What was the reason for doing that? Upsides or downsides. Does it do anything for throttle response at very low RPM?

I think it's better to have some primary opening before the secondary opening. This makes driving around the pits and lanes smoother.

To expound on your question, I do not know of any advantage to a 1 : 1 opening. I think it would make the car hard to drive at low speeds.

I do have a 1:1 linkage on my carb. I went to that when I was strictly footbrake racing. It was suggested by a very successful, long-time foot-braker. His thought process was that it takes a few thousandths for the foot & pedal to go from stage RPM to WOT, and during that period of time any variation in throttle motion would have more of an effect on the 60' times with a progressive linkage.
The man was constantly looking for thousandths in consistency, (not performance). Honestly, I don't know if it tightened up the variance of my short times at all. But it did make 2 very noticeable differences: 1. My 60 foot times were definitely quicker by .01-.02 and you could feel it leave a bit harder, and 2. It was definitely more responsive to small part throttle opening as well, (just like what Larry was saying - Driving in the pits it is rather touchy, and going from Staging RPM to Launch RPM you gotta be VERY smooth with your foot!).
Carb is an Alcohol 1050 Dominator. To make it work for footbraking, remember you have a big "hole" in manifold vacuum to cover up at the hit, so two 50cc accelerator pumps and some massive shooter nozzles are required, especially on alky. (Just gonna say, you can't buy shooters as big as mine, but you can easily make them).
The pump cams and pump lever adjustments needed some fine tuning also.
With all that being said, I mostly use the Trans Brake these days, so none of that setup is necessary at all, but it doesn't cause any harm, so I left it, and I can easily go from FB to T-Brake or back.

Very good post. I, too, am a footbrake racer and leave at very low (1500-1600) RPM. I am fairly successful........many trailer checks on my garage walls. Recently I bought an E-85 carb. It was delivered with .040 shooters, 50 cc pumps and brown cams. First test was very bad stumble. I changed shooters to the largest I had which was .050 front .047 back. Went to a race this past Saturday. It was useable in the afternoon heat (85*) but by 4th round temp had fallen to 68* and that hit was much more noticeably bad. My guess is that in the heat the carb is functioning somewhat below acceptable but the cooler temp required EVEN MORE fuel to cover the hole and boom..... stumble......the sixty fell off by .035 and you know what happens then. Couldn't run the #. ATM sent .052 and .055 shooters to me along with 1:1 link to correct the problem. I'm gonna try the .055 first and see how that does. Thanks for your insight

We run a Holley Sniper throttle body injection & it comes with a 1:1 linkage. While I have no issues with it the pits or racing. There's a good many users that complain (throttle is too sensitive) about it when using it on the street. Holley makes a progressive kit for it & users state it corrects the sensitive throttle issue.

2BKING

When I tried to 1-1 I just didn't like how it acted. To me it was like the difference of a light switch and a dimmer. LOL

I am running BLP base plates now and they are much more progressive than even the plates that came on my carb (Barry Grant Gold Claw RS). It is probably slower to 60' but it helps keep me green and it's easier to cover the hit with pump shot. I am on alcohol and my car is very light so my set up may not be for everyone.

62 Galaxy, I really don't know anything about E85, other than it makes your exhaust smell terrible. So I don't know if the ideal AFR for it is closer to gas or alky. But I would agree with you trying the 55 nozzles first, and see if that helps. Especially as we move into cooler weather in the fall. What size is that carb? And what is your CID? Only asking because the closer one is to being "over-carbureted", the more pump shot you need. And also consider the pump cams - With a progressive linkage, you'll want a smaller volume, longer duration pump shot on your secondary. If "under-carbed", same with the primary. With a larger carb, you want a larger volume, shorter duration pump shot for the primary, and with a 1:1 linkage, same for the secondary.

One more tip for anyone who footbrakes and leaves at a low RPM - In cooler weather, or later rounds as the ambient temp falls, opening your idle mixture screws just a 1/4 turn will often cover a slight stumble at the hit. I got lucky a couple years ago, in a race that began on a very warm Fall afternoon. Car and driver were both on-point. They had Pro-Mods mixed in that evening, and I don't thing a single pair both made it down track in any round. Multiple oil downs. By 4th or 5th round it was after midnight. Temp had dropped a bunch, it was hard to keep engine temp up with all the sitting during track cleanups. Fortunately my opponent went red, because my car stumbled just a touch, but enough to ruin my pass. Another long cleanup, so I did the idle mixture adjustment.
No stumble next round, or the one after.
I left the carb the same for next race day, and in the heat it was blubbering fat and wouldn't build heat, so I had to lean it back down. But I have the notes in my logbook, and my checklist for night racing.

quote:

Originally posted by FootbrakeJim:
62 Galaxy, I really don't know anything about E85, other than it makes your exhaust smell terrible. So I don't know if the ideal AFR for it is closer to gas or alky. But I would agree with you trying the 55 nozzles first, and see if that helps. Especially as we move into cooler weather in the fall. What size is that carb? And what is your CID? Only asking because the closer one is to being "over-carbureted", the more pump shot you need. And also consider the pump cams - With a progressive linkage, you'll want a smaller volume, longer duration pump shot on your secondary. If "under-carbed", same with the primary. With a larger carb, you want a larger volume, shorter duration pump shot for the primary, and with a 1:1 linkage, same for the secondary.

347 sbf, 650 ATM Innovation carb. Not over carbed, some say a touch under carbed. E85 uses about one third MORE fuel than gasoline. I think the 55's will work. They are blanks that are drilled to exact diameter unlike stuff from Holley. Block off one side of Holley shooters and blow thru the other side. You'd be surprised as to the variation you feel with the same numbered shooters. I might go up 2 numbers on jets also as the air gets cooler. I'll keep the idle mixture screws tip in mind. Thanks for you help.

Should not need nozzles that large on a 650. 1050's will use 37-47 range, 850 generally 35-42 range. I've used stock gas nozzles on 660 E85 carbs. The trim screw is an excellent way for quick adjustment at the track. Temperature is very important and I don't stage at less than 180 on E85. Butterfly adjustment on t-slots to .020 or less to get full use of the slots. Sounds like the idle restriction needs to be some larger to supply the t-slots more before the mains start. Very small e bleeds and mab around .024-25 could help also. Depending on engine vacuum sometimes bleed air is needed to keep t-slot margin correct. Too large nozzles may cover the bog but risking a very lazy 60.