Exhaust System Sounds Like A Drum

[c3po]

I have a 1983 Chevrolet El Camino with a 305 V-8 that has dual exhaust and a Quadrajet Carburetor. It was cold tonite, about 30 degrees out, and on the way home the carburetor backfired a few times and the exhaust kept making a drum sound. Could this be:

1) Vaccum Leak
2) Choke not adjusted right
3) Bad Cap and Rotor
4) Bad Spark Plugs
5) Bad Vacuum Advance

She starts up fine and when I got home, I popped the hood and turned the lights off and while the car was running I could not see any arking on the spark plug wires.

I am wondering if the mixture for the carburetor needs to be retuned because of this winter gas.

The only other thought is a leak at the carburetor gasket. I did have my valve seals replaced on the passenger side and my buddy cleaned the spark plugs and put them back in.

I plan on replacing the cap and rotor as well as the spark plugs later this week, something has me thinking this is a choke adjustment problem.

Is there anyway I can start the car and watch the choke to make sure it is working right.

Any help would be appreciated.

 

[-Clayton-]

Did it only backfire through the carb when you got on the throttle?

 

[c3po]

Yes it did Clayton, but when it was parked it still sounded like a drum, meaning the exhaust sounded like a drum.

When it backfired I was cruising along and the car was not totally warmed up, meaning the temp gauge had not gotten to 180 degrees.

 

[-Clayton-]

Have you re-adjusted your choke for the colder weather? Sounds like a lean mixture, Check for vacuum leaks, Check carb adjustment,
And ignition timing, Make sure your vacuum advance is working properly as well as the centrifugal advance in the dizzy.

 

[c3po]

Clayton, I have not, and I am not positive on how to do this, usually I just play with it every year and then it is fine.

The problem is that I play with it 2 times a year so I am not sure what I am doing.

My expertese on cars is changing simple parts and fluids.

 

[tom slick]

put your hand over each of the exhausts while it is running. it should steadily push your hand away with the pulse of the engine. if it doesn't push your hand away when you hear the "drum sound" then you've got a miss. If it is a true dual exhaust then the miss is happening in that corresponding bank of cyls. the miss could be anything in the ignition system or anything in the valve train.

 

[c3po]

Tom, it is a true dual exhaust, with shorty headers and a x-pipe. I should also add that I may have a leak at the intake manifold.

One of my intake maniflod bolts had some oil that came through the intake manifold bolt, it was only 1 bolt, but if oil came through it, then I may need to retighten it, since this could be a source of my backfiring problem.

 

[-Clayton-]

Here's how I check too see if the chokes operating correctly when the engine is cold before you start it for the first time remove the air cleaner assembly and manually open the throttle at the carb the choke should then shut before you open the throttle 1/4 of the way. Then start the engine go watch the choke flap it should open slowly as the engine warms up.

Does it have an electric choke or the hot air kind?

 

[c3po]

Clayton, it has an Electric Choke.
I should point out that I once had an intake manifold with EGR, since I have historic plates on now I have an intake manifold with no provision for EGR, so no EGR valve.

I am thinking the choke since tonite is the 1st time it has gotten below freezing, lately it has been in the 50's at night.

 

[-Clayton-]

Check for vacuum leaks first, Still sounds like a lean miss. Then do a tune up if needed, And everything else I mentioned in my earlier post.

 

[c3po]

Clayton, I will check for leaks 1st and then check the choke.

The car starts up fine and idles fine. The cap and rotor have been on there for 45,000 miles, so it may be time to change those items.

 

[mechtech2]

Sounds lean, so maybe it's a choke adjustment.
They used to have to be adjusted seasonally.
Retarded ignition advance will do this, as will a jumped cam chain.
But you have to dive n to ALL the vacuum lines and check the carb or intake for vacuum leaks.
This is a basis for anything else.
Also, your intake manifold may not have the heater/fuel vaporization like a stock one. This is the exhaust passage from head to head through the intake that is vital to heat the cold fuel droplets and get them vapoprized. This can not be underestimated, and is very important.

 

[c3po]

mechtech2, the intake manifold does have the exhaust passage in the intake manifold.

Here is how today went:

1) Adjusted choke, same problem
2) Checked all vacuum lines and fittings
3) Found a loose carburetor bolt, will have to be heliarced

My Dad's friend was a mechanic for UPS for 35 years and was in charge of 300 trucks every night, so he is no slouch.

He said 1 loose carburetor bolt cannot cause this problem. He told me to check my vacuum advance diaphragm.

Maybe I did this wrong but I blew into it and sucked on the vacuum line and it did not hold, I know this sounds crazy.

So I then capped off the vacuum advance at the distributor, and the port at the carb, and then started the car, no more:

1) Backfiring
2) Exhaust making a drum sound

Am I on the right track, car does not have a hole lot of power, but then again I did not get into it, and of course the vacuum advance is capped off.

I plan tommorow to:

1) Heli arc thread on the intake manifold
2) Replace vaccum advance diaphragm
3) Replace Rotor
4) Check condition of the cap.

Could the exhaust passage in the intake manifold be plugged, and could the weights on the distributor be messed up.

 

[tom slick]

you can heli-coil the intake manifold much cheaper and easier.

certainly sounds like the vacuum advance is shot.

If you are going to replace the rotor then replace the cap also, why mix and match parts of different ages. If you rarely drive it then don't worry about the ignition system unless you have a problem after fixing the other two problems.

 

[kingrob]

I've had this problem on the numerous Quadrabogs I've owned through the years. That bolt can and most likely be the source of your problem. Next stop would be the vacuum advance. Another problem to look out for, though I doubt it could be the source of the backfire, would be that the linkage is shot and you're only running on two barrels (if it's a four barrel carb). This could be the source of the drum sound in the exhaust.

 

[c3po]

tom slick, I am also going to do the cap as well.

How can my vacuum advance be shot, I have no doubt that it is, I have only 45,000 miles on this new distributor.

I know that the cap and rotor usually need to be replaced. Is the vacuum advance diaphragm another maintenance item.

 

[tom slick]

was it new or rebuilt?

how many years is 45,000 miles?

If you suck on the vacuum hose to the advance and you can keep drawing air through then it is bad. you can also remove the dist. cap and watch the advance mechanism move as suction is applied.

 

[c3po]

tom slick, this was a D.U.I. Davis Unified Ignition Distributor, it is a new unit.

I kept sucking on the vacuum hose to the advance and I kept drawing air. I was doing this for a minute, and then gave up.

kingrob, this is a 4 -Barrel carb so I will check the linkage. But most of the time I am cruising so I am only using 2 barrels.

My thinking with the carb bolt is that since it is loose, it is letting air in, kind of like a vacuum leak.

 

[tom slick]

certainly shot.

 

[c3po]

Certainly shot is right, I took the new 1 out of the bag and hooked a vacuum hose to it, and I could not blow air through it.

I could not even suck any air out.