what do I need to know about the carb and engine to properly jet my carburator?

 

a vacuum gauge and the ability to read the plugs. a timing light. Look up your carb and see how it was set from Holley

 

A couple of things come to mind.

- Is it a DP or Vacuum Secondary. DP has no rear power valve and requires more jet than with vacuum secondaries with a PV.

- A few specs on your engine, helps compare to others.

- Manual or auto trans.

I have a 750 Holley DP, 65 Power valve, #73 pri. & #77 Secondary jets. Stock '70 LT-1

 

Take the list number and go to the Holley site, put it back like it was when it left Holley, then work from there.

Neal

 

Some double pumpers have 2 power valves. I don't know how common they are or aren't, but I do know some do have them.

I don't really know a lot about carbs, just learning. I installed a permanent air fuel gauge in the car, will install a vacuum gauge as well. Watching the Holley videos where they go through all the components and also reading some tuning guides has given me a bit of a start. Also got a friend who knows carbs to help me the first time to get a halfway decent tune so I have a starting point. If you have something that runs terrible and you are just learning, it does make it hard to know which direction to head in.

My 750 DP was actually too lean to begin with when it first went on the car brand new out of the box, was stumbling too easily and at WOT it was leaning out too much, ended up going up a jet size to get it running nicely and safely, and I think in the future I'll go back down in jet size once I get all the other parts of it working well.

 

Correct jetting is highly dependant on the particular engine combination. I have a 770 Holley Street Avenger jetted #67 primary, and #69 secondary and its still a little fat. I installed an air/fuel ratio gauge because I couldn't believe I needed to lean it that far from stock. I highly recommend the gauge. Makes tuning WAY easier.

In my experience Holley tends to jet their carbs rich, although my carb is five jet sizes lean from stock and that is a little more than usual.

 

Before you get into jetting , get it running right , Usually out of the box the items that need attention are squirters , air fuel adjustment , idle and transition set up.. Once you have those areas dialed in I would bet the jets are fine or so close you can leave then alone.

 

X10 w/diehard

Most start fooling with power valves or jets that dont need to be fooled with.

 

This is very good advise.

 

Although I'm not sure what you mean by transition set up. If you start with the carb the way Holley set it up from the factory. It will run. From there it is fine tuning. You will set the float level, idle mixture, and the idle speed. If that's all you want to do, fine.

Next I would deal with the jetting. The traditional method is to read the spark plugs, but I recommend the air/fuel mixture gauge. Makes reading plugs look like something out of the stone age, but its just one more tool. You have to make your final judgement based on how the car is running. Once the jetting is there, if you have a flat spot when you open the throttle, you can play with the squirter nozzles and accelerator pump cam(s).

That's about the limit of what you can adjust on a Holley without making irreversible modifications. Remember, you can always go back to where you started if you screw it up. (Ask me how I know this.)

 

The transition slots are a huge part of setting up a Holley . That allows the carb to come off the idle circuit to acceleration seamlessly in normal driving and also affects drive ability when at a steady speed as you advance the throttle .

In some cases once that transition is set you will find yourself unable to get a good idle speed, and that will require you to adjust the rear butterflies to get the idle your looking for.

How can you tell if the transition is set wrong , a simple way is you will notice the idle air bleeds make little difference when you screw them in or out. That is because your idle is using the transition slot to maintain idle. I have on some cars set my transition slot based on air bleed function , not the best way but it has worked fine.

JETS are last item , almost always a street motor with decent power will be perfectly happy with the stock jets.. Driving issues on an out of the box carb are ALMOST never a jet issue but almost always a set up issue..

 

I agree with most of what has been said here, however to say that a street motor will be perfectly happy with the stock jets may be stretching things a bit far.

Holley and most others manufacturers setup their carb jetting to run at sea level.

If you live in Denver Colorado or near the Rocky Mountains like I do, then I don't think the stock jets are going to work very well out of the box.

I would suggest a little research with Holley and similar manufacturers to clarify about the jetting of their carbs and how things such as altitude, temperature, humidity etc. affect the jets to be used.

JMHO.

 

I hear you, but I don't think I would suggest someone that is just learning about carburetors should start fooling around with the transition slots. These are not normally considered adjustable on a Holley. Transition slots are part of the idle circuit and just as the name implies, are a transitions between the idle circuit and the main circuit. Idle air bleeds are the air source for the idle circuit. I think you are referring to the idle mixture screws. Guys have problems when they put a huge cam in the motor and have to open the throttle blades farther than normal to get the engine to idle. This uncovers the transfer slots and basically screws up the carburetor calibration. As you correctly point out, the giveaway is when the idle mixture screws no longer seem to have any effect because the engine is pulling fuel from the transfer slots at idle. I've never run a cam that big so I don't have any experience with tuning for them, but I understand it is done by drilling holes in the throttle blades, or sometimes putting pieces of wire in the idle air bleeds. Anyway, my point is this is not something a beginner should be messing with. This is for advanced engine tuners only. I think if the OP starts by setting up the carb the way Holley had it, then sets the float level, idle mixture and speed, and jets in that order, he will be fine.

 

My bad I did use the word bleeds I meat mixture screws ..

Setting the transition slots is as easy as any other adjustment , and YOU do not drill into butterflies or add wires to attain a correct idle because of a big cam.

I did mention how to determine if your idling off the transition and a simple fix , as well as how to get the correct idle using the rear barrels.. Maybe you did not read all that I wrote ..

I bet many issues are due to a Holley owner having no clue the transition slots exist then almost any other issue. And a huge reason for people sending a crab back as bad.