Racing Fuel Systems

I've been reading here and on the previous site, absorbing information and thinking I've got a handle on most of it. I ran Thermoquads on my car before borrowing and rebuilding a Holley 3310, then picked up a 750 Mighty Demon. When I found that carb stumbling at part throttle I began reading up on potential issues, and hearing the HSAB was too big as supplied, I swapped out its .039's for .025's which apparently cured the issue.

Researching further on the function of the bleeds in the metering block air wells, I haven't seen anything definitive addressing just how much they are exposed or covered under all operating conditions. The first illustration I saw (not in these forums) describing their function showed the fuel level just below the top orifice and the fuel bowl level even with that point, the second half of that illustration showed the fuel level low, uncovering all of the bleeds, and the fuel bowl level again matching that point, indicating highest demand operation. I don't think that was correct, maybe it was just an oversight or just a convenient form of depiction.

As I see fuel levels in a carburetor, and just addressing the main circuit, there are 3 separate compartments--the fuel bowl, the main well and the air well. Each has its own exposure to atmosphere, the bowl via its vent; the main well via (primarily) the booster and an emulsion bleed in its own circuit (near the high point according to some illustrations); and finally, the air well's exposure is via the HSAB. There is a "kill" bleed or anti-siphon bleed between the top of the air well and the main well but it appears too small to have major effect during normal operation. Please correct any of this I've got wrong.

Now for the big chance to get things wrong. At rest with a full fuel bowl, and for that matter it should apply when the carburetor's only being run on its idle circuit, fuel level in all 3 compartments is the same with atmospheric pressure equal in all 3. The fuel level is the same because it flows from the bowl, via space behind the main jets, to the main well, and from there it leaks into the air well. The air well does not "see" the main bowl directly, only via transfer first to the main well and then leaking in via the small bleeds. For starting point's sake let's say the fuel in the air well is just at or below the top bleed orifice. It looks to be about there anyhow, eyeballing the window in the bowl of my MD 750.

Once the main circuit is activated via drop in pressure above the fuel in the main well, it pulls fuel from the bowl as its main resource and in part maybe influenced by lesser restriction from jet/PV on one side as compared to small holes submerged in the air well. Still, it seems it should draw fuel down in the air well, eventually uncovering more e-bleeds over time. At this point the fuel in the bowl should be feeling consistent atmospheric pressure and the fuel in the air well should be seeing some measure less than atmospheric since some is being drawn by the top bleed. There doesn't look to be any sort of venturi effect on the e-bleeds from all the fuel passing up the main well, but is there at least some draw from it passing by, beyond comparative atmospheric pressure present in the air well vs that in the main well?

Throughout operation the fuel level in the bowl should remain relatively constant via fuel supply system, but what happens to fuel level in the air well under various conditions, including WOT pulls?

Welcome aboard! You already have the TQ, best 4bbl made!! You obviously have good knowledge of carb operation. Any thoughts on the size & position of the holes in the secondary discharge tubes on production TQs. Have never been able to get an answer on how/why the size & position is chosen.

The best description of how the emulsion tube & bleeds work will be found in the late, great John Passini's Weber book, ISBN: 9781855207592. He has forgotten more than the rest will ever know.....A small book packed with knowledge.

Read through this. It should add to knowledge and discussion

https://www.tapatalk.com/groups/british ... -t294.html

Before going much further on your journey this modification is worthwhile to be able to fully adjust the IFR and e-bleeds on your Mighty Demon. I have used shouldered threaded bleeds but socket head are as effective. Threads are 6-32.

kgb.

TQ jetting. If you have, or can get hold of, the Mopar Performance book it has jetting tips on the TQ. One application with a single plane intake, reqd 169 sec jets. So you were going in the right direction. It also recommended the sec blades be 88* at WOT for a single plane & 81* for a dual plane intake. The WOT position of the air valve also changes the A/F ratio. This is the beauty of the TQ. Simple changes at the track.

Something I have done for years on production TQs which noticeably increases performance: solder shut the holes in the sec discharge tubes; then cut off the bullet ends at an angle, similar angle to the Comp Series TQ [ that is where I got the idea from ]. Run a 3/16" drill bit inside the tubes to make sure solder has not blocked them. I use a 80W electric soldering iron & 18g resin cored solder.

My cousin jumped down this rabbit hole working on his carburetor, told me about moving the IFR and I've read a lot here about the benefits of it. I'm currently in the place of "not messing with what works" since the MD idles down to 850rpm very nicely although I'd appreciate if it were a bit smoother. I'd like to move it and replace with a smaller one, trying to get the cruise levels leaner and more in line with what the TQ gives me. Not that I put a lot of street miles on it anymore, and I've lost the advantage of vac advance for those conditions as well, I can always put the factory design tach back in for anything other than testing on the street which is pretty much all it got last year besides the bracket racing. I've no doubt I'll get around to the relocation.

I have A Mopar Performance Engines book (8th edition by Larry Shepard), not sure if it's THE MP book as it doesn't address the TQ very much. Most of what I've learned about them is from the Carter Carburetors/Super Tuning and Modifying book. I was pretty comfortable with it up until I saw what it was doing as it wound out WOT, it's a very nice runner otherwise. At idle, with either carb, my combination doesn't pull much vacuum--well under 10", yet both carburetors idle and run pretty well. I won't know how bad they're actually running until I make some improvements as I find on this forum, then wonder how it was so acceptable for the past couple decades. Doesn't seem the TQ in particular should behave as well as it does when the combination is this far from what they designed it to work on.

Regarding what's going on in the Holley/Demon air wells, without a sensor of some sort to record details of fuel level and vacuum it doesn't look like anyone will be able to declare for certain how and when fuel levels change throughout its range of operation. Not that it's a bad situation, I do like to understand what I can about things like this and am okay when I run into a wall and have to proceed with knowing that it does work without knowing exactly how.

When I read that replacing the .039" MAB with a .025"-ish version would cure the MD's particular ill I just did it and got the result I was after. I knew very little about e-bleed placement and sizing, and believing what I'd read and then experienced about the MABs influencing intitation points for main circuit activity, I assumed from illustrations I'd seen of small holes drilled into the main fuel circuit passages near their high points within the carb body that those were also fed by the MAB. I have not yet found anywhere reading on this forum about that being so, all air going from the MABs to the main circuit must exert its influence solely via the crossover bleeds and e-bleeds within the air wells.

As I record A/F under various conditions, or even just monitor while driving around, I'll be curious to know that change X results in reading Y. Something like changing jets or power valves, or changing an IFR or blade settings/mixture screws should be easy things to estimate and confirm. Numbers, placement and sizing of e-bleeds and the MAB that feeds them is a bit beyond me at this time of ignorant bliss, or heading toward it.

Tell us what your combination is.

You are over analyzing things which leads to little accomplished or mistakes from your assumptions.

I believe you mentioned that you have David Vizard"s Holley Carburetor book . Read page 17 . That is what you need to know.

E-bleeds are the last thing to alter on most carbs when tuning with a AFR/gauge. And usually do not require adjustment.

I don't believe you said what version 750 Mighty Demon , Annular or Downleg , was it new or used, Holley or Barry Grant .

Changing from .039' HSAB to .025" would have a second effect to fuel ratio at higher rpm WOT. You didn't mention that.