/HD and see but am not sure the tip will go all the way in the seat before hitting the next size tap.

One thing we have plenty of here in Louisiana, it's gunsmiths! Thanks.

61porsche said:
The idle mixture valve isn't metric! I just miked it. Try for 3/16 32 tap for machine screw. Sears Craftsman should have that size. Try for one with a square end vs. pointed. If the Home depot is closer- there's a Klien electrical multi tap tool might want to look at that. Looks like a multi tip screwdriver , but taps.

Possibly on the metal machine screw- file it down at the tip and cut grooves in it like a tap. Go slow.

One obscure thought- gunsmith if you know one close.

Loctite has a thread repair compound with a release agent if all else fails.
Click to expand...
 
Zeniths

Very recently, there was someone selling a pair of Zeniths on the mye28.com board which could probably be had for cheap $.
 
I couldn't find them on my328 board could you link me? there is also an old dusty pair that came on eBay today.


TodB said:
Very recently, there was someone selling a pair of Zeniths on the mye28.com board which could probably be had for cheap $.
Click to expand...
 
comparison

Yep. Original mixture screw just like yours. If you want- double check the thread count. Taps are sized by thread count per inch. Since the total thread is less than an inch you're going to have to guess a bit. On the high side, I've only seen 32 or 40.

At first I thought it was a wire guage tap- less than 2 mm. But it's 4.67 which tells me it's not metric. Then I hit the converter on the mike and it came up dead on 3/16.

There is a void at the end of the brass to the hole in the carb throat or said another way- the mixture screw cone is in free space except for the seat in the throat.

Gun shops come up on every kind of machine screw there is. Good ones make them up. I know there are plenty of good old boys who use flashlights in the bayou. But you better be speaking French!
 
I measured the thread count and it is approx 4-4.5 threads per 1/16" so that would be around a 64 per inch. I bought a 40 tap and comparing the threads it is not close to being fine enough. You're right - it does look like 3/16" diameter and not metric. Gunsmith said they couldnt do it and suggested a machine shop that does custom work but those are few and far between here post-Katrina. I dropped the screw off with Jim Wheeler at Bimmerworks who is very helpful and he is going to show it to a model builder friend to see if he can machine it as well as look around for an old Zenith carb base lying around his shop (which is full of older BMW stuff collected over the years). For this seat and screw he also suggested the nuclear option which is to get the car running and screw it in one last time and stop it when it reaches a decent idle. I still will try to clean the seat threads before that option, as well as source a donor carb as backup. The seat seems to be pressed in to the carb base and unlikely to be removed without destroying it.


61porsche said:
Yep. Original mixture screw just like yours. If you want- double check the thread count. Taps are sized by thread count per inch. Since the total thread is less than an inch you're going to have to guess a bit. On the high side, I've only seen 32 or 40.

At first I thought it was a wire guage tap- less than 2 mm. But it's 4.67 which tells me it's not metric. Then I hit the converter on the mike and it came up dead on 3/16.
Click to expand...
 
Hit an Ace Hardware and find a nut that fits the screw properly--then you'll know the proper tap size. Exception is if it's one of those rare pitch sizes. Use oil/gas pipe thread tape if you're concerned about air leaking past the screw's threads.
 
Fixed!

Thanks to a very generous forum member here, I received a donor throttle base. Both mine and the donor had butterfly plate screws that were not coming out so my only option was to perform transplant surgury on the mixture screw seats. Problem is, this seat is pressed in and not designed to be removed. I slathered my stripped one with Gibb's oil and locked on the vice grips and gave it a twist - the seat unstuck and began slowly coming away from the body as I turned. 30 min later and lots more Gibb's I had it out. Next I did the same on the donor seat, this took me an hour of twisting and pulling to get out! I cleaned up the donor one and tapped it into my carb base. I reassembled the carb and am now getting mixture adjustment once again. So the lesson here is that these seats, while pressed in, can be removed if careful and patient. What was a grim situation has been resolved - I am no longer dead in the water - Thanks Jerry!
 
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