[gray73]

hi im having a problem with a 1965 mustang. the car has been sitting for a few years. at first it would not start, so i changed the fuel pump and some new gas. i got it to start and run but it backfires back through the carburetor when you give it any kind of gas even the slightest tuch. but when you first start it up some times you can give it a good pump on the gas and no problem. i did a complete tune up and it runs smoother but still backfires through the carburetor. it has a holly carburetor not sure the size or any thing. i do know there are many things missing around the carburetor. im changing it back to stock 2100 autoline. but can anyone help me out with the backfireing problem. i also need to know if anyone knows where i could get a photo or diagram of all the parts that hook to the carburetor and where they go. because when i get the new carburetor i have now clue whats all missing such as vacuum lines, fuel lines , throttle rod, etc. thanks for all of you help in advance

[shoe48]

Might look for Vaccum leaks , Hoses that are cracked or missing , the Vaccum advance is hooked up in the right port ? Timing is set right ,, 6 degrees BTC if I remember right ? Timing chain could be loose or getting worn,, You can check this by turning the crank shaft with a wrench with the distitor cap off then see how far you have to turn the crank before the rotor moves , go one way the reverse it the other the slack will show then , The Accerator pump in the carb could be failing , look in the carb and move the throttle ,,,see if it is squiting gas in to the carb ,,

[Old man]

It is unlikely to be carb or being lean from vacuum leaks.

If it blows back through the carb either an intake valve is hanging open or not seating, or something is firing a cylinder on the intake stroke.

Since the intake stroke is around 90 degrees OR further ahead of the ignition point, it can not be a timing issue.

You could have a multitude of problems.

If the engine seems OK other than the backfire:

1.) the points could be bad
2.) the bushings on the plate that the points are mounted on or the distributor shaft bushings could be badly worn
3.) the distributor cap bad
4.) a loose wire or bad connections on the low voltage side of the coil from the key through the coil to the points
5.) plug wires cross-firing (like poor wires in a bundle or pile) or you have two wires swapped
6.) bad intake valve or valves sticking.

There is a remote chance the carb is far over sized and opening so much at low speeds the engine fires back through a cylinder from hot exhaust gasses during a valve overlap time at the start of an intake cycle or that the cam has jumped time, but this would be an engine that bogs badly or runs terrible.

This is where you need a dwell meter, compression tester, and timing light to sort things out.

Check the firing order and plug wires first. Put a timing light on it and watch the timing as you rev the motor with the vacuum advance disconnected. If the timing is moving all over the place erratically you probably have bad points, a bad distributor bushing, or bad wiring on the 12V system to the coil and through the points to ground. The condenser normally can not cause this, it cause weak spark not wrong spark.

Most often when this has happened to me it has been plug wires or bad valves.

Tom

[gray73]

well thats the problem there are no vacuum lines any where on the car. i have no clue how they go and there are things missing. well i just found a vacuum diagram online. but the way it looks im missing intake manifold fitting. the carb it self is missing stuff. i for got to mention earlyer the the car acts like it wants to die some times. it starts to bog down. i will check the timing chain tho. i took the carb off already. im also having a problem with the choke plate staying open and closed. how do i take care of that.


the cap is brand new just baught it today. i thought it could be the points but not sure im goin to change them anyway just incase. i checked the wires 3 times just to be sure. just did the tune up today also and the car ran way better at idle. but soon as i pushed on the throttle it boged a little and backfired and at to much it just kept backfiring worse and boging

thanks chez

[gray73]

can anyone tell me the name of the fitting that goes on the intake manifold. or is there even one on it. im trying to put vacuum lines on a car that didnt have them and im so lost at doin this. 1965 mustand 289 2bbl carb. can anyone tell me where i can get this at. im uploading a pic of what i found pertaining to the way the vacuum is ran.

thanks again in advance

[Old man]

It is blowing out the exhaust or intake up through the carb?

There is a huge difference in reasons. I thought you indicated up through the carb, rather than popping out the exhaust. Which is it doing? Through the carb or out the pipe?

As for vacuum lines, it pretty much doesn't matter where you get the vacuum from in an old 60's stang.

They don't use delayed vacuum, just straight manifold vacuum. So any hole in the intake that is near the plenum will provide proper vacuum. Since there is no air or exhaust passing into the intake on a 60's stang, the size of the hoses won't matter much either. The vacuum doesn't flow, it just sucks and moves parts.

The vacuum to the distributor pulls the plate the points are on pulls forward to advance the spark. I think it is about 20 degrees, you should look in a book to be sure. At high vacuum (low pressure) the cylinders aren't fully filled and the engine runs better (higher efficiency) with additional advance.

When you open the throttle the manifold pressure increases and the timing retards. This prevents spark knock and lugging with open throttle at low speeds by pulling the timing back when the vacuum goes away.

As the engine revs more, some weights will start dialing in more advance. This is to correct for delay in the burn at higher engine speeds and make more power at high RPM.

By not having vacuum lines you lost the low speed idle smoothness and some fuel efficiency at very light throttle. It will not cause backfiring or loss of higher RPM or wide open throttle problems unless you cranked the timing way off.

Plug a line from the distributor to any open manifold port near the plenum of the manifold and you will be fine. No need to worry. You should see the engine speed up at idle when it is connected and be able to use less throttle when cruising at constant speeds with the vacuum advance connected.

I think you have more issues than that though. You need to beg borrow or steal a dwell meter and timing light if you have a old stang.

I was GM and an engineering supervisor at a company that manufactured dwell, timing and other test equipment. We made almost everything Sears and Snap on and others sold, but we went out of that business when electronic ignitions and foreign imports took over. :-) I hope someone still sells the stuff.


Tom

[gray73]

i should have been more spacific backfiring / popping out of the carb. the more gas you gave it the worse it got.

i will try plugging in to the manifold for vacuum. where could i get the fitting to put in the manifold. i talk to advance auto and they couldnt find anything here. im working on putting all the vacuum lines on. i ordered a new distributor and a new carb and have all then lines just waiting on the carb to come in. i will also redo the timing to be sure. the person who had it before had it all messed up.

[Old man]

Be sure the plug wires are in correctly, dressed properly, in good shape, and the distributor cap is good.

You could have cross-firing. You could have bad valves. You could have distributor problems.

Tom

[gtjoe]

i'm not saying I'm an expert or anything, but whenever I had an engine backfiring throught he carb it was timing related. I would start there. Keep in mind that the timing marks may not be accurate. Old-time mechanics used to swear by setting the timing with a vacuum gauge instead of a timing light.