440 backfiring

[custom880]

O.K. This one is driving me crazy. I have a 440 in my 66 Polara. The 440 is a 75 motor with points ignition. I just put a stock cam and chain on new. The engine had a purple shaft 484 but it was just to much for a big C body convertible. I needed to quiet the car down and make it more of a cruiser. The 440 started right up after the new cam was put in. I broke it in just like the manual said. The car ran fine for a day and a half. Now it is backfiring throught the carb like crazy. I figured it was a timing problem so I went back and redid everything. Idle was fine, rpms in park seemed o.k. When you start out it seems o.k. untill you hit around 2 grand then it spits throught the carb intermitently. Brought it back, re-timed everything. It did the same thing. I brought it back and changed the dwell a little and advanced the timing just a bit. same thing. Took it back to specs now it idles rough. I pull all the spark plugs and they are black and fouled with carbon. Dry, black, fluffy. The exhaust is grey and lots of it. So I figure it is running rich. I lean the carb. same thing. New cap, new points, new rotor, wires are already new, new condenser, same thing. Newer Accel coil. Double check the cam to make sure its at TDC. Checks out with Number one at TDC. Check for vacume leaks. can't find any. driving me nuts. The car ran fine with the 484 cam and ran fine after the break in. I am thinking that the 700 carb is to much for the engine now, but it should be able to handle it. No power. If you try to jump on it it spits and dies. Ideas? Carb? Timing? Distributor? What the heck am I missing?

[ehostler]

disconnect and cap off the vacuum advance. See if that makes any difference.

USe a dial back to zero timing light and determine total advance and when it comes in by.

700 CFM is not too much for a 440. I've run 750 vac secondary on a 383 that still ahd the 2bbl cam in it and it was strong.

I rule out the carb, as it was working fine, before you swapped the cam.

take a bottle of soapy water (409 works) and with the engine runnig, spray it around the intake gasket surfaces. If the idle RPMs change, then you know that you have a vacuum leak.

[pishta]

cam plate moving, causing the points to open when they shouldnt. watch the plate move when you suck on the vacuum advance tube, it should be smooth and not jerky, if it needs lube, use graphite. Worth a look. check your gap while you are down there. also run the thing in a dark garage and look for sparks, maybe arcing.

[dave571]

You've checked all the basic stuff, it has to be a hard fault.

Pull the intake, rockers, pushrods.

Slip out the lifters one at a time, and look at the face of them.

I'd be very surprised if you won't find one of the lobes chewed. Probably one of the exhaust lobes.

Doing the break in according to the manual, doesn't mean the cam can't fail. It only means less chance of it failing.

Cam failure is an epidemic these days with motor oil formulations that are missing the zinc etc that the old oils had.

I had never seen a cam break in failure in my career, until the last 2 years, where I've seen 4 in my shop alone. Other shops telling me the same story.

[custom880]

Thanks guys for the help. I am really trying (hoping) that I don't have to take this thing apart again. The gaskets get a little pricey! I will look for vacume leaks. The black fouled plugs make me think it is carb. I have disconected the vac advance and everything worked like it should. Also with the advanced connected the tming mark moves like it should when the rpms increase. The timing mark isn't jumping around on the balancer, so I know the chain is good. The carb was just rebuilt last month (by proffesionals) and like I said with the bigger cam it worked good.

[scdanny]

I had a simular problem a couple of yrs. ago. 440 GTX Car began to back fire after it had been running about 20 min. Turned out to be the coil. The coil was fine until it warmed up in about 20 min. Freaking drove me nuts til I figured it out. Good luck & let us know.

[ehostler]

If it's not doing this when the vacuum advance is disconnected (as you just stated), then the problem is that you are getting too much advance (from the vacuum pot) at the point when you begin to backfire through the carb.

That is mostly a cruising RPM or an RPM range that you are decelerating through. That is because the vacuum advance does not operate while you are accelerating.

If it is an adjustable vacuum advance unit, you need to back it off. If it is not an adjustable unit, just leave it disconnected and capped.

[custom880]

Looks like I have found the problem. The Eddy carb (700) seems to be dumping gallons of gas in the engine. I thought about it slept on it and the thing that kept coming back was the black carboned up plugs. Today I grabbed one of my old rebuilt 600 carbs (not the greatest by any means) and put it on. I re-gaped the points and fired the car up. The carb seemed to be better, so I took it out. The car burped once cleared it's throat and settled down. It ran good but very underpowered. I thought that maybe something had blown out after the burp so I brought the car back in and switched the carbs back. The 700 spit, backfired, died, just as soon as you put a load on it. I put the smaller carb back on and it ran fine. So for now I will use the smaller carb untill I figure out what is wrong (if anything) with the bigger one. Thanks for all the suggestions and help.
Bob