So my question is, why do most, if not everyone, say that for a multiple carburettor set up, (thinking mainly about in-line but could apply to other set-ups), you must have extended throttle shafts ? Looking through the multi-carb picture thread on here, I see that most do set up with extended throttle shafts and the secondary’s in a tri-power set up will be fixed on one side and the primary activates one secondary from the other side. What I would like to know is what is the reason for this, if there is one. Surely with extended shafts, there is more chance of vacuum leaks. The only advantage that I can see is more room for throttle linkages. I would be interested to hear your comments or opinions. Thanks, Clemens.
Not sure why people do them. I have run the factory dual quad set ups on both SBC and 409" cars, as well as 3 X 2 setups. Used either the factory setup or aftermarket ones. Never had to use extended shafts.
It’s necessary in my case. Gotta get the far two over enough. Although there is a bit of flex that I would rather was not there.
Slightly off topic for the intent of the original question...but... Most of my linkage will be mounted on standoffs on the left side rocker cover. Two bell cranks, one short rod, and one long rod. All connected to the stock Holley throttle arms. Mike
I hung a lot of linkage and springs on the side of mine so the long shafts were a plus. There are extensions on the two far side carbs that come across and go through the support. This is a progressive set up versus the straight set up Dreddybear shows for four two barrel carbs. I don't know how extended shafts affect vacuum leaks as long as they are fitted to the body of the carb correctly. The system you show Clem looks to be OEM vs hotrod.
One thing not mentioned yet is the extended throttle shafts are typically oversize by 0.003" or so. That way your worn out throttle body base plate can be reamed to a good as new condition. Other than that, I believe the extended throttle shafts just make linkage easier, as well as more universal.
Just to clarify a little, - I was referring to ‘extended’ as in coming out of both sides of the carbs, (left and right side), hence 2 points of wear and 2 potential vacuum leaks as the throttle shaft or base wears. please keep the comments and pictures coming, as I need good ideas. Thanks ! .
Do you think that you are going to drive your vehicle enough miles to wear out the shaft bores, or the shafts themselves ?
This made from mostly Ford 3-2 setups for the 406 and parts from a SBF too. There is a slider in the front and rear carb. The slider for the rear carb is actually on the center one. Pedal feed is attached to a cross feed on the right of the rear carb mounting studs but all the linkage is on the left side. Works great but took a lot of work and mistakes to get correct. There are no extended shafts. You can see it’s all done with screws and nuts on the existing arms attached to the shafts when the carbs were assemble new.
^^^^Thanks, this is really helpful to me. Gives me a bit of confidence going forward when I see some of these close up photos, that I’m on the right track.
Expending the throttle shaft doesn't change the number of wear points, the shaft still rides in 2 bushings on either side of the throttle passage in the lower housing or base of the carb, with the same potential of vacuum leakage. There might be some additional force causing wear on the 2nd bushing due to action of the linkage, but there's no certainty of that.
Possibly, but in reality, probably not, but that wasn’t actually my question. I have absolutely no idea how many miles or hours that would take, perhaps your experience could help answer that. The person that rebuilt my carbs used nylon or plastic sleeves, so quick wearing of the sleeves possibly will be an issue for me. I have put my original question from my first post above your comment/ question just to clarify. I had also wondered if extended shafts either side was to lessen the force on the shafts, but it seems unlikely. From the answers so far, I’m thinking it was mainly done for simplicity of running the linkages and perhaps simplicity of adjustment. it seems some carb rebuilders are adamant that you must use extended shafts, while the photos above surely suggest otherwise.
A flatened 3/16" line (or 1/8") one end with a hole drilled in it and a internal thread on the other end with a heim or clevis makes linkage look store bought. The Ford restoring suppliers make repop sliders.
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