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350 cfm Holley 7448-1 idle too rich

General engine tech -- Drag Racing to Circle Track

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benno318
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350 cfm Holley 7448-1 idle too rich

Post by benno318 » Thu Aug 29, 2019 3:24 am

Attempting to tune a customers inline 6 186ci he has a (supposed) brand new 7448-1 350cfm 2 barrel. the metering block is dated "17"

I found a few vacuum leaks and fixed them, but the thing requires the mixture screws J-U-S-T opened ever so slightly for best idle quality. this of course makes a bit of a bog off idle and generally unpleasant at part throttle.

I cant really stall the engine by seating the mixture screws. the idle speed screw is set low enough (approx. 700rpm by the way) that the transition slots are not even uncovered.

it has pcv and blocking this didn't help the idle mix situation although I didn't drive it like that.

timing is anywhere from 10 to 20 initial and tried with and without vacuum advance (ported and full vacuum) best vacuum at idle is 15" - power valve is 8.5 and also tried a 7.5 with no noticeable difference.

had a few old holleys about and one that was already dismantled, I borrowed that metering block (dated "05") with his 61 jets and 8.5 powervalve. idle mix screws liked about 3/4 to 1 turn out on that one, so to me that says there is something different in the metering block.

then I tried my go-to, lend to anyone, old as the hills 4412 500cfm 2 barrel. mixture screws almost 1 turn out, drives perfect, smooth in transition, no flat spots or bogs - does everything right. I suggested he takes the "new" carby back but apparently the guy he bought it from was hard to deal with and is now not responding to any contact. this guy also "tuned" it after fitment to "get it right"

now I also have an old 7448 from 1982 so I am thinking ill try that metering block on his carb next.

anyone else struck this kind of thing? is it a calibration change of the "newer" metering blocks, or has this guy got a dud?
thanks in advance everyone

1972ho
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Re: 350 cfm Holley 7448-1 idle too rich

Post by 1972ho » Thu Aug 29, 2019 10:05 am

Are your idle air bleeds adjustable and have check there size and do you know the size of idle feed restrictions and is it adjustable those are some things you can check that will change how far open the mixture screws can be open.

ustahava67
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Re: 350 cfm Holley 7448-1 idle too rich

Post by ustahava67 » Thu Aug 29, 2019 10:10 am

New Holley carbs are all over the place in calibrations.... You will have to gauge EVERY hole in the metering blocks and main body to determine the differences and nail down a good tune. It sounds as though you have a known good calibration in the 4412. I would use that as a starting point
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Walter R. Malik
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Re: 350 cfm Holley 7448-1 idle too rich

Post by Walter R. Malik » Thu Aug 29, 2019 10:19 am

benno318 wrote:
Thu Aug 29, 2019 3:24 am
Attempting to tune a customers inline 6 186ci he has a (supposed) brand new 7448-1 350cfm 2 barrel. the metering block is dated "17"

I found a few vacuum leaks and fixed them, but the thing requires the mixture screws J-U-S-T opened ever so slightly for best idle quality. this of course makes a bit of a bog off idle and generally unpleasant at part throttle.

I cant really stall the engine by seating the mixture screws. the idle speed screw is set low enough (approx. 700rpm by the way) that the transition slots are not even uncovered.

it has pcv and blocking this didn't help the idle mix situation although I didn't drive it like that.

timing is anywhere from 10 to 20 initial and tried with and without vacuum advance (ported and full vacuum) best vacuum at idle is 15" - power valve is 8.5 and also tried a 7.5 with no noticeable difference.

had a few old holleys about and one that was already dismantled, I borrowed that metering block (dated "05") with his 61 jets and 8.5 powervalve. idle mix screws liked about 3/4 to 1 turn out on that one, so to me that says there is something different in the metering block.

then I tried my go-to, lend to anyone, old as the hills 4412 500cfm 2 barrel. mixture screws almost 1 turn out, drives perfect, smooth in transition, no flat spots or bogs - does everything right. I suggested he takes the "new" carby back but apparently the guy he bought it from was hard to deal with and is now not responding to any contact. this guy also "tuned" it after fitment to "get it right"

now I also have an old 7448 from 1982 so I am thinking ill try that metering block on his carb next.

anyone else struck this kind of thing? is it a calibration change of the "newer" metering blocks, or has this guy got a dud?
thanks in advance everyone
IF the throttle plates are closed to much, you probably have either a vacuum leak still somewhere, to much initial ignition advance at idle or simply an unmatched carburetor for the application.
If that carb has a slot restrictor on the metering block face side of the main body, (sometimes called a "channel restriction); remove it.

For the way it is now ... lengthening the slot downward and more idle air bleed, (or less idle fuel feed), would be the best recourse.
http://www.rmcompetition.com
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bill jones
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Re: 350 cfm Holley 7448-1 idle too rich

Post by bill jones » Thu Aug 29, 2019 7:48 pm

---It seems to me that the 7448 2 barrels had a "reverse idle circuits'---meaning that the idle mixture screws are adjusting the air of the idle circuit rather than the fuel of the idle circuit.

--this means you can't just switch to some other common meter block unless you change the internal circuitry of the 7448 to become a common fuel adjustment circuit rather than an air adjustment circuit.

Walter R. Malik
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Re: 350 cfm Holley 7448-1 idle too rich

Post by Walter R. Malik » Fri Aug 30, 2019 11:00 am

Walter R. Malik wrote:
Thu Aug 29, 2019 10:19 am
benno318 wrote:
Thu Aug 29, 2019 3:24 am
Attempting to tune a customers inline 6 186ci he has a (supposed) brand new 7448-1 350cfm 2 barrel. the metering block is dated "17"

I found a few vacuum leaks and fixed them, but the thing requires the mixture screws J-U-S-T opened ever so slightly for best idle quality. this of course makes a bit of a bog off idle and generally unpleasant at part throttle.

I cant really stall the engine by seating the mixture screws. the idle speed screw is set low enough (approx. 700rpm by the way) that the transition slots are not even uncovered.

it has pcv and blocking this didn't help the idle mix situation although I didn't drive it like that.

timing is anywhere from 10 to 20 initial and tried with and without vacuum advance (ported and full vacuum) best vacuum at idle is 15" - power valve is 8.5 and also tried a 7.5 with no noticeable difference.

had a few old holleys about and one that was already dismantled, I borrowed that metering block (dated "05") with his 61 jets and 8.5 powervalve. idle mix screws liked about 3/4 to 1 turn out on that one, so to me that says there is something different in the metering block.

then I tried my go-to, lend to anyone, old as the hills 4412 500cfm 2 barrel. mixture screws almost 1 turn out, drives perfect, smooth in transition, no flat spots or bogs - does everything right. I suggested he takes the "new" carby back but apparently the guy he bought it from was hard to deal with and is now not responding to any contact. this guy also "tuned" it after fitment to "get it right"

now I also have an old 7448 from 1982 so I am thinking ill try that metering block on his carb next.

anyone else struck this kind of thing? is it a calibration change of the "newer" metering blocks, or has this guy got a dud?
thanks in advance everyone
IF the throttle plates are closed to much, you probably have either a vacuum leak still somewhere, to much initial ignition advance at idle or simply an unmatched carburetor for the application.
If that carb has a slot restrictor on the metering block face side of the main body, (sometimes called a "channel restriction); remove it.

For the way it is now ... lengthening the slot downward and more idle air bleed, (or less idle fuel feed), would be the best recourse.
IF this is a REVERSE IDLE system carb ... forget everything I said.
http://www.rmcompetition.com
Specialty engine building at its finest.

Tuner
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Re: 350 cfm Holley 7448-1 idle too rich

Post by Tuner » Fri Aug 30, 2019 12:38 pm

7448 has a 'normal', as in not reverse, idle system. If for some reason it has a reverse idle metering block with the blunt tip adjusting screws and .110" curb idle screw metering orifice it would act like the OP describes, could only tolerate the mixture screw barely cracked open. The standard 'normal' Holley idle system usually has an idle needle tip orifice about .063", however some are a bit larger or smaller, + or - about .010", so .050" to .070", but usually .063 (or 1/16"). If it is a reverse idle block on a normal idle circuit it will act like the OP's problem.

benno318
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Re: 350 cfm Holley 7448-1 idle too rich

Post by benno318 » Sun Sep 08, 2019 4:37 pm

just a follow up for anyone interested...

so I put the metering block from a 1982 model 7448 onto the otherwise brand new carb, long story short the mixture screws wanted to be about 1 turn out here, so a definite change from the original, but there was a big dead spot that would not recover just off idle. much worse than the new carb is actually.

I did a detailed comparison of the 3 metering blocks, one from 1982, one I had from 2005, and the 2017. all different. although I left the paper I detailed all of it on, at work - but ill share my findings here soon.

I ended up re-kitting that complete 1982 carb and as expected, it works PERFECTLY.
mix screws 1 and a half turns out for best idle, no flat spots, hesitations, gags, just like the new one should be.

thanks to everyone who posted for all the advice and suggestions

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