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The Search for a "Good" McCulloch BDC carb duckbill - by Jeff Campbell |
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Elder Rubber Company
(ERC) has built new duckbill tooling to produce gasoline resistant duckbills. These new duckbills will be made from a BUNA-N material, and will be available for the 2005 season. |
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Track Testing Results |
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On track running of the new ERC duckbills went well. Feedback is that the motors responded well and ran very normally throughout rpm range. Measurements taken before and after running showed only about
1% swelling, a 12 fold reduction over previously available EPDM rubber duckbills. |
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Gasoline Soak Test |
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36 hour gasoline/Klotz swell soaking test results show a reasonable 9% swell in these new ERC duckbills. That is a huge improvement over the 40% seen with SBR and EPDM rubber materials!!!! |
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Old Portion of Duckbill search article: |
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Yes, all 4 of the duckbills in the above photo were the same size prior to soaking, after a 6 hour soak, the EPDM and SBR duckbills soaked in Methanol/Klotz(KL100), showed no signficant swelling compared to
dry duckbills. The duckbills soaked in Gas/Klotz(KL200) swelled a lot, the EPDM part was the worst at 43%, the SBR duckbill swelled 39%. OK - if you are running methanol, readily available EPDM duckbills used in
automotive brake master cylinder applications should work fine. Where to get them, there are several distributors selling small quatities, for one try Elder Rubber (just ask for the McCulloch kart motor duckbills, they will know what you are asking about).
I did spend considerable time talking and meeting with Vernay, they are the ones that actually make all the 6 "tit" duckbills for the automotive industry, part number VA3131. They
sell them in EPDM and SBR rubber, but refused to give me a quote for fuel resistant materials, even at high volume, they wanted no part of it. If anyone else wants to try, good luck! |
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An alternative to the duckbill? |
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I pulled this nozzle check valve out of a Tillotson 239B carb. These check valves feed fuel off high speed needle directly into carb venturi, they are used on most of the Tillotson HL and HR
carbs used for Kart applications. This particular check valve is 1/4" in diameter, has a very small light weight while plastic check ball inside (probably nylon). The small size of the ball and design of the
valve makes it a possible replacement that could reasonably match the dynamic performance of a duckbill valve. |
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Another simplier option I discovered after making the nylon adapter sleeve, was to use a piece of 3/16" vinyl tubing (one size smaller than standard kart fuel line). Heat the tubing in hot water first, then
push in the nozzle valve and shove into carb body. NOTE: to date, I have not had time to fully test this nozzle valve duckbill replacement, nor have I heard from anyone that has been successful in getting it to work. |
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More alternatives to the duckbill |
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There are two more alternatives, that I'm currently investigating and will publish the details once evaluation and testing is completed. One alternative is to modify the BDC gas carbs to run methanol, which
would allow the use of readily available EPDM or SBR rubber duckbills. The second alternative being investigated is to actually eliminate the duckbill circuit entirely. This involves plugging the duckbill circuit,
redrilling the carb, and dropping the pop-off pressure significantly. This modification would most likely result in less than optimum performance of the carb, but might be a reasonable compromise to get the old stock
carbs running again true to vintage form. |
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