Wrong gas cap causes starting/running issues?

[1971RR]

I have the major issues outlined in the Ignition Electrical section. I was curious if the incorrect gas cap can cause the problems I'm having. I got a new cap, but I think it may be for a 72, which I think had an evaporative system while the 71 did not. I seen to build what I believe to be, excessive pressure in the tank and fuel line once the car had been running a while. It gets very hard to start and wants to fall over at higher RPM. Can I drill a small hole in the cap to relieve this pressure? I still have the old had cap I was using, which was off of a 71 Ford F-100. It kept the had where it belonged but just didn't fit well. I'm going to try that when I get home. Just wondering what a non vented cap might do.

 

[godfatherofchry]

if the cap fits good I would drill the smallest hole you can.I know I had the same trouble with my 66 charger with a healthy 440 wen I was racing drilled a hole and solved the problem..the only trouble I see if the cap is in the back under hard take offs it mite puke some out like the Chevy's do............Artie

 

[1971RR]

Thanks Artie! I'll give it a shot. The cap fits perfect, nice and tight, which I really hope is my problem. I will experiment with the "basically covers the spout" gas cap from the Ford. If it runs ok with that then I'll try the drilled hole in the new cap. Interestingly enough, the other cap that was recommended, and seemed more appropriate for my car, didnt fit very well so I went with the one I have now.

 

[Slantsixdan]

Drilling random holes in the gas cap is the wrong answer. Think about it: Did the factory drill random holes in the gas cap? No. Do the cars those undrilled caps are installed on normally start and run correctly? Yes. So the lack of a randomly-drilled hole is not the problem. Find and fix the problem!

All '71 cars (and '70 California cars) had an evaporative emission control system (ECS) with a pressure-vacuum cap. Some of the caps are physically different between '71 and '72, but they are functionally identical. When the ECS is not working correctly, yes, you can get starting and running issues as a result. Read here to understand how the systems work, then make sure all the parts of your car's system are present and in good condition, hooked up correctly and working properly. If that does not fix the problems you're having, they lie elsewhere.

 

[Meep-Meep]

Certainly that will alleviate the problem, but I'd be digging into why the factory venting system isn't functioning. Fuel evep or not, there has to be some from of tank vent. I have a similar problem with my 67 Ranchero but have yet to look into it. It does seem to run OK though.

 

[1971RR]

The two cap pictured were my options when I ordered the caps from AdvanceAuto. The thinner one is the one I went with as it fit tighter and it was listed as "April and Older" while the thicker one was listed as "May and Newer". Now the second one looks more like the one that was on the car many many years ago, but it did not fit well at all. Hence the reason I went with the thinner one. I also did put a new tank on it last year. The new tank looked identical to the old one and had the four fittings coming out of the driver side of the tank. I put them in the same locations as they came off. After reading your other response, I've always wondered what the "tube to nowhere" was in the engine bay of my car. Seems it may have something to do with an evaporative emmissions system. I attache a pic of the car as it came out of storage. If you look at the left side of the engine bay, just above the A/C line drier (next to the coil), there is a line coming from the drier that makes a left and then heads to the rear of the car before diving down befind the fender well. Just before it makes that dive you can see the mystery, open ended tube to nowhere. Been like that for the 26 years I've had this car.

 

[Slantsixdan]

Judging by the aftermarket valve covers and air cleaner, I'm guessing the breather is incorrect (or missing; can't see because its location is hidden by the fuel filter and heater hoses) I'd say the "mystery" is down to someone having hacked away at a system they did not understand.

Your carburetor has a bowl vent fitting, 11/32" diameter, near the top front. There should be a hose running from it to the crankcase breather. It looks as if there is not. What is that fitting ducted to, if anything? Is it capped?

 

[1971RR]

Judging by the aftermarket valve covers and air cleaner, I'm guessing the breather is incorrect (or missing; can't see because its location is hidden by the fuel filter and heater hoses) I'd say the "mystery" is down to someone having hacked away at a system they did not understand.

Your carburetor has a bowl vent fitting, 11/32" diameter, near the top front. There should be a hose running from it to the crankcase breather. It looks as if there is not. What is that fitting ducted to, if anything? Is it capped?

On the right side of the engine behind the A/C compressor there is a breather cap on the valve cover. The PCV is on the left valve cover, which is obscured by the fuel filter. You can just make out the PCV hose running across the valve cover before it heads to the base of the carburator. The carb in the picture is a Carter AFB. It has since been replaced with a Edlebrock 1406 which is essentially the same carb. As far as the hacked system goes, what you are seeing in the pic is the way I got the car in 1985. That tube by the drier has always been like that. I was never concerned with what it was as the car always ran great. Since I replaced the tank and everthing else, I must've upset the balance of the force or something...

 

[1971RR]

Anyone know where I could get my hands/eyes on a fuel tank or fuel system schematic for this model year b-body?

 

[72RoadrunnerGTX]

Pretty sure the cap change-over took place end of April 72’ production along with the redesign of the tank and tank vapor separator system. As mentioned, there is no functional difference in the two styles of caps. They are both non-vented with vacuum/pressure relief. If you drill holes in the caps you will likely have fuel spillage under acceleration with the neck being so low. The later caps are double action, meaning it requires two 90 degree counterclockwise rotations to remove the cap. The first action unseals and releases any normal pressure build-up with a second set of tangs designed to prevent the cap from getting out of hand at pressure release. The necks are different as well and as you have discovered the wrong cap will not fit correctly, use the cap that fits/seals to the neck. Slight tank pressure build up is normal or it wouldn’t have a over pressure release cap.
I would check the tank vapor separator system and hoses, be sure the hard vent line running from the separator to the engine compartment is not plugged off. As mentioned for 71’ should be connected the breather cap, using the crankcase to store at rest vapor. 72’ and later it’s connected to the charcoal canister.

 

[72RoadrunnerGTX]

Here is the 72' version, shows the charcoal canister, the tank vent system depiction is not acurate but basic tank layout is the same for 71'. The four tank vent nipples are on the driver’s side of the 71-throuth April 72’ tank with a lager vapor separator in the left rear wheel well. May 72’ and up these nipples are at the upper front of the tank with a much smaller separator mounted above the diff.

 

[1971RR]

So the mystery tube in my pic isbody the tank vent that is supposed to be hooked up to the crank case breather somehow? If this line is plugged, it can possibly give me the hot start/run issues I'm having? My tank vent manifold setup is not near as nice as that pic. The four hoses run to the manifold then disappear into the body. Can I blow air through th lines to see if th there are any blockages?

 

[Slantsixdan]

In the '71 system as originally set up, the fuel tank's one and only vent line (coming off the 5-port cylinder in the trunk) and the carburetor bowl are both vented to the engine crankcase via two small fittings on the crankcase breather. You no longer have a factory-type breather, which means those two fittings are absent, which means the fuel tank and carb bowl are vented to ????? or their vents are blocked off. Still waiting to learn about your carb bowl vent as asked in post #7 of this thread.

Your easiest and best way forward is going to be to get a charcoal can and a purge valve and put the system "back" together as described in the link I provided in post #4.

 

[1971RR]

as far as I know the bowl vents are contained within the air breather assembly.

 

[1971RR]

I'll try and post some pictures of everything tomorrow.

 

[72RoadrunnerGTX]

The 71’ b-body fuel tank vapor separator.

Mounted in the rear of the left rear wheel well, the hoses exit the tank and pass through the frame rail to the nipples at the bottom of the separator, a metal shield covers the hoses outboard of the frame rail. The fifth connection (at the far right in the picture) connects to the hard line running to the engine compartment. As long as the other end of line is not closed off to atmospheric pressure, I’m not following how the tank vent system could create a hard starting condition and a high speed “fall off”. I understand that you are running an aftermarket carb, may not have the enclosed bowl vent valve fitting. Without an original style breather cap with the correct 71’ hose nipple configuration (3 of them) you won’t have anywhere to connect these lines short of rounding up some 72’ charcoal canister parts as mentioned.

71' system

 

[1971RR]

Here are some pictures I took of the various systems. Please dont hate on the wiring. The car sat for 19 years. I'm still cleaning it up. Once I know its running consistent, I'll route/hide wires. Dont forget to checkout the knarly kickdown bracket.


Engine Bay. FrankenRunner


Filler Neck


Tank Vent Lines


Mystery Tube Engine Bay. Tank Vent I'm guessing.


Separator Inlets


Carb Edlebrock 1406 (Same as the Carter AFB)


Carb Side


Carb Top

 

[1971RR]

If I get off at a decent time today, I'll run a coat hanger down the vent tube in the engine bay. Could be a dead critter in there.

 

[72RoadrunnerGTX]

These pictures show that your aftermarket carb is not equipped with a bowl vent valve and fitting. The tube questioned is the vent line from the tank separator and it appears to be open to atmospheric pressure. I would also suggest that you re-route that fuel pump to carb line away from the exhaust manifold, appears to hang directly over it. That would tend to pre-heat the fuel before reaching the carb contributing to a possible heat soak percolation problem. Factory hard line routing had it running up the front of the motor around the distributor to avoid manifold heat.

 

[1971RR]

Looking at the engine bay picture, I should run the fuel line up the front and between the distributor and A/C compressor?
I believe the carb bowl vents are those two cresent shaped pockets right next to the air cleaner flange, so they would be vented directly back into the carb via the barrels. Since my car is FAAAAAR from a numbers matching Satellite or Roadrunner, I dont know that I will worry too much right now about a correct carb.
With my tank venting to atmosphere, I can possibly assume that there may be a blockage in that line somewhere? If I have to, I can run the PCV on the other valve cover and get a new crank vent breather with a couple of nipples and run the tank vent to one nipple and the other to, I dont know, the vacuum line right above the right bank air/fuel mixture screw? Thats the only other open vacuum line I have left on the carb.