Carb backfires

Recheck the plug wires and firing order 13726548.

Hi
Now, there may be something wrong with something after standing unused for long. But I would suggest at least initially to assume nothing is fundamentally wrong.
To me, the backfires and the rough idle and the fact that it ran slower and slower and then stalled, is typical for too late timing. And since you redid the points, you have changed the timing, that's unavoidable. So start by connecting a timing gun, set it to maybe 14 degrees. And/or when it's running, have the bolt loosened so you by hand can move the distributor to quickly find where it runs best.

If this works, then you can fine tune, allow me to recommend my book. And if you're in a hurry, I'll be happy to send you the section you need by email.

Just wanted to add, the points have to be adjusted as well to get the proper dwell angle, and that has to be done before changing timing as the dwell affects the timing. Would probably have been easier to start the car and then change the ignition parts later, just for other readers...

Mikael,
Thank you for the reply.
I am not familiar with this Forum, and can't see how to reply directly to you, but hopefully you will see this.
Google research seems to tell me the following: most likely causes are either lean mixture,vacuum leak and/or ignition timing.
I think I have several open ports to carb, so I will look to plug them. I think timing is more likely.Will check firing order - I think it's ok.
As far as ignition timing goes; how do I check with timing gun if engine won't run? Do I do static timing with # 1 cyl on compression stroke?
When it briefly runs, it seems to be running smoothly, which suggests to me that timing may be close enough and that problem is leaning out?
Anyway, will check all that you suggest.
Thanks again, and maybe Danish heritage connection will be an assist to solving this.

Bill Nelson

What carburator have you on your engine ?
A Holly ?
maybe it's the gas is overflow, that gives also a slower and slower RPM till engine is stopped idle.

I have this problem all the time.
Simon

Danish heritage solves everything!

Yes, you can check timing even if it won't run. Just use the timing light as if it was running, it'll flash every time it wants to fire, and then you read the result on the balancer.

Or, if you feel you're close, do this. Loosen distr bolt, turn distr 1 mm clock wise. That should give you a bit earlier timing, maybe enough to get it running.

When it's running, you have to adjust points to get proper V8 dwell angle, IIRC it's about 28 degrees, but if anybody knows, chime in please (I have no points, electronic). And then set the timing. Best to do as in my book, best to do to have optimum steady vacuum, but rough rules are:
as early ignition as possible with two restrictions: no detonation and not too hard to get running on the starter.

While I agree with Mikael that late (VERY Late) timing can do this, make sure you perform Joules' verification of plug wires.

Please post up when you have verified it, so we won't keep thinking to ourselves... "the guy's plug wires are mixed up"..

#1 is the Passenger Front cylinder. #5 is the front driver side.


NOTE: This is important because Ford motors have two different firing orders, that can easily be confused.

Rocky

How long is a long time? The carb probably needs a rebuild also. Especially if it's original.

PM sent

Mr. Nelson, be aware that the valves and ports in your 351-C are the same size as in a big-block Ford 460- and are larger than in some versions. So the valve-train is heavy and typically spinning all that weight, rapidly wears the timing chain- especially a stock 'silent chain'. About 20,000 miles is all one can expect to get from a reasonably driven Cleveland, before a fresh chain is needed. A 'stretched' chain may actually jump a tooth or two during start-up. A 351-C is not an interference engine so no further damage will occur, but will either act like yours or worse- not start at all.

In addition, the stock harmonic balancer inner hub and outer ring (where the timing marks are) are NOT bonded together, only pressed in place with a thin rubber strip separating them. So after decades of time, the rubber damping strip in between the iron parts shrinks. That allows the inner and outer parts to shift, changing the timing mark toward the 'Retard' direction. I've seen timing marks 180 degrees out of true time from this; obviously, setting timing without checking for true Top Dead Center on such an engine will give fearfully wrong ignition timing.

So first, one should verify the basics when resurrecting a long-dormant engine. For instance, original rubber fuel lines are often brittle and will crack with engine vibration, causing a fuel fire; the 22 gallon gas tank is mere inches away from the carb.... Stay in touch with this problem; long-distance trouble shooting is guesswork at best so we'll likely all learn something.

Set your timing @ 16 at idle,the back fire is the accelerator pump is not squirting fuel, adjust the arm to no play, if it sat you need to rebuild the carb. the diaphragm in the accelerator pump may be bad. It is not to much fuel, it's not enough.

Sounds like you have one or more intake valves stuck open. A quick compression check would confirm.

This is one of the double edged swords of troubleshooting car problems on the internet.

You have a lot of experts, giving you very valid inputs on possible conditions that could be occuring.

You now have to roll up your sleeves and start ruling out what it is not!

Calculatus Eliminatus!

Good luck

Rocky

My 2 cents. I did the same thing to my 72 after 19 years of storage. I did what you did except I also rebuilt my carb (an AFB .. not Holly). It started just like new .. no fuss, no mess .. it surprised me, I expected more drama. If there was gas in the carb when it was stored there is always the potential of some residue plugging or restricting something.

One more hint - as if you needed any more: If the carb is a Holley, backfire may rupture the power valve, causing an internal fuel leak and very rich condition.

Did you see my PM?

An engine that is running and the carb suddenly runs dry will backfire.

Generally severe enough to blow out a muffler and possibly enough to put a hole through a piston or bend a valve.

All in the system that would effect fuel delivery to the carb into the bowl should be verified good FIRST.



What are you using for a fuel pump? You need to put a number on the fuel pressure regardless of the type of pump, i.e., mechanical or electric.

Carb needle and seat needs to be verified that it opens and closes. They tend to stick closed after the engine is run and the carb is allowed to go dry by evaporation from sitting. That takes only a couple of weeks.

The float needs to be verified that it hasn't collapsed. The brass floats should be changed to the solid plastic nitrofil floats for safety.


After you do all that, then you set the float levels ACCORDING TO THE DIRECTIONS FROM HOLLEY.

Once these steps are done, THEN you look for vacuum leaks at the carb, at the intake manifold gaskets and at the vacuum hoses.

One place to look for a vacuum leak on a Pantera that is rarely mentioned is at the power brake booster.



You can have a defective check valve OR you can have a rupture to the internal diaphragm due to age.

Even the steel vacuum tube that runs under the car could have rusted through.



These are just some of the situations that will create a backfire.

There are more. You can have a cross wired spark plug wires, seriously fouled spark plugs (then you need to find out why), and a malfunctioning distributor can cause it under certain conditions as well.



Probably what you should do is make a checklist and go down the list one by one like you would do on an aircraft with your co-pilot?

Guys, with all due respect, this thread is miserable. We have a new guy with a problem. If you read his description, it stands out that he has changed the points and not set his timing (or dwell angle) afterwards. So it would be a miracle if the timing is right. And late timing gives the symptoms he has. All agree? So this is probably 80% chance the issue. So worth checking first IMO. Wouldn't you? Would you start to look at whether the timing chain had jumped a tooth or do compression test to monitor for a stuck valve BEFORE you set the timing right? I think not.

It looks like it becomes a competition in imagining reasons for backfire, regardless of his symptoms. Most of them with less than 1% likelihood. And some of them, if you READ all his symptoms, with 0% likelihood. That really doesn't help him, that confuses him I could imagine.

It's often the simple things, start there, eliminate them, and then go to the more complicated stuff.

First off, it isn't a debate. Second it isn't a contest to see who is right.

It is a process to deal with beginning with the most serious and the most consequential.

The level on knowledge and understanding by the individual is unknown, and not encouraging.

Under that condition, the first thing that should be advised is, don't point the barrel of the gun at yourself and pull the trigger.

Of course that is just my opinion. Others may have differing perspectives?

quote:

Originally posted by No Quarter:
Guys ... it stands out that he has changed the points and not set his timing (or dwell angle) afterwards. So it would be a miracle if the timing is right. And late timing gives the symptoms he has. All agree? So this is probably 80% chance the issue. So worth checking first IMO. Wouldn't you? ....

It's often the simple things, start there, eliminate them, and then go to the more complicated stuff.

Agree 100% if the points were changed w/o regard to setting timing afterward that is the 1st place to begin

from experience it's a miracle to set points with a feeler blade and have the dwell in spec with an electronic dwell meter. at least for me & I'm not exactly all thumbs over here

my recommendation is always to pitch the points & buy an electronic replacement module, short of a more serious ignition upgrade

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stored engines can have several problems when brought back on-line, valves & seats can rust / pit / corrode causing less than perfect sealing ability



valve springs also sustain damage from remaining compressed and can break when the engine runs after sitting a long time. i caught this during a clean-up / reassembly of a stored engine. i barely noticed that something didn't look right and it took a closer exam to find the spring broke after i assembled the head