Ok so in a effort to improve my gas milage I have been working on everything from ignition to rearend gears.

This week end I dropped the jet size in my Holley 600 vacuum secondary from 66 to 64. It has been running a bit black in the exhaust pipe so I thought they were to big.
Now when I start excelerating it has a bit of a surging to it nothing major no loss of power or anything but it didn't do it before.

What's you thoughts? any one ever had this problem?
I can't hardly believe tha dropping 2 points on the jets would have caused it.

jd

 

You could try a different power valve. They're rated at whatever vacuum they open at so I'd go a step larger on the # to compensate for the lighter jet. It'll open sooner.

The power valve is located in the metering block and you'll find the number stamped on it. For some reason they always end in .5 so look for 4.5, 3.5, etc.

Bigger squirter? Seems a little extreme tho.

You can also alter when the vacuum secondary kicks in with a spring change, but I've never owned one so I don't really know any particulars.

 

I will have to give that a try. I did notice today that it is at it's worst (still not to bad) when cold. After it warms up it is barely noticable. I like tinkering so I may just give the power valve a shot to.

Thanks

jd

 

careful not to go too lean, the fews cents you save(and thats about what smaller jets will save you) in gas could cause you to loose the motor from running to lean, rich is always safer.......just my 2cents

 

That is one of the things I worry about. I thought going 2 sizes at a time was safe. I think that the 64 I put just may be as low as I go. I am checking the plugs this weekend to see how they look.

jd

 

good luck JD, I like to tinker too...but ended up burning a hole in a piston one time, so now I never go leaner than the carb came with..engineers spend alot of time and $$$$ to come up with the right fuel/air mixture, and its hard to do at home, I know we all want to cut down on gas but be safe.....Jim

 

jd-you might look into readjusting the idle mixture screws, they affect the off idle and low speed mixture more than most people are aware. Also make sure that your accelerator pump arm/cam/lever adjustment is correct, ANY play in those parts can cause a slight stumble.
The power valve should not affect light acceleration, it only opens under a fairly heavy load(low vacuum) or rapid throttle opening.
I think I would start with the acc. pump linkage, the 2 size drop should not get you out of the black tailpipe range that fast.

 

Thanks Putt

I have not touched any of the adjustments after installing the jets I will try that before getting to crazy with anything else.

jd

 

I've messed up more Holley's than I care to think about, but IMO you should always change at least 2 jet sizes just to know you've actually changed something. A skinny 65 can be right next to a fat 64 making the difference virtually undetectable.

The idle circuit, while still contributing a small amount of fuel, is effectively out of the loop as soon as you get enough venturi vacuum to get on the boosters. It is important for that initial throttle response but once the butterfly clears the slot on the side of the throttle plate at about 1/8th throttle or less, it's no longer effective as a tuning tool.

In my experience, as long as you don't have intake leaks, damaged screws, maladjusted butterflies, worn throttle shafts, or any other malfunctions, they run 1 1/2 to 2 turns out.

The Power Valve lets extra fuel bypass the mains when vacuum drops below a certain level and that's the number on them. A 10.5 allows the extra fuel to flow anytime intake vacuum drops below 10.5. A 9.5, etc. Assuming a fairly stock cam you're probably running 16 or so at idle but opening the butterflies any amount reduces the vacuum until the motor catches up. FWIW, they say you should size your valve at least several inches below your idle vacuum.

For instance I have a Mustang with a mild cam that only pulls 11 inches so obviously a 10.5 wouldn't be the right size. It'd be on all the time.

Power valves are an excellent way to overcome the lean mixtures that result when pulling a load up a hill like Putt said, (low intake vacuum and the motor can't catch up because of the load), but they affect throttle response too.

What else? LOL!

Pump circuits! You can get 30cc and 50cc pumps. There's a cam on the side of the throttle shaft that comes in several different flavors. They're even color coded, but don't use 50cc cams on 30cc pumps. Some of them even have a second locating hole that let's you move the cam half a hole for fine tuning. Heck, I know a carb guy that cuts his own.

Squirters come in different sizes too but as long as you aren't stumbling when ya give it the gas I wouldn't look for any changes in the pump circuit.

Float level has an overall effect. The higher the float level the richer the mixture will be at any operating range and the first thing that needs to be checked is the fuel pressure!

Intake vacuum guages also have a pressure side that can be used for this. 4-6 psi is about right for a Holley and it should stay that way no matter what rpm the motor is turning. At higher rpm's factory fuel pumps are notorious for running all over the needle and seat and raising the fuel level. You might consider tinkering in a fuel pressure regulator just for fun. They don't cost much and then you'd know.

Holley's have the best float level adjustment setup I've ever seen. There's a locknut just above the inlet that, once loosened, allows you to raise and lower the whole needle and seat assy. A brass screw on the side of the bowl opens a hole that you use to determine the level. I like to set mine to where I can knock a splash out of the hole with a bump of my hand.

You can get clear plastic to replace the brass plug, theory being you can see the level thru it, but they get cloudy and I broke one once. Pissed me off. Ain't going there again.

And of course you should always be monitoring the plugs. I put a set of headers on a motor once and one session later found these little silver globules on the plugs. I do believe that was a bit of my pistons.

BTW, some pistons are way more sensitive to abuse than others. These new-fangled hyper-eutectic's won't take much at all so I stay with forged. I know me. I'll usually mess around until I mess it up.

FWIW, a good way to see what's really going on inside the venturi is to flash a timing light down there and then rev it up. Something about the way the light flashes allows your eyes to actually see the curtain of fuel start to come out of the boosters. The accel pump shoots a cone that curves into the airflow and then disappears. Way cool.

And I wonder why I have no eyebrows left.