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Mechanical Help Please (fouled plugs/erratic idle/hard start)
#1
I ran into an issue the other day when taking the Mach 1 around the block. I just recently got my car operation (full resto) and am troubleshooting and tuning everything now. Prior to this day, the car started, idled, and revved up well, but would get a slight surging effect when I drove it over an idle speed (so far have only made it around the block).

During my test drive on this particular day the engine started surging horribly and sounded like it was missing. I limped it back to the house and started diagnosing the issue. It would barely start and had an erratic idle. I assumed I fouled the plugs due to having it miss all the way back to the house (1 block away). I pulled them and, as expected, they were full of carbon. Cleaned and reinstalled the plugs and then tried the restart. I was able to get it started for a few seconds but it was rough at low RPMs and idle. I immediately brought the RPMs up to about 3000. It began to smooth out and run normally at this RPM. I plugged the vacuum advance from the distributor and checked the timing. It was at about 40 total (changed a bit from my initial setting) but could not get the initial because the car would no longer idle. I backed it down to 36 total. I was able to get total set to 36 and the idle then evened out at about 750 (where it was initially set) for a while after I ran it up getting the total timing. The initial was about 14 but hunting slightly. When running it up I had the vacuum gauge hooked up and was steadily getting 55 cm Hg at 4000 RPM.

I dropped it down to an idle and let it run for a while. Once it started to cool off the idle started bouncing around between 500-1000 again. I messed with the idle set screws thinking maybe they were too rich and causing the erratic idle. It didn’t make any difference. I then decided maybe there was a vacuum leak somewhere. I plugged the only other two vacuum lines (power booster and PCV to front of carb). I used the started fluid method and came up with nothing, and because I let it cool off a while the idle and low RPM range got worse. It would barely stay running and sounded just as bad as when I started trouble shooting. It still smells and acts like it is running too rich (but I know/think the timing is very close to where it should be).

It is now looking like it may be a carb issue (too much fuel at low RPM causing erratic idle and fouling out the plugs). I do have an upgraded cam, but nothing too crazy (see part number and specs below). I took the carb apart and checked the floats. They were at the stock 7/16 setting and everything looked good (all stock settings on the carb). Should I look into changing the springs and possibly the primary jets to restrict fuel flow? Do I need to swap plugs rather than just clean and replace?

So as it stands…the car runs like hell, doesn’t want to idle or run at low RPMs, the plugs are fouling from too much fuel and misfires…other than that everything is great! Any advice and technical knowledge would be appreciated.

Paul

Car specs...
351c bored 30 over
Comp cam: FC 268H-10, Hydraulic Flat Tappet, Adv. Duration 268/268, Lift .494/.494
Edelbrock 650 cfm
Edelbrock Performer Intake for 351c 4v
Stock Ignition system (new distributor and coil)
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#2
My first suspicion is the carb. If I read your post correctly, the car was sitting for a while so there may be a build up inside the fuel metering passages.. Try some seafoam in the gas tank for a while which may clean things out before tearing into the carb itself..

All things being equal, mechanical items don't usually change after sitting for a while..

The person who says it cannot be done should not interrupt the person doing it. 
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#3
The carb was new out of the box about 3 weeks ago. I have started it regularly sense installed (every 3 days or so). It idled, started, and ran good until now.

Paul
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#4
Gunk in the fuel line may have gotten into the float needle, holding it open, and causing too high of a fuel level, resulting in a too rich condition and flooding. There may also be something loose internally in the carburetor, new doesn't mean it was assembled correctly.



“If you can't explain it simply, you don't understand it well enough.”
--Albert Einstein
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#5
Be careful spraying starting fluid on the engine while running. It is very flammable I would use carb cleaner, less flammable, less chance of fire from spark from a bad plug wire.
Double check your plug wires since you had the plugs out again. Cleaning used plugs is just as good as new unless they are burnt.
I would take your gas line loose up near the carb and crank the engine over and catch the gas in a glass container and see if you see any trash coming through. Like stated might have got some trash in carb.. Hope you did put new filter on.


When a man is in the woods and talks and no women are there is he still wrong??
Tongue
David
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#6
I would check vacuum connections first, then electrical connections in the ignition system. What type of distributor?

Use a voltmeter to verify voltage at the ignition coil is 10+ volts.

1971 Mustang
Grabber Lime with black deluxe interior
302 engine with C-4 transmission and 9" rear end with 3.50 gears
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#7
Carolina_Mountain_Mustangs;272418 Wrote:Be careful spraying starting fluid on the engine while running. It is very flammable I would use carb cleaner, less flammable, less chance of fire from spark from a bad plug wire.
Double check your plug wires since you had the plugs out again. Cleaning used plugs is just as good as new unless they are burnt.
I would take your gas line loose up near the carb and crank the engine over and catch the gas in a glass container and see if you see any trash coming through. Like stated might have got some trash in carb.. Hope you did put new filter on.

I will give that a try. It has all new lines, new tank, new pump, new filter, new plugs, new plug wires. Everything in new. If I pull the carb apart again, where should I be focusing on looking for junk? When I took it apart the first time the bowls were full of fuel and looked clean (but I was not as focused on looking for anything, just checking floats).
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#8
Chris Womack;272420 Wrote:I would check vacuum connections first, then electrical connections in the ignition system. What type of distributor?

Use a voltmeter to verify voltage at the ignition coil is 10+ volts.

I replaced the coil after this happened and checked the volts, it was at 12.7 or so. Coil wire was checked as well and had steady voltage. All vacuum lines were sealed trying to isolate the problem and I did not find and leaks (they are still plugged). It has the stock distributor with single vac advance (plugged for troubleshooting).
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#9
Check the fuel level in the bowls, make sure it is at the specified level. Check the needle valve(s) for the float(s), make sure there isn't anything in the seats keeping the valve(s) open. Check for debit floating around in the bowls or in the bottoms.



“If you can't explain it simply, you don't understand it well enough.”
--Albert Einstein
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#10
Don C;272424 Wrote:Check the fuel level in the bowls, make sure it is at the specified level. Check the needle valve(s) for the float(s), make sure there isn't anything in the seats keeping the valve(s) open. Check for debit floating around in the bowls or in the bottoms.

Took apart the carb again. Fuel level in the bowls was good. Nothing blocking the needle valves (needles and springs looked good). There was a very little amount of debris in the bottom of one bowl. I sucked it out, but doubt that it had anything to do with the issue (very fine and minimal amount, just a few specs). Pumped some fuel into a clear bottle, fuel was clean. When looking down into the intake there was still a bit of standing fuel on the top.
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