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-   -   carb jetting on an edelbrock 750 (http://www.hotrodders.com/forum/carb-jetting-edelbrock-750-a-224853.html)

69novassguy 10-09-2012 04:53 PM

carb jetting on an edelbrock 750
 
ive been struggling with misfires for a long time now and finally realized im running so rich that it fouls the plugs. has anyone had this problem? and what size jets/metering rods should i use for a 350 sbc?

cobalt327 10-09-2012 05:24 PM

Quote:

Originally Posted by 69novassguy (Post 1597663)
ive been struggling with misfires for a long time now and finally realized im running so rich that it fouls the plugs. has anyone had this problem? and what size jets/metering rods should i use for a 350 sbc?

I've heard more complaints of the carb being too lean out of the box than too rich. The first thing I would do is baseline it back to stock and see what it needs from there, if anything.

Owners manual for Performer and Thunder series Edelbrock carbs. It has tables showing how the carb was delivered originally as well as what changes are needed to go richer or leaner.

Another good source of info is Edelbrock carburetor tuning.

FWIW, a ZZ4 SBC ended up needing:

primary jets- 0.113"
secondary jets- 0.110"
metering rods: 0.073" x 0.037"
silver (8 in/Hg) metering rod springs
pump rod in the top hole

But the thing is, every engine, vehicle and situation is different. Changes in the parts used, fuel, gear ratio, weight, how the vehicle is used, even ambient conditions, etc. all can have an effect on what the carb will need to work the best.

Not to mention the ignition timing. What's often blamed on the carb is in reality the timing/advance curve being wrong for the engine. The timing should be set up first, then dial in the carb. More on timing can be seen here. The article was written on the GM HEI, but the timing info holds true, regardless of the type of distributor/ignition.

69novassguy 10-09-2012 05:30 PM

thanks the timing is good and the carb is tuned and ive been going all over asking around and im being told that the 750 might be too much carb for it, but i have the carb cal kit from edelbrock so im wondering which rods and jets to use....ive been looking at the manual and the graphs and charts are helpful but ive never had to do this before...

cobalt327 10-09-2012 05:53 PM

Quote:

Originally Posted by 69novassguy (Post 1597673)
thanks the timing is good and the carb is tuned and ive been going all over asking around and im being told that the 750 might be too much carb for it, but i have the carb cal kit from edelbrock so im wondering which rods and jets to use....ive been looking at the manual and the graphs and charts are helpful but ive never had to do this before...

First be sure the float level is correct and that there's no leakage past the needle and seat from trash or wear.

Then if it's still too rich, baseline the carb to factory specs. Still too rich? The tables show what changes to make to lean it out but you need to know if it's rich at light throttle cruise or under a load, or what.

The 750 is a bit large for a mild 350 sbc, but it'll run fine if it's tuned in correctly because the secondaries open on an "as needed" basis- not like a double pumper, where the secondaries will open up if you floor it regardless of what the engine wants. The primaries are larger than a 600, but that's not so much of a problem that it will always run too rich. What you may notice is a bit less crispness to the off idle response due to the larger primaries- which work to slow the air/fuel velocity a bit.

It will help to know the specs of the engine- cam specs, what type intake, the timing as already mentioned, the compression, etc. Everything that's known about it will help.

You said the timing is OK. What are the numbers associated w/it (initial, total, all in by rpm, vacuum advance amount)?

69novassguy 10-09-2012 06:46 PM

it runs rich at idle and at WOT and it is at factory spec...i got it about a month and a half ago replacing a holley 600...i looked at the charts in the manual and was a little unclear on what to do. i dont understand what 1 2 or 3 stages rich or lean mean and that being said i dont know what i want it to be at...

69novassguy 10-09-2012 06:49 PM

i have a torker II INTAKE performer rpm heads, all of the electrical is fresh and the timing is set correctly...when i bought it the guy said it was cammed but didnt know what kind and it clearly sounds like it, not sure the compression

F-BIRD'88 10-11-2012 08:25 AM

The 750 is not toomuch carb for it.
If it is "rich" at idle the motor needs more timing at idle.
Cammed motors need a lot of idle timing. Especially with a edelbrock carb.
Fresh plugs, check ignition for a fault.
what is the timing curve. ? manifold vacuum at idle? idle rpm?

techinspector1 10-11-2012 04:55 PM

Tee off at the bowl inlet and run a small copper line back to the firewall, then up past the hood lip to the cowl. Mount a mechanical 0-15 psi fuel pressure gauge on the cowl so you can see it through the windshield while you're driving. This carb doesn't need any more than about 5 psi of fuel pressure and I'm bettin' that the pump is puttin' out more than that, which causes the fuel pressure to overpower the needle and seat in the bowl and blow raw fuel into the intake manifold. Use tie wraps, duct tape or whatever you have to use to mount the gauge to the cowl. It's only temporary until you get the pressure under control.
http://www.processdepot.com/product-...category_id=27
And by the way, the Carter "Muscle Car" pumps will do a great job if you want to use a mechanical, engine-driven pump. Electric pumps will need some type of pressure control valving in the line.

F-BIRD'88 10-11-2012 10:17 PM

Quote:

Originally Posted by techinspector1 (Post 1598297)
Tee off at the bowl inlet and run a small copper line back to the firewall, then up past the hood lip to the cowl. Mount a mechanical 0-15 psi fuel pressure gauge on the cowl so you can see it through the windshield while you're driving. This carb doesn't need any more than about 5 psi of fuel pressure and I'm bettin' that the pump is puttin' out more than that, which causes the fuel pressure to overpower the needle and seat in the bowl and blow raw fuel into the intake manifold. Use tie wraps, duct tape or whatever you have to use to mount the gauge to the cowl. It's only temporary until you get the pressure under control.
P13K2
And by the way, the Carter "Muscle Car" pumps will do a great job if you want to use a mechanical, engine-driven pump. Electric pumps will need some type of pressure control valving in the line.

Carter electric pump #P4070 does not need a pressure regulator at all.
The fuel pressure is just right for a edelbrock/carter carb.
(6psi)

69novassguy 10-16-2012 07:10 PM

i have a regulator on there now and its putting out 5.5 psi...ive jetted it down and changed the metering rods, and my #2 plug still fouls out...and should i advance the timing or retard it?


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