750 Edelbrock issue

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Dartswinger70

Dartswinger70

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I am in the process of tuning my 340 with 750 edelbrock. It will idle at 800 rpm and then the idle gets low, if I rev the throttle it returns for a while then does it again, this is sitting in neutral in the garage. I am suspecting a problem with fuel pressure, however I am wondering if anyone else has had or seen this problem with Eddy carbs. The only vacuum ports I am using are PCV and vacuum advance, car has manual brakes. There isn't any vacuum leaks, car is pulling 16 in hg on a vacuum gauge. I am thinking my next step is check fuel pressure, fuel filter, and then get into fine tuning with jet kit at some point. The exhaust is "smelly" no matter how I set the carb rich or lean.
 
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Yes.

Check return spring pressure. I noticed that sometimes you can push the throttle closed just a tiny little smidge and the idle drops.
Readjust the idle trim screws. They may have a play in this.

Are you using a O2 sensor?
What is your distributors timing and advance curve?
 
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i'm going to save you weeks of frustration.
Sell the Edelbrock 750...bolt on an Edelbrock 650 Thunder and have fun!
 
ir3333 said:
i'm going to save you weeks of frustration.
Sell the Edelbrock 750...bolt on an Edelbrock 650 Thunder and have fun!
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Funny you say, I have a 600 Holley that I ran on my 318, I could put that on just to see if I can get the exhaust to not smell rich. The thought of going to a 650 crossed my mind..it is going to be a street car application.
 
With Edelbrock’s, the slightly smaller carb size I like for its response and the way it acts.
 
the Edelbrock 750 has been giving some hot rodders fits for many years. I had tuning problems with the carb years ago and at that time there were many posts about the various problems with idle, hesitation, rich and inconsistent A/F mixers at different rpm.
I talked at length with an Edelbrock tech and was told by design it responds poorly to engine vacuum and can be difficult to tune on some engines. Many guys obviously have good success with them but i know many who along with myself didn't!
 
Yes, I was thinking about getting the tuning kit. But right now i may throw my 600 Holley on it and see what it does as far as getting the exhaust fumes down or a lean best idle setting. With the 750 i see a dark gray cloud when i rap the throttle. I figure its rich, not burning oil
 
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you can google the carb manual on the internet and buy individual metering rods and jets and using the tables go one or two steps leaner economically, but best would be to install an oxygen sensor and A/F meter for accurate results.I have done that and once tuned you can use it on many vehicles.
...i'm betting you can just bolt a 600 or 650 on, set your idle and needle valves and enjoy
 
I am going to try my 600 Holley, and see if I get rid of the "fumes" I may consider a 650 Eddy. I read somewhere that the 750 is for 408 and lager CU Inches as recommended by Edelbrock .
 
If you have black sooty exhaust that makes your eyes water then the carb is too rich at that stage (and/or you may need to advance the timing). Changing might help but that's really guessing. Your going to likely tune whatever carb you put on.

Any more details on your 340's build like cam, comprssion, intake, porting, etc? Current timing initial and total and when, Gears, Converter, etc? The more we know the more we can help.

Guys run 600 to 950cfm carbs on stock stroke 340's, but it all in the build and use.
 
Fuel pressure too high, no more than 5.5 lbs. Common.
Float level set too high. Set it 1/16 lower than they say. Common.

After those two things are perfect start looking at step up springs.
 
Sounds to me like the metering rods are bouncing. 16hg is still lower than what those carburetors come "sprung" for. I think they are more tuned to what a stock engine pulls.....like around 21hg. I'd go to a stronger spring.
 
RustyRatRod said:
Sounds to me like the metering rods are bouncing. 16hg is still lower than what those carburetors come "sprung" for. I think they are more tuned to what a stock engine pulls.....like around 21hg. I'd go to a stronger spring.
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The engine is 69 340 block, .030 over with KB 243 pistons and a Summit Camshaft, Hydraulic Flat Tappet, Advertised Duration 276/288, Lift .441/.441, X heads and MP electronic ignition, not all that radical of a build IMO . But yes I think its running rich no matter where I set the mixture screws.
 
Dartswinger70 said:
The engine is 69 340 block, .030 over with KB 243 pistons and a Summit Camshaft, Hydraulic Flat Tappet, Advertised Duration 276/288, Lift .441/.441, X heads and MP electronic ignition, not all that radical of a build IMO . But yes I think its running rich no matter where I set the mixture screws.
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It doesn't have to be very radical. Where's initial timing?
 
RustyRatRod said:
It doesn't have to be very radical. Where's initial timing?
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I haven't checked it but I remember setting it at 12 BTDC. It starts up nice doesn't fight the starter etc. I remember it didn't like the 5 BTDC book spec setting.

I was setting timing about a year ago I could go back over it.
 
Dartswinger70 said:
I haven't checked it but I remember setting it at 12 BTDC. It starts up nice doesn't fight the starter etc. I remember it didn't like the 5 BTDC book spec setting.

I was setting timing about a year ago I could go back over it.
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A couple of points/explanations.
I'm going to assume this is in a '70 Dart and that was the shop manual you got the initial from.
1. The spec in the book will work OK in combination with the exact distributor that was a '70 340 that year; and that carburetor set to the exact specs.
2. The rpm of that initial timing is critical, particulalry for the late 60s - early 70s Clean Air System (CAS) cars.

12* BTDC is a good starting point. It will be worth finding out at what rpm that 12* is measured at.
For example it's possible that the 12* was measured at 850 rpm, but the distributor begins advancing at 750 rpm.

WRT the Edelbrock 750, they have some known issues which sometimes to often cause linconsistancy but mostly in the high speed system. I'll find the threads where this has been discussed.
 
Mattax said:
WRT the Edelbrock 750, they have some known issues which sometimes to often cause linconsistancy but mostly in the high speed system. I'll find the threads where this has been discussed.
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This summerizes the two big differences in parts between the way they were designed (Carter) and how Edlebrock makes them:
1. Booster is too short for the venturi.
2. Cluster may have any of several versions of well tubes.
AVS2 800 carbs
 
1970 340 Timing.
1970 Factory Shop manuals indicate a pretty high idle rpm. 900 rpm is what I'm reading.
That 5* was to help reduce certain emissions at idle and run with a fairly lean mix for the same reason. The high idle (900 rpm) sort of made up for the loss of power.

If you have more info about the distributor on the current engine, great.
Regardless, it will be helpful to measure timing from the slowest speed it will idle, and then ever 200 or 250 rpm up.
Not sure what you have in terms of timing light, tach and timing marks/tape. If its limited then just get measurements to the extent you can.
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edit. Thought this rang a bell.
Discussed this last August. Might have got missed in the smoke.
1968 340 timing

Its an unknown distributor. Most important thing related to the idle is knowing the rpm the timing begins to advance. If you measure it, and find its 12* at 850 rpm, then when the idle seems to drop off, measure it again and see if its still at 12* and what the new slower rpm is.

Let me know if you need a hand. If its not too far, maybe I can drive over one evening.
 
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I bought a new 750 edelbrock a couple years ago. I bought the strip kit and did all the tuning on a mild 383 with an air/fuel meter. No matter what i did i could not get rid of an off idle dead spot. I mean DEAD, if you held the throttle right off idle it would die every time. I tried everything, something is just wrong with the carb. I restricted the idle air bleeds, drilled the idle fuel restriction a little, adjusted the mix screws to perfection, fattened up the accelerator pump shot, all of which helped a little- but it still does it. Ive worked on carbs for 30 years, never seen anything like it.

The idle, cold starting, hot starting, highway cruising, WFO, all work great- the car got the best miliage ever.... but that dead spot will not go away.

Switched to an old 9000 series carter and it works great.
 
ir3333 said:
i'm going to save you weeks of frustration.
Sell the Edelbrock 750...bolt on an Edelbrock 650 Thunder and have fun!
Click to expand...

Listen to the man, he's right.
Something about the 750 has given people fits. I've tried a 600, 650, and 750. Never could get the 750 just right.
The 600, and the 650 run great on a SB.
 
Dartswinger70 said:
I haven't checked it but I remember setting it at 12 BTDC. It starts up nice doesn't fight the starter etc. I remember it didn't like the 5 BTDC book spec setting.

I was setting timing about a year ago I could go back over it.
Click to expand...

If all it has is 12, I bet that's probably the whole issue. Pull about 16-18 in it and see how it is then. You'll likely have to adjust the idle.
 
The only problem mentioned so far is smelly/variable idle. That will have nothing to do with booster design because the booster is not part of the idle cct.
- These carbs will take a LOT of fuel pressure before they flood, so I doubt it is excessive fuel pressure.
- Although unlikely, a pulsating PCV can cause idle havoc, especially if the cam was installed retarded. PCV should be disconnected & port plugged to test.
-Could be leaking needle & seats causing a high & varying fuel level, which would cause the problems listed.
-Seat may be loose in the body or gasket under it missing.
-Float level should be 7/16".
-Floats should be checked for leakage; make sure they are not bent, binding on body & free to move on their pivots.
-Check the idle feed tubes in the boosters for blockage & blow out the idle system with compressed air.
-If more than 0.040" of transfer slot is showing at idle, then that could be activating the main system & causing the problem. One thing other than cam duration that can cause excessive pri t/blade opening is if the secondary blades are not fully closing. This acts like an air leak, pri blades need to be opened further to access extra fuel from the t/slot. Fix is to loosen sec blade screws & reposition blades to fully close. Screws are staked; staking should be ground off to facilitate screw loosening/removal & Loctited once adjusted.
 
To add to what our friend down under Bewy said, not enough initial timing can cause all of the things he outlined. Too much transfer slot exposure, causing it to be on the main circuit at idle and pulling extra fuel in through the boosters. It's getting extra fuel from SOMEWHERE. It's VERY easy to pull some extra initial timing in and see. I'd pull in 18 and not mess around and see if it improves. Here's a clue. If you start pulling in more timing and the engine RPM increases, there you go. It's tellin you it wants more. It costs nothing and you can put it right back where it was if id doesn't work. But I bet it will.

I also agree on his point to the fuel pressure. Holley carburetors will stand close to 10 PSI before they push the needles off the seats. Those floats have a lot of leverage.
 
Thank you all for the info. I'm not sure when I'll get back at working on it,possibly Friday night. I'll post what I find. If I end up going with another carb etc.
 
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