Zenith carb

Mike

Mike

Well-known member
How do you repair a Zenith carb idle air screw seat in the upper half of the carburetor? The treads are fine, and I tried several different idle adjustment screws and they all leak air, the idle jet is tight and secured.

thank you,

mike
 
should be a 10-34 thread size.maybe it was re-sized wrong. never had that problem. may try some Teflon pipe tape??
 
The treads are good, the idle screw does bottom out onto the seat, without the spring and cup. I have tried several idle air screws no help l.
Mike
 
1. If you are closing the idle air screw (gently ... the screw is only brass, whether plated or not and deforms easily).

2 Then blowing in idle jet to test idle air screw seat you MUST at the same time also plug the idle mixture port inside the throat with your finger.


3. If it still leaks then the idle mixture needle is too short or the spring on the needle is too long and collapsing before it seats.

To test for condition 3: Remove the spring and "cap" if it has one and put the idle air screw back in before testing again.

Look for the coils of the spring touching each other. A shorter spring will fix this.

Also maybe some one drilled out the seat larger than the needle diameter.
 
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Gary WA said:
should be a 10-34 thread size.maybe it was re-sized wrong. never had that problem. may try some Teflon pipe tape??
Click to expand...

Contrary to what some books say, the threads are in fact M5 - 0.75 metric threads, not 10-34.

If you notice that the needle and or jets make a squeaking noise and turn hard when screwing them into the casting, the reason is the use of the wrong tap.

Note if the threads are "sloppy" then they will not squeak and not bind in the casting.
 
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I had a carb with the same issue, turned out that the threads near the bottom of the hole were rusted so the needle never fully seated. Once I cleaned the threads with a tap it worked as it should.
 
Sparky said:
I had a carb with the same issue, turned out that the threads near the bottom of the hole were rusted so the needle never fully seated. Once I cleaned the threads with a tap it worked as it should.
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Very true ...

I also have seen where someone in past had run the tap too deep and destroyed the seat; so that it does not seal.
 
Benson said:
Contrary to what some books say, the threads are in fact M5 - 0.75 metric threads, not 10-34.

If you notice that the needle and or jets make a squeaking noise and turn hard when screwing them into the casting, the reason is the use of the wrong tap.

Note if the threads are "sloppy" then they will not squeak and not bind in the casting.[/QUOTE That's correct my mistake 5mm 75 pitch. My first mistake this year!
Click to expand...
 
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