Over Carb’d?
Thread Starter
Racer
Joined: Nov 2006
Posts: 258
Likes: 1
From: Temple, TX
Over Carb’d?
Hey everyone, so I did a ton of searches before purchasing a Holley 750 dp mech secondary electric choke for my 74 - 454 4 spd Muncie. This replaced the summit branded 600 cfm vacuum secondary that was on the car when I bought it. I replaced the carb at the same time as doing a full tune up, upgrading original distributor, and adding headers / exhaust. From what I can tell, my NOM 454 is your mostly stock setup with a mild RV cam and high rise edelbrock performer manifold. With the old carb I could spin the tires with ease, with the new Holley it seems like I’ve lost a good amount of low end torque; however, it does feel stronger up top. I like to drive to town and back and have some fun on back roads, did I pick the wrong carb? I’ve adjusted my A/F using a vacuum gauge connected to manifold vacuum and all other adjustments seem spot on. I sold the summit carb after I got the new unit, should I switch back to a smaller carb? If so, what do you all use? Not interested in a Quadra jet and don’t want to start that type of thread. The street demon seems to get good reviews on this site, and I’ve had good experience with two summit brand carbs on two c3’s, maybe I should stay with a 750 but go to vacuum secondaries? Thanks for all of your help!
Thread Starter
Racer
Joined: Nov 2006
Posts: 258
Likes: 1
From: Temple, TX
Forgot to mention that I’ve spoken with two Holley techs that did a calculation and said that the max flow of this motor is under 600 cfm, but it seems like most everyone is running much larger carbs and the quads flowed over 800cfm, so what am I missing?
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,118
From: Crossville TN
Q-Jet carbs are 'engine-capacity' regulated, so they can't dump too much fuel in the engine. Vacuum regulated secondaries on non-Q-jets operate similarly and don't have those problems (if tuned well). Mechanical secondary carbs need to be sized properly for the amount of air/fuel needed by the engine: too small--no top end; too large--no bottom end.
Race Director
Joined: Jan 2000
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From: Corsicana, Tx
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Definitely not overcarbed. A 750 DP is about the most universal carb ever made. It works on everything....what is timing etc? Something is definitely off if a 600 vacuum feels stronger at the low end.
JIM
JIM
Thread Starter
Racer
Joined: Nov 2006
Posts: 258
Likes: 1
From: Temple, TX
We (mech & I) could not really see the timing marks much anymore, so we set it where it sounded best, advanced it until it had hard start issues then backed it off. Never heard it ping though.
Old Pro Solo Guy
Joined: Dec 2017
Posts: 8,094
Likes: 4,449
From: Marlton NJ
One of the best things you can do for a motor is to customize the timing advance curve to a more performance oriented curve, not a factory one. It usually really helps the bottom end and driveability in the mid-range. Contact Lars for his excellent instructions. As I see you"upgraded" the distributor you already made some changes in this area. Roughly 10-15 degrees idle setting, 32-36 degrees at 3200 depending on compression, and only 10-12 degrees from the vacuum can. Vac can on manifold vac, not ported. That should get you in the ballpark.The DP is "OK" for the street if tuned properly, but not required, especially on what sounds like a mild motor, and is much easier to foul-up the tune. Personally I would put the vac sec carb back on until you have the distrib tuned, so that you are only changing one thing at a time.
Le Mans Master
Joined: Mar 2004
Posts: 7,502
Likes: 1,508
From: Little Rock AR
Vacuum secondary carbs are more forgiving on the street than a double pumper. I suspect this is a contributing factor here.
If the new carb seems "flat", as compared to the older vacuum secondary carb, the acceleration circuit on the new carb may be just tuned too rich for your airflow. Feels "lazy" because it is too rich in that circuit. The vacuum secondary carb helps prevent that issue but you can tune that circuit on your new carb to better match your needs.
Lars has a Holley tuning paper he would email you if you want to work on it.
If the new carb seems "flat", as compared to the older vacuum secondary carb, the acceleration circuit on the new carb may be just tuned too rich for your airflow. Feels "lazy" because it is too rich in that circuit. The vacuum secondary carb helps prevent that issue but you can tune that circuit on your new carb to better match your needs.
Lars has a Holley tuning paper he would email you if you want to work on it.
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Le Mans Master
Joined: May 2006
Posts: 9,993
Likes: 1,136
From: League City Tx
750DP is the best all around carb for a mild big block PERIOD... It will need some jetting changes, accelerator pump adjustments and basic idle mix screw adjustment. That carb should hit 10X harder then that old vac sec carb....
You probably need to go through it, do a good tune up, dial in the timing and adjust the carb some. Getting it on a chassis dyno with a wideband O2 would be very beneficial for dialing in the carb jetting if you don't have a wideband O2 meter in the car.
Keep tinkering with it...It probably needs to have some jet pulled out.
You probably need to go through it, do a good tune up, dial in the timing and adjust the carb some. Getting it on a chassis dyno with a wideband O2 would be very beneficial for dialing in the carb jetting if you don't have a wideband O2 meter in the car.
Keep tinkering with it...It probably needs to have some jet pulled out.
Last edited by ajrothm; Jul 16, 2019 at 01:55 PM.
Old Pro Solo Guy
Joined: Dec 2017
Posts: 8,094
Likes: 4,449
From: Marlton NJ
Oh I agree with you 100% AJ. But the OP changed so many things at once I just think he would be well advised to get the timing corrected first, and then spend time tuning that 750DP. One tuning problem at a time....
Last edited by leigh1322; Jul 16, 2019 at 02:15 PM.
Le Mans Master
Joined: Aug 2010
Posts: 5,339
Likes: 546
From: Central Texas
Originally Posted by crazy4c4
Hey everyone, so I did a ton of searches before purchasing a Holley 750 dp mech secondary electric choke for my 74 - 454 4 spd Muncie. This replaced the summit branded 600 cfm vacuum secondary that was on the car when I bought it. I replaced the carb at the same time as doing a full tune up, upgrading original distributor, and adding headers / exhaust. From what I can tell, my NOM 454 is your mostly stock setup with a mild RV cam and high rise edelbrock performer manifold. With the old carb I could spin the tires with ease, with the new Holley it seems like I’ve lost a good amount of low end torque; however, it does feel stronger up top. I like to drive to town and back and have some fun on back roads, did I pick the wrong carb? I’ve adjusted my A/F using a vacuum gauge connected to manifold vacuum and all other adjustments seem spot on. I sold the summit carb after I got the new unit, should I switch back to a smaller carb? If so, what do you all use? Not interested in a Quadra jet and don’t want to start that type of thread. The street demon seems to get good reviews on this site, and I’ve had good experience with two summit brand carbs on two c3’s, maybe I should stay with a 750 but go to vacuum secondaries? Thanks for all of your help!
That carb needs to be set up for your combination imo. Sounds like a little tuning on pump shot and accelerator pump adjustment would help your low end.
Stick with what you have. It should work well.
Craig