General Category > Quadrajet Carb Talk and Tips

Largest N/S you've used?

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77cruiser:

--- Quote from: Cliff Ruggles on September 07, 2017, 04:31:42 AM ---I made a .160" for a customer running a FAST Class car once.

I use a .145" seat at 7.5 psi my own carb, but it works equally a well with a .135" seat as long as I keep the fuel pressure at least 6 psi......Cliff

--- End quote ---

That must be close to the edge with 7.5. Does your fuel pressure drop as soon as you shut the pump off?
I had a mech. pump that put out 7.5 & would hold & it would flood a bit on hot restart.
When I set the press. to 6 psi. I watched when I shut the pump off & it dropped back pretty fast, so no flooding on hot starts.

Cliff Ruggles:
7.5 psi is fine on later q-jets.  Having to use low pressure is a myth that comes from the poor fulcrum/hinge pin location of the early units.

The fuel delivery system is also a player here.  I've seen a few "dead head" systems that would induce flooding at lower pressures.

I use a Comp 140gph pump at 14psi and a regulator with a full bypass.  I've cranked it up past 10psi just to test things out and never once the first indication of wanting to flood.

A high flow mechanical pump w/o a bypass seems to be the worst set-up in terms of flooding from what I've seen.  May have something to do with the fact that the fuel flow stops and starts with the valve action inside those pumps. 

Many years ago I tried to run a Carter high performance mechanical pump, it was around 120gph nearly as I can remember?  Anyhow, it was a complete POS and my carb absolutely hated it.  Not only did the pressure fluctuate it wanted to flood on with hard braking and sometimes with quick cornering, it wouldn't keep up with my engine at the track on hard runs.  I tossed it out and went with the Comp 140gph pump and bypass and never had the first indication in over 2 decades now that it wants to flood for any reason. 

The new system is also superior in fuel delivery and never once any hint that the bowl is going low or empty on a hard run.  The Carter HP pump wouldn't even think about keeping up with my engine, and back then it made about 100 less HP than the engine I'm using now...FWIW.....Cliff

77cruiser:
Looking at the wide band log it looks to be leaning out at the top of the gears, so maybe I'll up the pressure a bit.
BTW I ran 11.93 @ 112.7 & 4200 lbs.

Cliff Ruggles:
For sure you're getting moving pretty good to run into the 11's at 112mph.

Last time I was at the track went 7.18 @ just under 97mph.  That was nearly a tenth quicker than most of my 1/8th mile outings and after the 3rd run I figured out why.......the fan belt was missing!!!  In similar weather it typically runs 7.24-7.30 @ 95mph and some change.  That's around 11.20-11.30's in the 1/4 mile just over 120mph for those who are wondering.

Despite it being in the 90's and high humidity the engine never overheated until I pulled into the pits after the last run.  I heard it "burp" some coolant into the catch can a few seconds after I shut it down and a little steam come out from under the hood.

The engine developed a coolant leak after that so I haven't raced it since, thinking it may have pushed out a head gasket.  Turns out it was leaking from the intake water crossover instead and migrating down the passengers side head gasket and dripping between the two center bolts.

It finally blew the crossover gasket clear out couple of weeks ago and no leaks after replacing it, so we dodged a bullet there for sure......Cliff

77cruiser:
I'd say you dodged a bullet, especially on a 90 deg. day. Hmm a tenth with the belt of 11.8, won't try it on purpose.

https://www.youtube.com/watch?v=HFnRG7Rqc0E

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