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Racing Fuel Systems Forum for carb guys. 2023-09-21T18:09:47+01:00 https://racingfuelsystems.com/feed.php?f=5&t=380 2023-09-21T18:09:47+01:00 2023-09-21T18:09:47+01:00 https://racingfuelsystems.com/viewtopic.php?t=380&p=2621#p2621 <![CDATA[Re: 750 Mech Secondary IAB sizing question]]>
The carb idle configuration differences (2 corner/4 corner) is what I was missing and things makes sense now.

Scott P

Statistics: Posted by SP70Nova — Thu Sep 21, 2023 6:09 pm

]]> 2023-09-13T10:48:51+01:00 2023-09-13T10:48:51+01:00 https://racingfuelsystems.com/viewtopic.php?t=380&p=2584#p2584 <![CDATA[Re: 750 Mech Secondary IAB sizing question]]> Statistics: Posted by cat herder — Wed Sep 13, 2023 10:48 am

]]> 2023-09-13T10:39:20+01:00 2023-09-13T10:39:20+01:00 https://racingfuelsystems.com/viewtopic.php?t=380&p=2582#p2582 <![CDATA[Re: 750 Mech Secondary IAB sizing question]]>
Based on my notes on what I've read over the years, there seems to be different schools of thought on this:
1. match the primary side IAB
2. size similar to a vacuum secondary IAB since the time spent in or transitioning through the transfer slot will be very short.
I understand the need for a smaller IAB on a vacuum sec carb since there is no accel pump shot available for the secondary's.

I'm lost on this because mechanical or vacuum secondary will usually have similar size secondary IAB these days.
Example : my 770 quick fuel vacuum secondary with 4 corner idle has .031" IAB primary and secondary.
Of course there are exceptions.

Statistics: Posted by rgalajda — Wed Sep 13, 2023 10:39 am

]]> 2023-09-13T10:23:14+01:00 2023-09-13T10:23:14+01:00 https://racingfuelsystems.com/viewtopic.php?t=380&p=2581#p2581 <![CDATA[Re: 750 Mech Secondary IAB sizing question]]> Statistics: Posted by rgalajda — Wed Sep 13, 2023 10:23 am

]]> 2023-09-05T17:40:34+01:00 2023-09-05T17:40:34+01:00 https://racingfuelsystems.com/viewtopic.php?t=380&p=2547#p2547 <![CDATA[Re: 750 Mech Secondary IAB sizing question]]> Statistics: Posted by SP70Nova — Tue Sep 05, 2023 5:40 pm

]]> 2023-08-29T17:16:20+01:00 2023-08-29T17:16:20+01:00 https://racingfuelsystems.com/viewtopic.php?t=380&p=2522#p2522 <![CDATA[Re: 750 Mech Secondary IAB sizing question]]> Statistics: Posted by rgalajda — Tue Aug 29, 2023 5:16 pm

]]> 2023-08-28T19:35:51+01:00 2023-08-28T19:35:51+01:00 https://racingfuelsystems.com/viewtopic.php?t=380&p=2519#p2519 <![CDATA[Re: 750 Mech Secondary IAB sizing question]]> Statistics: Posted by SP70Nova — Mon Aug 28, 2023 7:35 pm

]]> 2023-08-27T01:46:56+01:00 2023-08-27T01:46:56+01:00 https://racingfuelsystems.com/viewtopic.php?t=380&p=2516#p2516 <![CDATA[Re: 750 Mech Secondary IAB sizing question]]> Statistics: Posted by 1969Camaro — Sun Aug 27, 2023 1:46 am

]]> 2023-08-26T09:57:19+01:00 2023-08-26T09:57:19+01:00 https://racingfuelsystems.com/viewtopic.php?t=380&p=2515#p2515 <![CDATA[Re: 750 Mech Secondary IAB sizing question]]> Statistics: Posted by rgalajda — Sat Aug 26, 2023 9:57 am

]]> 2023-08-25T21:06:59+01:00 2023-08-25T21:06:59+01:00 https://racingfuelsystems.com/viewtopic.php?t=380&p=2514#p2514 <![CDATA[Re: 750 Mech Secondary IAB sizing question]]> Statistics: Posted by 1969Camaro — Fri Aug 25, 2023 9:06 pm

]]> 2023-08-25T17:26:22+01:00 2023-08-25T17:26:22+01:00 https://racingfuelsystems.com/viewtopic.php?t=380&p=2513#p2513 <![CDATA[Re: 750 Mech Secondary IAB sizing question]]> Statistics: Posted by SP70Nova — Fri Aug 25, 2023 5:26 pm

]]> 2023-08-22T20:21:03+01:00 2023-08-22T20:21:03+01:00 https://racingfuelsystems.com/viewtopic.php?t=380&p=2504#p2504 <![CDATA[Re: 750 Mech Secondary IAB sizing question]]> Statistics: Posted by 1969Camaro — Tue Aug 22, 2023 8:21 pm

]]> 2023-08-22T18:44:11+01:00 2023-08-22T18:44:11+01:00 https://racingfuelsystems.com/viewtopic.php?t=380&p=2503#p2503 <![CDATA[750 Mech Secondary IAB sizing question]]>
Based on my notes on what I've read over the years, there seems to be different schools of thought on this:
1. match the primary side IAB
2. size similar to a vacuum secondary IAB since the time spent in or transitioning through the transfer slot will be very short.
I understand the need for a smaller IAB on a vacuum sec carb since there is no accel pump shot available for the secondary's.

My questions has to do with the the fuel curve of the secondary's:
Since the accel pump is available and is tied to the throttle movement, except for when the throttle is held in the secondary transfer slot. 1. Would the extra richness/small size of the IAB cause issues/effect during the opening of the secondary's?
2. Could this be enough to mask issues with the accel shot (duration/amount)?
3. When would you match the primary IAB besides when the carb linkage ratio is 1:1?

I had some time on my hands recently and started thinking is the main reason for this. I haven't had time to go play/test to see if I could feel a noticeable difference between the two IAB setups. I'm just looking for thoughts/opinions.



Here's my original topic for more details on the car/use from a couple years ago:
http://racingfuelsystems.com/viewtopic.php?f=5&t=282

Q-750 mech sec w/Downleg Boosters (bought new in 2010)
Setup out of the box (factory)
MABs: 30’s pri & 30’s sec
Main Jets: 74’s & 65 PV - prim and 84’s & no PV – sec
IABs: 72’s pri & 72’s sec
Idle Mixture Screws: 1-1/4 turn out
Floats set to bottom of bowl window

IFRs: 33’s pri & 33’s sec - Located in upper Idle well
Emulsion Stack - both primary & secondary metering blocks
Position Restriction
Kill Bleed .028”
E1 .028” - open
E2 .028” - open
E3 .028” - plugged
E4 .028” - open


This is what has been adjusted/done to the carb so far (I'm using a 4779-2 750DP as a base line setup):
MABs: 26’s pri & 26’s sec
Main Jets: 70’s & 85 PV - prim and 80’s & no PV – sec
IABs: 74’s pri & 27’s sec
Idle Mixture Screws: 1-1/2 turn out
Floats set to middle of bowl window

I lowered the IFR's and modified the emulsion stack to more of a typical/original Holley emulsion setup:
added TSRs to main body - .081" both primaries & secondary's
IFRs: 31’s pri & 31’s sec
Position Restriction
Kill Bleed .028”
E1 .028” - open
E2 .028” - plugged
E3 .028” - open
E4 .028” - plugged

HEI Dist w/ MSD 6AL (Analog) & external E-core coil
Timing:
Initial 15
Total 37 all in by 3000
Vacuum Advance - Manifold sourced 10° at Idle - Vac pot advance restricted to 10°.

Thanks,

Scott

Statistics: Posted by SP70Nova — Tue Aug 22, 2023 6:44 pm

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