Backfiring after installing new heads.

[Ornamental]

The 340 started backfiring after I installed new heads on it.

The old heads were most likely the stock ones from '72, the new ones are fully ported, and milled down to get a 10.25:1 compression.
The current carb is a Eddy 1406, 600 cfm el.choke (The new Holley HP 750 that I bought is among other things being shipped by sea instead of by air, thanks to a screw-up by the shipper)

The engine were running nicely earlier that day, but after parking in a slight uphill for a while, and then starting it, it began to backfire and bog when under load.

I wonder if I'm running lean?
Should I try to adjust the mixture screws as explained here:
http://neptune.spacebears.com/cars/stories/carb.html


Here is a video where both the engine running on the road, and the backfires can be heard:

[MEK-Dangerfield]

You didn't mention engine size, but think about it this way. After porting the new heads, you have more air-flow, which needs more gas to go with it. A quick check of your spark plugs will probably tell you what to do. I'm guessing they look white. Your idle mixture screws might need adjusting, but you probably need larger jets.


  Mike

[Ornamental]

Thanks Mike!

Now I'll just have to get some larger jets, and learn how to change them too.
This article makes it look possible even for me:
http://www.carcraft.com/techarticles/carb_tuning/index.html

[Moparal]

Maybe something went through the carb. You said it ran good then parked it on a hill. Or even a plug wire touching the exhaust. Or possible a valve might be to tight sticking on you. That would be bad. Maybe got some water in the gas. Maybe the cap is arching . You need to check it out. a 600 cfm would still run good, just not as strong. Something went a rye in your tune up some how. Could even be a bad coil or condensor.   Hope you get it figured out soon.The more it backfires, the more you take a chance of timing chain stretch

[torredcuda]

Did you retorque/tighten everything after you ran it a while as you may have a vacuum leak causing the lean condition?

[Ornamental]

Did you retorque/tighten everything after you ran it a while as you may have a vacuum leak causing the lean condition?

No, so this might be the reason.

[nivvy]

pull a plug first......... then go from there! 

[hemiken]

Double check that you tightened the dizzy as it may be loose and spun a little

[Aussie Challenger]

Double check that you tightened the dizzy as it may be loose and spun a little

  Check for air leaks, spray aero start around the usual areas, if no luck pull a couple of plugs. Unless it is extremely lean it would only 'ping', more likely to be a tight valve, however I would also check inside the distributor cap.

[moper]

Backfiring to me is out the pipes or popping in the mufflers. is it popping out the carb? At idle? light throttle? all the time? just full throttle?

[Ornamental]

Thanks for all suggestions!

I pulled a plug, looked good,
But the plug wires were an altogether different story. One plug cap were loose, it didn't sit on the plug. Another had been a little too close to the header, so I changed the plug wires.
Also, the intake bolts were retorqued.

After this, no more popping together with bogging.

But I do have backfiring out the pipes, this occurs after I've given the Chally a bit of throttle, and goes off the gas pedal, then I get some nice shots out the rear.

[MEK-Dangerfield]

Thanks for all suggestions!

I pulled a plug, looked good,
But the plug wires were an altogether different story. One plug cap were loose, it didn't sit on the plug. Another had been a little too close to the header, so I changed the plug wires.
Also, the intake bolts were retorqued.

After this, no more popping together with bogging.

But I do have backfiring out the pipes, this occurs after I've given the Chally a bit of throttle, and goes off the gas pedal, then I get some nice shots out the rear.

Believe it or not, this is progress.  You said you pulled a plug, but you didn't mention how it looked. It still sounds like you might be running lean, or have a leak in the exhaust system somewhere.   


  Mike

[Ornamental]

Believe it or not, this is progress.  You said you pulled a plug, but you didn't mention how it looked. It still sounds like you might be running lean, or have a leak in the exhaust system somewhere.   


  Mike

Hey, no bogging and much less of those nasty noises are most definetively considered progress by me.
And yeah, I'm pretty sure that I have a leak between the header collectors and the pipes.

The plug:



When I took that picture, the Chally had only been run about 15 miles on the new heads and ditto plugs, which are Champion RN9YC.

[Aussie Challenger]

  Plug looks reasonable, however RN9YC may be too cool a plug for your colder climate and normal driving, I would be using RN11YC in Champion and keep the 9's for more spirited driving.
  Personally I don't like Champion as they have a tendancy to break down, I used to use them in everything I built and tuned but now use NGK.   

[hemiken]

I am thinking the plug looks too wet and you could have a sticky needle and set or fuel pressure gets too high......................