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Holley 600 getting poor mileage

roadmonster

roadmonster

Member
I've had the same carb for 8 years always getting 15 mpg. Slow start with puff of smoke got me looking at it and the secondaries were wet with leaking gas. Quick rebuild, everything is great now but 12-13 mpg. Carb and gaskets are perfect, float levels, idle, everything ok, but mileage sucks. No smoke, plugs are great, excellent performance but rotten mileage.

Car has been well sorted out for several years, that is, I'm not messing with it and throwing a lot of variables in.

Details: 1850-4 (4160 type) 600 cfm. K&N filter, stock main jets and secondary plate, #31 shooter, yellow secondary spring, orange pump cam in #1 hole. Accelerator pump diaphragm and secondary diaphragm are new. Starts, idles, runs great.

Weiand manifold, TH350, 2100 stall converter, 3.00 Ford axle, Comp 252H cam, quick advance (all in 1600). This combination has worked GREAT for several years but the mileage is suddenly awful.

What am I missing?
 
Pull the air cleaner off, start it up, and then look down the venturis. Are you seeing any liquid fuel above the throttle blades, at idle? You shouldn't.

Let it come up to normal operating temperature, shut it off, and then look down the venturis. See any liquid fuel? You shouldn't.

In short, if you did not change any of the variables that are actually metering the fuel, then it has to be leaking it out onto the ground.
 
Is there a little bit of fuel coming from the accelerator nozzles? Could be the pump lever needs to be readjusted. Isn't there a ball that acts as a stop for fuel nozzles that relies on a spring to block off fuel ? Right under the nozzle body it self ? Have you replaced the nozzle body gasket ?

It is summer. Been driving the pee out of it ?

John

Opps, sorry, secondary. Check fuel level in bowel.

You probably need a tunnel ram injection. See my latest tunnel ram article.

Or not. It's not all that great.
 
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No leaks anywhere. Float bowls adjusted dry and checked wet. No drop in performance, just mileage. And don't get me started on tunnel rams. A sure sign the owner has no clue.
 
I have the same carb on my Ford 302, also leaked fuel, but only after the car was run for a while. I found out the heat from the manifold was causing the problem, place a fiber spacer between the carb and the manifold - this solved MY problem with the fuel leaks and maybe the mileage (didn't have the car that long until I found the leak).
 
Fuel pressure [while running down the road] , power valve , timing [vac advance], metering block gaskets , metering plate gasket, that's all that comes to mind...
dave
 
More info, if it helps. Original leakage was secondary needle and seat. That's cured with new n/s, power valve, gaskets, o-rings, diaphragms are all new. No leakage in secondary bores or anywhere. Timing and vac advance are spot on, and I did a lifter adjustment just to be sure. No doubt the engine runs great, just gets lousy mileage.

I rebuilt it once before about 6 years ago, so it should be like new with a fresh kit. It does not puff smoke when starting as it did with leaky carb, but the 15+ mpg never came back.
 
Also remember that roadmonster alluded to a rebuild in the near past. What the rebuild consisted of and the exact date and mileage were never revealed.

John
 
Sorry Roadmonster, we must have responded within a minute of each other. Have you done a compression test and a cylinder leak-down test yet ?

John

P.S. Different tires on the back ?

P.P.S. Trans showing a little, teeny weeny bit o wear ?
 
Same tires, tranny ok, no reason to suspect compression loss (which shouldn't affect mileage anyway). I changed heads 2 years ago to camel humps and lowered the compression with laminated gaskets, so it breathes better, also very slight loose converter (~2100) but these changes did not affect the mileage.

I think I will adjust the secondary float lower so the needle and seat will remain closed longer. It's the only thing I can think of now. It did sputter in a quick turning take-off last weekend, and died once after a hard stop. So it's quite possibly float-related in spite of by-the-book adjustment.
 
The floats can be adjusted wet until gas comes up to the plug. I do it dry with the float bowl off (more accurate), then check it wet on the car.

Plugs were not too fouled, but I went to longer R45Ss when I rebuilt the carb. R45Ts are just slightly shorter.

The fuel pump has never been a problem and shows no leakage whatsoever. This is a mild 300-320hp torquer that should not use a lot of gas. Internal leak would show up immediately as hard starting and missing. Engine runs fine.

Now that I think of it, I did go to the second yellow secondary spring a while back. I'd been using the super-light yellow which opens the secondaries quite early. I thought holding off the secondary opening might be good, but maybe not. Maybe it requires more throttle opening, thus reduced mileage.
 
Not that it will correct the mileage, but Holley states specifically, gas level should be below the sight hole. Fuel pump leak, if mechanical, could go into the pan, not sure on chebby, I am all blue oval. What rpm do you cruise at? Maybe your speedo is incorrect and the math is not accurate. It is very possible you do not have any of the internal carb leaks Mike mentioned, but you really need to know where and how to look for them. Just because you do not see a stream of gas does not give your carb a clean bill of health. Did you use the power valve included in the kit. What is your manifold vacuum at idle with the T stopped in drive?
 
There is one more variable that I don't think anyone has mentioned -- your gas. Some areas of the country will change the blend of gas for summer vs winter use. Also...has the % of ethanol changed in the gas your buying, or did you change brands? My daily driver will run on E-85....and I've tried E-0, E-5, E-10 and E-85 to test fuel mileage. There is a difference, the higher the ethanol %, the lower the fuel mileage. If nothing has changed with your car, carburetor, etc., it my just be your local dealer has increased the ethanol % in the gas your buying.
 
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