Carb Recomendations

SVMCOBRA · #1 (**permalink**) 03-21-2005, 01:36 AM

Folks, I need some recomendations on a carb for my 351W. The motor has been bored 30 over, 10 to 1 compression, world product heads, Comp cams solid roller and Torker 2 intake. Cam spec's are Gross lift 586, advertised duration 288 I&E @ .050 244 I & E, lobe lift 366 I & E on 110 deg lobe seperation with 4 deg advance set in cam. I have had a Holley 770 street avenger on the car and it seemed to be a little to much carb. It was very rich even after jetting it down a couple of sizes on the primary and secondarys and would bog when full throttle was applied. I then switched to a 570 street avenger and it runs much better but it has a stumble anytime you increase speed normally or at WOT. I had a 65 mustang with a warmed over 289 and ran a 750 vac secondary with none of the above problems and also ran a 650 double pumper on the same car. I want to get this resolved so I can start putting some miles on the car with out fear of being to rich or to lean and hurt the motor.
Thanks in advance, Ray

mj_duell · #2 (**permalink**) 03-21-2005, 03:50 AM

My suggestion would be to call Barry Grant (www.barrygrant.com). I am not and expert, but I would figure you would need a 750cfm Speed Demon with vacuum secondaries. I had similar problems with my holley but after I switched to my Mighty Demon they all went away. It is also very easy to tune. Just my .02 cents.

--Mike

Hotfingrs · #3 (**permalink**) 03-21-2005, 04:30 AM

Put bigger accelerator pump squirters in...Don't know what you have in there now, but something in the low 30's would be in order.

trularin · #4 (**permalink**) 03-21-2005, 05:14 AM

(cubic inches x maximum RPM)/3456 = CFM needed.

Round UP to next size.

Bruce Edwards · #5 (**permalink**) 03-21-2005, 05:21 AM

The carb should be fine for your set up. What size jets are you running? How does it idle? What is your idle reading on a vac gauge? What is your timing set at?
Give us a bit more and we can help zero in on what may be the problem.

rob frink · #6 (**permalink**) 03-21-2005, 07:07 AM

..........

Randy Rosenberg · #7 (**permalink**) 03-21-2005, 10:15 AM

I'm really happy with my Pro-System carb. I had all sorts of drivability issues, and so I sent my carb to Patrick http://www.pro-system.com/ . He calibrated it to my engine/driveline specs. I run great. Easy to start, idles smoothly, transition between circuirts smoothly, no hesitation at all at anytime. I highly recommend them.

YMMV,
Randy R...

TonyMadrid · #8 (**permalink**) 03-21-2005, 11:45 AM

I always turn to Don Gould, he's the guru on Demon carbs. You can e-mail him at bigdaddy@4secondsflat.com or visit his site 4secondsflat.com

Provide him with all your engine specs and he will tell you what the optimum carb for your application would be.

flipper35 · #9 (**permalink**) 03-21-2005, 05:12 PM

I second Tony's choice. Not only knows his stuff but has some high level access to BG and gets to see stuff most distributers don't. Anyone that runs low 11s in a 66 'Cuda with 318cid has to know a bit.

SVMCOBRA · #10 (**permalink**) 03-21-2005, 05:15 PM

Ok, a little more info. My timing is set at 16 deg btdc, add the 4 deg set in the cam and that puts it at 20. Curb idle is set at 1100 rpm, anything lower than that and its is to choppy. Idle vac is 15( will double check this but this is what I remember) .
The 570 holley has 54 jets on the primary and 65 jets on the secondary. PV is 85 and pump shot is 35. The 770 has 69 jets on the primary and 72 jets on the secondary. PV is 65 and pump shot is 25. Idle and vac was the same when I had the 770 on the car. Ignition is a MSD 6AL with Dura Spark dist. I do have the vac advance hooked up. Valve are adjusted at .20 on intake and exhaust. The car starts fine hot or cold and once warmed up idle is fine.

blykins · #11 (**permalink**) 03-21-2005, 06:00 PM

770 sounds like way too much carb for a 351. I'm running a 750 Mighty Demon on my 425ci Windsor.....

brainsboy · #12 (**permalink**) 03-23-2005, 06:44 AM

You have something wrong with the 770. You said its running too rich. When you go with a carb thats bigger then what you need the air flow through the carb gets slower, this causes lean conditions not rich, so something is wrong with your carb. After you jetted down you noticed it bogged because your already lean and now your going even leaner then before. The first thing I would check in the 770 is the power valve. Sounds like it's leaking or opening up on you.

As far as the 15" of vacuum that seems way off for your setup and camshaft.

SVMCOBRA · #13 (**permalink**) 03-27-2005, 09:03 AM

Well guys I called Barry Grant and they suggested a 650 Mighty Demon. After checking out I Don Gould's website I plan to call and order one from him.
Thanks Ray Hedden

Bruce Edwards · #14 (**permalink**) 03-27-2005, 12:16 PM

Your choice but why not get the one you have working and save yourself the cost of a new carb?
I use 750's on the 351's I build and have never had a problem. Yours should tune to the engine.

SVMCOBRA · #15 (**permalink**) 03-27-2005, 02:31 PM

Bruce, should I use the 570 or 770?? I really would perfer to not buy another carb. I have talked to a friend of mine and he is going to try and help me out. I thank everyone for their input and will let you know what I do.
Ray Hedden

Bruce Edwards · #16 (**permalink**) 03-27-2005, 03:18 PM

If you are running a built up 351W and the 770 is a vac second use it. I install 750 Holley's on all the 351's I build and have never had an over carb problem.
If you are running double pump carbs I would sell them off and get a vac second. Vac seconds are much better for the street and D.P.'s are better for the track.
Again just my view on the subject.

SVMCOBRA · #17 (**permalink**) 03-27-2005, 03:36 PM

Thanks Bruce, yes they are both vac secondary carbs. I will replace the gaskets on the 770 and try it again as it has been sitting on the shelf for about 6 months.

Ray

Bruce Edwards · #18 (**permalink**) 03-27-2005, 03:58 PM

Just take your time with it and make sure you get every passage clean.
They are not hard to overhaul.

Jack21 · #19 (**permalink**) 03-27-2005, 07:16 PM

I'm using BG Speed Demon 650DP. After playing with the Holley this way, and that, I gave up. BG's just seem to work better with hotter cams.

Smaller engine, heavier car, auto trans, mild cam: use vac sec carb. Secondaries don't open until engine demand needs extra fuel.
Larger engine, lighter car, manual trans, hot cam: use DP carb. Secondaries open when you put your foot in it.

Bruce Edwards · #20 (**permalink**) 03-27-2005, 07:45 PM

With a lighter car the engine demand is low and as such the vac second will work better. If the demand is low and you step on the double pumps with low air speed in the intake the engine can not help but stubble even if you do not feel it. Just because it sounds fast does not make it so.
Case in point the old Quadrajets Even though it was not a double pumper and had an upper air valve to improve the air speed, like the Carter or now the Edelbrock carbs, if the valve was not set correctly and fell open when you stepped on it you got the ever famous QuadraBOG.
The way Holley and some other carbs get around the bog with out an air valve is by adding the extra shot of fuel thus the double pump carb.
Again great at high speed when the air flow is up and really sucks when air speed is low. As pointed out in a post the vac will only pull in what the engine can handle and thus you get a much smoother powerband. As the air flow goes up and the demand goes up the seconds open up so you do not get the big dump of fuel when you step on it and the resulting bog.
Doubles, as I have said before, are great for the track when you never let the rpm down and the engine needs the extra shot of fuel from the rear seconds when you pull out of the corners or for that hole shot off the line at the 1/4 mile strip when you are coming off the line at 3500 rpm or more.
For street use stick to the vacs. Less work to tune, better fuel mileage and just a hell of a lot more fun.
Again my feelings on the matter from 30 years of building engines and replacing a lot of double pump carbs on street engines with vac seconds.
Just a question to close out this post for any that may not see it this way. Look at any multiple carb factory engine and what do you see more than half the time? From 427 Ford with two Holley's, 440 or 340 sixpacks, to 427 Chevy what do you find? Vac seconds.
Ever ask yourself why that is?
By the way works even better on big heavy street cars for all the same reasons.