Street Rat

I'll get the Alki needle and seats. Thanks for the advice.

I'm still waiting on some parts to hook up the carb return line to the 3/8" steel return line. I used some aluminum hardline adapters. I found out they don't work very well on steel line. So I've got a brass hardline adapter coming tomorrow.

flyeri

Finally got mine finished and drove it Saturday. The orifice is .0625", and the pressure bounces a couple psi at idle. The flow back to the tank at 1500-2000rpm is a weak stream with little to no aeration. Pressure at 1500 rpm is solid 5.5psi.

When I got home at the end of the day the pressure gauge was a solid 0. When I took the gas cap off there was a very slight positive pressure. Now I have not had that before that I recall. I filled the tank in the morning and drove 50 miles or so to a car show and let it sit in a sunny parking lot for about 5 hours. Temp was 90-92 degrees. On the drive home my wife said her car showed 98 degrees. The tank temp was 102 degrees as was the frame of the car.

So my question is this. Was the positive pressure caused by the warmer fuel returning to the tank or was it from the high outside temp. If the tank was hot and the frame not, I would think it was from the fuel return. But since the frame and tank was hot I tend to believe it was the outside temp. Your thoughts are welcome.

Street Rat

What are you using to vent the tank? Do you have a vented Cap? A dedicated vent valve or line?

gkull

Vettes like my 79 were built with a tank vent return charcoal canister back to the air cleaner and kind of the vacuum system the cap was sealed. Kind of the go green idea

The tank has vent lines

I'm not sure about where my stock return goes into the tank. I would hope near the bottom.

But a hot day would vaporize gas in the tank

flyeri

1968 and it has a vented cap only. Allows air in, not out. It works properly.

I did put in a smaller jet, .050", and I will drive it again on a typical drive for an hour and see if it still makes pressure. I know it does not if the fuel is at ambient temp when I start out. I started out Saturday adding 12 gallons of cool underground gas that increased by 40 degrees or so in a matter of a couple hours. Also have insulation ordered to put around return line as it runs back behind the right head and down the firewall a couple inches behind the headers. It has to be picking up some heat there too.

I could definitely tell a difference having the return with the smoothness and consistency of the idle in stop and go traffic with the engine hot.

Clarification .050 jet: I made jets to fit in a -6 to -4 reducer out of 10-24 brass screws so I could easily try different size orifices.

cardo0

There is no set number/size for that but a small 0.02" hole is a good place to start. If find you need larger you can always drill it larger but it's much more difficult to make the hole smaller again.

cardo0

FYI a hole is not an orifice, it's just a restriction. An orifice has engineered dimensions with bevel angle to create a specific differential pressure. But all we need is a restriction to keep back pressure in the system.

It ain't rocket science.

flyeri

It's just a word. Actually a hole is not a restriction either. All the material surrounding the hole would be the restriction, The hole would be the lack of restriction.

But I will try the .050 and if that does not work I will make a .020 and work my way up if necessary.

cardo0

Well then u cant say a orifice is a restriction either but it is. Any material form that reduces the diameter of the tube/pipe is a restriction.

The concept to remember is that an orifice is an engineered device for flow measurement and not normally used for a restriction creating backpressure. U will find plenty of U-tube homemade tools calling holes orifices and expecting accurate results. One example is a homemade leakdown tester using a hole drilled into a blob of JB weld as and orifice to mimic a purpose built leakdown tester with an engineer orifice designed for the correct size of cyl vol measured.

Dont make it rocket science just call it what it is.

Street Rat

This discussion reminds me of my college days in Instrumentation Technology class.

Street Rat

This morning I finally got to install the fuel return bypass line. I used a #6 line off the back of the fuel log. It's routed to the original 3/8" steel fuel supply line from the tank. I had to get a special hardline adapter to adapt the #6 line to the 3/8" steel line. I was trying aluminum adapters for the hardline. I found out they DON'T work on steel lines. I used a brass adapter with a nitrile rubber ferrule inside. I drilled and tapped a hole in the fitting for a 1/8" brass pipe plug. I drilled a .060 hole in a 1/8" brass pipe plug and installed it in the hardline adapter. This gives me the option to change the restriction size if need be.

I adjusted the fuel pressure to 6 psi with the engine idling. So far I haven't got to drive it or take photos. One heck of a rainstorm came up as I was finishing up. More to come...

Street Rat

Took her out for some hard acceleration runs this afternoon. The fuel pressure never dropped below 5 psi. I am still going to try gkull's recommendation to change to the larger needle and seats in the float bowls.

Getting happier .

cardo0

Congrats and thx for the feedback.

Street Rat

Thanks cardo0.

I'm going to do my best to finish out the story on the threads I start. I get the best answers from you guys. Others might need some of this info as well.

Still going to tinker with the needle and seats and take some photos. But it won't stop raining here. My car is an outsider with a cover on her.

Still not as bad as the poor folks getting flooded in Louisiana. Praying for all effected over there.

Street Rat

The rain finally stopped so I got a chance to work on the car some.

I fabricated a return line with a .060 restriction after getting the correct fitting to hook up to my 3/8" steel line. Previously the 3/8" fuel supply line.

I installed the needle and seats that were recommended by gkull and set the float levels.

I set the initial fuel pressure at 6 psi. It never dipped below 5 psi when driving it hard.

Here are some photos of the return line installation.