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 have a 1967 Olds 442 with 400 engine. I had the original Quadrajet rebuilt locally. The guy told me that new mechanical fuel pumps put out too much pressure and cause these carbs to flood. He says they are designed for no more that 5.5 psi. I have a new Delphi mechanical fuel pump on it and the carb does flood a little bit. I tested the pressure and it is about 9 psi. The pressure immediately drops when I turn the engine off, another olds I have with a 330 and Quadrajet does not drop like this and the fuel pressure is 7.5 to 8 psi. l Previously I had an Edelbrock 1406. The instructions with the Edelbrock said to use their fuel pressure regulator to get it down to 5.5 psi. Should I put a fuel pressure regulator on my Quadrajet? I remember driving these vehicles years ago with no problem, but they did occasionally flood out causing fires etc. Too much fuel pressure in the old times?
The mechanical fuel pumps are not supposed to put out that much pressure (7 psi max for QJets), but some of them do. Years ago I replaced the fuel pump and had flooding issues (pegged my gauge at 10 psi); brought it back and got another pump that did the same thing. I put a regulator on and ran that for years. That fuel pump eventually failed; I bought a new one and it put out the correct pressure, so I removed the regulator and all has been fine for many years now.
I suggest you try a pump from a different manufacturer before installing a pressure regulator.
Interesting. There are two basic pumps that will fit just about any second generation Olds V-8 engine - with return line and without. The Carter line is M6108 (without) and M6109 (with).
AC Delco and Airtex also make Olds mechanical fuel pumps.
Last edited by Fun71; December 4th, 2018 at 06:51 PM.
The original GM part number for the fuel pump for a 1967 442 is 6416836. Aftermarket equivalents are the Carter M4516, Airtex 40523, and Bosch 68620. All of these are available on-line for about $20 - $25 each.
I believe that I recall reading in the Cliff Ruggles Quadrajet book that you should always check the inlet needle with a vacuum pump before installing it because there are many bad ones. I would say better check that first.
Another by product of high fuel pressure is a higher float level. It's easier for the float to shut off fuel at 5-6 psi than at 8-9 psi. Higher psi translates into higher float level.That's probably why Edelbrock requires the regulator. The older Carter WCFB four barrels are really specific 4.75-5.25 psi. Although most carbs can handle more pressure, the manufacturer's do have specifications on recommended fuel pressure for optimum performance and economy. Excessive fuel pressure also causes aeration (bubbles). Good luck.
Interesting. There are two basic pumps that will fit just about any second generation Olds V-8 engine - with return line and without. The Carter line is M6108 (without) and M6109 (with).
AC Delco and Airtex also make Olds mechanical fuel pumps.
AC Delco is just a name these days. You'll find an Airtex pump in the AC Delco box and pay a premium to boot. Airtex is the manufacturer.
Also, I have a digital fuel pressure gauge in my 442 and it goes as high as 10 psi occasionally only for a short period and there is never a problem with flooding. It was built by a pro builder. I say look at that inlet needle and valve.
Took me a long time to figure this out, but aftermarket mechanical fuel pumps for gm are almost always 9 psi, I found one Delphi for a short block chevy at 7 psi but all the rest are 9 psi. Optimum for the qjet is between 5.5 and 7, and 6.5 will do nicely for late 70's onward, with lower pressure required for earlier years and less hp. There are plenty of performance fuel pumps at lower psi and it will make a big difference. Don't get rid of your OEM fuel pump unless you are certain it is bad. As soon as I installed a regulator all my performance problems disappeared. Someone had installed an aftermarket fuel pump which was putting out 9 psi and I dropped it down to 6.5.
December 4th, 2018, 04:03 PM
