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 Post Posted: Sun Jun 24, 2012 10:03 am 
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Hi Gang, I've been registered here for a long however I never posted much. I've been interested in the 2 circuit conversion for several years just never did anything about it untill now. I'm about to jump in to convert two 1250's for use on a tunnel ram manifold... I have repeatedly read the dedicated threads and I have been in contact with Jmark and Bruce[nomad] who I met at the BLP Carb shootout.. I still have been unable to understand why the intermediate circuit [3rd cir] has such a profound effect on the idle circuit while the car is just idling? Or does it? If a properly tuned 2 circuit can sit in the driveway on top of a race motor and idle so cleanly that the once blackened exhaust ports turn light gray why cant a 3 circuit with aftermarket metering blocks do the same? I believe I understand the impact of the intermediate circuit under light throttle or "cruising conditions and can see how that would or could impact the fuel curve,and I understacd the premise of the undersized main well with the idle feed tube restriction issues on jetting. But it is the idle quality of the idling race car that has me puzzled. Won't lowering the idle feed jet to improve or stabilize fuel flow through it and the TSJ install to meter flow to the slots,on 3 circuit blocks result in the idle quality improvement seen with the 2 circuit conversion? I'm not trying to be skeptical but the more I read the more questions I have , I absolutely believe the 2 circuits mods work I just dont understand the reasons especially the ones to clean up the idle.. A .040" Idle feed is a .040" idle feed wether its in a 2 or 3 circuit aftermarket block...I think ...lol any wisdom here would be appreciated.. I will keep you posted how this process unfolds.... Thanks for your patience with a newbie....Mike


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 Post Posted: Sun Jun 24, 2012 10:49 am 
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Great question, I'm wondering the same thing...........

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 Post Posted: Sun Jun 24, 2012 10:51 am 
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Mike, it's my understanding that the 3 circuit carburetors were originally designed to work with an IR intake that Chrysler built for their Hemi engine years ago. That 3rd circuit was suppose to help solve problems with reversion pulses from that IR intake.

It's also been said that the additional circuit can help with "shift recovery" and I know that some carb builders have some that have as many as 4 circuits in'em, ie Braswell comes to mind as I think I remember Doug Schreifer of Braswell posting a picture of one here a few years back.

But back to your question about idle, I'd venture to guess that idle quality CAN be achieved with a 3 circuit carb but remember that as soon as you go opening the throttle then THAT is where the problems start as the placement of that 3rd circuit dump tube, especially in the old style carbs which causes fuel flow at relatively small throttle openings.

The other major drawback is the tube in the mainwell that you mentioned but in a dual carb configuration I don't think that is an issue. Someone can correct me if I'm wrong on that but it's my understanding that when you have 8 venturi feeding an engine instead of just 4 then fuel delivery isn't a problem.

Several have reported that they have successfully blocked off that 3rd circuit in a stock metering block but I'd have to see some AFR data to believe that.

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 Post Posted: Sun Jun 24, 2012 3:53 pm 
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Idle quality has nothing to do with the intermediate circuit, the location of the idle feed restriction has everything to do with it. Idle feed location also impacts the operation of the main circuit as well, there was some great info on the Innovate site but unfortunately it's been closed. The idle fuel delivery is more stable and consistent on a race engine with low idle vacuum when the idle restriction is submerged. Along the way someone had an application that the higher location made an improvement, probably not to the idle quality but in the difference in the emulsion it feeds back at higher RPM's. With improved knowledge in engine design and metering blocks with a larger selection of emulsion holes and position messing up the idle circuit operation is no longer needed to have the same effect.

The intermediate circuit issue is that there is usually no need for it, with the increasingly larger engines IF the idle, transition, and main circuit are tuned correctly and passages properly sized you get a smooth transition from one circuit to the next with no "hole" to have to fill with an intermediate circuit. So an intermediate usually make the engines run excessively rich in the range it provides most fuel, usually seen in the burnout and sometimes on the return road. This leads to black plugs. Why add fuel if it doesn't have to be there, and fuel that is NOT atomized anywhere near as well as fuel passing through the booster?

The other issue with the standard 3 circuit blocks is the restriction from the idle feed tube in the mainwell. Some are more of an issue than others, and engine fuel requirements from larger engines or oxygenated fuels make the issue show up faster. So then you lean the intermediate to clean it up, that takes away more fuel from higher RPM's that you can't make up for with larger jets. At this point the mainwell becomes the restriction, this messes up the proper operation of the emulsion, so you end up on a merry go round.

If you prefer to maintain an intermediate circuit my choice is the BLP wide body blocks. They feature an idle circuit like the 2 circuit blocks have, you can place the idle feed in the lower position. Mainwells have no restriction to prevent the intermediate circuit from being plugged if necessary.


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 Post Posted: Sun Jun 24, 2012 8:48 pm 
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Jmark and Ken Thanks for the replies... I do not see any advantage to having a 3rd circuit to provide any supplemental fuel even with the BLP wide body option. I think the extra fuel would unnecessarly complicate tuning the WOT fuel curve. Having that extra tip in of fuel with the 3rd circuit when you move off idle doesnt help the drag race car or the street driven car...sooo who needs it.
. Mark I understand your explanation that; pulling idle fuel from the lower or "submerged" location improves fuel flow through the idle circuit of a low vacuum race motor but isn't it ultimately then the size of the idle feed jet and the IAB that determines the A/F quality at idle? Is smooth flow through a .031" restrictor going to produce a cleaner idle than a pulsing or less than smooth flow through a .031'' restrictor jet with equal corresponding air bleeds?.. It would seem that that the less than optimum flow would yield a leaner idle quality??. My 1250's exhibit the classic symptoms of pig fat spark plugs at idle and part throttle however the WOT fuel curve can be richened sufficiently down track to loose MPH. They are equipped with BLP 3 circuit blocks, the angle channels are at .160" and they have the idle tubes in there. The idle feed is at the top of the blocks and currently have .031"
jets and the IAB is .045"...The engine is a little blubbery after the burn out which I attribute to the 3rd circuit extra fuel and the fat idle circuit calibrations although those numbers for the idle don't seem to far out.. Can the idle feed on the BLP 3 circuit blocks be reocated to a lower "submerged"
location like the 2 circuit BLP blocks pictured by Scott Smith in his conversion build up? I suspect not lol. The 3 circuit blocks dont have those two holes available directly beneath the emulsion stack as the 2 circuit blocks do... Thanks again for your help... to recap: the location of the idle feed[bottom] is critical to good idle circuit performance and very few aplications really need that 3rd circuit....lol Mike


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 Post Posted: Sun Jun 24, 2012 10:09 pm 
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Mike, I believe the idle restriction can be lowered on the wide body blocks. The existing passage is too big for the 6-32s we normally use for emulsions and idle restrictions in the 2 circuit blocks. I believe an 8-32 may work. I've a new set of wide bodies out in the garage. I'll look in the morning and post back here.

By the way, welcome to Motorsports Village. We're glad to have you aboard.


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 Post Posted: Mon Jun 25, 2012 7:29 am 
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Mike, it's the inconsistency of the fuel delivery that make idle worse when the idle feed is up high. When the metering blocks have idle feed tubes the only way to lower the restriction is at the bottom of the tube like the earlier blocks had. I believe they got away from that to save costs, a straight tube is easier to make. Wide body blocks have the idle circuit the same as a 2 circuit block, and those can be relocated easily. I was able to use 6-32 at the lower position on my big carb for the idle feed, it has the wide body blocks with the intermediate plugged, except for the panic pull to get enough fuel in the corner runners at the challenge... And we all know the fun that engine was.

Consistency of the fuel delivery is also seen in a carb with a lot of emulsion in the mains, you get slugs of fuel then pockets of air, but the amount of each is not the same every time. With the delivery erratic you don't idle as well, and the same if the mains have too much emulsion. You can cover some of this for the mains with the intermediate, but then you have poorly prepped fuel out of the discharged tubes compared to the boosters.

On an engine with a tunnel ram, you don't gain HP because you are feeding the engine twice as much air, you gain most because you are able to more equally distribute fuel to the cylinders. Since the engine does not see a large change in airflow, the metering blocks with idle tube are passing much less fuel than on a single carb engine. You don't see a flow issue through the main well in this case, and can easily jet the engine to lose time even with an oxygenated fuel.


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 Post Posted: Mon Jun 25, 2012 7:34 am 
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The NACA paper 49 that discusses this can be downloaded from the NASA archives at the following link.

http://naca.larc.nasa.gov/search.jsp?R= ... tor%252049


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 Post Posted: Mon Jun 25, 2012 8:01 am 
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Thanks Guys .....Carby school is in session !! Bruce sent that paper on Saturday and I'm just getting into it...... So You guys think the 3 circuit wide body blocks are good for this conversion?


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 Post Posted: Mon Jun 25, 2012 9:09 am 
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Mike, as Mark said you can plug off the intermidiate with the wide body blocks. In essence, you can try things both ways. That said they require a modified accelerator pump arms and longer bowl screws.

I don't know it we talked about this but, are you running an O2 of any kind?


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 Post Posted: Mon Jun 25, 2012 9:49 am 
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The wide body 3 circuit blocks will work fine with any Dominator.


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 Post Posted: Mon Jun 25, 2012 10:42 am 
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Okay I just checked. The idle passage that needs to be drilled for a 6-32 is larger than the #29 drill needed for 75% thread engagement. A #22 drill fits perfect in the existing passage and only needs to be tapped for 50% thread engagement.

I start every hole with a pilot tap and finish with a bottoming tap so the set screw can seat and seal on the bottom thread. It only takes a thread or two with the pilot tap to get things going. Then you can go right to the bottoming tap.


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 Post Posted: Mon Jun 25, 2012 11:13 am 
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Bruce ..We do not have any O2 sensors at this time..We have a racepak for normal data but nothing to monitor A/F ratios except a really big box of crappy looking 3932 Autolite spark plugs =; Thanks Mike. Glad to be aboard


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 Post Posted: Mon Jun 25, 2012 11:38 am 
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mike fabish wrote:
Bruce Thanks Mike. Glad to be aboard


So what's the cost of adding O2 capabilities to a Race Pack logger? :-k

I run that same AR3932 plug in my engine on methanol. What kind of life do you get out of them on gas? Reason I ask is my engine will go through two sets a season?? O:) NGK -10s last longer but the engine doesn't seem to make as much power with'em.

Obviously Bruce didn't notice that you joined here back in March of 2008? O:) ;-)

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 Post Posted: Mon Jun 25, 2012 3:22 pm 
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Ken, I'm not sure of the cost of the O2's maybe some one knows a good ball park figure...I'll check with the dealer we bought the Racepak from.... When were not messing around "tuning" I'll go a couple of races on a set of the AR3932's... I do use warm up plugs to get the engine up to temp and then change them out to our "race" set.... The warm up usually soots them up pretty good, I've tried 3233's and 3234's as warm up plugs and they dont stay clean for long...We don't usually drive the car up to the line , or drive it back, usually tow it back and forth with the golf cart....


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