How to Fix a Leaking Carburetor

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Lurker Carl
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How to Fix a Leaking Carburetor

Postby Lurker Carl » Sun Apr 01, 2018 12:51 pm

Nothing other than an empty fuel tank will keep a gravity feed fuel system from ever leaking. That’s because when there’s fuel in the tank, this system is pressurized. If the fuel petcock on the sediment bowl is defective, the float needle valve is the next line of defense. Everyone assumes a dripping carburetor is the result of a leaking needle valve. It’s not always the case.

I had three Cubs with constant fuel leaks ranging from annoying drips to massive floods. Between the three, there were leaks from the sediment bowl and petcock, fuel lines, carburetor gaskets and (of course) out the drip hole under the choke butterfly. Unless the tanks were empty.

First I repaired or replaced the sediment bowl assemblies and repaired the fuel lines and connections. Then I rebuilt the carburetors using IH kits, replacing idle tubes and floats as needed. I also performed some JB Weld repairs and straightened warped castings so everything mated and matched when assembled. I installed the carburetors and poured 5 gallons of gas in each Cub’s tank.
I made 3 fuel level gauges using Tygon tubing and hooked them up to the bowl drains with brass fittings. All three carburetors flooded and were dripping gas 12 hours later. The gauges showed two carburetors flooded, the third had high fuel level and just starting to leak. No signs of crud fouling the needle valves.

I borrowed advice from a variety of folks and looked closer at the pesky carburetors. Aside from the float needle, I found 5 other areas where the carburetor can leak gasoline. The most obvious are the fuel line compression fitting and threads at the inlet, the main jet gasket and the bowl drain threads. They are outside and you can see them seeping or dripping. The two sleepers are the gaskets sealing the main discharge nozzle and the needle valve cage. Both are inside the carburetor with their threaded portions submerged in gasoline. The discharge nozzle will seep fuel into the main bore and out of drip hole. A cage gasket will allow fuel to slowly overfill the bowl as if the needle valve were leaking. Sealing the threads will stop these two gaskets from leaking.

I hooked up 1-2 psi compressed air and used leak detector on the needle valve and cage to see where the leak came from. One leaked air between the gasket and casting, the other leaked between the gasket and cage. There is plenty of corrosion on this casting and the gasket can’t seal these pits and crevices. The third carburetor did not leak here, but leaked at the fuel discharge nozzle gasket instead. So, I need to seal the threads and gaskets to stop leaks.

If the teflon tape is too wide, trim it down to a better size.

Trim off the overhang. You don’t want loose ends to become another problem. Teflon tape did not work very well on the cage threads, so I use chemical sealants instead.
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When using a chemical sealant, you only need a little. Apply on the male threads, only one thread around the item and about 1 thread down from the tip. You don’t want any excess oozing out to cause problems later. I use a toothpick to apply it. Follow the instructions on the package. Don’t over-tighten these items when you install them. You run the risk of deforming the gaskets. Or worse, stripping threads.

Don’t use any old sealant to stop leaks in your fuel system. Read the instructions carefully. Don’t confuse the terms ‘gas’ with ‘gasoline’. In this case, gas is propane or natural gas. Many household sealants dissolve in gasoline. Test your sealant by applying it to a bolt, screw the nut over the sealant (let cure if necessary) and soak in gasoline overnight. Take it apart the next day and you’ll know if it is the right stuff.

After sealing the cage and discharge nozzle threads on all three carburetors, I let the sealants cure for 24 hours, reset the float level and reinstalled them on the Cubs. We took a merry trip around the neighborhood and parked with the fuel level gauges reinstalled and the petcocks left open again for 12 hours. Two carburetors leaked about ¼ teaspoon gasoline, the third’s fuel level was close to full. I removed all three carburetors and retested for leaks. No leaks at the cages, so the goop on the threads is working.

I replaced the IH needle valves and cages with the ‘off the shelf’ Viton needle and cage assemblies. Goop on the threads, 24 hours to cure, reset the floats and repeated the test again with very different results. Two carbs had no leaks – one had high fuel level and the other was perfect. The third one leaked 1 teaspoon.

Off with the carbs again. I reinstalled the IH parts and repeated the test - same results as the first time. Then I removed the IH parts and reinstalled the ‘off the shelf’ parts and repeated the 12 hour test with them again. One perfect, two in the early stages of leaking. Drove them around again, now only one leaking after 12 hours. I don’t think the Viton needles are consistently seating in their cages. At least the IH needles were consistent, but consistent in leaking. These Viton needles are a crap shoot as to whether they leak or not.

Then, I remembered a Kohler carburetor kit Albie recommended at the 2002 Cub workshop, so dug out my notes and found that info. I ordered three genuine Kohler carburetor repair kits (#25-757-02S) from Farmer’s Exchange ($7.30 each). I only used the needle, cage and gasket from the kit as the rest of the parts aren’t needed.

The left and center valves are brass with a Viton tip, a soft synthetic rubber. The IH valve on the right is solid metal, perhaps stainless steel. The Viton tips had a larger angle cone than the IH needle. All three valves have the similar clearance between the needle and cage wall. The inlet holes in the cage are all the same diameter.

Both the IH and ‘off the shelf’ kit valves are similar in size and both weighted 1.7 grams. The Kohler valve is much smaller in length and weighed only 1.3 grams, 25% less than the other two.
I sealed the Kohler threads, assembled, reset the float levels and let cure for 24 hours, installed carburetors, drove around the neighborhood and tested for 12 hours with the fuel level gauges installed.

It’s a shame I couldn’t find just the three parts I needed instead of the whole kit. I’d bet someone needs those excess gaskets without the needle and cage! I did not try an ‘off the shelf’ brand of the Kohler carburetor kit, having been burned with the TSC/Valu-Bilt kits. Once bitten, twice shy.

Success! After 12 hours, no leaks and no change in fuel level in all three carburetor bowls. So, I took all three for another drive and parked them for 7 days with the petcocks open. All three had NO LEAKS after 7 days. I think the reduced weight of the Kohler needle valve is critical to stopping the fuel leak. A lighter needle has less resistance to the upward pressure exerted by the float, thus producing a tighter seal against the downward pressure of the fuel. I’ll see how well the Kohler needles behave over time. Just like the petcock, the needle must make a perfect seal to prevent fuel from leaking while maintaining the seal against the downward pressure of the gasoline. Over-tightening the cage will deform the gasket and may deform the brass cage as well – changing the round hole into an oval hole. It’s a lot to ask of these little parts.

I’m still going to shut off the fuel at the petcock. Cheap insurance.
"Chance favors the prepared mind."
- Louis Pasteur

"In character, in manners, in style, in all things, the supreme excellence is simplicity."
- Henry Wadsworth Longfellow

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