1973 302 Backfiring. Can't figure out why.

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MikeW302

Member
Joined
Mar 15, 2015
Messages
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Location
Maryland
My Car
1973 Mustang
302 2V V-8
I've got a 1973 Mustang that ran perfectly fine when I last drove it a month ago. THe only things that's changed since I last drove it was the unbearably cold weather. Now, when I try to drive it, it backfires. The idle is just fine. When I put it in gear it sounds a bit rough. When I have it under load, like a hard acceleration or going up inclines, it backfires fairly rhythmically through the carburetor. The timing is fine. I recently cleaned the carb. I tried adjusting the Idle Mixture Screws. I double checked the fuel lines. The ignition system seems to be in good shape. I've only put 6000 miles since the last tune-up. Nothing seems to work. I can't figure out why.

 
MikeW302,

+1 with Don C. Also, check your vacuum advance on your distributor. Also, when was the timing chain last changed or inspected?

mustang7173
Checked the vacuum lines and found nothing out of the ordinary. I haven't touched the timing chain in the 6 years I've owned the car. But I haven't had any problems with timing at all.

 
If you use a timing light and the timing is still where it should be (within a couple of degrees), the timing chain hasn't jumped a tooth.

If the brake booster and vacuum advance diaphragms aren't leaking and the PCV isn't stuck open the next most likely thing will be plugged main or accelerator circuits in the carburetor. And, talking about the carburetor, make sure the mounting nuts are snug. Another possibility is that the intake manifold is leaking around shrunk gaskets, cold weather can do unexpected things to seals and gaskets.

 
as others have covered, could be a vacuum leak.

if the engine is left for a while to warm up then driven is the problem still present: that would point to the choke not functioning correctly maybe stuck open.

now you need to determine if the backfire is coming back through the carb or out the tailpipe it means 2 different things.

Carb backfire is lean condition and points to a problem with the fuel mixture not enough fuel. clog in the fuel system possible or problem with accelerator pump circuit.

tailpipe backfire is usually timing or problem with ignition system.

there could be massive built up of carbon inside the heads from low speed driving or stuck choke causing a rich condition.

first figure out if it is a carb or a tail pipe backfire.

second inspect the engine vacuum system for a poped off hose or nipple, if none is found above or below the car, keep it on the list

check trans modulator hose on the transmission if you have an automatic.

third inspect the ignition wires, check seating of ignition wires make sure they are fully seated in the distributor cap and on each spark plug, they can vibrate of or loosen up, inspect motor mounts if you find pulled or loose wires.

Points or electronic ignition? electronic ignitions can go bad in the cold i had this happen to me. might want to change back to points for diagnostics.

you should check timing with a light that will confirm initial timing and you can move the pickup around wire to wire and see if you have a misfire on a spark plug other then #1

going back to second point. cap off all vacuum accessories to the motor for diagnostics and see if she runs good with no backfire then you know you have a blown vacuum accessory in the system. Plug PCV for diagnostics.

return one system at a time until you find the vacuum leak. use a vacuum gauge to see how things are.

so severe cold,,, possible stuck choke, possible stuck pcv allowing a massive vacuum leak, defective electronic ignition system as more possible issues.

next up vacuum leak in another system(possible mouse damage) issues with ignition system loose connections, corrosion issues like engine grounding wire making bad contact.

Fuel filter clogged, clogged up carb, something could of broke loose inside the fuel system clogging everything with junk, water in fuel

last go over if something did change right before you parked it that you may of forgotten about.

additionally a backfire can produce additional damage. you could of started with a simple ignition issue, then the first backfire could of blown out a reed valve and or a vacuum servo motor, including damage to the brake booster.

i had something like that happen to me; Engine was backfiring from the start and caused additional damage to the system requiring a new booster, new vacuum canister and check valves.

 
I have had the ignition condenser mimic many other things. When the condensor failed, it would back fire when it wanted to, blew out mufflers. Back fire on acceleration, usually after I'm a block down the road. It would start working OK on its own. Sometimes cut out completely on each trip out. I replaced it all by itself. Never acted up again. I'm not one to just change parts, but on troubleshooting everything kept checking out OK.

On another car I had the ignition coil fail. It checked good every time. I could only get half power from the engine. Sometimes it would backfire. Just FYI.

 
I agree with Judge, before I replaced my breaker point distributor with a Duraspark I had the same symptoms and found the distributor's condensor was bad (cheap/easy fix).

What type of ignition do you have?

 
It is backfiring through the carburetor.

I checked the vacuum lines with a vacuum gauge. All plugged in, no leaks. Everything is perfect.

I checked the PCV. That was fine as well.

I checked the timing. That is also spot on.

I checked the distributor cap. No condensation.

I checked the ignition wires. They're all seated properly on the cap.

I cleaned out the carburetor with carb spray.

I'll be replacing the fuel filter once it comes in stock.

All the bolts and screws on the carb are good and tight.

 
it looks like the problem is going to the carb

but the ignition could still be an issue.

backfire through carb first thing to look at is the accelerator pump circuit.

pull the air cleaner.

get a flashflight.

start the car let it idle.

with car in park or neutral, point the flashlight down the carb throat, look at the throat off to the side just in case of backfire and try to get a line of sight on the pump shooter jets that are aimed at the ventris, blip the throttle linkage by hand and watch the accelerator pump jet come out of the shooter. was it a nice clean spray? did it come on the instant you blip-ed the throttle? did you see 2 or 4(4bbl) streams going to each ventri? it is possible a piece of crud is stuck inside clogging the shooter or pump shot circuit. do this blip a few times , shut down,, inspect the intake manifold top or under the accelerator pump housing for leaking or weeping fuel. that is a sure sign that the diaphragm is torn and needs to be replaced. if you find an issue then you will need a rebuild kit and then take the carb out for servicing.

cleaning out a carb may require blowing out a circuit with compressed air to get a piece of junk out.

you could remove the carb drain it into a pail and inspect the fuel that comes out, if there is junk in it then you know it clogged up the circuits and your fuel filter is no longer doing its job.

if you still find nothing:

pull the distributor cap again and look at the points inside are they worn do they have build up on them

look at the rotor, is it worn, possible replacement.

you can have a phantom issues something looks good but isn't:

ignition wires replace.

coil replace.

condenser replace.

a coil can go bad and a condenser can go bad and start all kinds of crazy problems.

i have also seen cross fire occur and drive you nuts. You go into the garage at night open the garage door turn the lights off, lift the hood and start the car, if you see lightening firing all over the engine bay you found a problem.

i have also had an issue with bad gas in the winter water gets into the tanks at the gas station and you fill up with bad gas.

if you drove the car for a half hour and after it warmed up everything is fine then it would point to ignition, electronics problem. a bad condenser or coil or electronic ignition does this.

 
The wires are 6 years old, but only have about 6000 miles on them.

It's been under a cover for most of the bad weather.

I've tried letting it warm up - no good. It always backfires under load (fairly rhythmically too), but not in idle. The points and the rotor looked just pretty fresh.

I'll take a peek down the throat when I get the chance.

 
if this is a points ignition: i know you checked timing, but did you check dwell?

if the dwell is out of wack then when you rev the motor the engine timing can go anywhere, it could be fine at low rpms but the minute you rev it the contacts could go totally out of alignment or the timing could increase too much on top of the mechanical advance.

something else could be a problem with the vacuum advance. the diaphragm might of blown. that would cause a vacuum leak only off idle.

the cars used ported timing, that means at idle your working off the mechanical advance only and vacuum at the advance is 0HG.

the second you come off idle the vacuum advance comes on.

does it backfire if you slowly raise the throttle?

try plugging the vacuum advance as a test and blip the throttle and see if the backfire stops.

If you are running points i highly recommend changing to electronic ignition.

the problem is today the replacement condensers and points are all made in china and complete JUNK!

the vintage stuff from the 1970s or 80s is the only good stuff left and you just can't buy it outside of ebay or a swap meet.

as a real life example i ran points back in 2005 every 100 miles i would come back and check timing, the timing was good, but the dwell was WAY out of spec, i would readjust the points and the dwell was perfect, 100 miles later and again the dwell was out of wack completely the point of contact wasn't even wearing on the distributor cam it was like the spring was getting weaker and weaker on the points and the car would misfire when accelerating. i changed to electronic and while i had major issues at the start overall the reliability is much higher.

long short,,,, check dwell if you have points and the timing is correct.

inspect the accelerator pump just in case.

try pluging the vacuum advance and see what happens.

besides taking the carb apart and inspecting you start getting into swaping parts till you find the problem.

Condenser and coil swap out first, then wires and if you do the wires inspect the plugs see what they look like and the gapping on the plugs needs to be checked.

there are more things that it could be.

you also don't want to start chasing your tail,,, you do all the work only to find out the gas went bad..... when you put the car to sleep did you put stabil in the fuel tank?

I had one car i poured stabil in the tank,, in the spring the car ran horrible i pulled the plugs and the porcelain on every plug was PINK!!!! changed all 8 plugs and dumped the fuel in the tank, problem solved. I learned new ethanol 10% gas takes a long time to go bad and there is no need for a winter additive, just fill it up and cap the tank good for months of storage.

 
I had a condenser go bad and it backfired through the carb and acted like it jumped timing. I checked everything else first, even tore the timing cover off, but everything checked out. An old Ford guy took a look at it , changed the condenser and it fired right up. Made me feel like a dummy!

 
I had a similar problem with my 72 and after a great deal of searching and testing we found the two month old yellow top reproduction coil was failing intermittently. I'm not saying this is connected to your problem and from everything I've read I think the problem is carburetor related. I wish you luck.

 
I peeked down the carb last night and the revved the engine. The jets looked fine. I swapped out the fuel filter, took it for a test drive and it was still backfiring. I popped open the distributor. I think it actually is electronic (see attachments). It all looks clean. Not sure how to check the vacuum diaphragm. Also not sure how to check for leaks in the intake manifold or carb short of taking them apart, for which I have neither the time, tools, nor skills.

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