Runs great for 30 minutes then not so well

[Bill Blue]

They are "not so smart" if the spring mechanism doesn't work for any reason. Mainly corrosion is the culprit I always spray a little WD 40 into the tiny hole on the spring mechanism, and then push in and out many times to lubricate the inside of the cap. Had many overflows of gas on a hot day because of a faulty spring mechanism. Lesson learned.

Thor 1211 stated that any air movement when opening the cap is bad. Not so. It is very appropriate for some pressure to be present. Note that I did not state they will operate forever with no problems.

Bill

The gas cap could be the problem. After sitting for 24 hours, there is zero pressure when opening the cap. When running just for a few miles there is noticeable pressure when opening
RootesRooter; yes the dizzy came with a new cap

Gasoline will develop pressure when "stirred up" and your findings are completely normal. Vacuum, however, is something you should never encounter.

Bill

 

[pruyter]

If you want to rule out that vacuum in the petrol tank is the problem you can test it by driving the car with the fuel cap not locked and for instance a half full tank so that no petrol can be spoilt. If the problem not occurs after the drive (with the number of miles which were droven before the problems showed itself) then you know wheter vacuum is the problem or not.

Regards,

Peter

 

[Tom H]

Or maybe drive until the problem occurs and then pop the gas cap and see if problem goes away.

 

[Van Bagley]

Yesterday I found that the dizzy was not gapped correctly. I gapped it .15 and the motor ran perfectly from a cold start. I drove the car for about 6 miles without any missing. Flawless. Then all hell broke loose. Same thing, backfire, missing, random response. I stopped the car, opened the gas cap and I let it sit for about an hour. Started up again and it ran for a few minutes before getting so bad I had to flat bed it back home. I am now of the opinion that it has a bad relationship the the coil.

 

[RootesRacer]

Yesterday I found that the dizzy was not gapped correctly. I gapped it .15 and the motor ran perfectly from a cold start. I drove the car for about 6 miles without any missing. Flawless. Then all hell broke loose. Same thing, backfire, missing, random response. I stopped the car, opened the gas cap and I let it sit for about an hour. Started up again and it ran for a few minutes before getting so bad I had to flat bed it back home. I am now of the opinion that it has a bad relationship the the coil.

Did you replace the condenser yet? Cheap, easy and more likely to fail than the coil.

Also...
Years ago on my sunbeam, I would drive it and it would spontaneously cut out and after a lot of troubleshooting discovered the coils primary terminals were loose. The rivets had gotten a little loose over the years.

 

[Van Bagley]

I have not changed out the condenser thinking that I have less than 100 miles on a new distributor. I will do it today as well

 

[RootesRacer]

are you even sure its ignition related?

The pump valves may be leaking and the pump may not like pumping when hot.
The carbs may be boiling fuel.

If you have a glass sediment bowl on your pump, watching the fuel pass through can tell whats going on.
I had bubbles entering my bowl you could see and I discovered that the pump was sucking some air
on an unsealed tank fitting.

Ive seen hot carbs vapor lock when good and hot too.

 

[MikeH]

My money’s on the coil. My 65 Mustang did the same thing. Run fine until it heats up then no power and missing. It died and got towed home. Next morning, started right up.

 

[Tom H]

Yesterday I found that the dizzy was not gapped correctly. I gapped it .15 and the motor ran perfectly from a cold start. I drove the car for about 6 miles without any missing. Flawless. Then all hell broke loose. Same thing, backfire, missing, random response. I stopped the car, opened the gas cap and I let it sit for about an hour. Started up again and it ran for a few minutes before getting so bad I had to flat bed it back home. I am now of the opinion that it has a bad relationship the the coil.

Does not sound like the gas cap is the problem, BUT, did you leave the cap open when you restarted it?

 

[RootesRooter]

I don't quite understand the discussion over gas cap venting. As a car warms up, I can't imagine a vacuum being formed in the tank that would keep fuel from traveling up the line. As for a pressurized tank, my '69 Alpine fastbacks had no such venting. As the car and/or the weather warmed up, the tank would become so pressurized that popping the cap would let out a huge WOOOSH of air. Although the fastback Alpines stopped running for many reasons over the years, lack of steady fuel supply was never one of them.

I would try RootesRacers's suggestion to observe the fuel bowl for bubbles, or replace the coil per MikeH.

One more question: when all hell would break loose and it started to run badly, would it still idle OK or was it misbehaving at all rpms and/or acceleration?

 

[Bill Blue]

The Alpine will build considerable pressure. A few years ago we were having temps in the low 90's and the cap was "singing" constantly. We stopped at our evening destination and I decided to relieve tank pressure. When I unlatched the cap, gasoline boiled out, soaking my left arm and shoulder. That really surprised me as we had over a hundred miles on that fill.
Bill

 

[MikeH]

Were they sealed systems? I remember not filling up all the way in the summer because of expansion causing gasoline to run out of the vent of the tank. Not referring to an Alpine here.

 

[Mike O'D]

Not a sealed system. There has to be a way for air to get in to replace the gas as it's being used. The caps are vented to allow for that. I don't know the internal construction - is there a "check valve" that keeps positive pressure from getting out, or is it just a simple vent hole? If it's a simple vent hole, then I don't see how there could be any build up of pressure or vacuum.

 

[Bill Blue]

From the sounds coming from my gas cap, I'd guess the cap (and gasket) is spring loaded. Its action is the same as the modern radiator cap.
Bill

 

[beamdream]

I have not changed out the condenser thinking that I have less than 100 miles on a new distributor. I will do it today as well

You realise that you can mount a new condenser on the outside of the distributor ?

I would only do this as a temporary measure, but at least avoids opening the distributor. Just connect the pigtail lead to the low tension wire between coil and distributor and earth the condenser body anywhere, this will bypass the existing condenser. Just a couple of minutes work to prove or eliminate a faulty item.

 

[Mike O'D]

Bill - It would make sense, except the reverse of a rad cap - relives suction, but holds closed with pressure. OK, now I'm going to have to take one apart!

 

[hartmandm]

From what I can tell, my series V gas cap is spring loaded to keep a tight seal at the gasket when the cap is closed, but the vent hole allows air to pass in either direction all the time.

Mike

 

[Bill Blue]

Bill - It would make sense, except the reverse of a rad cap - relives suction, but holds closed with pressure. OK, now I'm going to have to take one apart!

Mike which one are you describing? "Relieves suction, holds closed with pressure". Really makes no never mind, it describes both. When the radiator cap relieves suction, it pulls coolant in from the overflow tank.
Bill

 

[Bill Blue]

From what I can tell, my series V gas cap is spring loaded to keep a tight seal at the gasket when the cap is closed, but the vent hole allows air to pass in either direction all the time.

Mike

View attachment 22817

Mike, I'd say your cap is defective. Why have a spring loaded gasket seal if the vent hole is open all the time?
Bill

 

[MikeH]

Being a 1960’s car with no emission standards, that the vent in the cap is just that, a vent. Neither holds vacuum or pressure. I would think the gasket was just to prevent fuel from sloshing out.

found at https://www.tanksinc.com/index.cfm/page/ptype=results/category_id=160/mode=cat/cat160.htm

Up through the 1960's, most vehicles used vented gas caps. This is simply a gas cap with a hole in it. Unfortunately, this hole would allow the fuel to splash out when accelerating or turning a corner.