backfiring

dvcobra · #1 (**permalink**) 01-30-2005, 08:37 PM

I have a 351w and it has been backfiring when I accelerate and then let off on the gas quickly. If I decelarte or throw it into neutral it doesn't backfire. My total advance @3000 rpm is at 34 degrees. I checked with a vacumn gauge and it is indicating I might have late ignition timing or possible vacumn leak. Vacumn gauge has a low steady reading.
I am trying to learn how to make these adjustments (timing and carb)so I can do it at the track myself, any help would be appreciated.

I have 2 questions;
1)This may be basic but does late ignition timing mean I need to increase to 35 or 36 degrees or drop down to 33?
2)Any recommendations on how to track down a possible exhaust leak?

Mike Simard · #2 (**permalink**) 01-30-2005, 08:58 PM

Backfiring at throttle lift indicates an exhaust leak. probably the header gasket. Header bolts do like to loosen up and fall out, it's possible you could cure your problem by just tightening them.
BTW, your timing, 34deg@3000 sounds good. It would be nice to have a 20 deg at idle with 14 deg movement if you could get that little from your distributor.

Steve R · #3 (**permalink**) 01-31-2005, 09:42 AM

A "backfire" means it is popping out the carburetor. Is this the trouble you are having, or is it popping out the exhaust?
Steve

sdcobra · #4 (**permalink**) 01-31-2005, 09:51 AM

agree with earlier comments that this is likely an exhaust issue, probably loose header or burned thru exhaust gasket

had similar symptoms/cause on S/O motor...exhaust gasket was burned thru

trularin · #5 (**permalink**) 01-31-2005, 10:21 AM

I agree with Mike. If you are sealing the header gasket, try the ULTRA COPPER RTV. It works very well and fills voids.

Had the same problem and used the RTV in my fix. I recommend it.

dvcobra · #6 (**permalink**) 01-31-2005, 02:07 PM

Steve, to answer your question it is poping out the exhaust. Everyone thank you for your help. Header bolts all seem to be tight even though I have had that problem in the past. Installed Earls Header pressure seals, which has helped. I will look where side pipes attach to headers.
Thanks everyone

Steve R · #7 (**permalink**) 02-01-2005, 07:08 AM

dvcobra,
I just wanted to make sure we are all on the same page. Did the new header gaskets fix the problem?
Steve

Michael C Henry · #8 (**permalink**) 02-01-2005, 07:13 PM

I fought what I thought was a carb problem from late summer.Fouling plugs, backfires,missing . It would initially run fine then once i had some confifnce I'd try to get the car to the freeway.It would start to miss ,so I'd turn the back towards home and the missing and popping would get worse.I expected the engine to die at the traffic lights, but it would idle just great .I finally bypassed the Cd ignition box .^That cured it so I replaced a faulty ignition box.

Jahred · #9 (**permalink**) 02-01-2005, 11:08 PM

I'm still of the opinion that popping out of the exhaust is normal, particularly with straight thru pipes. Frankly, it adds a little excitement after dark........I figure if its not popping, you must not be pushing enough gas through that carb......time to add another four barrel........:-)

dvcobra · #10 (**permalink**) 02-02-2005, 09:47 PM

Thanks everyone I will be working on it this weekend and let you know. Does anyone have tricks to locating an exhaust leak if it is hidden or hard to find?

trularin · #11 (**permalink**) 02-03-2005, 05:59 AM

Black soot on the block is a hint.

dvcobra · #12 (**permalink**) 02-05-2005, 01:08 PM

Do you apply the Ultra Copper RTV to both sides of the gasket or do you eliminate the gasket and use just the RTV? (learning the hard way) Those of you that quessed "exhaust leak", you are the winners. I was checking all the header bolts that were visible/accessible and missed the two back by the the fire wall. One of them was gone and the other loose. tightened everything up and took it for a drive. worked good at first, then all of a sudden started backfiring again. I think the gasket may have been damaged so I will replace it and keep you all posted.

trularin · #13 (**permalink**) 02-05-2005, 01:20 PM

Ultra Copper on both side of a new gasket.

Might as well do both sides.

Had the exact same thing happen to me.

CowtownCobra · #14 (**permalink**) 02-05-2005, 02:31 PM

Just started test driving a new motor. It started popping through the exhaust. Went back and retightened the header bolts. Problem fixed. Timely thread.

Tommy · #15 (**permalink**) 02-05-2005, 06:19 PM

One more thought from one who also experienced this problem. If you put too much sealant between the header and the head, the relatively low torque on the header mount bolts will cause the header to stand out a little farther than it would with just a gasket. On my car, combination of the too thick gasket, the weight of the headers and side pipes, and the motion of the car caused the header to begin to rock back and forth until it began to leak. That was in spite of the fact that the header mount bolts were safety wired. I now use just an exhaust gasket on my 351W.

trularin · #16 (**permalink**) 02-05-2005, 07:26 PM

Dave, Glad to hear you are back on the road.

CowtownCobra · #17 (**permalink**) 02-06-2005, 06:38 AM

trularin, hope your weather improves soon and you can get back on the road.

I'm trying to figure out why a loose header causes a backfire. It sounds like raw gas is exploding in the exhaust. But why would a loose header cause that??

kountzecobra · #18 (**permalink**) 02-06-2005, 07:23 AM

Dave, loose header lets the exhaust suck in oxygen.

trularin · #19 (**permalink**) 02-06-2005, 07:38 AM

Dave's right, is allows burnable air to enter the flame-out portion of the exhaust and reignite.

Thanks, I am waiting for just about any good weather. In Michigan, that's a wait.