I race an '81 Camaro on a half-mile sealed clay, D-shaped oval in the Pure Stock class. The engine is a 350 outfitted with flat-top pistons, stock intake, and exhaust manifolds, .390-/.410-lift cam, 993 heads with stock valves, and a 750-cfm Quadrajet. A three-speed manual transmission and 5.57 rear gear get the power to the rear wheels.
A veteran racer installed a set of used DA secondary metering rods in the Quadrajet, which is a rebuilt unit intended for a '72 Camaro. After this simple swap, the carb was deemed ready to race. Today I inspected the rods and found significant wear on one rod where it contacts the hanger, and the other rod has a slightly bent tip.
The car performs up to my expectations, but the air/fuel mixture seems to be rich. Spark-plug and exhaust-pipe color is consistently black. The plugs are also slightly damp.
Do you have any advice on secondary metering rod and primary jet selection?
Let me see if I can help you here. On the basis of the information you've provided, several thoughts come to mind.
First, the installation of used metering rods and a hanger assembly runs the risk of parts being previously damaged. Bent metering rods may hang in their respective jets, possibly causing improper fuel enrichment, and mixtures that are either too rich or too lean. In fact, depending upon the extent of metering-rod damage, rod movement may be sufficiently impaired to cause extreme rich/lean conditions somewhere in the rpm range.
You should replace the metering rods and hanger assembly with new parts right away. Next, make certain the vacuum piston and its bore are free of any nicks or burrs that may impede movement of the piston. Then, see if you can obtain a couple of different sets of metering rods. The recommendation would be to opt for ones that provide leaner intermediate steps. This will help part throttle torque and response in your race car. Keep in mind that the idle enrichment is a source of fuel that never stops once it is activated. The idle circuit will reach its maximum delivery point and remain there throughout the rpm range.
For calibration (including jet selection), a good plan is to jet and select corresponding metering rods for the best power. Then you can use a spark plug heat range that allows reasonable plug life. Stated another way, this means calibrate fuel for power and put a plug in the engine that will live. However, do not put a plug in that will run so hot as to encourage detonation.
In any case, the suggestion is that you use only new metering rods, new hanger assemblies, and new jets. Experimenting with the hanger "cam" can also be a fine-tuning method. As a starting point, see how well a "K" cam will work in your Quadrajet.
I hope this will help you with your issues in the carburetor.
Help Me, I'm Rich
I have a question I hope you can help me with.
I have a Chevy 350 with .030-over K-B flat-top pistons, steel crank, Pink rods, and a camshaft with the following specifications: 247 degrees of duration at .050 on the intake and 255 degrees at .050 on the exhaust. Lobe lift is .350 on the intake and .360 on the exhaust. The Lobe separation is at 106 degrees. The gross valve lift is .525 on the intake and .540 on the (lobe-lift-time rocker ratio) exhaust. The engine is equipped with a 750 Edelbrock carburetor. The plugs are GM R46 and the ignition is stock with a super coil.
While the car is idling and I stand behind it, the exhaust burns my eyes. Why is this? I am puzzled. I hope you can tell me what is going on, and I look forward to your help.
Thanks for taking the time to answer my letter.
Mark A. Casey
Thanks for the question, Mark. From what I read in your letter, it sounds like you're getting a dose of unburned fuel, or at the very least you are getting the results of an overly rich fuel mixture condition. Lets talk a little about the basics of this situation. As valve timing begins to incorporate increased amounts of overlap, low-rpm combustion efficiency (especially at idle) tends to suffer. This means the engine is only partially burning its induced mixtures, which means it is then passing some amount of raw fuel out the exhaust.
Remember, carburetors are merely pressure-differential devices. Generally, atmospheric pressure sits at the carburetor's entry, and fuel-metering signals (pressure differentials) are established when lower-than-atmospheric pressure exists beneath the carburetor. Since valve timing affects cylinder pressure, increasing valve overlap tends to cause ragged idle quality (and mixture burn) at idle and low rpm, absent any appreciable engine load.
Since you didn't indicate idle rpm or amount of spark timing, it's difficult to be specific about your problem's cause. If the engine runs properly otherwise, then you can probably eliminate the possibility of a stuck carburetor float, improper float level, or intermittently fitting needle and seat.
There are a couple of simple things you can do for a preliminary check to try to isolate the problem. First, check the air bleeds to make sure they are clear and not plugged. Next, turn the fuel needles in, thereby reducing the amount of fuel getting to the engine. If this cures the problem, your job is done. If you go too far and stop the fuel flow completely, the engine will stall. If, however, the engine continues to run (at idle) with the needles turned all the way in, then there is a fuel leak somewhere supplying the gasoline for idling. Check for leaks. One possible leak is in the power valve, where the diaphragm may be broken. If that is the case, fuel will be passing by, and this could be the problem.
There is also the fact that you may need a professionally reworked carburetor to produce the idle characteristics you require. The professionals purposely do a lot of work on the carburetor to get the engine idle as lean as possible. The idle circuit can be tuned by opening up the idle air bleeds, and holes can be drilled to arrive at the desired effect. Doing that kind of thing is not recommended for the amateur--unless you know exactly what you are doing, you can further complicate the problem. By the way, that's why trick carburetors cost so much and work so well.
I hope this has pointed you in the right direction. Good luck out there racing around!
Mopar Or No Car
I race on a quarter-mile flat asphalt oval track. I drive a '73 Duster with a 360 block and 340 heads and intake. It has a three-speed stick. Track rules allow us to run a 4412 Holley, stock intake, stock exhaust manifold, and up to a .490-lift cam. My 360 has a .474-lift Comp cam and 9.2:1 compression ratio. I also have a 340 with X heads, a stock intake manifold, a .484-lift Mopar cam, and a 10.8:1 compression ratio. Which motor will enable me to develop the most power on this type of track? (Which motor would better enable me to stuff the Chevys?) If it is a toss-up, please give me the pluses and the minuses of both motors. I look forward to reading your answer in the most awesome go-fast-turn-left magazine there is.
It's Mopar or no car for my wife and me!
Name and Address Withheld
Taking a look at the two engines, it would appear on the surface that the short answer to your question is that the 340 would be the better of the two. I say this because of two factors. First is the compression ratio of the 340, which is 10:8:1, versus the compression ratio of the 360, which is 9.2:1. The second factor is the heads. The X heads of the 340 are designed for improved flow characteristics over stock versions, and I think it's safe to say that X-head configuration is superior.
Having said that, let me expand a little more. I am assuming that the 340 has the right cam for what you are trying to do. Unfortunately, I am not provided with the specifications of the cam, so I cannot advise on that issue. But all things being equal, and if the cam is right for the motor, then I would say the 340 with the X heads and the compression ratio of 10:8:1 would be the first choice.
By the way, if you wanted to change cams in the 340, there are some choices we have that offer you a broader rpm range to run in. With the kind of compression ratio the you have in the 340, we can select a little larger duration cam, which would give you a broader power range. By doing that, it should help you get around the track better on a flat track and in higher-rpm environments.
Now, let's talk a little about the 360 engine. This motor is not bad as a second choice, but there is a scenario I can think of that might make it your first choice. It may be used as a first choice if you are running at a track that has a poor surface and you do not need as much torque as you get from the 340. In that situation, the 360 might serve you better.
I hope all of this has helped you, and whichever engine you wind up using, stay safe and have fun.
Spring Into Action
I have a couple of technical questions I can use some help on. I am fairly new to racing, and I wondered if you could help me with some baseline spring and weight settings for my car. I am running an old Ford unibody car in a pretty much Stock class. It has front coils and rear multileaf. Total weight of the car is approximately 3,250 pounds. As the car sits right now, it is running 52 percent front weight. I am running it on a quarter-mile paved oval with 9 degrees of banking in the corners. As I said, I am looking for somewhere to start from. Any help would be appreciated.
From the information you have supplied, there are some basics that can be suggested, however it's not possible to give you an exact because more information is needed. That having been said, let's take a look at your situation and see if we can't give you some direction.
Because you are running on a relatively flat track, here is a general setup suggestion for your car. This is, of course, just a suggestion for a baseline, and it's important that you take your car to the track for a little open-practice testing to see how this works. It is sometimes hard to be a desk jockey and tell you what works best, and nothing can take the place of testing and practice. Also, looking at this setup, I know you say its stock, but you should take a look at some aftermarket shocks for this car, I think you will be happier with the performance over the stock shocks.
On the hypothetical side of this, you'll have to get a feel for the car to achieve the right combination. For instance, if you feel it's rolling over too much (going into the corners) then you need to work a little more on the setup. If you are like a lot of new drivers, you probably will take the car into the corner as far as you can, nail the brakes, and then try to power out of the turn. This happens a lot with new guys, but after practice and races under your belt, you will develop a smoother entry into and exit out of the corners. The point is, as your driving style matures, you will also be doing some finesse-type changes on setup to adapt to the changes in your driving skills. It's a natural progression that comes from seat time.
With experience, you will develop a smoother entry and exit into the turns. After that, along with practice time, you should become adept at knowing what you need and then making those changes.
Childress Race Cars
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