Backfire through carb, Ran during the summer...backfiring now?? Options

[Project66]

I kind of threw this together the other night but here is my "new" car. It is a 70 cuda with a newly built 512.

http://www.cardomain.com/ride/3952107/1970...mouth-barracuda

Parts...
512 cubic inch motor(440 source)
Edelbrock cylinder heads and intake manifold (60929 & 7193)
MSD Pro-billet ready-to-run distributor (8387)
MSD Blaster 2 coil
Roller cam (I forgot the specs)
Holley 750 double pumper carb (w/ electric choke)
TTI headers and exhaust
Jegs power brake booster and master cylinder

Okay so I had this car pretty well put together during the summer. I got it to start and idle and sound freakin AWESOME towards the end of the summer before I left to go to school.

Fast forward to now, I get back and try to start the car and it backfires through the carb. WTF?!?! (like a "crank...crank...POW!" then gas or fire spews up through carb) I cannot figure it out at all. Timing shouldn't be an issue since I had it running pretty well in the summer. So it puts me back to air/fuel mixture.

Cold weather=run it rich
Warm weather=lean it out

Am I barking up the wrong tree? I was really looking forward to driving it after this really stressful semester. Any help would be GREATLY appreciated.

Let me know if you need any more info. I might be able to post a video tomorrow or something.

[Furios66]

I had a similar deal with my car a while back. I had a vacuum plug that cracked so bad it acted like no plug. Car would back fire when trying to start it. It would eventually start but not with out a fight. Worth a look

[moparmania51]

I have a 383 / 496 stroker kit from 440 source. Mopar 452 aluminum heads, Comp Cam, Malory Unilite, Barry Grant Six Shooter, Sanderson......I had issues like this and it all had to do with s@*T in my gas. I can't remember how many times I had these carbs apart to get the junk out of them. Worst part is, everything is new
/ gas tank / braided lines, 2 filters before carbs. JUST A THOUGHT

[67Mopar]

I would play with the timing a little first and see what that does for you; it may be firing a little to early. It could be the old gas doesn't want to fire the way it should. Get it running and burn the old crap out of there... You can always go back and tune it in again when you have more time.

[440roadrunner]

I would CHECK timing since someone could have messed with it. It only takes a minute or two.

To do this, pull the no1 plug and stick your finger in the hole. Bump the engine over until you START to feel compression on your finger. You may have to "go round" a couple of times. WHEN you start to feel compression, STOP, and watch the timing marks as you bump/ or wrench the engine further. DO NOT put the engine on TDC, but rather WHERE you want the initial timing to be. Where did you set the timing? 15 BTC? 12? 20? Stop the marks wherever that point is.

Now go up to the distributor. Carefully mark the no1 tower position from the cap on the top rim of the dist. so you can see "where you are" with the cap off. If the timing has not changed, the rotor should be approaching or at the mark you make, and the reluctor wheel should be in the middle of the core of the pickup coil, or nearly so. Remember that a BB/ RB distributor rotates CCW, so this means that to ADVANCE it you turn the dist body CW.

CHECK the plug wires that they and the firing order is correct. Someone may have played with you. Make sure that someone did not swap a plug wire for the coil wire!!!!

Then run a compression check. You may have something went wrong, a valve hung open.

If neither of these gets you anywhere, pull one valve cover at a time and inspect the valve train cranking AND running. See if any valves appear to be not opening much --I hate to say--a wiped cam lobe

What did you do to break in the cam, what did you do about cam lube?

[74CudaDave]

Is it gas or fire `spewing up through the carb'?If it's gas, it could be flooding. After trying to start it, shut the key off and take a look down the carb throat, do you see alot of fuel? Your carb might be stuck or some junk may have gotten into it jamming the floats open.

[greg b]

Sounds like the gas. I filled up with new gas without treating it and let it set for 2&1/2 months. I had the same problem you discribed. I drained the gas, filled up with clean gas and it ran fine. Since then I always use gas treatment if it's going to set for an extended time.

[Richard T]

Hi, Ive found the holley carbys to be troublesome if nothing has changed but time then maybe the power valve diaphram has perished or failed as that will always cause the same problem as you describe.
Rick

[racerhog]

Fuel went bad......

[joeart]

check the distributer cap for corrosion or moisture and pull the spark plugs and check to see if they are wet or black.check coil wire at coil and not shorting to post on coil.then make sure needle and seat is not stuck in carb.carb will be flooding inside carb throat.do not look down carb with it is backfiring.disconnect coil wire while checking with your face far away from carb.use a mirror to look down carb.power valves in carbs do not like backfiring and may need to be replaced if runs rich at idle