MopaR&D
Nerd Member
Hey all just reporting in on my experiences so far with this brand-new carb I just bought. It's the 750 cfm model ("Holley") Street Demon with the plastic bowl. I didn't even bother running it with OOTB jetting because everywhere I read people were saying they are set rich from the factory and on top of that I'm at ~5000 ft. above sea level. So with the carb I ordered Holley P/N GA4708 which is the tuning kit; this is actually the kit for the 625 cfm carbs but that's fine for me because I needed to lean mine out from stock. I replaced the primary and secondary jets with the next biggest size down which was quite a step; stock is #89 primaries and #86 secondaries, I put in #82 on both ends. I left the factory metering rods to see how the AFR split from cruise to power looked.
Initial testing shows that it's a pretty good carb although it seems to be more sensitive to adjustments than my old 625 cfm Carter AFB. I am running a wideband O2 sensor with AFR readout (no way I could do this without it) and like a Holley carb the idle mixture affects the AFR not only at idle but also at low-medium cruising speeds; but like a Carter changing the metering rods also changes the idle mixture a bit lol. I first was getting around 13.5:1 at cruise and 13:1 at WOT but found my idle was super rich. After leaning out the idle it had a bad hesitation when opening the throttle, moving the acc. pump arm to the top hole fixed that. I put in the next size bigger metering rods which put the cruise around 14.5:1 but it made the primary power circuit too lean (15:1) as the steps are closer together in diameter. I made up a chart of orifice sizes using different jets and rods and it looks like my best bet is to drop the primary jet farther down to a #76 and go back to the factory rods as they have the largest split between power and cruise.
Things I don't like: 1) choke linkage arm must be disconnected to remove pass. side step-up piston and metering rod and it uses teeny-tiny C-clips; 2) the primaries don't open as far as I'd like before the secondaries start to kick in, I straightened out the linkage arm and it's a little better but now the secondaries don't open 100% (more like 90%); and most of all 3) in order to take off the top of the carb to change jets or whatever the carb needs to be removed from the engine completely to take out the 6 screws holding it together from the bottom.
The throttle response is noticeably better than my old Carter though and I'm sure will only get better once I dial things in some more. It makes quite a howl too when you open up the secondaries which is cool and the adjustable air door takes out the slight hesitation I used to get with the non-adjustable AFB. And most importantly it doesn't lay over as much as the RPMs climb with the extra airflow capacity (this is on a 10.5:1 360 with Magnum heads and small 213/220* @ .050" Voodoo cam). It does feel like this carb is a bit too big for the current combo and the way I'm trying to tune it for a lean cruise BUT a bigger cam is in the works and I know that will make it need all the airflow it can get. Also when it's time to hit the track I have a 750 Mighty Demon (double pumper w/ no choke) which will be jetted only for max power and response.
Overall I would recommend this carb to anyone for what it's intended for (performance street use) but like with any carburetor swap IMO you will be leaving A LOT on the table as far as power and especially fuel economy goes if you just take it out of the box and bolt it on.
Initial testing shows that it's a pretty good carb although it seems to be more sensitive to adjustments than my old 625 cfm Carter AFB. I am running a wideband O2 sensor with AFR readout (no way I could do this without it) and like a Holley carb the idle mixture affects the AFR not only at idle but also at low-medium cruising speeds; but like a Carter changing the metering rods also changes the idle mixture a bit lol. I first was getting around 13.5:1 at cruise and 13:1 at WOT but found my idle was super rich. After leaning out the idle it had a bad hesitation when opening the throttle, moving the acc. pump arm to the top hole fixed that. I put in the next size bigger metering rods which put the cruise around 14.5:1 but it made the primary power circuit too lean (15:1) as the steps are closer together in diameter. I made up a chart of orifice sizes using different jets and rods and it looks like my best bet is to drop the primary jet farther down to a #76 and go back to the factory rods as they have the largest split between power and cruise.
Things I don't like: 1) choke linkage arm must be disconnected to remove pass. side step-up piston and metering rod and it uses teeny-tiny C-clips; 2) the primaries don't open as far as I'd like before the secondaries start to kick in, I straightened out the linkage arm and it's a little better but now the secondaries don't open 100% (more like 90%); and most of all 3) in order to take off the top of the carb to change jets or whatever the carb needs to be removed from the engine completely to take out the 6 screws holding it together from the bottom.
The throttle response is noticeably better than my old Carter though and I'm sure will only get better once I dial things in some more. It makes quite a howl too when you open up the secondaries which is cool and the adjustable air door takes out the slight hesitation I used to get with the non-adjustable AFB. And most importantly it doesn't lay over as much as the RPMs climb with the extra airflow capacity (this is on a 10.5:1 360 with Magnum heads and small 213/220* @ .050" Voodoo cam). It does feel like this carb is a bit too big for the current combo and the way I'm trying to tune it for a lean cruise BUT a bigger cam is in the works and I know that will make it need all the airflow it can get. Also when it's time to hit the track I have a 750 Mighty Demon (double pumper w/ no choke) which will be jetted only for max power and response.
Overall I would recommend this carb to anyone for what it's intended for (performance street use) but like with any carburetor swap IMO you will be leaving A LOT on the table as far as power and especially fuel economy goes if you just take it out of the box and bolt it on.
and I just did the last jet change on this same tank of gas.
from me lol.
