Backfire help

[Hugh Mongus]

I finally completed my engine break-in. Now that I can get in it a little, it backfires anytime i tromp on it. As long as I don't give it more than about half throttle it runs great, but any more and it pops and farts through the carb. I have tried different metering rid/step up spring combos, changed the rear jets out from .98 to .101. I have tried changing the timing 8, 12, 14, & 16. The plugs are a near perfect tan color.

GM crate 350 (260hp)
Eddy 600cfm (1406)
HEI (accel coil)
New plugs/wires

Looking for some ideas. Thanks in advance.

[LSUMurse]

I have the same setup and the same problem, though mine doesn't always do it once I reach operating temp.

Subscribed.

[geezer#99]

Likely it's in your carb. The secondary air valve counterweight is too heavy causing the lean bog and missfires. Common on those eddy carbs. Always had that stumble even back in ancient times when that design was used by carter carbs afb. The cure is removing some material from the weights. How much you ask? You gotta experiment.

[Chckls88]

Hey a new guy here. I have a 72 c10 with a 800 demon carb and my truck was built pretty much just for the drag strip/any street race I can find. When I first broke in my engine and got everything rolling I ended up havering to put a curve on the distributer becouse I couldn't get the range from the factory curve. Call a shop if you want they only charge between 50 to a 100 to do a curve and adjust your carb

[hugger6933]

Where is the timing? Did you do an all in type thing at 3000? or did you set the base then add vac? Back years ago when fooling with those old hotrod motors and bracket cars we would set the timing at say 36 degrees all in. Meaning we would rev it to 3000 so all advance would be figured in the picture and set it and forget it. The small block liked a little less but not much a little playing might be in order. there. Also when going backwards to recheck it seemed pretty close to spec on when the dial on the back of the light was turn back off. Jim

[Hugh Mongus]

I have only set the initial timing. It is currently at 14 degrees. I can try setting the overall timing to see if that helps.

[Chckls88]

Lol the real fix is using the eddy as a fishing weight and putting on a demon or something better. I use to love eddy carbs but as soon as I went over 650cfm they were worthless to me granted that's my opinion and everybody has a preference no offense but eddys are known for backfiring when there is even just the slightest timing change

[Chckls88]

My advice would be go to youtube and look up some videos and see if there are some similarities and trouble shoot from there you will probly find something pretty darn quick

[gvw5400]

I have an unpleasant thought. You mentioned just finishing breaking in the engine.
Did your cam survive break in?
I have had two engines backfire through the carb when a lobe on the cam wore flat.
I hope all you have is a carb issue, but this may be worth checking.
All the best, Dirk

[kwmech]

Maybe see the above post^^^^^

this was my first thought, but you also may have a tight intake valve. I'd check those first, set the timing at 8 btc for now. See what happens

[Hugh Mongus]

I talked to Edlebrock tech today. They said the 1406 does not work well with the RPM manifold. He told me I should use the 1405 with the RPM manifold.

How would I check to see if the cam has issues? I followed the break-in rules perfectly.

[gvw5400]

Pull the valve covers, disconnect the HEI power feed, have a buddy crank the engine over
as you watch the rocker arms. Look for one or more that don't move as much as the others. This is the easiest non technical place to begin.
All the best, Dirk

[TheSeeker]

I thought the only difference between the 1405 and 1406 was the choke. could be wrong lol

[geezer#99]

Originally Posted by TheSeeker

I thought the only difference between the 1405 and 1406 was the choke. could be wrong lol

1405 has richer primary jets and richer metering rods. Not much, just a little more. Secondaries are the same though.

[Hugh Mongus]

Originally Posted by TheSeeker

I thought the only difference between the 1405 and 1406 was the choke. could be wrong lol

I asked this exact question to the Edelbrock tech guy. He said the two carbs are completely different. I am going to replace my 1406 and go from there.

[airdale94]

Does your distributor have a adjustable vacuum advance?

[Steve Cole]

Mine does the same-thing. I have a 1407 on top of a RPM Air Gap manifold. I just got t running so I haven't done any troubleshooting yet. I don't even know where the timing is set yet. Let us know if the carb switch fixes it.

[Hugh Mongus]

Originally Posted by airdale94

Does your distributor have a adjustable vacuum advance?

No

I replaced the 1406 with a 1405 and it fixed my backfiring. It runs great now.