Choosing the right carb

-
B

bigtooth

Well-Known Member
Joined
May 17, 2011
Messages
245
Reaction score
9
Location
Vermont
I would like to hear you're suggestions for a new carb for my setup for my duster.
I've got a 360 magnum crate engine 390hp, 4 speed non OD, but with the low first gear, 3.91 rear, but may try a 3.23.
The car is strictly for the road. I have a 750 holley vacuum on it, and am not the happiest with it, and want to go with a double pumper.

I have one recomendation that I go with a Quick Fuel Q series 650, but i'm conserned because mopar recomends a 750 for this engine, will the top end trade off be worth the gain on the bottom using a 650?
Also, no choke on that carb either...

What are some of your opinions?

Oh, and happy thanksgiving!!!
 
I am running a Q750 mechanical secondary on my 390CID stroker. I absolutely love that carb. It is reliable, easy to tune, and not that expensive for what it is.
You'll like it a lot better than a vacuum secondary.

http://www.4secondsflat.com/Carb_CFM_Calculator.html

That will help you calculate a size for a carb. Or you can call Quick Fuel and they will give you a recommendation for a size.
 
Thanks rnaz, did you buy your carb from FBO and have them set it up?
 
No. I bought mine from Summit and set it up myself. It was really easy to set up.
 
A good rule of thumb is VS for auto car and MS for manual car. I would think a 650 would suffice. RPM range also is a factor, if you don't plan on winding it past 6000-6500 RPM then the 650 should be fine.

I recently downsized to a 600 cfm on my car and it performs much better, throttle response is much better also.
 
What about your current carb makes you unhappy?

Personally, I wouldn't put anything smaller than a 750 on it. Just my preference.

Those calculators are great for finding the smallest carb you want to run.

Ask Jason with the RX7 about his fun with carbs and the huge difference a good fuel system and bigger carb can do for perfrormance. Those calcs call for a 600ish carb, ran 12.80, put on a carb with a proform 830cfm body and it runs real close to 12.00 now carrying and extra 150-200 pounds. No other significant changes.
 
The calculators are a guideline that do the math for you. Call QFT and talk to them. They know their product, and know engines in general.

I also wouldn't go less than a 750. I'd also highly recommend the annular boosters too. They atomize the fuel a bit better, and that helps improve your overall drivability on the street.
 
IIRC, Jason had a QF 650 on his car prior to swapping out to the proform mainbody...
 
i have a 360 with edelbrock heads and thumper cam it responds well to the qf 650 ss mechanical secondaries. my car is 727 auto with 3.91 gear. i called don at fbo and gave him my vaccum reading at idle in park.love this carb
 
I'm with Crackedback. It sounds to me like you have the right carb. on their. It just may need set-up.
 
Just helped a buddy install a msd atomic. No, the ads and instructions lie. Their fuel pump won't work, you need one from; can't remember, fuelpumpsinc.com.? You need a return line, other venders sell them, built into the normal gas gauge set-up. now, this has cost almost 3k by now.

But what a set-up, what a difference in accell and just plain drivability. You can fine tune it with it;s own little hand held( buy the 6' extension cord, so the passenger can tune it while driving). and get their billet dist, and let the computer set it's own curve.

Like I said, we trialled and errored it, untill we figured it out. But what a sweet set-up.
 
crackedback said:
What about your current carb makes you unhappy?

Personally, I wouldn't put anything smaller than a 750 on it. Just my preference.

Those calculators are great for finding the smallest carb you want to run.

Ask Jason with the RX7 about his fun with carbs and the huge difference a good fuel system and bigger carb can do for perfrormance. Those calcs call for a 600ish carb, ran 12.80, put on a carb with a proform 830cfm body and it runs real close to 12.00 now carrying and extra 150-200 pounds. No other significant changes.
Click to expand...


READ MY LIPS, er, text..........Don't put too much faith in those carb calculators. I defended my decision to buy my QFT 650dp for months before finally deciding to try a Proform mainbody (830 cfm) that Rob (Crackedback) loaned me. With the 650 I ran a best of 7.78 @ 86 (calculated to 12.30's since I primarily run 1/8 mile), but by the time I got the new 830 mainbody dialed in I ran 7.61 @ 89, 12.04 @ 111 (actual not calculated).

I personally wouldn't get anything smaller than a 750, look into the HR 750 or 850 dp carbs from Quick Fuel. This is what mine is, although the 650 version, and is a great bang for the buck. Completely tuneable like the Q series carbs, 4 corner idle, screw-in replaceable everything, just no fancy billet blocks or HP style mainbody. The cost between the 2 is about the same and both are cheaper than the Q series carb.

Out of curiosity, what don't you like about your current carb, specifically?
 
Patrick at pro-systems carbs has a sheet you fill out with all your specs. Send it to him and he can set you ip with a very good carb for your motor. I know several people running his carbs and they are top quality. And they also have a used selection they can sell you and change to perfectly fit your motor. Patrick is getting my call when my lincoln goes back together.
 
Sounds like most of you guys are leaning towards the 750. I understand that track times would be better with a larger carb, but this is a street only car. But it sounds like most think i'd be better off even on the low end with a 750? Sorry i'm new to this...

My reasons for wanting to try a different carb has a lot to due with the fact I AM so new to this. I've played with the holley vaccum secondaries quite a bit, new diaphram, quick spring change kit, lighter springs, heavier springs. Using the paper clip test my secondaries never seem to open much at all.
There is also a bad stumble just after idle till ~2k rpm, then it takes off. The thing I havent messed around with is the squirter nozzle and pump cam. But i'm not convinced that is it because I can feather the throttle right around the bog and it continues to run rough right there even after the initial acceleration off idle.

I've had a 6.5 power valve in it and now have a 9.5, I think I noticed a bit of difference after the switch, but its not quite there, I don't think.
I was also under the impression that without a mechanical carb with my 4 speed i'd be leaving some performence/drivablitity on the table. I guess i'm a bit quilty of having the 'just throw $$ at it to make it better' mentality.

initial timings at like 16 BUT I run my vacuum advance off of manifold vacuum so it idles with close to 30 and I allow 18 in my MSD distributor.

best vacuum readings I can get at idle are just shy of 12"
 
Which springs are on the advance mechanism? If you have the heavy ones that came on the dist from the factory, get rid of them and use a light and medium spring from the MSD kit/baggy.

Wrong gasket, bad PV gasket/too tight, clogged passages/air bleeds. It would be much easier to figure out with an a/f or O2 meter.

Go through the basics on the carb, float level, idle mix screws in to highest vacuum, etc. Try setting it and leave the vac advance disconnected and plugged for now.

The difference between a 650 and 750 down low, when tuned correctly, is negligible. Fuel economy regarding carbs and vac/mech secondaries is directly related to your right foot. :)
 
You must log in or register to reply here.
-

Back
Top