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Old 08-06-2016, 07:33 PM
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625cfm street demon carb

I recently purchased a 625cfm street demon carb for my 1973 Camaro LT, which has its stock 350 engine, and the damn thing won't accelerate, it idles like a champ but when I hit the gas it just sputters and shakes and doesn't accelerate and prior to putting on the carb the car ran fine. If anyone has any experience with street demon carbs or my current problem any advice would be appreciated.

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Old 08-06-2016, 10:41 PM
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Did you buy the carb new or used?
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Old 08-07-2016, 08:28 AM
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what kind of intake manifold??
did you change the intake??

where is the timing??
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Old 08-07-2016, 02:47 PM
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its a carb number 1901 correct..

http://documents.holley.com/lit701rev10.pdf

OH crud.. these people writing this manual have left out a complete circuit..

and they don't show it in the manual..

a few questions..

it idles perfectly... great..

open the throttle SLIGHTLY.. SLOWLY... what happens.. does it stumble.. misfire.. pop back thru the carb.. you are opening slowly enough that the accelerator pump has no effect in this..

let me describe..

at idle.. vacuum thru the round idle feed ports pull fuel thru the idle feed restrictions deep inside the carb.. this idle fuel is limited by the tapered idle mixture screws. there is no limiting of the fuel volume at this idle setting by the idle feed restrictor for the most point.. it does effect it somewhat but not a lot unless its way off.. the idle air bleeds.. limit the amount of fuel being pulled out of the idle feed restriction..


just off idle is where the low speed circuit is. it covers from just as you open the throttle to around 2400 RPMs depending on the carb size and engine displacement.. all the fuel from idle to around 1900 RPMs comes thru the idle transition circuit.. from 1900 to 2400 rpms.. the idle transition circuit decreases its flow as the primary main circuit increases its fuel out of the booster venturi..

you can tell with a timing light aimed down the venturi as you slowly increase the engine speed to see what point the fuel starts flowing from the booster venturi..

i have not had one of these carbs on my work bench yet.. it looks very much like an thermoquad.. but i am not sure.. what i am sure about is that all carbs.. have idle transition circuits.. its the only way they will work properly..

huge hint.. on other carbs... with a cold motor.. i count the turns to lightly seat the idle mixture screws.. then i take them out.. i give a 1 second blast of carb cleaner into the hole followed by a blast of compressed air.. these two done right can blow dirt out of the idle feed restriction..

here is the thermoquad idle circuit



have a look at this AFB/AVS image..




have a look at a motorcraft/autolite at idle..



see the blue spot that says transfer slot... as the throttle blade opens.. it exposes more of that to vacuum and pulls a progressively increasing amount of vacuum thru the idle transition circuit.. pulling more fuel..

here is the same thing in a 2G rochester



here is the same circuit in a solex



if you have issues when you slowly open your throttle. you could have issues with the idle transition circuit.. even if it idles perfectly..


whats interesting.. look at the rusty sediment in the float bowl of this thermoquad..



that carb had less than 20 miles on it since i rebuilt it.. i had given instructions to the owner to install a 10 micron fuel filter directly before the fuel pump.. crank the engine 3 times for 15 seconds a time.. into a clean clear plastic bottle. they bought a clear replaceable fuel filter..the crud went right thru it completely clogging the idle circuit and primary main circuits.. if you think i am kidding.. i went over and installed it and did the test myself ...

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Old 08-09-2016, 10:15 PM
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I installed a new Street Demon 625 on my mild Olds 350 last year and it started and ran smoothly right out of the box. However, my mileage was only 14 mpg versus 18 mpg I had been getting on a QJet. After some study, I bought and put in leaner primary rods and jets that should have reduced the fuel flow by about 22%. After this change my mileage is about 18 mpg and performance improved a little - better than the QJet I think.

So, the stock SD 625 is set up for a high performance small block. It will need tuning to your engine.

As for your problem, I don't know what could cause the lack of "go". Like Techinspector asked, is this a new or used carb?

John
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Old 08-10-2016, 09:56 PM
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Maybe something to check is the secondary air valve spring. If it is loose or set for easy air valve opening, the air valve might be opening upon light to moderate throttle and causing a bog. Read the section on "Secondary Air Valve Tuning" on page 10 of the instruction manual that comes with the carb, or go to the Demon website to pull it up.

John
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