When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
I bought a 1970 Cutlass a few months ago. I fired it up a couple times this winter and ran it for about 30 minutes each time without issues. Last week it was warm enough to take it out for my first ride. The fuel gauge indicated 1/4 tank when I received it so on my first ride I stopped in town and topped off the tank. I drove it for about 20 miles on the first trip and it ran great with no issues.
A few days later I wanted to go for another ride and it fired right up but it didn't run long and seemed like it wasn't getting enough fuel. The only thing new was filling the tank with gas, so I thought something might have got in the fuel line.
I disconnected the fuel line that runs from the tank to the mechanical fuel pump. I tried to blow through the line with my mouth back into the tank but it felt like blowing through a straw with a 1/16" diameter. I tried to clear the line with 10 psi of air pressure but that didn't clear it. I bumped it up to 20 psi of air pressure and I could hear the tank bubbling so I knew it was clear. I could then blow through the line with my mouth with no resistance.
I waited an hour for things to settle and it fired right up, but only ran a couple minutes before stalling again. The line was plugged again.
I dropped the tank today and the sock filter on the sender has a couple good tears that are probably letting crud in.
The gas was perfectly clear when I siphoned the tank with no noticeable rust and only a few pieces of crud were caught in the filter as I filtered the gas into portable gas cans. I suppose the big pieces of crud wouldn't fit in the siphon hose anyway.
I don't have a good way to look inside the tank beyond what I can see looking into it with a flashlight. I don't see any rust on the bottom, but there are a few big pieces of crud here and there in the tank.
The car is mostly rust free and has an undercoating. The fuel lines right off the tank are marked "FUEL INJECTION HOSE", so a previous owner has done some work in that area.
At this point I'm thinking I'll just replace the sender. Do you see or know of any reason why I should replace the tank? I'm a bit concerned there might be issues higher in the tank where I can't see, but that is only based on me having the issue after I filled the tank. Nothing else visually is really supporting that.
You can use an inspection mirror while shining a flashlight on to to see inside the tank. If you have a radiator shop nearby, take it to them and see if they will clean it out. I'm not a fan of sealers but others here have used them with success.
You can use an inspection mirror while shining a flashlight on to to see inside the tank. If you have a radiator shop nearby, take it to them and see if they will clean it out. I'm not a fan of sealers but others here have used them with success.
Thank you for the advice Eric. I tried the mirror route, but my mirror is 2" and 1-3/4" would be a friction fit through the hole in the tank. I'm going to try to get a smaller mirror when I'm in town today.
There are no radiator shops nearby. My Dad uses a shop closer to him, and he really likes their work, but in each case he's had to drop his item off, wait a few weeks, and then go back and get it. It's an option, but not a close option for me.
New tanks are relatively cheap -- under 150.00 mail order if it will set your mind at ease. I dont think you have a tank problem. While its off i would put in a new sending unit. Your problem in the first place may have been a "collapsing" rubber hose? I would replace all of the rubber from fuel pump to tank. Those two tank hoses in your pic dont look like they need replaced. If you get a new tank -- clear-coat it before installing to keep it from rusting.
Here's an update from where I left off. I was able to get a 1-1/4" mirror on clearance at the hardware store for $0.50. Heckuva deal. I couldn't see everything inside the tank, but everything I could see was clean and rust free. I wasn't comfortable intentionally putting water in the tank, so the boys and I did our best to flush it really good with a lot of gas.
I installed the new sender and reinstalled the tank. I fired it up, but after a couple minutes it wanted to stall when I would let it idle, although now I am always able to keep the car running with the throttle. Before I replaced the sender I couldn't keep the car running with the throttle.
I also installed a fuel pressure gauge when I put everything back together in case it might help with more troubleshooting.
Right away this morning, for fun, I disconnected the fuel line from the pump and then I hooked up a 5' length of hose from the pump to a portable gas can next to the car. The car fired right up and idled at 800-900 rpms, which is where it wants to idle in park when it is going to run correctly. The fuel pressure gauge was slightly swinging between 5 and 6 psi. I let it run for about 5 minutes and it sounded great so I turned it off.
I then decided to try some more testing to see if I could isolate what wasn't working. I disconnected the hose from the pump to the portable gas can and then I ran a 16' hose from the pump back to the sender with the hose just resting on the floor. The car started, but it was idling at about 600 rpms, and I knew it would want to stall after too long. The fuel pressure gauge started off about 5-6 psi but after awhile the rpms started to drop (about 400 rpm) and then the gauge swung wildly between 3-8 psi. I could keep the car running by adjusting the throttle so it would maintain about 1,000 rpms with about 5-6 psi of pressure but as soon as I let it idle it would go back to 400 rpm and the fuel pressure gauge would swing wildly.
I disconnected that and then ran the 16' fuel hose into the portable gas can at the back of the car. It pretty much did the same thing.
I disconnected that and then hooked up the 5' fuel hose back to the portable gas can by the car hoping the result from the morning would return, but it didn't. The car would run with some throttle, but would want to stall if left to go back to idle.
I then reconnected the fuel system from the tank to the pump using the lines in the car. It did the same thing. I could keep the car running by using the throttle. 1,000 rpm was 5-6 psi and 1,500 rpm was 6-7 psi, but as soon as I let it idle the rpms would drop to about 400 and the fuel psi would swing wildly between 3 and 8 psi.
I let it sit for a couple hours and then tried again with my son. The car fired up at 600 rpm idle. Smooth, but not high enough where it likes to run when it's going to work correctly. It had about 5-6 psi at 600 rpms for about a minute. The rpms started to slowly drop and the psi went to a wider 4-7 psi swing. We put it at 1,000 rpms but it swung even more wildly between 3 and 8 psi. 1,500 rpms was even worse. We let it idle and it was pretty wild between 2 and 5 psi. We did one more run to 1,500 rpms but it was wildly swinging between 1 and 5 psi.
Did you replace the small filter at the carb inlet? Didnt hear you say that?
other things to try?
1. can you borrow another carb to try to rule out csrb trouble?
2. add an electric fuel pump and regulator to the system (still using the manual pump). The elec pump just pushes fuel straight through the manual pump.
Try running the temp fuel line along the chassis next to the original fuel line from the tank to the pump. The mechanical fuel pump does not have enough vacuum to maintain proper head pressure. In normal operating conditions the fuel gravity feeds to the pump. You may have a weak fuel pump.
Did you replace the small filter at the carb inlet? Didnt hear you say that?
The car has a Demon carb. I replaced the fuel filter (Wix 33033 20 micron) that sits between the pump and the carb, so that is brand new.
Originally Posted by Chesrown 67 OAI
1. can you borrow another carb to try to rule out carb trouble?
This is a good idea, but it might take some doing, so this might need to wait a bit to try.
Originally Posted by Chesrown 67 OAI
2. add an electric fuel pump and regulator to the system (still using the manual pump). The elec pump just pushes fuel straight through the manual pump.
I know Amazon has some inexpensive options I could try to test this idea. I appreciate all the ideas.
Try running the temp fuel line along the chassis next to the original fuel line from the tank to the pump. The mechanical fuel pump does not have enough vacuum to maintain proper head pressure. In normal operating conditions the fuel gravity feeds to the pump. You may have a weak fuel pump.
Thanks again Eric. I'm up for trying anything, but I had the rear of the car up in the air all day except for my last test. When I was done testing the 16' hose connected from the sender to the pump I think I was going to do the 5' hose test again and went to disconnect the 16' hose from the pump and once I got the hose off the fuel started flowing through the hose onto the floor. I reconnected the hose at the pump and then decided to instead do a test using the 16' hose from the pump to the portable gas tank sitting on the floor at the back of the car. Your idea, however, is definitely worth a try.
Chesrown 67 OAI suggested trying an electric pump, and I might just do that to rule out the pump.
At this point, I would say that you've proven that the problem is unrelated to your fuel pump, fuel line, or fuel tank.
Personally, I've driven across the country and back with ⅜" rubber hose from the tank to the pump, with no problem, so the hose is not doing this.
So long as your engine is turning RPMs that are within spec., your fuel pump is delivering the specified pressure. The only thing you haven't done is to confirm that the fuel pump delivers the specified volume, but considering that you do get the right pressure, there's really no need to.
You need to look to other causes of your problem, that is, carburetor or electrical.
The first obvious thing to do would be to swap that aftermarket carb for a proper QuadraJet, and see whether the problem persists.
What kind of ignition do you have? Points? HEI? Electronic breakerless conversion? MSD?
The only thing you haven't done is to confirm that the fuel pump delivers the specified volume, but considering that you do get the right pressure, there's really no need to.
You need to look to other causes of your problem, that is, carburetor or electrical.
The first obvious thing to do would be to swap that aftermarket carb for a proper QuadraJet, and see whether the problem persists.
What kind of ignition do you have? Points? HEI? Electronic breakerless conversion? MSD?
- Eric
Thank you Eric. The car has a HEI.
Something carb or electrical could be the underlying issue, but I think I should look at the fuel pump a bit more. The car will run smoothly and has never been in danger of stalling while the fuel pressure gauge swings 5 psi and higher, but when the rpms go down and the psi goes down below 5 psi (low part of the swing) it wants to stall. If the car wanted to stall with 5 psi or more then I would rule out the pump.
The fuel pump pressure goes down when the RPMs go down.
You don't need a specific fuel pressure to keep the engine running, all you need is a full float bowl, which you will have at idle with even a low pump pressure.
You are barking up a convenient tree, but it is the wrong one.
When you have convinced yourself that the fuel pump is not the problem, you can move on.
Try this: Suspend a half gallon of gas from the hood, with a piece of hose from the bottom of the bottle to the carburetor, no pump involved, and see what happens.
Or, just try it as a thought experiment.
Try this: Suspend a half gallon of gas from the hood, with a piece of hose from the bottom of the bottle to the carburetor, no pump involved, and see what happens.
Or, just try it as a thought experiment.
- Eric
I will try to get a small gas tank with an outlet at the bottom (I'm thinking a push mower tank) and try your experiment, but it might take a few days to get that together.
As a thought experiment I think it will go like this: The bowl always has some gas in it when the engine is stopped. The car nearly always fires and starts in less than one second of turning the key, so I know it almost always has some fuel in the bowl, unless I let it run until it dies, then I will need to crank it for 10-15 seconds to get some fuel in the bowl. As long as I don't let it die it will fire and run almost instantly. It will run smoothly for a bit, until I'm assuming the bowl is low, and then the rpms will drop.
The manual I was able to find online for the carb says it wants 6 to 7-1/2 psi of fuel pressure. The book I have indicates the Demon carbs were designed for 6 to 8 psi of fuel pressure. The fuel pressure is required to open the needle valve.
Gas isn't water, but I don't know the calculations for gas, so I will need to substitute water. If I hung gas from the hood it would be about 40" above the carb. This would create about 1.45 psi in the line. That pressure would be insufficient to open the needle valve, so the bowl would not refill and the engine that was started and running on the fuel in the bowl would stall.
I am not familiar with the Demon carb, but carburetors I have been familiar with (including Strombergs, 2-Jets, QuadraJets, Bings, and Webers) have not required any pressure to open the float needle - the float drops as the fuel level drops, and, one way or another, lifts the needle off of the seat. Perhaps the Demon is different.
I am not familiar with the Demon carb, but carburetors I have been familiar with (including Strombergs, 2-Jets, QuadraJets, Bings, and Webers) have not required any pressure to open the float needle - the float drops as the fuel level drops, and, one way or another, lifts the needle off of the seat. Perhaps the Demon is different.
- Eric
Beat me to it. The problem is usually that fuel pressure is too high and forces the needle open, flooding the carb. Low pressure is only a problem when you need to rapidly fill the float bowl (such as during wide open throttle operations).
The problem is usually that fuel pressure is too high and forces the needle open, flooding the carb. Low pressure is only a problem when you need to rapidly fill the float bowl (such as during wide open throttle operations).
Exactly. To recapitulate my earlier posts: "running out of fuel" at idle, with pretty much any measurable fuel pressure, means either he is not running out of fuel at all, or that he has an internal carburetor problem or blockage.
I'm in agreement with the two of you. If he can keep the engine running by giving a bit of throttle so the RPM stays up, it obviously isn't a problem with too little fuel, which means the following assumption is incorrect.
Originally Posted by Happy Trails
It will run smoothly for a bit, until I'm assuming the bowl is low, and then the rpms will drop.
I am not familiar with the Demon carb, but carburetors I have been familiar with (including Strombergs, 2-Jets, QuadraJets, Bings, and Webers) have not required any pressure to open the float needle - the float drops as the fuel level drops, and, one way or another, lifts the needle off of the seat. Perhaps the Demon is different.
- Eric
I couldn't scrounge up a gas tank, and I was getting antsy to try your suggestion, so I drilled a 1/2" hole in a one gallon water jug and pushed a 5' 3/8 fuel hose into it to use it as a temporary tank. I was a bit surprised it didn't leak and actually worked pretty well.
Anyway, I hung the temporary tank about 5' up in the air in front of the car and connected it to the carb hard lines that feed the two bowls, bypassing everything else.
The Demon carbs have sight windows in each of the fuel bowls and next to each window are three line markings: center, above, and below. Before I started I noted that the primary bowl was showing no gas at all, even when I rocked the car side to side. The secondary bowl was right at the center marking in the sight window.
I then poured some gas into my temporary tank. After a couple minutes the secondary bowl fuel level came up a little. I still couldn't see anything in the primary bowl. After another couple minutes (four total) the secondary was a little higher still but not yet to the top line. Gas finally showed up at the very bottom of the primary bowl sight window. After a couple more minutes (six total) the secondary bowl was full (past the top of the sight window) and started coming out the screw at the top that is used to adjust the float level. The primary was also rising and after 8 minutes total it was to the top of the sight window past the marks and then leaking from somewhere in the middle or bottom area that wasn't immediately obvious, but it didn't rise anymore and didn't leak through the screw in the top.
I was able to siphon out the gas so now each bowl has gas to the top line in the sight window. I wasn't able to siphon any more gas out.
At this point, before I do anything else, I should clean the carb, use a rebuild kit, and adjust the floats to spec. if necessary. Thank you for the advice and for the patience.
Sounds like you're narrowing the problem down to a float, needle/seat, or fuel inlet problem.
You'll have it fixed soon.
- Eric
Well, I think I'm back in business. It took awhile to get the carb rebuild kit, but I did eventually receive it last week. I did the rebuild yesterday.
At some point a previous owner put a large 5"x13" K&N air filter on the car. I don't know much about the K&N filters, and I replaced it with a paper filter, but perhaps they over oiled it because the carb was covered in oil. Pretty much every surface exposed to air was covered. The inside wasn't too bad, but the fuel needles in the bowls were bad (very dirty). The primary needle was black and covered in black fibrous material. The rebuild kit came with new needles so that was nice. I had to do quite a bit of scrubbing to get everything clean because the carb cleaner wouldn't just spray off the oil.
I don't think the floats were set correctly either. I had to use dental pics to remove the needle assemblies because they were screwed in past the nut that is also used to remove the assembly. I reinstalled and adjusted per factory specs.
Today I reattached the carb and hooked up the temporary tank again. I put gas in the tank and within a couple seconds the sight windows showed the bowls were full to the line and holding steady. Looks like the needles and floats are working correctly now and the level in the bowls doesn't appear to go down.
I reattached the fuel line and fired up the car. Before I rebuilt the carb the fast idle never worked or it was disabled. I adjusted the choke when I rebuilt the carb. The car fired right away and this time the fast idle worked, but it was pretty high at 1200 rpm. I let it run for a bit and then blipped the throttle and it dropped down to 500 rpm, which I think is a bit too low for park. I adjusted the idle mixture screws and now it idles in park at about 750 rpm. It's in the low 30s here, so it's too cold to drive right now, but I did put it in drive and it idled at about 500 rpms or so.
It might still need a few more small adjustments, but it appears to be running quite well now. Hopefully we get some warm weather soon so I can get it on the road (up to 8" of snow in the forecast for this week, so it might be a few weeks). Thank you again for all the advice.
Maybe you said it wrong, but the idle mixture screws are to get the idle mixture at the correct lean/rich mixture, not to increase or decrease idle speed. There should be an idle stop screw that adjusts idle speed independent of the mixture screws. I have always adjusted the mixture screws to get the highest smooth idle, then back off the idle stop screw to get the proper RPM.
Maybe you said it wrong, but the idle mixture screws are to get the idle mixture at the correct lean/rich mixture, not to increase or decrease idle speed. There should be an idle stop screw that adjusts idle speed independent of the mixture screws. I have always adjusted the mixture screws to get the highest smooth idle, then back off the idle stop screw to get the proper RPM.
I was trying to do it by ear and rpm, but I have subsequently changed my mind and decided to break down and buy a vacuum gauge and do it the right way as far as setting the idle mixture screws. I didn't want to adjust the idle stop (the butterfly positions are correct based on factory specs) until I knew the idle mixture screws were dialed in and I've come to the conclusion I can't know for sure they are right without the vacuum gauge.
What has been bothering me a bit was before I rebuilt the carb the screws were all turned out about 1/2 turn. The factory specs say 1 to 2-1/2 turns out should be best, but some cars can work well with less. By ear 1/4 to 1/2 turns out sounds best, but that might not be the right setting. Thank you for the feedback.
Last edited by Happy Trails; April 25th, 2017 at 01:13 PM.
As someone stated previously, I am not familiar with Demon either. A Quadrajet preliminary idle screw adjustment (varies from car brand to car brand) but is generally 4 to 6 turns out from the needles lightly seated. Then adjusted for best high idle speed, or highest vacuum reading, then adjust idle stop screw to set the idle. The idle mixture screws are needles on a Q-Jet that leans or richens the mixture. The idle stop screw in effect, opens or closes the throttle to get to the right idle RPM. I don't know, but would think the Demon would be similar.
As someone stated previously, I am not familiar with Demon either. A Quadrajet preliminary idle screw adjustment (varies from car brand to car brand) but is generally 4 to 6 turns out from the needles lightly seated. Then adjusted for best high idle speed, or highest vacuum reading, then adjust idle stop screw to set the idle. The idle mixture screws are needles on a Q-Jet that leans or richens the mixture. The idle stop screw in effect, opens or closes the throttle to get to the right idle RPM. I don't know, but would think the Demon would be similar.
Demon works pretty much the same way with four mixture screws to adjust. When I get the vacuum gauge I will adjust the mixture screws for the best vacuum and then adjust the idle stop if necessary. Thank you again.
So, I hooked up the vacuum gauge today and fired up the car. I walked around to the engine and noticed the fuel pump was leaking gas on the floor. I stopped the engine.
I checked the hoses and retightened everything down at the pump, wiped the pump clean, and fired the car up again. Pretty much immediately gas starts running down the pump and drips on the floor. The leak is at the seam where the top section meets the bottom section.
So, could I have done something wrong to cause this? I'm assuming these pumps can't be fixed, correct? It looks like they are $20-30 online, so I'm assuming I just need to replace this if it has failed.
The diaphragm probably sprung a leak. Inspect your oil for fuel contamination also.
Thanks Eric. The oil on the dipstick does smell a bit like gas, and the oil level is now reading about a full quart above the full mark. I'll change the oil while I'm at it.
I changed the fuel pump. The new pump swings steady between 4.5 and 5.0 psi at idle. The wildly swinging pressure readings before had me suspicious something was wrong with the pump. It will probably be helpful to know what normal is sometime down the road.
I changed the oil and now the oil pressure is staying quite a bit higher even when the oil is hot. Before it would get down to about 10 psi at idle, but now it never gets below 20 psi. Could just be the new oil, but having some gas dilute the oil probably didn't help.
I used a vacuum gauge to set the idle mixture screws. The best I could get at idle was 12" of Hg. Everything on the carb seems to be dialed in now.
I took the car for a 20 mile drive and it ran great for the trip. I think I have my fuel issues beat, for now. Thank you for all the help and advice.
April 6th, 2017, 06:56 PM
