JamesHigdon Posted October 6, 2011 Report Share Posted October 6, 2011 I’ve built probably half a dozen Quadrajets and have never had any serious issues until we started running a known good carb. on a ’68 Vette. The carb. (an early 80’s non-electic APT setup) has run great on two other mild small blocks and starts off good on this Vette. The issue is when the car gets hot I lose idle quality and off-idle response, my guess at this point is the car is getting hot enough under the hood to cause the fuel to boil in the bowl. I know C3 Vettes run hot under the hood and I know GM has had issues with quadrajets over-heating and causing idle issues (anyone ever seen the carb. cooling fans issued under a TSB for 3rd Gen Camaros!). I put a box stock Edelbrock on the car and a lot of the issues we’d been having with it went away but with a caveat; when we went with the Edelbrock we plumbed in a 3-port GM fuel filter with a fuel return line(we didn’t run an external filter on the Quadrajet). Many early C3 Vettes where provisioned from the factory for or equipped with a three port filter and a return line with most Vette guys explaining their purpose as being to help reduce vapor lock. My question is does anyone have experience with vapor locking Quadrajets or the benefits/draw backs of running a fuel return line on a GM carbureted vehicle? My assumption is that whatever fuel dead-heads against the needle and seat is bled off but otherwise nothing changes, why would this help vapor lock issues? Link to comment Share on other sites More sharing options...
jwmbishop Posted October 6, 2011 Report Share Posted October 6, 2011 It's not really vapor locking - it's boiling - but along the same cause - just different solution. Vapor locking is when the vapor barrier prevents the liquid fuel from entering the low pressure area of a pump. Quads are funny in that it is not just underhood temp - but intake plenum temp with the underhood that can cause boiling and vapor locking. Using the thinnest phenolic spacer you can find helps - but alas duntov's cronies did not give us much room there in the vette. Prop the hood open an inch and drive the gremlins out of it. if the problem persists you will know to look elsewhere. you are correct it dead heads - but then sits there over time absorbing more heat. By returning you are reducing the time for heat transfer AND cooling the inner bowl more by introducing cooler fuel. The reason boiling effects idle and off idle first is the incoming air cools the fuel being introduced so it denses back up. Remember fuel is delivered by mass volume but works from weight - and mass volume changes with temp. When you add the return via the three port filter you can use bypass regulator (note: there are two kinds of regulators - bypass and pressure - Holley 12-803BP is a bypass) mounted well back on the return line to hold the pressure to the VERY needed 7psi. If running a mechanical pump it should be high volume when going to a return system. One thing that may help - a working q jet is not as simple as working holley. qjets are demand feeders and holleys are force feeders. verify you have a good vaccuum curve, no bouncy needle on vacc gauge etc. right centrifugal and vacc adv curve all that good stuff. especially if stock engine - even though the qjet should go from engine to engine and work - because it is demand driven if there is ANY thing wrong with the signals it sees in the vacc - it will do all kinds of unpredictable stuff. Link to comment Share on other sites More sharing options...
PAPA Posted October 7, 2011 Report Share Posted October 7, 2011 James Does the car still have the Air-Dam under the Rad? You have no idea how important that little bugger is as far as keeping the engine bay cool as well as the Rad. If its not there then go over to the salvage yard on Old Grissom (He has tons and tons of Vette stuff) and get one. You might also try blocking of the exhaust crossover at the intake. As far as carb insulators go.....Edelbrock # 9266 for the Edelbrock...Gm # 3969835 for the Holley and #3969837 for the Q-Jet. Link to comment Share on other sites More sharing options...
HiTech Posted October 7, 2011 Report Share Posted October 7, 2011 had the same problem on my 327 in my olds starfire ....really tight in there with no cooling room ..i ran duct from the wheel well ....aimed it towards the carb area ..then rapped my fuel pump and fuel lines going to the carb .with water heater wrap from sutherlands ...... ...that problem drove me crazy for some time before i did that .....solved it . Link to comment Share on other sites More sharing options...
Hoop98 Posted January 16, 2012 Report Share Posted January 16, 2012 had the same problem on my 327 in my olds starfire ....really tight in there with no cooling room ..i ran duct from the wheel well ....aimed it towards the carb area ..then rapped my fuel pump and fuel lines going to the carb .with water heater wrap from sutherlands ...... ...that problem drove me crazy for some time before i did that .....solved it . Always get a pressure/vacuum gauge and hook it up on the suction side. If you have any sustained vacuum while running you have a restriction between the pickup and pump. This lowers the boiling point of the fuel and cause vapor lock. Technically vapor lock is a suction side issue as the pump can't pump vapor. Boiling the fuel in the bowl or line is a problem, just not vapor lock. Link to comment Share on other sites More sharing options...
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