Vapor lock problem on a 1956

ron56

Active Member
I've run into a problem that has me scratching my head. I've rebuilt my car from the front to the rear and it has run fine for around 1,000 miles over two years. I have electronic ignition, Holley 1841 carb, 312 with auto. The other day I got about 10 miles from home driving on surface streets and all of a sudden the car started running rough. I got turned around and headed home with the car only able to run around 30 mph. At no time did it backfire just wouldn't run but if I gave it a accel pump shot the motor would pick up and then die back down. So I'm thinking that I have a fuel pump problem. I got a electric pump from CASCO so I mounted it in the body brace bracket by the front door. It would pump fuel through the stock pump (which was new at the rebuild). It this point I'm thinking I'm good. A few days later with the outside temp a few degrees warmer I started out and this time when about 15 miles on surface streets the engine started to act up the same. I switched on the electric pump and no change. The motor would still pickup with a accel pump shot but then drop back to around 30 mph. Again made it home ok. I checked the fuel filter and it was free flowing, I pulled the bowl off the Holley along with the jet plate and blew out all the passages but there seem to be nothing wrong. Checked the needle and seat, I couldn't see anything wrong. The bowl was as clean as it could be. The float level was fine. Again the new electric pump filled the bowl so it's working. The car again ran fine but I haven taken it very far. If the problem is vapor lock I wonder why the electric pump didn't help. Does anyone have any suggestions? One more thing when the car was running poorly I could rev the engine very high without any issues so I'm pretty sure the problem isn't ignition related especially when a pump shot will pick the engine up.
 
I have a simular problem whereas the car will only run on the electric. I replaced the tank, the 2 filters in front of the electric pump, the fuel line to the pump, put in a pre-tested new pump, changed the filter in the bowl and the inlet filter. I mention all of this because I have a new theory. I think the first filter (a inexpensive in-line) may be causing enough restriction to not allow the mechanical pump to pull fuel. That change is scheduled for tomorrow Also there have been a lot of bad fuel pumps being sold. I got my pre-tested one from Prestige. I know at least the vacuum portion is working. Got caught in a sudden rain storm the the old vacuum wipers worked fine.
 

Tater Salad

Well-Known Member
One more thing when the car was running poorly I could rev the engine very high without any issues so I'm pretty sure the problem isn't ignition related


Ummm if you're not under load, why would you rule out vacuum, or vacuum advance?
 

ron56

Active Member
One more thing when the car was running poorly I could rev the engine very high without any issues so I'm pretty sure the problem isn't ignition related


Ummm if you're not under load, why would you rule out vacuum, or vacuum advance?
I'm basing it on the fact that the engine will pickup with a accel pump shot. Yes, that in itself will drop the vacuum, but so will a poorly running engine when I would try to go up a hill. Also if it was somehow vacuum advance related not sure why the problem only shows up after running for 10 miles.
I should add a couple more details:
New stainless fuel tank and lines when I rebuilt the car. The gas cap is drilled to make sure it vents and I checked and the hole is clear.
 

Tater Salad

Well-Known Member
Do you still have the exhaust valve in the exhaust on the passenger side functional? And if so, does it open properly?
Ive not looked back at my notes, but is the 1841 still the Holley tea pot? They do have issues boiling fuel from the heat if the exhaust valve isn't open
 

ron56

Active Member
I removed the heat riser valve when I rebuilt the car. The Holley 1841 is a 465 cfm carb not a tea pot. It's like the model 4160. The engine has a 1957 manifold and I replaced the distributor with both vacuum advance and mechanical features. I made it from a truck distributor that came with a tach drive. It's a cast iron body and I adapted all the advance parts over to it from a 1957 car distributor. It's very rugged part.
 

Ward 57

Well-Known Member
Have had kind of the same problem. Plenty of fuel, replaced the condenser I think it's the coil. Got one original coming but the package seems to have been lost. Classic symptoms of a failing coil.
 

ron56

Active Member
I would agree except how do I explain that the engine picks up with the accel pump shot every time I do it. I believe that if the problem is with the ignition the engine wouldn't pick up. I have never experienced vapor lock when it happens will the engine finally stop? My car keeps running for a good15-20 minutes with the same symptoms. No change that I can tell.
 
I would agree except how do I explain that the engine picks up with the accel pump shot every time I do it. I believe that if the problem is with the ignition the engine wouldn't pick up. I have never experienced vapor lock when it happens will the engine finally stop? My car keeps running for a good15-20 minutes with the same symptoms. No change that I can tell.

THat would be my thinking too. Sounds like it's starved for gas till it gets the shot from the Acel pump. Bad ignition tends to get worse the higher the rpm but yours runs better (faster) as long as the acc pump gets it some gas. Do you have a clear bowl fuel filter where you can see if it's getting fuel? My inclination is to think the problem is in the carb but it could be a fuel supply problem I suppose. How hot is it where you are... is it hot enough vapor lock is a real possibly on a 10 mile drive? I'm thinking vapor lock isn't your problem.
 

Ward 57

Well-Known Member
I would agree except how do I explain that the engine picks up with the accel pump shot every time I do it. I believe that if the problem is with the ignition the engine wouldn't pick up. I have never experienced vapor lock when it happens will the engine finally stop? My car keeps running for a good15-20 minutes with the same symptoms. No change that I can tell.
When the coil heads up or is starting to degrade it acts up just like that. Poor mileage, stumbling, and flat out dyeing. Once cooled a little bit it will get you home. The throttle pump just helps the richer mixture ignite with a weak spark.
 

Critterpainer

Well-Known Member
After the changes you have made, I would suspect a weak condenser or coil. They are quick to change out and will allow you to rule that out as a possibility. Usually vapor lock stalls the engine until it cools a bit. A quick way to prevent vapor lock (sorry but this is a true redneck repair) is a gallon of Diesel to a tank of gas. It will not cause the engine to smoke and will help to rule out vapor lock as the issue.
 

Critterpainer

Well-Known Member
Adding just a little diesel to a tank that is full of gasoline will help to prevent vapor lock. Is it something to do on a daily basis. Of course not, but as a diagnostic tool it is a quick test. If you fill the car with diesel thats different issue altogether.
 

Ward 57

Well-Known Member
After the changes you have made, I would suspect a weak condenser or coil. They are quick to change out and will allow you to rule that out as a possibility. Usually vapor lock stalls the engine until it cools a bit. A quick way to prevent vapor lock (sorry but this is a true redneck repair) is a gallon of Diesel to a tank of gas. It will not cause the engine to smoke and will help to rule out vapor lock as the issue.
I just replaced my coil but now it's raining but it does start and idle better. Will post again once I can take it for a test drive.
 

Ward 57

Well-Known Member
Years ago, my dad accidentally put diesel in his 50s Dodge pickup. Although it ran, it didn't have enough power to make it over a railroad crossing.
Been there done that. My cousin found a full gas can in the garage and put it in my Model 'A'. It too ran but smoked like crazy. I sniffed the can and yup it was diesel that was intended to start a yard debris fire ( when you used to be able to do that ). Drained the tank and no harm done.
 

ron56

Active Member
A update on my vapor/??? problem. I've done some more testing and here's what I'm finding:
I had a bit of a chat with Chris Ames (the water pump spacer guy) about vapor lock problems and he told me a couple of things that helped him. One was insulating the line from the fuel tank to the frame rail and the other was to wrap the left side exhaust pipe. So I took his advice and insulated the line from the tank to the frame rail with some DEI stuff. I also a installed fuel pressure gauge. On a mid 90 deg day I drove for 1hr and 40 minutes stop and go mixed with 50 mph portions. First thing I noticed was the fuel pressure started out at 6 psi at idle and then as I drove it would drop to about 4 psi at idle. After it got good and warm the fuel would drop to 2 3/4 to 3 psi under hard accel. After about 1 hour of driving I drove home and pulled the car right up to the garage door and left the car in gear and after about 20 minutes the the temp gauge was to the far right. The car still ran fine. So at this point I think I no longer can blame anything in the ignition system. I did make two changes after the two trips when I had my problems. I added the fuel line insulation and I did remove the carb bowl and the jet plate on the carb and blew out all the passages. I'm sure nothing was wrong because the fuel circuit, jets to the venturi are large passages, and if the much smaller air bleeds were plugged that would only make the car run rich.
With no other changes I drove the car for 45 minutes and then back with the same results. Fuel pressure didn't drop below 2 3/4. If I run the electric pump the pressure will stay at around 4 1/2.
Then I added the exhaust wrap on the left pipe from the start of the hump over the rear end to almost the end of the tail pipe. I then repeated this same trip and this time the fuel didn't drop below 3 psi. The air temp was still in the mid 90's. I'm thinking that Chris's suggestions did the trick but I still don't understand why when I first had the problem and then added the electric pump, why turning the pump on made no difference on how the car ran.
I'm wondering if the drop in idle fuel pressure from when I first start the engine from 6 psi down to around 4 said anything about the fuel pump. It will go down to 4 psi within a few blocks, too short of a run to heat the fuel I would think. I tried to buy a new pump from our t-bird suppliers but they don't seem to have any in stock. Wanting to put the problem to bed I ordered a new pump from a O'Reilly auto parts store. The brand name was Precision Fuel Pumps which looks to be a store brand. When I opened the box the first thing I noticed was it was a pump with Philips screws which I heard are the problem pumps. I moved the pump lever back and forth and figured it was defective right out of the box. I went ahead and mounted it anyway and the car would run on the electric pump but not on the mechanical pump. Not wanting to order another of the same fuel pump I had them order a Delphi pump ( to the tune of about $50. extra) hoping that this will be a pump from a different manufacture. I should get the new pump next week.
Does anyone have any idea what the fuel pressure would be during a hard accel? The only spec I see is 4-5 psi at 500 rpms.
 

Ward 57

Well-Known Member
I really don't think it's a vapor lock problem. These cars are not known for that. Had a beetle that did and the symptoms were different. You should have more pressure no matter what. Have you looked at the source of the fuel and not the pump? It could be the pick-up in the tank or a crimped fuel line. If you still have the original pump, have it rebuilt. I have heard nothing good about aftermarket pumps.
 
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