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Carburetors

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Cool53:
Moose, this really is fun. Mid road and Mike, are savvy guys. This is fun.
We'll talk about low budget. My truck ran a 12.3 with pump gas and no juice utilizing a 750 Holley my friend picked up at a yard sale for $10. The next few weeks were spent mixing jets. But before I get carried away, I did cut the choke housing off and try to make it look a little better with some sanding rolls and a die grinder. And I worked on the distributor. The carb is never, ever going to feel right until the timing curve is right.
Now here again I might get some stones thrown at me. Whenever possible you should run a vacuum advance and if you have an extra forty bucks put in an adjustable unit. Ignition timing follows engine load, and engine load follows manifold vacuum, or vice versa really. I know everything you read says connect the vacuum advance to ported vacuum, but in most cases it is not correct. The ported vacuum signal comes from a small hole located above the throttle plates (idle position). In stock type carburetors this port would be moved up and down relative to car weight, engine size, rear axle ratio, and so on. That's why there were so many carburetor numbers for carburetors that appeared identical. Manifold vacuum represents true load and will move the timing advance accordingly. With a good vacuum advance you will have the ability to adjust diaphragm spring strength and there should be an advance limiter included in the kit. Timing on most of our cars will be between (with this Artesian Well Water we call gas) set at a total of 32 -34 degrees at about 3,000 rpm (good starting point), without vacuum. But with the vacuum advance connected you may find your engine likes as much as 56 degrees at high vacuum, low load. But as soon as you crowd the throttle the vacuum drops, the timing drops back to mechanical total, and you don't break any rings. Read and follow the directions for your vacuum advance kit.
Now you can play with weights and springs in TGIF distributor. Often a car will be lazy in rpm ranges under 3,000 because the weights are dragging or the weight return springs are too strong. So you need to pick up a weight advance kit, too. You still have only spent chump change and have made big changes in how well your car runs.  Make sure when you are setting timing that your TDC mark is absolutely correct. I pull the pushrods on #1 and check TDC on every car I set up. Here come those broken rings again.
Now that your distributor is near where it should be you aren't going to believe how much better your carburetor works. 95% of the carb issues that come here are mixing problems.
Now for the carb. If I had an older carb I would put new reusable gaskets. The hardest part of working on an old Holley is getting the old gaskets off. Sometimes it's almost impossible to get the float bowls and metering blocks off. Be careful not to damage anything. If you have a 4160 carb or a 3310 style with a metering plate in the back end rather than a metering block with jets then there is a kit to upgrade the carb to jets from Holley. Again, not expensive, and a must.
Rich is safe, lean can be damaging, so you can go to the Holley site, or most other carb sites, and see what they are running on their carbs as delivered. Often you can just use the original jets and work from there. As Midroad and Mike have stated you will take your car out and drive it,with a handful of jets, and lean until power just starts to drop off, at wide open throttle, then you can come back and fine tune idle and transition fuel. If you have a carb with vacuum secondaries you can buy a bag full of springs for the secondary dash pot and change them until you get it feeling just the way you want it of you can buy an adjustable unit from Quickfuel that adjusts simply with a screwdriver. That's the best. I forgot the power calve. Rule of thumb is to pick one that is half of what your idle vacuum is. Big cam shafts, smaller numbers. If you have an older carb you can buy a kit to install a check valve in the base plate in the event of a backfire. Carbs since the late '90's already have them.
Fuel pressure should be about 7# for street cars, 7.5 for racing. Set the floats so fuel just spills out when idling. Too low and you'll get a hesitation.
The fuel pressure gauges that are available seem to be all over the place, so you may want to check yours.
Your starting point for idle screw adjustment should be 1 1/2 turns out and they should always be even. This should be very close. If the screws don't seem to work then the throttle is open too far and you are pulling furl from the transition slots in the carb. This can happen if you have a big cam, that's why you'll see performance cars with holes drilled through the throttle plates. That allows more air into the intake with cranking the throttle open. You can do this yourself, but there is no going back if you go too far, other than installing new plates.
I developed a method for bypass air that is adjustable and works really well. If you want to know how to do that leg me know and I'll take some pictures and walk you through it.
When you are all done you may end up with a tip in hesitation. That is almost always going to be eliminated by adjusting the idle screws one way or the other. Idle and transition fuel come from the same feed, so if idle is pulling all the fuel there will be less for transition. Usually leaning the idle screws will get rid of the hesitation.
Whenever you change throttle position by turning the base idle screw(s) you need to readjust the accelerator pump(s). Use two 3/8ths wrenches so that you just eliminate any play when the throttle is closed.
I very rarely change an accelerator pump cam or associated parts for a performance or drivability issue, usually just on race cars. Holley did their homework.
Idle screws on Holleys adjust fuel glow, on Edelbrock, a Carter style carbs they adjust air.
I have to go run the dogs, 2 below zero this morning with a foot of snow coming tomorrow through Friday. Happy New Year everyone!

Cool53:
When tuning for mileage you may find that changing jets doesn't do much. If you are motoring on cruise at low rpm you are probably not even into the boosters ( main metering) and are just pulling fuel from transition. If you want to know where transition ends and main metering begins with your setup wrap tape around the boosters on he primary side and drive the car, somewhere that's safe with no traffic like a closed industrial park, and throttle up very easy and see at what rpm the engine falls off.  That's the point where your boosters starting supply main metering furl. If it is more rpm than where you cruise at then you would have to cut down on idle circuit furl supply to increase economy. You can do that by placing short pieces of different diameter music wire in the feed circuits until you get it where you want it.
Something I forgot to say earlier is you might want to disconnect the secondaries when tuning for power and jet the primary side until it runs well, then add the secondaries and finish the tune.

themoose:
Jef

There's definitely a lot of info about tuning a carburetor that I was not aware of...Thanks for the great post. You touched base on the importance of setting up your timing correctly and I was wondering if  as some would suggested do you ever need to deviate from the manufactures specified spark plug heat range or is that something that's not ever necessary? 

Moose

Cool53:
Good question Moose. Most of what I do is with aftermarket heads, so I use the spark plug family that the head manufacturer suggests, usually start in the middle of the heat range then check them after driving a bunch of miles. A light coffee color or light tan is what you are looking for. I tend to lean towards a hotter plug. For those that don't know the difference,ma hot plug has a deep trough around the center electrode insulator, a cold one is more shallow.
I like NGK plugs, have good success with them.
Make sure when you install plugs that you wipe the outside insulator as clean as you can, same for inside the plug boots. Contaminants can provide a path for the ignition when you put your big foot on the floor boards. I know a fuel car that was ruining engines just for that reason.

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