Progressive throttle linkage

[38 Hump]

Does anyone have experience with progressive linkage on a 2/94 setup on a stock 8BA with an offenhauser intake? On the road is great, around town runs rich. Originally set up by Jere Jobe with positive linkage.

[ken ct]

Progressive linkage does NOT work on a 2x2 setup.Ive modified more than 1 of them back to straight linkage which works.Do not be fooled by claims that progressive linkage works. ken ct.Its correct for a tri-power but not a dual setup.

[tiquer]

Originally Posted by ken ct

Progressive linkage does NOT work on a 2x2 setup.Ive modified more than 1 of them back to straight linkage which works.Do not be fooled by claims that progressive linkage works. ken ct.Its correct for a tri-power but not a dual setup.

I have bee4n wondering about that myself. Thanks for the info.

[OLD...BILL]

I'am with Ken, opereate it like a four barrel .....OLD....BILL

[Barlea]

If two two barrels would operate like a four barrel, no one would have invented the four barrel, Old...Bill. ..B.

[41ford1]

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From my personal experiance Ken is correct. The progessive linkage does not work well. Because of the vacuum leak caused by the "secondary" carb the system is very difficult to tune to idle decently. I even tried running the "secondary" with an active idle circuit and could not get a smooth throttle response. As soon as the "secondary" came off idle it stumbled no matter what PVs were in it or where the accelerator pumps were set. I am currently running the carbs with straight linkage. The jets are 47s (Ken's recommendation) and PVs are 3.5. Runs perfect. Smooth tip in and decent power.

[stromberg81]

Progressive linkage can be setup if the the intake has a open plenum area..
Duane.

[41ford1]

An Offy regular dual has a common plenum. Thats why I bought it. After two years of trying to get it to work the way I thought it should I gave up on the progressive linkage.

[Bruce Compton, Canada]

Hi 38 Hump : To answer your specific question, yes, progressive linkage can be made to work quite well on a 2X2 regular open plenum intake, although I would'nt attempt in on a super intake where the carbs are farther apart, and starvation becomes an issue. No matter what the linkage is on a multiple carb setup, by far the most important concern is the minimization or elimination of all vacuum and air leaks as well as the fit of the throttle butterflies in their bores. If you want your dual carbed engine to idle down around 500 RPM, this is critical. If you are O.K. with a 900 RPM idle then air leaks and butterfly fit are not so omportant. Another major concern with the 94 type carbs is the elimination of the vacuum leak created by the usually warped surfaces where the throttle body and main body meet. Any leak here will allow the manifold vaccum that is supposed to hold the power valve closed to escape which in turn lets the power valve to stay open full time. This very common problem is why most guys prefer the Stromberg carb with it's mechanical power valve over the 94 and it's vacuum operated power valve. Once these issues have been dealt with, you can turn your attention to the linkage.
I have a '36 with a modified 59L ( Merc innards) and ran a pair of 97s and a pair of 94s on a Thickstun PM7 intake at different times trying both direct and progressive linkage ( I really enjoy tinkering). The pair of 97s were bone stock, and the pair of 94s had 0.050" mains and 3.5 power valves. To be honest, I did'nt find much difference between the linkage setups although with the progressive, you could certainly feel the second carb coming in around half throttle. I basically found no difference between either linkage setup as far as performance and highway fuel consumption was concerned although around town with the progressive linkage I was more often than not booting it just to feel the second carb working. In my opinion both linkage systems have a common problem in that they both use the throttle shaft of one carb to operate the second carb which actually tends to twist the shaft of the primary carb. This is more evident with the progressive setup as you have to physically restrain the shaft of the secondary carb with a spring ,creating a situation that requires more force ( torque) to turn it. I found I was always tweaking the idle screws on the primary carb.
To sum it up,in my opinion, progressive linkage does work (just like a four barrel with mechanical secondaries), but requires a little more attention to the afforementioned details than a direct linkage setup. I should also state that I went back to the stock single Stromberg 97 on my car as the gas price up here is approaching $5.00/gal. Good luck : Bruce

[1952henry]

I know a guy in my club that used progressive linkage on a couple of different 2X2 setups (94s, plugged secondary pv). He was very happy with them. Probably was quite careful to get the secondary butterflies to seal.

Question for any or all...It has been touted on various sites about starvation with various manifolds, especially when someone asks about blocking off one carb and running on the other. The reply is, "Don't do it, you will starve the front (or back) cylinders."

Has someone first hand experience with this (proof)? Such as a comparison of plugs, burnt valve, melted piston, etc? I only ask because how does a six (or I-8) cylinder not starve #1 and #6 (or #8) with a single 1V feeding it?? Specifically speaking, a longer block like a Chrysler Spitfire, or the even longer IHC Red Diamond (2V, I think), Hercules 500+cid (1V) or Hall Scott 1000+cid. Surely the distance from the center-mounted carburetor to #1 and #6 (#8) must be greater than the distance from a front (or back) mounted carburetor to the farthest cylinder on a flathead. For that matter, what about the center mounted carburetor on a Packard V12 or Marmon V16? Or an inline 8 Buick (not the dual carb version) or Packard, etc. Again, there is quite a distance from the carburetor to the farthest cylinder.

If I remember correctly, someone once tried to answer this, someone else asked, but the fellow answering sounded more like he was running for office. In other words, he said a lot, but didn't answer the question.

[Mart]

My personal experience: I set up my first 2x2 setup with a progressive linkage and it was just ok. Then I re-jigged it for simultaneous opening and it was better. I didn't even bother trying a progressive linkage on the next one I did, just went for a straight linkage.

Mart.

[Bruce Compton, Canada]

Hey 52 Henry, you brought up a good point. Getting back to the original question though, 38 Hump did,nt say if he was using a "regular" or "super" Offy intake, but even on a "super" the distance from either end carb to the opposite end of the intake runner is substantially less than all of the engines that you mentioned. My guess on the "starvation" question is that unless an engine had a carb mounted directly over each individual intake port, then some cylinder must run comparatively leaner or get less of a charge than another. I'm assuming that the engineers arrived at a comprimise allowing the "close" cylinders to run a little rich and the "far" ones to run normal ( at least leaner than the close ones). In this particular case, I cannot imagine a stock flathead running anywhere near lean with two 94s no matter what the intake design or setup linkage is. One also has to question as to whether we are talking a "lean" condition ( too much air and not enough fuel), or not enough quantity of the correct mix, as once the mix enters the intake the ratio should'nt change. I'm certainly no expert on this, but am surprised that Uncle Max or Charlie NY have not commented here as they have tons more experience than myself. I also would not be surprised that they may have some knowledge of actual tests you request to prove or disprove this " starvation" theory. I'd sure be interested in finding out too. Cheers : Bruce

[1952henry]

Originally Posted by Bruce Compton, Canada

Hey 52 Henry, you brought up a good point. Getting back to the original question though, 38 Hump did,nt say if he was using a "regular" or "super" Offy intake, but even on a "super" the distance from either end carb to the opposite end of the intake runner is substantially less than all of the engines that you mentioned. My guess on the "starvation" question is that unless an engine had a carb mounted directly over each individual intake port, then some cylinder must run comparatively leaner or get less of a charge than another. I'm assuming that the engineers arrived at a comprimise allowing the "close" cylinders to run a little rich and the "far" ones to run normal ( at least leaner than the close ones). In this particular case, I cannot imagine a stock flathead running anywhere near lean with two 94s no matter what the intake design or setup linkage is. One also has to question as to whether we are talking a "lean" condition ( too much air and not enough fuel), or not enough quantity of the correct mix, as once the mix enters the intake the ratio should'nt change. I'm certainly no expert on this, but am surprised that Uncle Max or Charlie NY have not commented here as they have tons more experience than myself. I also would not be surprised that they may have some knowledge of actual tests you request to prove or disprove this " starvation" theory. I'd sure be interested in finding out too. Cheers : Bruce

Well, I'm certainly no expert either, but unless someone provides concrete proof that a "starvation" condition exists, I'm going to believe it a wive's tail. Or, in today's vernacular, an urban legend. Not that I feel the need to run a single on a dual, just curious. People used to say you couldn't swim after eating...but I'm still here.

[41ford1]

[Question for any or all...It has been touted on various sites about starvation with various manifolds, especially when someone asks about blocking off one carb and running on the other. The reply is, "Don't do it, you will starve the front (or back) cylinders."]
I had to block off the secondary on an Offy regular dual due to a bad float valve causing it to flood. Ran it that way for about a month. The plugs showed no evidence of the rear cylinders being lean. The lean condition will be dependant on the manfold configuration.

I agree, the secondary throttle plate seal is critical in order to get idle right. I used new secondary plates and an oversized shaft in three different carb bases. I could not get a good seal of the plates near the shafts. This is just my experiance. I ran out of carb parts to keep trying.

[Mike51Merc]

I put together a 2x2 progressive setup on my friend's car using 2 94s. We got a secondary carb base from Vintage Speed that has no idle circuits and is designed to completely close. We also plugged the power valve on the secondary.

The regular Offy manifold has transfer (equalizer) ports that work like a common plenum. As Charlie Price summed it up: If you're thinking about fuel starvation think about a straight six or straight eight with a little carburetor in the middle, feeding fuel to nearby and far away cylinders at the same time. They don't have starvation problems.

Bottom line is that it runs great, so much so that I'm now tempted to convert my straight linkage to a progressive.

[Bruce Compton, Canada]

Sounds like you were on the right track 41. I went through several combinations of throttle bodies , throttle shafts and butterflies to get a good seal. Also, I had to remember that the butterflies edges are cut at an angle ( 7 deg. I think) and if you put them in upside down nothing works right. Just as important is the fit of the shaft in the cast iron bore of the throttle body as even an oversize shaft in an out of round bore will create a lean condition. With a single carb setup, you can get by with a poor fit of the butterflies as well as slop in the shaft but when you put two worn carbs on the same manifold it becomes impossible to get satisfactory results. Anyway, I'm bullheaded enough that I can't resist tinkering with this kind of thing and I guess that if these old flatheads ran perfect all the time we would have nothing to do and less reason to have one, while discussing our good and bad experiences with other guys doing the same thing. Cheers : Bruce