Serious carb talk

[sam z]

Ok, I've crabbed here and there before about the carb I'm using. If anybody
forgot, I'll remind you: Holley #4779 750 DB mechanical secondary. Three annoying issues
right now: 1. The bog from a dead stop, 2. The smell of fuel (lots of it) after running the car hard and 3. The fact that it runs rich after constant tinkering and adjustment. Many of you have chimed in and hinted that this isn't the best equipment for street use, and Mopar Performance says quite similar. According to them its a racing carb. Internet searches have also revealed that this model has the bad tendency to run rich as I complained about a second ago.

I want this thing gone, and I want the RIGHT equipment for what I have and what I plan on doing.

This is what I've got:
1972 440 HP w/346 casting heads
MP 484 cam
3.91 sg rear
Holley street dominator intake (which Mopar perf. speaks highly of)
Headers
Automatic (as far as I know OEM)
Electronic ignition

I want this car to do a few things:
-Run properly and not friggin' stink like a gas station
-Launch and pull hard with the gears that I have

I believe that I have the ground work for a good performing car. The leak down test came back with excellent results when I had it done. I have no interest in changing the cam in the car at the present time. I realize it is older piece but it is still good equipment. If a torque converter swap is recommended I have no problem with this because the engine is coming out at some point anyway and I'll gladly put in another piece at that point if its beneficial.

Thanks guys. I'm looking forward to any and all responses.

 

[69hemibeep]

New power valve, If its bad it will flow extra fuel and give you a big bog. Before you buy one you need to know how much vacuum you have at the intake to figure out which one. I may be wrong but I think its half of the vacuum, its been awhile. Some one will straighten me out I'm sure. Or as you suggested change the carb. I have had allot of luck with the 750 vacuum Holley with the quick change vacuum kit.

 

[mac]

these are the things that i would think about. first, what is your compression ratio? that will help in getting a timing curve setup. i personally don't like the 484 cam. i think there are better cams that could match up to your build and purpose (street use). that cam won't come alive until 2500 rpms. what is your timing set at? (intitial and total at what rpm) depending on compession, you can really crank timing up. that would help with the raw fuel smell and make it a bit more responsive. :cents:

 

[sam z]

Mac, OEM specs say 8.2 on compression ratio. Everything I have discovered would indicate this
could be the case now, but can't be positive. A popular speed shop in Elma NY (you probably know it) did a lot of head work to the car in the early 90's, and that is when the cam was put in as well. Was compression increased? I can't say for sure.

 

[mcmopar]

You should definitely invest in a good converter and perhaps a shift improver kit. The 484 cam, as mac pointed out, comes it at a higher rpm than the stocker. If you are lucky your stock converter will stall until about 2000 rpm. You need a good converter with at least a 2500 rpm stall rating. This will help out with your low speed response. Your single plane intake is costing you low end torque, as is the low compression ratio, so a higher stall converter will really help your 3.91 gears get the car moving. Some good transmission tuning will go a long way toward making your car responsive on the low end.

As for the carb, I agree that a vacuum secondary 750, with a quick change secondary spring kit installed, would be the hot ticket for your application. I also agree that you should attach a vacuum gauge to your carb's vacuum port (where the line from the distributor's vacuum advance attaches to the carb base) so you know how much vacuum you have. This is important so you know what power valve to install. I run 9.5 - 10.5 in. of vacuum at idle so I installed a 4.5 power valve. My 750 vacuum sec. Holley is an outstanding carb setup that I am happy to keep despite having an 850 double pumper sitting on the shelf in the garage. A Holley or Street Demon would work well.

 

[mac]

Mac, OEM specs say 8.2 on compression ratio. Everything I have discovered would indicate this
could be the case now, but can't be positive. A popular speed shop in Elma NY (you probably know it) did a lot of head work to the car in the early 90's, and that is when the cam was put in as well. Was compression increased? I can't say for sure.

trying to figure out what your compression is would help greatly. if it is somewhere around 9:1 to 9.75:1 with steel heads would be a good start. do you have a dial back timing light? if you do, i would play with timimg to see how things change. this would be something to do with out investing in parts, just your time. :cents: what are your timing settings now?

 

[sam z]

Ok men, this is what I know:
I ran a vacuum check last summer with a friend of mine. The vacuum reading had indicated that I go with a lower number power valve. Quite honestly, the power valve number was around 4 or so (to the best of my recollection). I have read that the cam that I have will cause a lower number on the vacumm guage and hence this is about right (as far as I have described). It was a chevy guy that came over and assisted me on this process.

Mac, we altered the timing and it ran like s*&%. We ended up going back to the timing setting that it had been set to previously. I cannot tell you right now the setting that its at.

If I do make the leap and get a different carb, what makes / models are recommended?

Thanks guys

 

[mcmopar]

I'm not sure how you set your timing. This worked well for me on my 71 Charger SE 440 (484 cam, 3.55 gear, automatic) and also for the road runner:
To set the timing for max. performance you need to disconnect and plug the vacuum port on the carb. Then raise the engine rpm to 2000-2400 rpm with the curb idle screw to hold the rpm. Next set your centrifugal timing for 36 degrees before TDC. This is easiest with a dial back timing light but I just made my own timing tape (CAD is nice to have) with marks at 20, 30, 32 and 36 degrees BTDC and painted the timing marks on the damper since I don't have a dial back timing light.
Once timing is set, lower rpm back down to your curb idle speed (probably around 900-950 rpm with that cam). From here you have the choice of leaving the distributor vacuum advance plugged or (like I do) setting it up to run some distributor vacuum advance as well. The dashpot on the distributor has an adjuster down inside (down through the nipple) that uses a 3/32" allen wrench for adjustment. IIRC the total travel from full off (no vacuum advance) to full on (full vacuum advance) is 16 turns. I have mine set right in the middle - 8 turns. The car runs incredibly well with no pinging. If yours happens to ping (I'd be surprised if it does with that compression ratio) you can back off the vacuum advance by turning the adjuster to the left 1 full turn at a time, then road testing to see if the ping is still there. Adjust until the ping is gone. If the ping doesn't go away, then reset the adjustment back to 8 turns and reset your centrifugal timing again (don't forget to plug the vacuum line again) down to 32 degrees and try again. Once all is set up the way you like it, hook the vacuum line back up from the distributor to the carb if you intend to run some vacuum advance. If you just want to run the centrifugal advance without any distributor vacuum advance then leave the vacuum ports plugged. Timing takes time (no pun intended) to set up properly but once its set the difference in the way your car will run compared to the way it ran before will be like night and day.
When I had my '71 Charger SE it originally had a 383 and did 15.40 in the 1/4 mile. I swapped in a 440 from moparmoose Jerry's old Gray Ghost '66 Coronet that got clobbered by a doofus one night as he was on his way out to the strip. I put in a General Kinetics 284 degree 484 lift cam and lifter kit along with new springs and a decent set of heads done up by classracer (Jim Fitzpatrick - the guy doing Ray's heads). It ran stock exhaust manifolds, a Performer intake, a 440 AVS carb with 383 metering rods (for a little richer mixture), a 2-1/4" exhaust system with Cherry Bomb turbo mufflers and an 8-1/4" rear end with a Sure Grip and 3.55 gears. I only had money for a used 440 HP torque converter but had the 727 rebuilt with a 5.0 ratio kickdown lever and a mild shift kit. When I got it all back together I took it to the strip expecting to turn some good times but was very disappointed when it only ran 15.14! I was disgusted but Jim and Jerry encouraged me to hang in there. I did some reading and learned that there is usually 1 second to be shaved off the ET if you do some homework. I learned about timing the car and set it up for 36 degrees centrifugal advance at 2400 rpm. With nothing more than a timing change the car ran 14.74 next time out! That's 4 tenths for 0 $$$ outlay! Eventually I was able to get the car down to a 14.24 ET which, for the weight of the car and the setup I was running, was pretty good.
Right now you just need to do some tweaking. A timing change for max. performance and maybe an accelerator pump swap on the carb's secondaries will help eliminate the bog you are having. It will take time and you will have to spend a few $$$ for the accel pump cams but in the end, if you stick to it, your car will run well. If the carb is too much then get a new one with vacuum secondaries (Holley or Barry Grant Demon Series) and sell the double pumper to help offset expenses. Take it from someone who knows - its frustrating for a time until you hit that sweet spot. From there its all cake!

BTW - here are some carb choices:
Barry Grant Speed Demon Carb - 750 cfm w/electric choke & vac. sec. $419.99
http://www.jegs.com/i/Barry-Grant/132/1 ... tId=743583

Barry Grant road Demon - 725 cfm w/vac. sec. $356.99
http://www.jegs.com/i/Barry-Grant/132/4 ... tId=743601

Barry Grant Road Demon Jr. - 725 cfm w/vac. sec. and elec. choke $304.99
http://www.jegs.com/i/Barry-Grant/132/6 ... tId=754098

Holley 750 w/vac. sec. & elec. choke $291.99
http://www.jegs.com/i/Holley/510/0-8050 ... tId=743853
olley Quick Change Secondary Spring Kit w/spring assortment $26.99
http://www.jegs.com/i/Holley/510/20-59K/10002/-1

Holley Street Avenger 770 cfm w/vac. sec., elec. choke and quick change secondary spring housing factory installed. $389.99
Extra springs not included $11.99
http://www.jegs.com/i/Holley/510/20-13/10002/-1

 

[moparchris]

Hey Sam, I have that same carb in my possession and I run it on anything that needs it. I LOVE this carb as its an excellent all around carb and can be tuned to work well on any motor. I agree with Bob and check to make sure the power valve is still good and of the right size. It needs to be 2 sizes smaller than the vacuum reading at idle. This wont make it run rich on decel though. The 484 mp cam is an old school grind that has a lot of overlap and duration. It also has a tight lobe seperation. This causes it to have low vacuum at idle and to have a significant lope. This lope is what is causing the "gas station" smell. When a car misfires you get raw fuel in the tailpipe and raw fuel smells. Incomplete burn causes a high CO reading which we all know its odorless, well not totally since it usually is accompanied by some raw fuel in an engine. The carb is not entirely the problem. If you replace the carb and go through all the expense you will be VERY disappointed when it still smells and runs poorly. I would try a few things and I would isolate each problem and work on them one by one. The bog is created by the accelerator pump squirters and associated pump cam. The rich run on decel may not be cureable with the cam you have, especially if its a low compression motor. Send me a pm if you want me to give you some specific advice. I cant type all that good and its taken me 2 days to type this so far. Good luck.

Here is a pic of it on my stock 318. By the way it runs perfect, so I know you can get yours to work right.

 

[roadrunnerh]

I am not nearly as knowledgable as others who have commented on this thread, however, I would go with a dual-plane Intake Manifold either the Edelbrock performer or Mopar Performance M1 Intake.

Why are you running the single plane? This is a mild street car, right?

My stock '68 383 with iron manifolds, iron intake, and AVS with auto tranny and 3.91's runs 14.20-14.40's in my road runner.

 

[V269]

Moparchris, you say the power valve need's to be 2 sizes smaller than vaccum at idle. Does this mean that if you have say 14 inches of vaccum that the valve should be a # 12 ? I'm also thinking of putting the 750 dp on my car if I can ever get the guy to call me back. I too have run this carb before and had very good results.

 

[sam z]

someone please tell me how to use the quote feature so I can address the 600 good points that have been made one by one here.....

 

[Big John]

someone please tell me how to use the quote feature so I can address the 600 good points that have been made one by one here.....

If the quote is on another thread it may be better to post the link back to that thread.

You can also open the thread in another window and hit the quote button. Highlight the quote, right click and copy, then come back to this thread, right click and paste.

 

[69hemibeep]

Moparchris, you say the power valve need's to be 2 sizes smaller than vaccum at idle. Does this mean that if you have say 14 inches of vaccum that the valve should be a # 12 ? I'm also thinking of putting the 750 dp on my car if I can ever get the guy to call me back. I too have run this carb before and had very good results.

From what I remember you would use a 6.5 or 7 as close to half as they make. see what Chris says :cents:

 

[moparchris]

Moparchris, you say the power valve need's to be 2 sizes smaller than vaccum at idle. Does this mean that if you have say 14 inches of vaccum that the valve should be a # 12 ? I'm also thinking of putting the 750 dp on my car if I can ever get the guy to call me back. I too have run this carb before and had very good results.

From what I remember you would use a 6.5 or 7 as close to half as they make. see what Chris says :cents:

Bingo! They only make em up to 6.5 IIRC. It just gets critical when you get into a mow vacuum situation, sometimes if you use a PV that is the same as the vacuum at idle it will flutter and change idle speed and mixture. Tell me how I know this, it drove me crazy a while ago. The other thing to remember when your running a carb that is small for the engine your running it can start to pull vacuum at high rpms and will close the valve causing a massive lean condition and possibly hurt the motor. I highly doubt you will run into this situation, I very rarely run into this situation.

 

[69hemibeep]

If you have an older holley it just takes one backfire to blow a power valve. The newer ones have a port and check ball to save the PV :cents:

 

[mcmopar]

Yes, and you can buy a kit that has a drill bit and a restrictor so that you can install a power valve saver in older Holleys. I did this on a Holley I had on my '71 Charger SE.