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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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Last edited by Gordon Nichols
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It's not a "stubby", but I would say somewhere around 4" long?  Shorter doesn't gain you much in turnbuckle length, while longer needs a longer turnbuckle.  The rubber band is needed to pull the wrench fast to the bar AND when pushed up against the sides of the wrench it keeps it from moving sideways.  A set-screw would have been cool, but drilling into a hardened wrench is a no-go.

The center aluminum part of the turnbuckle is 3" (so I guess the wrench I used is 4"-ish) and I really needed a 4" version but that wasn't available at my ACE hardware store, so I opted, instead, to just buy one longer eyebolt (the brighter and longer one in the photo) to extend the reach.  Get the wrench on the bar, measure from the other end to the stud and that's the turnbuckle range you'll need.

Teflon tape on the eyebolt threads helped a lot in keeping everything from moving when the engine is running and gives it a nice "feel" when turning the buckle to change engine speed.

I've always undone the adjustment turnbuckles on both sides, set the idle speed to  1000 RPM with both carbs synced via snail meter, then reconnected the linkages and adjusted the turnbuckles, feeling for the same free play on both sides. Start the engine, then check for RPM and balance. If both are correct, I'm done. Is there are reason syncing at 2000 RPM is better than syncing at idle?

McGruff:  yes.  It eliminates the small difference you’ll get from that “free play” you feel on either side when tightening them up.  

Adjusting them with the engine off idle (I say 2K rpm but 1,500 and up is fine) will put force on both links as you adjust/tighten them and you can sync them closer. 

I increase speed with the tool to the point where the higher side matches the moving snail needle with a reference bar perfectly, then bring up the lower side to perfectly match needle and same reference bar on that side - it’s not going to be very much different, but it will be different when you start.  I have always noticed the engine running smoother after adjustment than if I just static balance them at idle.

Of course, that may just be “Mechanic’s Optimism”, too.  Jus’ sayin’......

Last edited by Gordon Nichols

It's not really that difficult, just intimidating the first few times (and Danny will back me up here...) - You have to be patient and do each step slowly and allow things to settle on each step.  A "Unisyn" tool is pretty much useless in my estimation (I have one, somewhere, unused), but a "Snail" gauge is indispensable.

https://www.cbperformance.com/...ts.asp?search=CFMCFM

Danny P. has posted a great article on here that tells you how to do it.  The hardest thing about the whole process is finding the mixture and air by-pass screws on the sides of the carbs if you need to adjust them - Really.  I use a mirror to see where things are and then just find them by feel.  The mixture screws are easiest as then stick out, but the bypass screws are a little more hidden and hard to get at.   Still, no big deal.

Besides, all your neighbors (and especially your wife) will wonder what the hell you're doing over there when you're syncing things at 2,000 RPM for five minutes in your driveway.    Or in your case, using the garage as a megaphone to blast that sweet exhaust note out into the rest of the neighborhood.   

Last edited by Gordon Nichols
DannyP posted:

I know the play is limited in our crossbars due to the heim joints, but the bar really needs to be pulled from the throttle cable attachment, all things considered. Especially true for people that have slop in the linkage.

I like what Danny says (again).  All of us are surely converted to heim-joint linkages by now, but one must also consider that the hex-bar has "torque flex" from one end to the other; enough to make a quite noticeable sync variable.

For example, pull off idle very slightly on one side's carb actuating horn and you'll notice the engine shake as the carbs don't come on in sync.  The other side's carb is pulling in late because of torque twist in the hex-bar.  On the other hand, pull from the throttle cable and the sync is smooth (presuming the sync was performed with the cable or its attachment point). 

Also, I like to make my first sync adjustment at just barely off-idle.  That is when you can tell if one carb is trailing the other.  Then go to 2000-3000.  If not sync'd there, then maybe some linkage geometry issues are at play.

My car has some fancy accelerator cable with a quick release on the end and uses a bracket as a stop.

I used Danny's idea of using the same attachment point of the cable and Gordon's use of the turnbuckle and came up with this.

Sync tool 1Sync tool 2Sync tool 3

Yes I know, I haven't done the heim joint upgrade yet.

The bracket that the cable housing stops on is also kinda cheesy. I'll put that on the list too.

 

 

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