392 jetting

dana · #1 (**permalink**) 04-20-2002, 05:47 PM

I have the fms 392 crate motor.
I received some bad advice and had the holley 750 double pumper 'tuned'. I thought the car was very loud in the beginning and then it started to 'pop' when downshifting. It was bogging down when I stomped on the go fast pedal.
I found out they put 77 jets in the primary and 78 in the secondaries.
I went out and bought a whole lot of jets. I tried the 73 in the primaries and it is a whole lot quieter( relatively) and seems to run smoother and has better throttle response.
The question is: how do tell which jets to use?
Should I just take it to a dyno and change them there?
Is there a decent combo I can drop in until then?
Carb rookie.

Jack21 · #2 (**permalink**) 04-20-2002, 09:02 PM

Carb jetting is as much an art as it is a science. It is a function of carb size, engine size, intake manifold type, and camshaft. Looking at your spec's, I'd say your primary jets are too rich, although a 750 is the correct size carb for a 393 ci engine.

Step one. Look in the Holley book and see what jets Holley uses in this carb as sold. Somewhere between +4, and -4 is what you should be using according to Holley, and it could go as much as -6.

Are you using a dual plane, or single plane intake? Dual planes provide a better vacuum signal, and tend to use leaner settings.

What kind of cam are you using? Hydraulic? Hydraulic roller? What is the duration? Hydraulic rollers pull more vacuum than flat tappet hydraulics. Shorter duration cams (under 220 deg intake)pull more vacuum than longer duration cams (over 230 deg intake). The stronger (higher) the vacuum at the carb, the more fuel is being ingested at a given jet size. Have you checked your idle vacuum with a vacuum gauge? If you're getting 11" - 13" vacuum at idle, the standard jet size as the carb comes out of the box new, to -2 jet sizes is a good starting point. If you're getting 15" - 16" idle vacuum, then -2 to -4 jet sizes is a good place to start.

What do your plugs look like? Black and sooty? Black and wet? Too rich. Grey and clean? About right.

The great thing about a Holley, you can tune it. The bad thing about a Holley (particularly a non-emissions calibrated like a DP), you have to tune it.

Andy Dunn · #3 (**permalink**) 04-20-2002, 11:57 PM

Here are a couple of holley reference sites so you can determine what your carb was out of the box.

http://www.holley.com/HiOctn/ProdLine/CarbList.pdf

http://www.mortec.com/carbs.htm

Here are some sample 750s and you can see how they vary from model to model

3124 750 (F)70,(R)76 8.5,8.5 4150
3247 750 (F)70,(R)76 8.5,8.5 4150
4779 750 (F)75,(R)76 8.5 4150
4779-1 750 (F)70,(R)80 8.5 4150
4779-2 750 (F)70,(R)80 6.5 4150
4779-3 750 (F)70,(R)73 6.5 4150
4779-4 750 (F)70,(R)80 6.5 4150
4779-5 750 (F)70,(R)80 6.5 4150
4779-6 750 (F)71,(R)80 6.5 4150
6109 750 (F)75,(R)76 8.5 4150
6709 750 (F)652,(R)76 6.5,3.5 4150
8156 750 (F)70,(R)83 6.5 4150
80186 750 (F)70,(R)70 6.5,6.5 4500HP
80186-1 750 (F)70,(R)70 6.5,6.5 4500
80378 750 (F)59,(R)72 2.5,2.5 4150
80378-1 750 (F)56,(R)73 2.5,2.5 4150
80528 750 (F)72,(R)84 6.5 4150HP
80529 750 (F)72,(R)84 6.5 4150HP
84779 750 (F)70,(R)73 6.5 4150

I would go back to default settings and then if you still have a bee in your bonnet, try going up and down 1-2 and see how it feels. You'll probably need a dyno or race track to really dial it in. If the carb is right for you engine, it should be pretty close out of the box.

hope this helps

Andy

carlo · #4 (**permalink**) 04-21-2002, 02:57 PM

Dana,you may want to check wich power valve or valves you have along with the cam steps on the pumps,there are a LOT of combinations you can end up with if some one bench alters a stock carb,you may also want to consider sight plugs for the bowls(fuel level) and a check valve (summit) to protect the power valve from being ruined by backfire.An excellent book on Holley carbs is available from Dave Emanuel, s/a books I believe, will show you how all the circuits overlap and work together.

niles · #5 (**permalink**) 04-21-2002, 04:35 PM

Real dumb question!
My engine was delivered with 750 cfm, vac secondaries.
Where do they put the carb model number; looked all over it; nothing correspondes with std model no's?
gn

DAVID GAGNARD · #6 (**permalink**) 04-21-2002, 07:07 PM

Niles;

This is not a dumb question at all...I know,I've had to ask it myself.....look on the outside of the choke horn or choke tower as some call it for the model #....if by chance it has been milled off,then it's anybodys guess,but most 750 vac sec. or of a standard model..

David

niles · #7 (**permalink**) 04-21-2002, 10:34 PM

Dave:
thanks!
It was hidden right on the side of the choke tower; 3310-4
gn

Hal Copple · #8 (**permalink**) 04-23-2002, 11:07 AM

Niles, i have a 396 Stroked Windsor, Holley Vac Secondary 750 sitting on a Performer RPM, on port matched Trick Flow Twisted Wedge heads. I had my car on a chassis dyno last summer, with an analyzer in the pipe.

I run 70 jets front, 80 rear (i took out the metering plate, put in a metering block, so i could swap out the rear jets easily). You do need a longer fuel inlet rail to do this, as the carb is about a half inch longer.

Anyway, my car has always run perfectly with this, good mpg, about 50 quarter mile runs, lots of road course laps (about 300). In the winter, since i drive all year here in SC, i put in front 71's to take advantage of the more dense, cooler air. I can trundle around in fifth at 1100 rpm and not stumble on acceleration. I had over 400 foot pounds of torque at the rear wheels on this set up, and the car drives fine. This is my suggestion.

I tired a bit richer in the fronts, by my town drivability deteriorated with each upward jetting.

dana · #9 (**permalink**) 04-23-2002, 06:59 PM

Hal,
I took your suggestions and went out and bought a whole bunch of jets. I had 77's in the primaries and put in 73's to start.
It is much better. Idles better now, now popping on downshifts and less black smoke.
I got a little hesitation when letting off then getting back on so I am going to try the 72's next. Suprisingly clean plugs for the over jetting.
One other thing, it idles a lot higher when the engine is hot, I tried loosing the idle screw, and it will not back down unless I force it down manually to the set screw. ??? only does it when hot

Mike Madonna · #10 (**permalink**) 04-24-2002, 09:28 PM

Did you see my reply to your post on SCOF regarding the throttle? If not, check the bushing where the accelerator pedal linkage comes out of the footbox. This will expand a little when hot. Give it a couple of shots of Silicone spray, that should help. As for jetting, my 750 Vac. carb (according to Holley) has 72 primaries. Your Mech. 750 may have had the same. Check Holley's site, they have a chart that shows what installed at the factory regarding jetting and power valve. My plugs are a little sooty/black. I believe this is due to the choke during the colder weather. Just a little food for thought....

Mike

dana · #11 (**permalink**) 04-25-2002, 04:40 AM

Mike,
I read your other email. I am going to try that next. I though my plugs were suprisingly clean for such an over jetting.
thanx
dana

Curt C. · #12 (**permalink**) 04-25-2002, 11:04 PM

What kind of ignition are you running. According to the folks at Barry Grant, a high power ignition system such as a MSD will leave the plugs a lot cleaner than with a stock ignition. Go check their website BGfuel.com or something like that. Truly the best way to tell is to get on a dyno with an EGT sensor.

I have a 351W crate motor and run the Holley 9379. I have play with the jets quite a bit. Surprisingly, the stock jets work the best for overall performance 68F, 81R. Car does run faster in the 1/4 with bigger jets, but is rich on the street.

Also, do not forget about the other circuits: idle, power enrichment. A carb can idel fat as hell but be extremely lean under WOT. It can be very confusing and frustrating. Get a Holley or BG tuning manual at the book store.

Curt

scr11 · #13 (**permalink**) 04-27-2002, 08:45 PM

I'm running a Holley HP series 750 DP on my 393. I'm using a Victor Jr. intake with Vic Jr. heads. After 3 hours on a Dynojet chassis dyno with the wide band sensor in my driver's side pipe, we learned the best jetting was 71's up front and 72's in back. My air/fuel mixture was a perfect 13:1.
The carb comes from Holley with 73's on all four corners.
The motor is so much more responsive and crisp, no black smoke out of the side pipes at all.

Scott, SPF#1230 / 393

dana · #14 (**permalink**) 04-28-2002, 05:40 AM

Scott,
I tried going down to 71 yesterday. It was hesitating, and had no power. I am now going back up, trying 72 next, if not back to the 73- that worked fairly well.
Then off to the dyno to lock in the secondaries and double check the fronts.
Dana

scr11 · #15 (**permalink**) 04-28-2002, 09:03 PM

Dana - Let us know how the dyno goes. I'm curious to find out where you end up on correct jet size.
Of course, I'm in Houston, so being at sea level dictates different jets from say, someone in the mountains.
Good luck!

Scott, SPF#1230 / 393

Mr.Fixit · #16 (**permalink**) 05-03-2002, 05:38 PM

Jetting on the dyno is not the same as the jetting the car needs in driving situations. The load on the motor is different, rates of engine accelleration are different, etc. Reading the spark plugs is the best way you can determine fuel need assuming you don't have exhaust gas temperature probes, or a dragstrip. Don't forget the powervalve. It should open at about two inches of vacuum less than what the motor idles at. The power valve is stamped with two numbers, yours will have a 6 and a 5 meaning it opens at 6.5 inches of vacuum. Make sure the diaphram is not ruptured on it either. Depending on your motor, gearing, and trans,it may like a 850 double pumper even more than the 750 vac sec. carb.