Thread: Edelbrock carb problems

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I have some weird issues with my carb it is a edelbrock 750 cfm just got rebuilt i put it on my small block chevy 350 and it idles fine and revs fine but when i get it under load it kinda bogs like it get not enough fuel or too much does anyone know what is going on anything will help thanks you guys.

Well, 750 is a little big for a 350, unless the engine is bumped up a bit HP wise, 600 would probably work better. But that shouldn't be what you are describing. Is the accelerator pump spraying as it should, what hole is it in? Has this carb ever worked ok with this combination? How about timing, is your vacuum advance working? Have you tried going up or down on initial timing?

It could be that 750 cfms is just dumping too much air into the engine for it to accept, when you get your foot into it. What all has been done to the motor? What intake are youi running? Might check your float level, could be too low.

Don

If you don't have the book that came with the carb, the Edelbrock site has the tune-up information for their carbs that is a very systematic approach to getting the calibration correct on the carb, might want to take a look at it.....

Hiya, I know from bitter experience the 750 is way too big for a small block 350. 600 is as high as it goes unless you got a blower. Put the 750 on a big block. It's sucking way too much air.

In my opinion, the 750 Edelbrock is a bit much for a stock 350, however; it can be made to perform very well. They do tend to run rich out of the box and more often than not, I’ve re-jetted and installed new metering rods to prevent them from single handedly supporting OPEC. Definitely download the installation/tuning guide from Edelbrock: http://www.edelbrock.com/automotive_...411_manual.pdf



I have the following questions:
1. Have you run this combination successfully before?
2. What modifications have been done to the engine?
3. You said it was just “rebuilt” – did this entail new jets and metering rods? (See my recommendations below).
4. What is you manifold vacuum?
5. Are you running a stock distributor? If so, is it HEI?
6. Which port do you have the vacuum advance connected to (assuming it’s a stock distributor of one requiring vacuum advance).

Like all carburetors, the Edelbrock performs best when the air to fuel ratio is 14.6:1.

All Edelbrock carburetors have three systems that meter fuel to the engine: Idle, Primary, and Secondary.

Idle delivers 100% of the idle fuel. It also meters fuel at off-idle throttle positions.

Primary (main) - delivers an increasing percentage of fuel as throttle position passes through idle and varies fuel delivery in response to air flow and manifold vacuum.

Secondary: The Secondary only delivers fuel when the secondary throttle plates and air valve are open. It ensures that fuel delivery varies with air flow. The secondary plates should start to open when the Primaries are about 65% open. The Primary and Secondary plates must arrive at the wide open throttle (WOT) stop at the same time.

Fuel pressure is critical for Edelbrock carbs as well. At idle, you should not have more than 6.0 psi. I set adjustable fuel-pressure regulators at 5.5 psi. With a stock fuel pump, the minimum fuel pressure is at WOT. Fuel pressure should not drop below 2.0 psi. If it does, the fuel pump is bad. It’s not uncommon for a stock GM mechanical pump and almost all aftermarket electric fuel pumps to put out more than 6.0 psi at idle – this will cause a problem and you will need to install a regulator.

Your carb is probably a 1407(manual choke) or a 1411(electric choke). If your carb is in its stock form (factory jets and metering rods), I’d recommend the following:

1407 – Install part number 1431, .107” main jet and part number 1445 .065” x .047” metering rods.

If you have a 1411, Install part number 1430, .104” main jet and part number 1450, .070” x .042” metering rods.

I always replace the airhorn gasket (comes in a kit, part number 1472, includes cab to manifold gasket).

Depending on your mechanical ability, these changes can be accomplished with the carb on the engine but bear in mind that there are tiny parts and if you drop any of them down the throat of the carb – let’s just say you’ll be in a bit of a pickle!

Good luck and let us know how you come out!
Glenn

Okay i put on a different edelbrock 750 carb on it and it ran way better so i was wondering what could be wrong with the rebuilt carb?

Any number of things could be responsible for the differences in the two carbs - i.e., the jets and metering rods could be different, float levels, step up springs may be different between the two carbs, pump drive springs, or perhaps you corrected a vacuum leak in the process of swamping them.

If your happy with the performance with the current setup - drive it.

Regards,
Glenn

Glen - sounds like you know your stuff. Please advise me on following:
I have a new GM crate HO with Edelbrock Hi rise RPM endura - was running a 1406 out of the box with no problem - changed carb "only" to out of the box Edl 18064 Thunder for "endurashine Purposes" - It fell on its face when hitting WOT before 2500rpm (after adjusting per Edlelbrock for fuel pump stroke and AVS and various settings) -sent carb back and dealer replaced - in meantime put on an old Holley I had - then adjusted and it screamed No Problem. Couple of weeks later and I installed the replacement out of the box factory 18064 and I have the same problem again. Love the LOOK but want it to run right too. Any Suggestions?
Thank You for your input on this forum.

Welcome to CHR and thanks for the kind words!

I really like the Edelbrock carburetors and have found them to be very dependable for daily drivers once they’re properly dialed in. I like Holleys too – but they sometimes require a bit more fiddlin’ that the Edelbrocks. That said; let’s look at your “opportunity”.

Before you do anything, verify, with a fuel pressure gauge that your pressure is between 4 and 5 psi. The floats also need to be at 7/16”, but typically they are spot on out of the box so I wouldn’t suspect them – especially in that this is carb #2 with the same issues.

The 1806 with the Endurashine is a different carburetor that the 1406 –not just in outward appearances. The 1406 is a 600CFM unit that comes stock with .098 primary jetting, a .075 x .047 metering rod and a yellow setup spring. The 1806 has .095 jetting on the primary, a .068 x .047 metering rod and an orange setup spring. The orange spring is stronger (5” vs 4” Hg) than the yellow spring and overcomes the vacuum sooner to push the metering rod to the open position.

I’d check the spark plugs and see how they look. They should be a light brown – kinda like coffee with some cream – in color. If they’re black or “dark chocolate” in color, you’re running rich. White or very light is too lean.

When you say, “….It fell on its face when hitting WOT before 2500rpm”, I get the impression that it’s bogging down and then catches up after stumbling – indicative of a rich condition. If that’s the case, it may be as simple as changing the setup springs from orange to yellow. It may also require you to go a step (or two) leaner.

The manual that comes with the carburetor is very good – but somewhat overwhelming the first few times one reads it!! The one that I have in front of me shows the 1806 on page 17. If the spring swap has no effect, I’d try going to the configuration shown as reference #16, i.e., .092 main jets and .065 x .042 on the metering rods.

You can buy the individual parts (jets and metering rods come in sets of two) from your local go-fast parts store, Summit or directly from Edelbrock. Part number 1425 for the jets ($4.39 from Summit) and part number 1416 for the metering rods ($9.25 from Summit)

Optionally, you can but a “kit” – part number 1840 ($49.95 from Summit) – that has an assortment of jets, metering rods and a complete set of setup springs. Remember that the top gasket (part number 1472 - $9.95 from Summit) is good for one remove and replace and after that, well it is suspect. If the gasket sticks at all, replace it. I buy the gaskets in a five pack (part number 1499 - $12.95 from Summit) as I got tired of needing one at 5:10 on a Saturday – ten minutes after the local shop closed!

Good luck with this one and once again, welcome to CHR

Regards,
Glenn

I agree with most of the responses. I had a 750cfm on my 350, and swaped it to a 600 cfm Edelbrock, (1406 model) and it runs like a watch. Too much carb, for a small block - unless your pumping 500 hp or better.

Use this formula for a guideline. I will amaze you how small a carb really needs to be:

CFM= CIDxRPM divided by 3456xVE

CFM = Cubic feet per minute
CID = Your engine's cubic inch displacement
RPM = MAX wide open throttle RPM, i.e racing. As a daily driver - the engine needs are much less
3456 = a constant
VE = Volumetric efficiancy. A stock engine is about 75%, mildly modded, 85+/-

An example(my engine):
CFM = 302x6000/3456x.85
CFM = 616 cfm.

Since I will seldom see, if ever 6000 rpm, I have a 600cfm Edelbrock 1405, modded to electric choke - and it does run very nicely

The Performer carbs absolutely do NOT like dirt, so a very good filter should be used

Glenn - very nice write up. I don't have a late IB, so am surprised the Endurashine is jetted so lean.

That formula is accurate, Dave.... but considering my background and upbringing as a dirt burner in South Dakota we just shortened it down to "Leaner is Meaner"..........

Lean is mean, Dave, that is until you start burning plug electrodes - but you knew that

Now I wish I could find that exhaust piping formula that I used to size for my car. Had I been left to my own guesstimate, would probably have used 2.5 instead of 2.25

Originally Posted by IC2

Lean is mean, Dave, that is until you start burning plug electrodes - but you knew that

Now I wish I could find that exhaust piping formula that I used to size for my car. Had I been left to my own guesstimate, would probably have used 2.5 instead of 2.25

I've got a link to buy the software for designing step headers, collectors, and the entire exhaust system.. Got to order it someday, it's only like $45.00. Shows the optimum length of each size in a step header, overall tube length, ideal collector size and length, etc. Definitely going to have it before I build another set of headers!!!!!

Here are some good reads:

Amazon.com: Scientific Design of Exhaust and Intake Systems (Engineering and Performance) (9780837603094): Philip H Smith, John C Morrison: Books