Tillotson carb

Russell Reay

Russell Reay

Well-known member
My 1930 car has a Tillotson X carb. It starts instantly and runs fine around the yard. After reading many threads about leaking carbs caused by faulty tank valves (and drivers) , I am conscientious about shutting off the valve. There has developed a gas smell in the garage with a stain on the floor, but not a puddle. The carb is covered with condensation with a gas smell, and it appears that the brass gas line fitting is cross threaded. Before I begin repairs, can someone comment on the likelihood of my being able to correct the threads.
 

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It looks crossed up from the image

Worst case scenario, you'd have to drill it slightly oversized and re-thread it. Then flair the next size up brass fitting onto the fuel line. Its definitely repairable, you might get lucky and just have to straighten out the threads on it too though
 
Various styles of thread inserts are available to fix broken threads. My favorite in the Helicoil. Measure the male threads on the fitting, diameter and threads per inch, and then search for an insert. You will have to drill and tap the hole. You may have to use a sealant when inserting the insert to assure that it is gas tight.

Alternately, look for a used carburetor on the internet that has good threads and then combine the two carburetors to make one good one.
 
Model-A.org says the threads for the fuel line are 1/2-20 UNF, see https://model-a.org/casting_threads.html.

1/2-20 by 1/2 long Helicoil inserts from McMaster Carr are $11.18 for 10, part number 91732A533. You will need the drill, tap, and insert tool. Kits with all the tools and 5 inserts are $96.12 from McMaster Carr, part number 98356A146. McMaster Carr is expensive but they have quality stuff. You could probably find what you need at a lower cost somewhere else.
 
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ebay has Tillotson X carbs from $50 and up. You would be buying an unknown carburetor. For my money I would install the Helicoil in your known good carburetor. It would be almost impossible to strip or cross thread the Helicoil. The Helicoil does not take up very much diameter whereas many of the other inserts have a much larger diameter that would eat into the boss on the carb and weaken it.

Double check the thread diameter and threads per inch on the fitting at the end of the tubing before you buy any insert. Buy a 1/2-20 nut at the hardware store to check the threads or a 1/2-20 bolt or both. Assure that when you drill for the Helicoil that the drill bit is straight into the boss. You could grind off a little of the end of the tap to make a bottom tap after you cut the treads with the stock tap.

If all this sounds a little overwhelming, take the carburetor to a machine shop and have them install the Helicoil
 
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nkaminar said:
...the Helicoil does not take up very much diameter ....of the other inserts have a much larger diameter that would eat into the boss on the carb and weaken it.....
Click to expand...
Hmm, I'm not convinced there's sufficient metal around the inlet for any threading of heilcoil or similar ... like Brentwood Bob I'd look out for a new/good used one .
 
If you decide to change carbs, do yourself a favor and find a good Marvel for Model A. I've found it to be the best running, easiest to tune carb for all around driving.
 
thanks for all the feedback. I haven't used a Helicoil for many years, but always considered that they were for larger threads. Will definitely look into it, though. Regarding purchasing alternatives --new core, used, etc--I also have a Zenith-2 in need of a rebuild, and the rebuild kit is already enroute. From all I have read , extra model A carbs and distributors are like an American Express carb--don't leave home without at least one of each.
 
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