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!