Edelbrock Thunder 850 tuning help

[440beep]

I'm trying to help my buddy tune his 70 Road Runner with a 440, iron heads, 4spd car. He's got an Edelbrock Thunder Series 850 carb on the car. The car drives nice but when he gets on it in 2nd and 3d gear, it starts misfiring. The carb is still setup as it came OOB. He doesn't want to dump money into tuning parts without "scientific support." I keep telling him it's a fueling issue now and we need to try changing the metering rods, and going to a rod with a smaller power section. My understanding is that the stock metering rods are 0.068 x 0.047. I want to try the 0.068 x 0.042 rods (or smaller) and see if that is the fix.

Am I going in the right direction, or is there something else we can try? He has the extra step up spring kit, so we can change those.

 

[Ranger16]

Are you absolutely sure this is fuel related and not ignition? Doesn't sound like fuel to me. What is your reasoning for diagnosing it as fuel?

 

[440beep]

His initial timing was set at 15, and all in at 35 at roughly 2500 rpms and this setup had misfires. Dizzy springs were both pink and mechanical advance curve had not been changed. I recurved one of my extra dizzys, put a two stage curve in and limited the mechanical advance to 16. Set initial timing to 19/20 and all in at 35 at 3000 rpms. Took it for a drive and had the misfires still. Backed the initial down to 15 with total reduced to 31 at 3000 rpm, but misfires was minimal. So maybe you're right and we need to keep dicking around with the timing curve.

Are you absolutely sure this is fuel related and not ignition? Doesn't sound like fuel to me. What is your reasoning for diagnosing it as fuel?

 

[AR67GTX]

I had some issues tuning the accelerator pump on my Edelbrock 800 but the problem was a severe bog (not a misfire) on giving it any gas. I also found upon a rebuild that the rods and jets had been messed around with and I set it back up according to the Edelbrock manual - but it never caused a misfire. So it sounds to me more like an ignition problem too. Does your distributor hold steady dwell when reving - worn bushings will cause a higher rpm misfire. Otherwise, possibly a bad cap/rotor, coil, etc.

In the event it is missing due to running lean (out of fuel) you might check any fuel filters. Seems strange it wouldn't be missing in 1st.

 

[YY1]

I've got a 440/833 with a mild cam.

I've had 2 Edlebrocks on it, both bolted on as they came out of the box.

1st was a 600, and it ran fine but lean surged at cruise.
No bog, no misfire.

2nd is a 750, no bog, no misfire but is rich at idle and off idle. Drives great, though.

Not sure if this helps, but I think I have both extreme OOTB scenarios covered and neither involves a misfire.

I do have a 650 AVS I'm planning to try, but I probably just need to tune the rich out of the 750, and save the 650 for my 400.

 

[lewtot184]

my experience with edlbrock 800's are that they are very rich out of the box. i suspect you have other problems such as ignition or an inadequate fuel pump.

 

[lewtot184]

I've got a 440/833 with a mild cam.

I've had 2 Edlebrocks on it, both bolted on as they came out of the box.

1st was a 600, and it ran fine but lean surged at cruise.
No bog, no misfire.

2nd is a 750, no bog, no misfire but is rich at idle and off idle. Drives great, though.

Not sure if this helps, but I think I have both extreme OOTB scenarios covered and neither involves a misfire.

I do have a 650 AVS I'm planning to try, but I probably just need to tune the rich out of the 750, and save the 650 for my 400.

i think the 750's are too fat. try a .104 primary jet with a 65x52 metering rod, and .098 jets in the secondaries.

 

[440beep]

Cap and rotor is new, as are plugs. As for holding "steady dwell," if you mean does it hold steady timing while holding the engine at 3k rpms, yes the timing holds steady and is not jumping around.

Fuel filter is a thought to check. Another funny thing that happens is when the car goes around a curve it misfires/stumbles. Funniest thing. I'm guessing that's a float level issue.

My buddy also stated his carb was NOOB, but when I forced him to check things we discovered he had swapped in the heavy step up springs. Swapped back to the orange spring (5 hg) and that helped a lot.

I had some issues tuning the accelerator pump on my Edelbrock 800 but the problem was a severe bog (not a misfire) on giving it any gas. I also found upon a rebuild that the rods and jets had been messed around with and I set it back up according to the Edelbrock manual - but it never caused a misfire. So it sounds to me more like an ignition problem too. Does your distributor hold steady dwell when reving - worn bushings will cause a higher rpm misfire. Otherwise, possibly a bad cap/rotor, coil, etc.

In the event it is missing due to running lean (out of fuel) you might check any fuel filters. Seems strange it wouldn't be missing in 1st.

 

[moparwacko]

I too am having Edelbrock issues with both AVS carbs i have. One 650 approx. 1 yr old on previous engine (stock 69 383 RR) - ran perfect. Installed a .30 383 with an unknown Purpleshaft cam in. Now has a mid range cruise surge I cannot get rid of. 1200-1600 rpm. Then purchased a brand new 800 AVS, runs so rich, i cannot stand to be next to it -lol. , absolutely same results. I've tuned many carbs in my day, but this has me baffled. I've tried umpteen jets, rods, and springs, etc., and have had the timing anywhere from +18 to +5 (barely runs) and seems happiest at + 14 initial.

• no vacuum leaks
• brand new mechanical Edelbrock fuel pump, filter(s) one at tank one at pump
• new plugs, wires, cap, rotor - all premium stuff Accel etc.
• about the only thing i have left to do is ask for help

Sorry, not meaning to hijack your post, but feeling your pain!!!

 

[440beep]

The midrange cruise surge sounds like a vacuum advance issue. Have you messed with that?

I too am having Edelbrock issues with both AVS carbs i have. One 650 approx. 1 yr old on previous engine (stock 69 383 RR) - ran perfect. Installed a .30 383 with an unknown Purpleshaft cam in. Now has a mid range cruise surge I cannot get rid of. 1200-1600 rpm. Then purchased a brand new 800 AVS, runs so rich, i cannot stand to be next to it -lol. , absolutely same results. I've tuned many carbs in my day, but this has me baffled. I've tried umpteen jets, rods, and springs, etc., and have had the timing anywhere from +18 to +5 (barely runs) and seems happiest at + 14 initial.

• no vacuum leaks
• brand new mechanical Edelbrock fuel pump, filter(s) one at tank one at pump
• new plugs, wires, cap, rotor - all premium stuff Accel etc.
• about the only thing i have left to do is ask for help

Sorry, not meaning to hijack your post, but feeling your pain!!!

 

[moparwacko]

No, I haven't. It's a MP conversion electronic kit installed in mid- late 90's?? Had trouble with orange box-since replaced with Jegs 40805 USA made. Distributor has maybe <4K miles use, but no I haven't changed weight Springs. I'm running full manifold vac off Edelbrock n
advance holds good from test.
Had an electric fuel pump prior so pushrod is also new.
Thanks in advance haha!!

 

[Ranger16]

Lewtot may have a point. I thought the 750 on my 383 was too rich. I rejetted it 2 stages leaner in cruise mode and one in the power mode. Mileage went up and the exhaust doesn't burn my eyes anymore.

That said, it never caused a misfire.

 

[440beep]

The vacuum advance is that silver pod outside the distributor and you can't change weights/springs in it. I forgot what size allen wrench the screw is, but remove the vacuum hose connected to it, and insert the allen wrench and locate the screw. Go lightly as there's a diaphragm inside you can rupture. It's pretty easy to find though. Turn the screw clockwise one full turn, reconnect vacuum hose, then go for a drive and see if car is still surging while holding steady throttle and just cruising, if so keep adjusting until surging stops.

For detailed instructions, follow this article to try and tune the vacuum advance better.

http://www.dodgecharger.com/forum/index.php?topic=14888.0

No, I haven't. It's a MP conversion electronic kit installed in mid- late 90's?? Had trouble with orange box-since replaced with Jegs 40805 USA made. Distributor has maybe <4K miles use, but no I haven't changed weight Springs. I'm running full manifold vac off Edelbrock n
advance holds good from test.
Had an electric fuel pump prior so pushrod is also new.
Thanks in advance haha!!

 

[440beep]

Well since we've messed around with different timing curves (with still one more to try) and still get misfires, doesn't it seem the secondaries should be fattened? Or is the Eddy 850 more than enough for a 440 OOB? It was stated ignition was the problem, so I'm assuming the reference is to the various fueling components and not specifically the ignition timing?

Lewtot may have a point. I thought the 750 on my 383 was too rich. I rejetted it 2 stages leaner in cruise mode and one in the power mode. Mileage went up and the exhaust doesn't burn my eyes anymore.

That said, it never caused a misfire.

 

[lewtot184]

the older 800's had .107's in back, plenty fat enough. the newer ones are supposed to have .101's, which i think would work ok. fuel pressure checks aren't that difficult to do. also, if the reluctor in the distributor is contacting the magnet silly things happen.

 

[moparwacko]

Yep, done that too.............it's a 3/32 allen head. I'll bow out of this thread and save it for an ignition topic. but as far as the Edelbrock goes, "new" 800 AVS are .113 and .107"s. with a 68/47 rod and orange spring. My personal best on my new 383 is the 650 AVS with .101 & 98's with a 65/42 and tossup between orange & blue springs. EVERY engine is different................aren't Edelbrocks fun. I'll figure it out sooner or later. Thanks for the info.

 

[lewtot184]

gotten to where i like those AFB's