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Five Cent Racing's Engine Throttle Hold Tool

I'm soon going to be dialing in the two adjustable links on my hex bar linkage - You know, those turnbuckle-like links going from the hex bar's end levers down to the throttle actuator arms on the carburetors.   The process is to release one link (either side will do) and balance your carbs at idle, then re-connect the adjustable link, run the throttle up to 2,000 - 3,000 rpm and hold it at one steady speed, then adjust the link on the lower draught side to make it sync with the other side.  Sounds easy, but made difficult by the engine rpm drifting around as your hold on the linkage arm or your partner's foot on the accelerator varies slightly, potentially throwing your adjustment off.

So, I came up with the Five Cent Racing Engine Throttle Hold Tool to give you a precise rpm and hold it there while you adjust the link and then tighten the locknuts.

The tool is made from a 1/2" wrench that fits onto the hex bar as a lever.  

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The wrench is connected to a common 1/4" X 20 turnbuckle and I wrapped the threaded ends of the turnbuckle eye-bolts with teflon plumber's tape to keep them from loosening under engine vibration - just enough to gently hold them in place but still allowing you to turn the center portion with your hand to make the ends go in or out:

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The other end of the turnbuckle is fastened to one of the engine's top case studs (I had one that happens to be protruding about 3/4" on the driver's side and have no fuel pump there so the area is open).  If your case stud doesn't protrude very far, get a turnbuckle with an eyebolt big enough to fit over the case stud nut, then use an 8mm nut and washer to hold it in place.  Whatever works.

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I found that the wrench on the hex bar can tend to wander along the bar under engine vibrations, so I added a rubber band around the hex bar just to firmly hold it in place.  That seems to work well and is cheap.  If you're desperate, you could even tape the wrench to the bar with electrical tape.  That'll surely hold it in place!

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How to use:

The following is best done with an engine already warmed up and sync'd at idle.  With the engine stopped, you assemble the throttle hold tool to the case stud and hex bar, slip the rubber band behind the hex bar and pull the ends out to catch on the bolt holding the wrench (see photo).  Loosen and back off both locknuts on the adjustable link on one side.  

Start the engine, letting it idle to smooth out and then adjust the turnbuckle so the engine begins to increase in RPM until you get to your linkage tuning speed (I just use 2,000 rpm but just about anything above a really fast idle will do).  Gently adjust the loosened adjustable link on the carb to make both sides pull the same rate on a "Snail" meter for carburetor air flow in each throat, left and right.  Once you get them even, re-tighten the lock nuts, re-check and re-adjust if necessary - it usually takes a few tries to get them even after tightening the locknuts, but your patience will be paid off with months/years of a smoothly running engine.

Once everything is adjusted, back off the throttle speed turnbuckle to return to idle, stop the engine (it's always good to let it cool off for a while by now) then remove the throttle hold tool and store it.  Admittedly, this is NOT a tool you'll use every day or even every year, but when you need to hold your throttle for a while, like setting timing or checking fuel pressure at speed or something like that, this is a very handy (and inexpensive) way to do it.  Total cost is about $10 bucks.  I toss mine into my tool back in the Frunk.

Sorry, I will not be selling these as a complete kit.  It's just a wrench a turnbuckle, some washers and a 1/4-20 bolt.  Follow the photos and you can build one, too.

Another nifty shop trick from the Five Cent Racing "Sanctum Sanctorum"

Sanctum

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  • Sanctum
Last edited by Gordon Nichols
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