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Classic Mustangs Tech Forum

Technical discussions specific to 1964-1967, 1968-1970, and 1971-1973 Classic Mustang. Discuss all tech related to in-line six cylinder and V8 powered Vintage Mustangs here.

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Unread 02-27-2010   #1 (permalink)
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Default Rough Idling and back firing 1968 Mustang 6 cyl

I have a '68 6 cylinder that is idling very rough (back firing). I thought I had bad fuel so I replaced the old fuel with new, still no better. I had the carb rebuilt, a little better but still not running like it should. Also, replaced fuel filter. I am at a loss. Please help someone. Thanks.
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Unread 02-27-2010   #2 (permalink)
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Questions...Have you had a lot of rain lately? have you looked at the dist cap for water, rotor for burnt spot, point gap? checked plugs wires for misfire? checked spark plugs, and while plugs are out check compression for the heck of it. And check for vaccum leaks.
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Unread 02-27-2010   #3 (permalink)
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welcome!!
sounds like the timing needs to be checked and adjusted. other than that, make sure the vacuum lines are going to the right places. no open lines.
the timing might be deeper than just rotating the distributor. you may have to make sure the crank and the cam are lined up with the top dead center of the #1 piston.
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Unread 02-27-2010   #4 (permalink)
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Default Need Help!! Rough Idling and back firing '68 6 cyl

Hello Mustang Blue,
Reading this thread it is hard to say what is the cause.

You thought you had bad gas - - -what symptoms caused you to think this?

Did you recently change anything prior to the start of the trouble.

When a problem arises - - we gotta play detective.
Ask yourself several questions.
1. - Did I do something recently that could have contributed to this?
new plugs? new tank of gas etc etc.
2. - When the problem started what did it do.

Back-fires are often caused by:
1. - Valve or timing issue or 2. - Incorrect spark - - Lean fuel condition.

Now we start to look at things.
I like to start with the simple things first.
1. - Are all the plug wires on?
2. - Any vacuum lines fall off?
3. - Has the car had a tune-up?

Next you gotta start checking things like timing and dwell. Both are simple if you have a timing light and dwell meter.

Next I examine the condition of the spark plug wires and the cap. Look for signs of age, wear etc.

Make sure the wires are on the cap in the correct order. (firing order).

The most logical step would be to replace points/rotor/condensor/cap - if they have not been changed in awile.

Now I pull a plug and look at the tip of the plug for signs of what is going on. Do the plugs look carboned/oily etc. This can often provide a clue as to what is going on.

It would help if you could provide a couple of more details such as

1. - How old are the plugs - wires - - cap - - - etc.
2. - Have you changed anything lately?
3. - What type of skills you have and if you have a timing light and dwell meter.

Don't be embarrased to say I have none of the above tools. We can work around that. For many years growing up as a kid, I had just the bare basic tools.

We can help figure this out. Don't be in a rush to assume bad things and don't just start replacing things. Chances are you will spend a lot of money and still have the trouble.

Let us help you diagnose this and come to a reasonable quess as to what is needed.

These are my thoughts for now - - - Print Dad
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Unread 02-28-2010   #5 (permalink)
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Thanks Print Dad for the info. The engine was rebulit about one year ago. The car was running fine until one cold day in November my daughter took the car over a friends house and left it parked outside for about 6 hrs and ever since then, it has not been running right. Normally the car is garaged kept. One other thing, a week prior to that, my daughter ran out of gas so i brought gas to her in a plastic can which was setting out in the yard for a while. That is why came to the conculsion water may be in the system. I drained most of the gas out and put new in with a solution called Heet, it ran fine for about 20 minutes, then back to rough idling, misfire, etc. I've checked the vacuum lines, they are fine. I replaced all the spark plugs, the wires are good. Replaced the fuel pump and filter. Is it possible for the timing to jump due to cold weather? Maybe I need to check to see if the timing is correct.
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Unread 02-28-2010   #6 (permalink)
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The car is mainly kept in the garage, but it has been caught out in the rain a couple of times. I had the engine rebuilt about a year ago, and everything was working fine. My daughter had it out one cold day in November and it sat outside for about 6 hrs and it has run right ever since.

Thanks for the info. I will check the timing.

Last edited by Snackmaster; 03-03-2010 at 11:34 AM. Reason: merge posts
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Unread 02-28-2010   #7 (permalink)
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I notice you said you had the carb rebuilt. Did whoever rebuilt it also reinstall it and tune it, or just rebuild it and give it back to you? No carb is ever adjusted perfectly for its engine before installation, even brand new out of the box. Backfiring is usually a timing problem, though.
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Unread 03-02-2010   #8 (permalink)
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[quote=Joes72/65;1948922]I notice you said you had the carb rebuilt. Did whoever rebuilt it also reinstall it and tune it, or just rebuild it and give it back to you? No carb is ever adjusted perfectly for its engine before installation, even brand new out of the box. Backfiring is usually a timing problem, though.[Yes, they just rebuilt it. No tunning. I've tried making the adjustments my self, but no luck. It's still running rough. I will try checking the timing next.]
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Unread 03-03-2010   #9 (permalink)
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You might think about installing a Pertronix ignition. It will replace the points and condensor and all fits under the distributor cap. My 66 has had butter smooth idling ever since we did that.

Just a thought.
 
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Unread 03-03-2010   #10 (permalink)
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Default Need Help!! Rough Idling and back firing '68 6 cyl

Hello Mustang Blue,
With this added info I think you may have found the problem.

When the car ran out of gas you probably sucked up dirt from the bottom of the tank.
This should be caught by the filters in the fuel pump and carb but you never know.

I would think the problem is the "old" gas you put in.

I would pull the hose that goes to the fuel pump and siphon out a lot of gas,
Then I would add "new gas " and a can of "dry gas". Give it alittle time and see if it
goes back to normal.

Your timing would not go off due to cold.

I would not do anything drastic.

Because you say the car was good for about 20 minutes, I would just try to get the old gas out.

If after running on the new gas and you still have a problem I would check the filter on the carb or in-line.

In my opinion it is not mechanical at this point.

Old gas even 2 months old can cause trouble if it is in a small container.



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Unread 06-01-2010   #11 (permalink)
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Hello All,

I have a question about the Ford 302 small block. I was told that you shouldn't bore a Ford small block no more than .030" over. Is this true? I took my small block 302 to .060" over. Will this cause me trouble with engine? Thanks.
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Unread 06-02-2010   #12 (permalink)
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Default Need Help!! Rough Idling and back firing '68 6 cyl

Hello Mustang Blue,
You should be fine with the 60 over. There may be a slight heating issue
so make sure the radiator is working well and all the other components of
the coolinmg system are good.

I have heard of people going 70 over but I think that is cutting too much.

In the old days, it was thought 30 should be max but many people are running 60.


LASTLY - - Re-reading the post, I still think it is gas related.
The carb could have sucked up something or in a filter.

Remember - - gas today has a pretty short life span.

If still having trouble - - blow out the fuel lines and frain tank and try new gas.
My thought is that a filter ma have gotten clogged.

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