C3 (1968) Fuel Vapor Return Line...Why?

[Henry J.]

As I understand it, all 1968 Corvettes equipped with Rochester Q-Jet carburetors (regardless of engine displacement) were fitted with a fuel filter, external to the carburetor, AND a fuel vapor return line running from the external fuel filter back to the fuel tank. However, all 1968 Corvettes equipped with Holley carburetors had neither the external fuel filter nor the fuel vapor return line. I need to understand what it is about the Rochester carburetor that necessitated this additional hardware.

The reason for my question is this: My 68 was originally (Holley) tri-powered. Therefore, it has no fuel vapor return line. Currently the car is equipped with an aftermarket Holley 4160 sitting on a single-plane Holley intake manifold. The car has an afermarket L88 hood for reasons of clearance. Although it's perfect, I hate the hood and plan to replace it with an original. After a long search, I acquired an excellent 427/390 HP intake manifold and an excellent Rochester carburetor (both correctly numbered and dated) that I want to put on the car. I want to determine NOW what operational problems I might experience if I attempt to run this set-up with the proper external fuel filter but with NO fuel vapor return line. By the way, for what it's worth, the car is a nice driver that is used only on summer days.

Thank you for taking the time to share your knowledge despite the fact that most of you think I'm crazy...I can't help it...I love the car!

[Patrick H.]

Re: C3 (1968) Fuel Vapor Return Line...Why?

Henry,

While I can't answer your question specifically (other than to say that as long as you think 60's era emissions is only a state of mind you'll be fine and it should work without a hitch), I'd just like to say that you're not crazy. I hope you're still looking for that tri-power setup, though.

Patrick

[Joe L.]

Re: C3 (1968) Fuel Vapor Return Line...Why?

Henry------

I don't really know why the external fuel filter/fuel return line was used on Rochester-equipped 68-69 Corvettes and not on Holley-equipped 68-69 Corvettes. I THINK that the purpose for the fuel return line (it was actually a fuel return line and not a vapor return line) had something to do with fuel vapor lock prevention. Beginning in 1970, most Corvettes and other Chevrolet cars got the fuel return line originating from the long cannister style fuel pump. The exception was 70-72 with LT-1. The 71 LS-6, which was Holley-equipped, did have the fuel return line from the long cannister style fuel pump, though.

I really do not think that the external fuel filter and/or fuel return line is fundamental to the Rochester installations. Rochester Q-Jets were used on other GM applications as early as 1967, or so, and I don't think that most of these used the external fuel filter or fuel return line. Of course, there may have been differences with those carbs to accomodate, but I really think that you can use a Q-Jet on your car without the need for the external fuel filter or fuel return line. I would definitely recommend eliminating the "carb heat groove" (but NOT the heat riser crossover passage) from your 68 L-36 manifold.

A problem that you will have in making this installation is that there is no pre-made and/or reproduction fuel line from fuel pump to carb that will exactly fit your installation. So, you will have to fabricate one. Or, you could use the 68-69 L-36 fuel filter and fuel pump to carburetor line with the vapor return fitting from the filter sealed off.

I applaud your decision to remove the tri-power set-up from your car. I really don't think that the "performance advantage" of the 3 X 2 system ever really justified the cost, complexity, or maintenence problems associated with this set-up. I believe that it was more of a marketing and sales "gimmick" than anything else. You will note that the really serious performance big blocks (i.e L-88 and ZL-1) did not use the 3 X 2 set-up and, instead, used a single 4 barrel carburetor. Hardcore racers and folks "in the know" would not be impressed with the 3 X 2 set-up. I'll bet that Zora knew that, too.

[Henry J.]

Re: C3 (1968) Fuel Vapor Return Line...Why?

Patrick and Joe, thank you very much for your comments.

Although I love the appearance of the tri-power set-up and was seriously tempted, I was advised by others that the tri-power set-up would be too finicky for my liking. That's why I chose the L36 arrangement for my restoration.

From your previous posts Joe, I already decided that I would block off the "carburetor heat groove" in accordance with your suggested methodology. I also thought it sensible to install a good reproduction fuel filter (complete with proper fuel lines) and simply cap the nipple on top where the fuel return line is supposed to go...just as you suggested now. Heck, maybe I'm not so crazy after all!

[Clem Z.]

Re: C3 (1968) Fuel Vapor Return Line...Why?

the Q jets had the return line to relieve the pressure on the needle and seat. since the Q jet had only one N/S it required a larger diameter hole in the seat,compared to a holley which has 2,and is toughter to seal under pressure. to prevent the excess pressure between the pump and the carb which caused fuel spill over and flooding the engine, the return line was added. since the fuel tank in a corvette is as high as the carb,if the N/S leaked thru it could fill the engine with fuel. if you do not have the return line you must use a fuel pump that is made for no return line as the other type will have too much fuel pressure because the return line is a calibrated leak that the extra fuel pressure made up for. a trick we used with the take apart fuel pumps was to drill a very small hole (.015) between the suction side and the pressure side to relieve the pressure when you shut of the engine. give it a try but if you have carb flooding with the engine shut off you will have to come up with a way to fix it. there was a item that was a auto parts piece that connected the input and output side of the fuel pump fittings together with a small bleed hole but i have not seen them for years.

[Clem Z.]

PS

the leak thru problem was greater after GM went to one way valves in the gas cap that only let air in but did not let the built up pressure out. if you have one of these type caps i would get a std type. i think the heat from the mufflers also add to the pressure in the tank by heating up the fuel.

[Joe L.]

Re: C3 (1968) Fuel Vapor Return Line...Why?

clem----

Your description of the reason now "rings a bell" for me.

[Henry J.]

Re: C3 (1968) Fuel Vapor Return Line...Why?

Thanks Clem. I'm going to have to think about this one for a bit before I decide how best to proceed.