Edelbrock 750 Tuning... - Page 2 - Chevelle Tech
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post #16 of 30 (permalink) Old Dec 13th, 18, 10:17 PM Thread Starter
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Re: Edelbrock 750 Tuning...

Quote:
Originally Posted by jeff swisher View Post
There are sometimes fine lines when tuning a carb . Raise the fuel level and the signal from the boosters does not need to be as strong to pull fuel so a rejet is sometimes needed.

I tweak and tweak on them..not as much as I used to as I can get really close with about 3 trips around the block.
One thing for sure is the engine needs to be warmed up as does the intake manifold .
Cold runners and engine and trying to test and tweak just do not go together.

Many times i have tuned on the highway. Took my jets and rods with me.
Long highway drive and pull over and change it and drive again and of course accelerate as quick as I can getting back on the highway

If you go Holley here are a few things I have done to them to make them pull harder on the top and mid range.

Depending on what air flow is needed i enlarge the venturies some and polish them.
I also thin the throttle shafts and either get small head screws for the throttle plates or machine the factory screw heads smaller.
I did this on 2 750's that would not pull as hard as my 750 AFB or my 750 Q jet.
One holley was a 3310 and the other a 4779 I also removed the choke tower.

I did not have any 800 or larger Holleys so it was not use them or try and make them flow a bit better and see if my top end improved.

After tweaking any of the 3 brands they all ran almost exactly the same.. to the point you could not tell i changed carbs.

I have never seen just a spring above the check ball in any of my AFB or edelbrocks..all mine had the brass puck and ball or a pointy puck and no ball.
The Speed Demon carburetors are supposed to have improvements over the Holley units. The venturi are smooth. I think it has drop leg boosters. The only thing I have struggled with on it is which power valve to use. Since I only have 7" of vacuum at idle, I have run them as low as 3.5. I had a 4.5 in there last year, but changed to a 6.5 when I put a kit in it (because I didn't have another 4.5). I didn't really notice any difference with any of them. I wish I had some jets - then I could change the primaries out to bigger ones. The stock jets should be 72 primary and 78 secondary.

On a side note, I did check the metering rods in the Edelbrock and they are .070/.037, so someone has changed them (the stock ones were .075/.047). The Speed Demon pulled harder before I had to repair the threads in the body. I may pull it apart and inspect everything before I put it back on.

Quote:
Originally Posted by zeke67 View Post
Sounds like the decision to move on has been made. Anyway, the difference in fuel height between the bowls is an issue. Sounds like float drop may be an issue, not allowing enough fuel to enter the bowl. The float drops but not enough to open the needle to meet the fuel demand. Jeff made an important comment. The hole location determines how quickly / slowly the pump shot is delivered. The squirter size determines how much. Not sure, just a guess, but maybe the bowl is running our of fuel for the accelerator pump to deliver.
That is what I am thinking. The level on that side is always lower when I take the top off. In fact, the fuel is always lower than the notch for the accelerator pump well. I really don't like the little screens that they give you to put into the seats - I think they restrict flow (but I could be wrong). There is a larger screen in the rebuild kit that I think could be used instead of the small ones (it looks like it would go into the fuel inlet opening behind the fitting). I put the 1468 accelerator pump in it (per Edelbrock tech line), which has a stronger spring on it. I also have the .043 discharge nozzles on there. But, I think it is completely related to the fuel level not being where it needs to be.

If I feel ambitious in the future, I may try the Edelbrock again. Perhaps some brand new floats would be in order. The ones that are in there are no longer brass colored - more like brown with some shiny silver spots.

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post #17 of 30 (permalink) Old Dec 14th, 18, 3:38 AM
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Re: Edelbrock 750 Tuning...

Quote:
Originally Posted by WakkoWarner View Post
Not confusing. What is confusing is why this carburetor just refuses to squirt fuel out when it first starts to move. I changed the float levels this afternoon. It did not dribble out of the boosters, but it just didn't run right under load. So, I adjusted them back, then removed the weight from the check ball. That did not help. I then put the spring back in over the check ball, but now it doesn't even want to squirt until it gets moved quite a bit.
At this point, I am tired of messing with it again. So, I will reinstall my Speed Demon tomorrow (I may have to find a power valve first, as I am not sure if the one I put in is new or not). I wish I could just get fuel injection.
Good choise Im not a fan of those E-clones, corrode like hell both inside and outside, a shame Edelbrock stopped making q-jet clones when they bought the afb.

Give that Holley clone some tuning and it will run Better than any carb except maybe a q-jet depending on the rest of the combination.
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post #18 of 30 (permalink) Old Dec 14th, 18, 3:39 AM
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Re: Edelbrock 750 Tuning...

[1] The 'screens' near the seats are anti slosh baffles to keep the fuel level as stable as possible.

[2] With The AFB design, when you accelerate the fuel runs to the rear of the carb; front of the float drops & admits more fuel. A great design because the carb fuel level gets replenished when the engine needs it most, under load, accelerating.

[3] Try lengthening the spring on the acc pump plunger. With the initial 'hit' on the pedal, that spring may compress slightly absorbing some travel. Result is no fuel for an instant from the acc pump nozzle.

[4] Look carefully at the acc pump linkage. Make sure that initial movement is transferred via the rod instantly to the pump arm. A bit of tweaking here might get faster initial movement.
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post #19 of 30 (permalink) Old Dec 14th, 18, 7:45 AM
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Re: Edelbrock 750 Tuning...

The screens he speaks of are in the seats and you can't see them.
You can remove those.
Those are insurance only. Think about it like this some jack wagon buys a carb and expects it to work out of the box.
His fuel filter is junk but still flows fuel so good enough. and his hacksaw cut steel fuel line is fine and dandy.

Does not matter that the sharp edge of that steel line shears some rubber off the fuel line he hooks to the carb.
Now think about all that junk he could be sending through the fuel line.
If those tiny screens were not there he would get debris between needle and seat and flood the carb.

He would then remove that piece of junk carb and send it back.
Install those tiny screens and have less carbs returned back to the place of purchase.

If you have a good fuel filter and smooth lines that do not shear rubber then remove the screens.

WakkoWarner what is your Vacuum reading at say 950-1000 rpm?

No matter the power valve you use in the Holley it will run the same if vacuum is above that point while driving down the road.
What is the vacuum reading while driving down the road?

I got a jet kit from advanced auto parts the Holley one and I went online and found advanced auto parts 25% off cupon and got free shipping to my door.
I also had a few bucks from them for purchasing a radiator and the kit to my door was like 36 bucks.

That is a lot of jets.
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post #20 of 30 (permalink) Old Dec 14th, 18, 9:27 AM
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Re: Edelbrock 750 Tuning...

Just 1 more thing
I have noticed recently ..the last 3-4 weeks that both of my 350" carbed engines have been running poorly.
I think they switched to the winter time fuel. as it does not smell the same as it has smelled and my upper RPM is not there like it was before and I get more hesitation than before.. but it could also be the weather is very cold here and I need to rejet.
Or could be a combo of the 2.
Prices have recently dropped at the same time my stuff began running poor.
More Butane in the fuel and maybe my engines do not like that.

Something to think about.
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post #21 of 30 (permalink) Old Dec 14th, 18, 10:34 AM Thread Starter
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Re: Edelbrock 750 Tuning...

Quote:
Originally Posted by Gtogeoff View Post
[1] The 'screens' near the seats are anti slosh baffles to keep the fuel level as stable as possible.

[2] With The AFB design, when you accelerate the fuel runs to the rear of the carb; front of the float drops & admits more fuel. A great design because the carb fuel level gets replenished when the engine needs it most, under load, accelerating.

[3] Try lengthening the spring on the acc pump plunger. With the initial 'hit' on the pedal, that spring may compress slightly absorbing some travel. Result is no fuel for an instant from the acc pump nozzle.

[4] Look carefully at the acc pump linkage. Make sure that initial movement is transferred via the rod instantly to the pump arm. A bit of tweaking here might get faster initial movement.
The rod does seem to move as soon as the throttle moves. I think it is the fuel level in the bowl that is causing the issue.

Quote:
Originally Posted by jeff swisher View Post
The screens he speaks of are in the seats and you can't see them.
You can remove those.
Those are insurance only. Think about it like this some jack wagon buys a carb and expects it to work out of the box.
His fuel filter is junk but still flows fuel so good enough. and his hacksaw cut steel fuel line is fine and dandy.

Does not matter that the sharp edge of that steel line shears some rubber off the fuel line he hooks to the carb.
Now think about all that junk he could be sending through the fuel line.
If those tiny screens were not there he would get debris between needle and seat and flood the carb.

He would then remove that piece of junk carb and send it back.
Install those tiny screens and have less carbs returned back to the place of purchase.

If you have a good fuel filter and smooth lines that do not shear rubber then remove the screens.

WakkoWarner what is your Vacuum reading at say 950-1000 rpm?

No matter the power valve you use in the Holley it will run the same if vacuum is above that point while driving down the road.
What is the vacuum reading while driving down the road?

I got a jet kit from advanced auto parts the Holley one and I went online and found advanced auto parts 25% off cupon and got free shipping to my door.
I also had a few bucks from them for purchasing a radiator and the kit to my door was like 36 bucks.

That is a lot of jets.
The rebuild kit comes with a larger screen that looks like it would fit behind the fuel inlet fitting, allowing one to remove the smaller screens. I haven't checked the vacuum at that RPM, but if past memory serves me correctly, I don't think it changed versus 750 RPM. I have no idea what it is while driving.

1971 Malibu, factory buckets/full gauges/console, 377 roller cammed small block, Turbo 400, 3.42 10 bolt posi (Buick), - All your base are belong to us.
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post #22 of 30 (permalink) Old Dec 14th, 18, 1:00 PM
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Re: Edelbrock 750 Tuning...

Brian, did you read the PM I sent you last night?

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post #23 of 30 (permalink) Old Dec 14th, 18, 2:55 PM Thread Starter
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Re: Edelbrock 750 Tuning...

Quote:
Originally Posted by Tom Mobley View Post
Brian, did you read the PM I sent you last night?
I responded to it this morning - you should have it.

On a somewhat positive note, I put the Speed Demon back on this morning after opening it up and putting yet another power valve in it. I checked my jets and they are actually 74 on the primary side and 83 on the secondary side. Once I got it warmed up, I got the vacuum gauge out and did a little tuning. I was actually able to get the vacuum up to a whopping 8" @ 750 RPM in gear. The vacuum doesn't seem to change between 750 and 1,000 RPM.

And since I had my tools out, I opened up the driver side door to find the source of a squeak (it was the rubber stops at the bottom - they had fallen out, so I put them back in with some screws/washers/star washers/nuts - I will have to order some new ones some day). I also decided to change out my door latch. The door opens a little easier from the inside now.
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post #24 of 30 (permalink) Old Dec 14th, 18, 6:54 PM
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Re: Edelbrock 750 Tuning...

Nope, I have nothing from you. please check.

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post #25 of 30 (permalink) Old Dec 14th, 18, 7:57 PM Thread Starter
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Re: Edelbrock 750 Tuning...

Quote:
Originally Posted by Tom Mobley View Post
Nope, I have nothing from you. please check.
I just responded again.

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post #26 of 30 (permalink) Old Dec 14th, 18, 8:10 PM Thread Starter
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Re: Edelbrock 750 Tuning...

I just drove across town and back late this afternoon (first chance I had to drive it after putting the Speed Demon back on). It is hanging up at idle a little bit - I may need to check the adjustment on my throttle cable. But, it is pulling pretty well (better than it did when I had it on there the last time).

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post #27 of 30 (permalink) Old Dec 17th, 18, 5:30 PM Thread Starter
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Re: Edelbrock 750 Tuning...

I called Edelbrock again today. I asked them about removing the screens from the seats to improve fuel flow to the left side of the carburetor. While they don't recommend it, they said it wouldn't hurt to try. I don't think it will cause any issues, as my Speed Demon has no such thing in it and the bowls are always debris free when I put a kit in it. I can put the large screen in right behind the fuel inlet nipple.

The Speed Demon just isn't cutting it for WOT power. So, if I feel ambitious this week, I will remove those screens and give it a shot. The new screens that came in the kit had the ends crimped down, so that they lost about 20% of their length, which might cause a flow restriction. We'll see what happens without them installed.

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post #28 of 30 (permalink) Old Dec 17th, 18, 6:58 PM Thread Starter
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Re: Edelbrock 750 Tuning...

Just so you can see the difference between the new and old seat screens, here is a picture (left screen in a pair is the new style and the right screen in a pair are the old style). The new ones are not only shorter, but they are crimped on the end (kind of hard to see the way they are sitting).
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post #29 of 30 (permalink) Old Dec 18th, 18, 3:39 AM
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Re: Edelbrock 750 Tuning...

Unless I missed it, you haven't told us much about this flat spot.

Is it just off from idle, taking off from rest, light throttle? Same, but heavy heavy throttle? Giving it throttle once up to cruising speed?
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post #30 of 30 (permalink) Old Dec 18th, 18, 4:37 PM Thread Starter
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Re: Edelbrock 750 Tuning...

I pulled the screens out today and put the large single screen in behind the inlet. When I took it for a drive, it almost cured it. When I pulled the top off of the carburetor, there was actually fuel in the accelerator pump well (which shows me that the seat screens were restricting flow). However, it still had a slight stumble, because it won't squirt strong from the get go. At this point, I just am through trying to make this old, used carburetor work. I put the Speed Demon back on and I will mess with the vacuum secondary spring to see if I can get it to pull a little better at WOT.

1971 Malibu, factory buckets/full gauges/console, 377 roller cammed small block, Turbo 400, 3.42 10 bolt posi (Buick), - All your base are belong to us.
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