I've got one for y'all... see if you can figure this out...

[clinebarger]

Originally Posted by 70clone

Throw that edelbrock away holleys are great but obsolete barry grant is the way to go.

What makes Barry Grant overpriced stuff better than a Holley or Holley HP?

[C20-67_N_MO]

Okay I gotta chime in here.

since I don't know how radical your camshaft is but if its in the 500+ lift range then you need to have a carb with mech. secondaries.

And like others are saying you will need a reserve vacuum cansister for your power brakes.

Good Luck.

[70clone]

Originally Posted by clinebarger

what makes barry grant overpriced stuff better than a holley or holley hp?

i used to think the same thing until i got a demon set it and forget it i had edelbrock (shiny rochestor) and was die hard holley my whole life but holley can be *****y sometimes they have a mind of there own sometimes. I bartered for the demon and it was night and day difference. Im not knocking holley im just saying they are a little behind in technology.

[68C15]

after reading all this I need to mention a couple things I have dealt with in my days of carb tuning for both street and strip.
edelbrocks/OE carbs are for street rods with mild engines
holleys/demons are for performance
granted there are exceptions to this as a butt-load of money can get you anything and make it work. I made a holley 600 DP highly drivable with 17MPG on a 6800# truck with a 400HP small block. I also made a Q-jet run a 406 @ 7500 RPM (very wild ride )

when building a vehicle or taking everything educated engineers spent countless months designing and throwing it all out the window in favor of more power you NEED to do it as a complete package. I mean complete, from air cleaner to the tires.
your motor sounds respectable, I like the heads and cam. but your converter is killing it. when you build up an engine you are basically increasing the amount of torque created on each and every power stroke. this in turn causes the converter to have a lower stall speed. I could go into the physics of this but the simple truth is you need to swap it out before you go any further, at all.

[LONGHAIR]

Originally Posted by C20-67_N_MO

Okay I gotta chime in here. since I don't know how radical your camshaft is but if its in the 500+ lift range then you need to have a carb with mech. secondaries.

That is just not so.....Mechanical secondary carburetors are really only "better" for wide open throttle situations, like drag racing..and vacuum secondaries can be made to work there too.
You will get better street driveabitily and fuel economy from a vacuum secondary carburetor.

Originally Posted by 68C15

when you build up an engine you are basically increasing the amount of torque created on each and every power stroke. this in turn causes the converter to have a lower stall speed.

This is not so either....Not only is this hugely over-simplified it is way to vague. This statement makes it sound like horsepower and torque are independant of each other, which is not the case either.

Torque is twisting force, but movement is not required. As an example: Apply 50 ft-lbs of torque with a wrench to a bolt that is already tightened beyond that point. Your 50 ft-lbs of torque is still being applied, but nothing is moving.

Horsepower has to do with movement in relation to time. You can use torque to make horsepower but it is not technically required. You can produce measureable horsepower with a hydraulic cylinder "pushing or pulling" with no rotation required.....same with steam, rotation not required.

With and internal combustion engine, you have both working together. The rotational force is converted to linear motion, through the gears to the wheels.

What really happens with higher performance engines is that they produce less torque at lower RPM as a trade for higher torque at higher RPM. This makes it harder for the engine to idle against a load. That is why a manual transmission has no effect, there is minimal load at idle, but automatics have to fight agaisnt the torque converter when idling in gear. A torque converter with a higher stall speed (AKA "looser") is easier to turn at idle because it "slips" more. This has 2 benefits, better idle is only the first. The second is in actual driving performance. This "slipping" allows the engine to rev-up to the engine's prefered RPM range quicker too.
There are trade-offs...you can go too high too. This is not helpful either. Too much slipping is "wasted" energy....and it creates more heat, which kills automatic transmissions.

I sure hope I didn't lose anyone there.....

[Pick8208]

I'm new here, but I've had a quite a bit of experience tuning Edelbrock carbs to run with big cams that make no vacuum. From what you describe, it sounds like you're too lean at idle, so the first thing to do is check for vacuum leaks at the carb, intake, and at all vacuum lines. If all that checks out OK, give this a shot:

1. Bring the engine to operating temp.

2. Using the idle speed adjustment screw, run the idle of the engine up to about 1,000-1,200 rpm (Edelbrock idle circuits overlap the power circuit quite a bit, and doing this will increase vacuum through the carb, which I'm sure is low, given you're running a 750 on an SBC).

3. Turn in one mixture screw until the engine starts to stutter a little, then back it off about 1/4 turn. Do the same thing with the other side. (I'm guessing the screws didn't do anything before, wherever they were set. Hopefully they're working now).

You'll want to try to get the most RPM out of it possible by just manipulating these screws.

4. Drop the RPM back down to about 550-700 RPM using the idle speed screw.

5. Get inside the truck, and put it in gear with your foot on the brake. How much RPM was dropped? Put it back in park. How much RPM was gained? If it's more than about 300-400 RPM, turn the mixture screws in about 1/4 turn and try it again. Try to get the drop/raise in RPM between park and gear as small as you can. Obviously, your stall converter will have a big impact here, and you might not be able to do anything about it... but it's worth a shot. Once you've finished, then take it for a drive and see if it's any better.

You may also want to increase the accelerator pump shot to help cover the transition from idle to power. Do this by moving the rod down one slot in the pump arm.

Also, just FYI, big cams that make no vacuum will always run pretty rich at idle, so regardless of how you tune it, watch for fouled plugs in colder weather. If you decide to go with any other parts, I'd suggest at least a 2500 stall converter and maybe a smaller carb.

[Ackattack]

Plain and Simple: Change your Torque Converter

After that then you can see where you are at with carb/ignition wise.

Back in the day when I ordered a cam for my engine, talking with the CompCam tech on the phone, they told me that the XE268 is all the bigger they recommend with a stock converter (so that's what I got). You have too big of a cam for your stock converter. Go find a good used one on craigslist, pull a few bolts and plug it in. I would expect 90% of your driveability issues will be gone.

[toddtheodd]

Originally Posted by 68C15

I could go into the physics of this but the simple truth is you need to swap it out before you go any further, at all.

Not an option...
I had to get this truck together in a hurry, and even though I spend a pretty picture doing it, I was tapped out when done.
Right now, this is my daily driver. So I have to figure out these other problems until I can afford the converter and price of putting the bloody thing in.

[Ackattack]

Originally Posted by toddtheodd

Not an option...
I had to get this truck together in a hurry, and even though I spend a pretty picture doing it, I was tapped out when done.
Right now, this is my daily driver. So I have to figure out these other problems until I can afford the converter and price of putting the bloody thing in.

Sounds like you've figured out a temp fix....use the choke. Save your money for the converter.

[toddtheodd]

Originally Posted by Pick8208

I'm new here, but I've had a quite a bit of experience tuning Edelbrock carbs to run with big cams that make no vacuum. From what you describe, it sounds like you're too lean at idle, so the first thing to do is check for vacuum leaks at the carb, intake, and at all vacuum lines. If all that checks out OK, give this a shot:

1. Bring the engine to operating temp.

2. Using the idle speed adjustment screw, run the idle of the engine up to about 1,000-1,200 rpm (Edelbrock idle circuits overlap the power circuit quite a bit, and doing this will increase vacuum through the carb, which I'm sure is low, given you're running a 750 on an SBC).

3. Turn in one mixture screw until the engine starts to stutter a little, then back it off about 1/4 turn. Do the same thing with the other side. (I'm guessing the screws didn't do anything before, wherever they were set. Hopefully they're working now).

You'll want to try to get the most RPM out of it possible by just manipulating these screws.

4. Drop the RPM back down to about 550-700 RPM using the idle speed screw.

5. Get inside the truck, and put it in gear with your foot on the brake. How much RPM was dropped? Put it back in park. How much RPM was gained? If it's more than about 300-400 RPM, turn the mixture screws in about 1/4 turn and try it again. Try to get the drop/raise in RPM between park and gear as small as you can. Obviously, your stall converter will have a big impact here, and you might not be able to do anything about it... but it's worth a shot. Once you've finished, then take it for a drive and see if it's any better.

You may also want to increase the accelerator pump shot to help cover the transition from idle to power. Do this by moving the rod down one slot in the pump arm.

Also, just FYI, big cams that make no vacuum will always run pretty rich at idle, so regardless of how you tune it, watch for fouled plugs in colder weather. If you decide to go with any other parts, I'd suggest at least a 2500 stall converter and maybe a smaller carb.

I'm making about 8 inches of vacuum at 1000 rpm... which if I'm understanding everything I can find correctly, is WAY bloody low, and is causing the metering rods to be in the "enrichment" stage, and so I need to find the "looser" springs that will allow it to be in the lean stage when idling.

I also found what you were talking about on the pump shot, and am heading out to screw with that...

Oh... and you're also right about the idle screws doing squat. I've screwed them all the way in, and the motor kept running...

[Pick8208]

Yes, lighter springs for the metering rods are also a good idea. They'll help keep the rods seated in the jets at idle. You can get a tuning kit for the Edelbrock carb for not a lot of money, and it'll have the springs you need as well as a collection of rods and jets. IIRC, the light springs are the Orange ones. I think those are the ones I'm running in my 468.

[toddtheodd]

Actually... Orange is supposed to be the one already in it... so I need to go to yellow or blue...

[PanelDeland]

I agree with the advice about the higher stall converter,but I also think you need to consider a smaller carb.For any but a ful out high rpm engine a 750 is just a bit larger for a 355.A 600-625 cfm carb will carry a better vacuum signal and give you better throttle response and less bogging.Even with the converter I think you will get better driveability from a smaller carb.It's just my opiniom but way too many people overcarb 350's and then wonder why their combo isn't making the power at a driveable rpm.

[toddtheodd]

Originally Posted by PanelDeland

I agree with the advice about the higher stall converter,but I also think you need to consider a smaller carb.For any but a ful out high rpm engine a 750 is just a bit larger for a 355.A 600-625 cfm carb will carry a better vacuum signal and give you better throttle response and less bogging.Even with the converter I think you will get better driveability from a smaller carb.It's just my opiniom but way too many people overcarb 350's and then wonder why their combo isn't making the power at a driveable rpm.

I totally agree and understand. I bought the 750 when I was young and dumb, and have regretted it since. I borrowed my inlaws 600 once and it was so nice... I had to give it back though when he needed it.

[toddtheodd]

Ok... with the "blue springs" in, it runs much better. Even idles around 550 now in D...
Still stumbles just the tiniest bit when I first give it gas, so I will pull the carb apart and put the bigger accelerator pump nozzles on.

I got lucky. There is an Oreillys in dallas on Forest Lane that is a "Hub" store, so they carry stuff that other stores don't. They had the step up springs AND accel pump nozzles for my carb. Hooray!
Once I finish getting this little problem sorted out, then I will either (hopefully) put a standard in, or get that torque converter...

[Arad68]

I know this isn't your case, but through experience, when you get the money, get a BG 650 demon, electric choke, mechanical secondaries. I went to that from a 650 holley, night and day difference. I picked mine up on ebay for about 60% of new cost. That is where I got a jet kit and gaskets too. Mine was truely set it and forget it. It even does well in the Las Vegas heat. The underhood temps get really high in our trucks.

The converter will make a world of difference too.
Sounds like you are on your way...Great luck to you dude!!!...Enjoy:

[C20-67_N_MO]

Originally Posted by LONGHAIR

That is just not so.....Mechanical secondary carburetors are really only "better" for wide open throttle situations, like drag racing..and vacuum secondaries can be made to work there too.
You will get better street driveabitily and fuel economy from a vacuum secondary carburetor.

I sure hope I didn't lose anyone there.....

Okay so far you have not lost me. But explain why you think vacuum secondary carburetor can but run on a camshaft with oh say .555 lift?

Not saying it can not be done but like you said it 'can be made to work'.

If so HOW?

Thanks!

[LONGHAIR]

Originally Posted by C20-67_N_MO

Okay so far you have not lost me. But explain why you think vacuum secondary carburetor can but run on a camshaft with oh say .555 lift?

Not saying it can not be done but like you said it 'can be made to work'.

If so HOW?

Thanks!

It's not so much about the lift of the cam as it is the total CFM (airflow) So more lift on a smaller engine may still flow less air than less lift and more cubic inches.

Double pumpers are not required to flow the correct amount of air, the difference is in how the secondaries are opened.

It's all in the tuning. A combination of the correct accelerator pump shot, secondary spring, and jetting.

[chevydog66]

My opinion is that your carb is too big, plus it's an Edelbrock. A 600-650 Holley or Demon would be perfect. And that your convertor is too small, get a higher stall speed. With that large of a carb you are losing the signal needed for proper atomization of the fuel and therefore using too much fuel, and will probably be fouling plugs, in normal street driving. If was a 1/4 mile truck that was always at W.O.T. then a 750 would be great.