Carburetor jet sizes...

My son and I are learning about the carburetor and my cousin (mechanic) was in town and we took it apart and changed the jets.
The engine is a crate 302 from Summit Racing (It’s made by BluePrint)

We put an Edelbrock 4 barrel carb on it.

It’s run okay for the last year and a half, but we took out the .098 size jets because my cousin said it was running “too lean”.
He put in .101 size jets and it had more power, but the gas mileage dropped from about 15mpg to about 11mpg.

Since she was my daily driver, I had my son put the .098s back in. She’s getting better gas mileage… but running not so well.

My cousin says “you’re killing the engine. She needs gas”

So, I’m going to put the .101s back in.

Thought I’d see what your experiences have been.

It’s important to remember that a carburetor is a Fuel / Air metering device. Most likely it was too rich with the .101 jets, but ran a bit better because it helps mask a bad timing adjustment.

The timing needs to be spot on before any changing of carburetor jets happens. It needs to be set properly by someone who understands what is supposed to happen. Centrifugal advance, vacuum advance, and initial are all as important as total advance. A good advance timing light is all you need, along with some way to know engine RPM.

Any re - jetting of the carburetor needs to be done with the help of a fuel / air meter, preferably on a chassis dyno. Otherwise you are just asking for trouble.

15 MPG is about right around town. With a Holley 600 CFM 4 barrel carb I averaged just under 20 MPG on a trip from Dallas to San Francisco and back with the AC on most of the time. Not sure if an Edelbrock carb is capable of that kind of performance but it ought to be close.

Remember one of the beauties of this carb is the metering rod. You can prevent it from being too rich at full power and increase fuel at low rpm by adjusting the rod. All this is well described in the Edelbrock tuning kit guide. Sounds like the jet is ok but either the rod needs to be changed or the spring or both.

Thank you. Great advice. My cousin, using a good timing light, got the timing down perfectly. So, I think we’re just needing to adjust the carb now.

He DID say that the Edelbrock was “overly complicated”. Maybe the Holley might have been a better choice.

That’s great to hear. My son was suggesting the metering rod changes. We’ll look into it.
Thank you!

What did you set the initial timing at? Most people go by the old shop manuals and set them around 6 degrees BTDC. That is ok, but on todays very slow burning gas you can ad more timing to your engine. Most of the cars I tune, both Ford and Brand X, react well to 12 degrees BTDC or more. When doing this you need to work with an adjustable vacuum advance that they sell now a days. You can “adjust” on a stock Ford vacuum advance, but it requires shims and spring changes. And the dual diaphragm vacuum advances are early emission control devices and retard the engine making it run worse. If you are a stock guy I disable the back fitting on the dual diaphragm vacuum advance with a small ball bearing in the hose.

More advance equals better mileage, less heat, and more power! You can achieve this without a dyno and you will thank you. Until you get the advance properly dialed in, do not change jet sizes or metering rods.

One of my biggest pet peeves is people that think a high compression engine will not run on todays gas. In my experience and I can back it up with some articles on dyno mule engines, todays 91 reacts very similar to old 99 or 101 custom supreme. Todays gas will not produce the same power, but it is very forgiving on detonation.


Edelbrock has a tuning guide that shows the combinations of jet and metering rod you need to make the carb leaner or richer. Take what Royce said to heart: get the timing and ignition right first. Then adjust the idle air screws, then you can start changing the rods and jets. My experience with Edelbrock is that they are set up for a Chevy 350 out of the box and too rich for the stock 289 or 302. Chevy runs a different strategy for vacuum advance than Ford and it makes a difference because the carb is a mechanical computer that uses vacuum as its primary input.

What I do is to follow the chart to replace the jets and or rods to lean the carb out. What you are looking for is lean surge when cruising at about 45 mph on level ground. Lean surge feels like a temporary loss of power sort of like hitting a head wind. Once you have that dialed in you know the lean limit. Typically I go to 2 steps richer and drivability and economy are both good.

My cousin says it’s set at 10. He restores old Ferraris in California… he’s a great mechanic. So I trust him.
Thank you for the info. This is great stuff!

Thank you. We will look at the metering rods next!

If the carb is correct for the application,the Edelbrock carb works well if everything else is in order.Gas mileage? Your right foot determines that.
If you want to take that Cougar and blow off new Corvettes on a road course you’ll need a lot more than that Edel carb.And they’re not overly complicated.Fact is they’re simple as long as you follow the instructions.
Don’t get smoked by some of this stuff.Common sense is the best tool you can have in the tool box.

Bill and Royce, I have a question. As I agree with your posts on getting timing and ignition “spot on”, how do you know when they are spot on. I can’t hear pinging anymore and have learned to read the spark plugs. This helps me and I have used this method along with curving etc. However, I am curious how you know when it is spot on? Just trying to learn and maybe help others along the way.

Merry Christmas to All

Any pinging is a clue you are not there yet. I like to run more initial advance than the stock specification, up to 15 degrees. However, you can get too much total advance that leads to pinging so you have to back it down. I also check to see how fast the timing is coming in. In my opinion, earlier is better so long as you are getting no ping at all. Typically with an engine that was designed to run on premium gas I get the best results by flipping the rotor drive to use the 11L or 13L window to limit centrifugal advance to 22 to 26 degrees. Total advance is initial plus centrifugal. So 15 plus 22 is 37 degrees. Then I fine tune vacuum when under cruise conditions. I look for any ping on slight hills or soft tip in of the throttle. Adding a spacer or tightening the internal spring (depending on type of vacuum module) will back that off.

In Arizona we get summer gas and winter gas and they are different enough to cause me to try to find a good compromise tune that is maybe not ideal for either one. If anything I may have to back off the timing in the summer just a degree or so. I am interested mostly in cruising and being able to nail it up the expressway off ramp when needed.

Exactly the same process I use. I have a bottle full of spacers and springs for the factory style vacuum advance. I prefer the newer internal springs ones. As you advance to the 15 to 16 degree mark you may experience “light load” pinging when climbing a hill. This is where you need to reduce the vacuum advance a bit. Best done on a hot summer day when heat and dry air can cause pinging.

On my all out motors I favor the MSD billet distributor with no vacuum advance. I adjust the springs for all in at 2,500 rpm and restrict the centrifugal to 19 degrees. This allows me to dial in 16 to 18 degrees of initial. Again, more advance without pinging equals more power, less heat, and better mileage! But my favorite saying is " There is more to life than gas mileage"!


I have my 600 Holley at 16 deg. Put in a Dyno and it was right on. Changed the jets one size and spring. Good luck

Great info all! Thank you.
We’re learning a lot!

With a good dial back timing light you can read the degrees of initial, total, centrifugal, and vacuum advance. I have found some small blocks like as much as 20 degrees of initial with up to 40 degrees of total advance, particularly when compression is low and an aftermarket camshaft is used. With a good timing light and a vacuum gage you can see what works best for your engine, but you still need to know where you started and how each part of the advance picture is working.

Timing can seem complicated, because there are a lot of factors. If you’re running smog equipment, it gets even more complicated because your engine will switch between ported and manifold vacuum for your vacuum advance.

For any street car with a mild cam, manifold vacuum works best.

Your mechanical timing is the baseline for your engine at Wide Open Throttle. It needs to be low enough that your engine won’t ping, but high enough to deliver crisp response. Typically, around 8-12 degrees is where it should be at idle. More importantly, you should end up around 34 degrees all-in by 2800 RPMs. The type of heads, and the cam you use can change both those values a bit. A 289 Hi-Po for example, prefers 38 degrees total timing, because of its high-lift mechanical cam, with a lot of overlap. The factory heads burn pretty slow. A typical build with a 5.0 HO cam and GT40P heads usually works best at around 28 degrees total timing. This is because the revised combustion chamber and mild HO cam result in a very fast-burn situation, and advancing the timing too much just means you’re hitting peak cylinder pressure too early, stealing power.

It’s important to understand that by setting your mechanical timing, you have vac advance disconnected and plugged. You are NOT tuning the engine to idle ‘smoothest’ when you’re setting your base (idle) mechanical timing. Remember, this is what your engine falls back to when you’re WOT.

Once you have it set, then you hook up the vacuum advance, and that should add another 16 degrees or so, when the throttle blades are closed, using manifold vacuum. When there’s strong vacuum in the manifold, there is very little air and fuel to compress in the chamber, so pressures are low. Because of these factors, the flame travel is very slow, and the explosion happens very slowly. Lighting things off 16 degrees sooner than you would at WOT, you’ll still reach peak pressure at the proper time to push the piston down, instead of the explosion chasing the piston down the bore, and spitting flames out the exhaust.

Using timed or ported vacuum turns off vac advance when the throttle blades are closed. It was used in early smog systems to ensure that there were flames in the manifolds to burn with extra air from the air injection system, and later to heat the catalytic converters. But ported vacuum causes underhood temps to be much higher (from the much hotter exhaust manifolds), and causes some drivability and tuning issues.

Using those values will get you in the neighborhood, but only driving the car and testing it (when the car’s engine is warmed up!) will tell you if it’s set perfectly or not. Once your timing’s set right, then you can work on fine-tuning the fuel system. If you have a choice, use a carb with annular boosters. They are flat-out superior for street use.

So: TL;DR: Use manifold vacuum. Mechanical with stock early heads should be about 10 BTDC, and around 34 total mechanical by 2800.

For any street car with a mild cam, manifold vacuum works best.

The above is a debatable subject. Pretty much all 60’s cars with vacuum advance used ported vacuum. That is how they were designed and engineered by the factory. Specifically the distributor curve.

When using ported vacuum the function of ported of the vacuum advance is to provide instantaneous “vacuum advance” as you pull away for a stop. Using manifold vacuum does the exact opposite as you open the throttle. The engine vacuum drops as soon as you open the throttle just as the engine wants more advance. On mild acceleration this may not be noticeable, but if you try to accelerate quickly from a stop the engine may suffer from a flat spot due to the removal of advance at a critical moment.

Based on that, under hard acceleration with 8 to 12 degrees of initial timing and say a drop to 5 inches of vacuum, the engine will instantly go from around 18 to 22 degrees of advance at say 1,000 rpm right down to nearly the initial timing setting. If using ported vacuum at with 12 to 16 degrees of initial advance the engine will not experience a drop in advance and at the same 5 inches of vacuum it may gain say 2 to 4 degrees of advance. And this is all happening prior to the centrifugal advance kicking in. The engineers designed the curves in the distributor to work with ported vacuum. To use the instantaneous addition of advance just as you open the throttle to transition to the centrifugal coming in.

Over the years I have experiment with increased initial and slowing down the vacuum advance with springs or shims or installing an adjustable vacuum advance. On stock and mild builds this has produced the best results in my experience.

The engines with dual diaphragm distributors and smog trees on the thermostat housing were designed by the engineers to reduce advance to slow down the burning of the fuel to more completely burn it. This was an effort to reduce emissions. I always found it hard to believe that by installing these performance robbing systems, that increased fuel consumption, that they decreased emissions. Seems to me that a 302 getting 15 miles to the gallon, vice one getting 10 or 12, would produce less pollutants. Go figure.

Use ported vacuum and more initial timing than the book says. Adjust out mild throttle pinging by adjusting the vacuum advance to slow it down. and I totally agree with Royce that a larger cam and lower compression will allow you to become even more aggressive with the advance.

One of the worse theory’s in engine building over the years is to go below 10 to 1 in compression ratios on these older V8’s. They will run just fine on todays gas and dropping compression turns them into toads for power.


Rob, it is definitely a debated subject. But the simple fact remains: High vacuum in the manifold means that the flame front takes longer to propagate in the cylinders. If you use ported vacuum, and have tuned your mechanical advance to be high enough for a smooth idle, then at cruise, when the vac advance kicks in, you will be over-advanced. If you dial it down to the point where it’s not too advanced for cruise with vac advance, then your idle is going to be problematic, because timing will be too low with the blades closed. It leads to high engine bay and header temps, along with poor fuel consumption, and a rough idle.

Your vacuum advance mechanism allows for you to have a baseline WOT setting that will never cause detonation or problems, and will advance your timing when mix is thin to compensate for low cylinder pressures and slow burn. A lot of experienced mechanics and engine builders don’t even feel that vacuum advance is necessary at all. For an all-out drag motor, it’s not essential, because the only ‘mode’ that matters is WOT, and the vacuum advance does not add more power.

However, for a vehicle driven on the street, vacuum advance adds economy, throttle response, and adds significant power at part-throttle operation.

Ported vacuum is a compromise best left to ‘factory smog motors’ which switch between the two using thermal vacuum switches and all sorts of kludgy things intended to show the Federal Government that auto makers were trying to clean up emissions. Or, in some cases, cars with a really nasty cam that do not idle well can benefit, because idle surge will make them unmanageable with manifold vacuum.

Burning fuel in the exhaust manifolds along with extra air is a very poor way to reduce emissions. It was a lousy idea when it came out in the mid 60s, and it’s even dumber today. I would ONLY set it up that way if you intend to show your smog-motor car at concourse events. It is possible for a well-tuned 302 (or 289) car to get around 25 mpg on the highway if you keep your foot out of it, with a well tuned ignition and fuel system.

Manifold vacuum, and a setting appropriate for WOT that does not cause detonation will enable you to prevent over-advanced Cruise, and eliminate issues like dieseling when you turn your car off too. Having your distributor properly curved, with an appropriate amount of vac advance dialed in is also an important part of this.

I know this is a bit long-winded in a thread asking about carbs, but I think the information is still very relevant. And just to re-iterate: Use a carb with annular boosters if you have a choice! They just work better for any street car. Better atomization, better power, better mileage.

I dont believe any responce here is long winded for anyone who tryes to tune their own cars as this kind of help is getting farther and fewer between. Bill thanks for the info as to what lean surge is exactly, just the opposite of what it sounds like !