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Classic Cars Carburetor Problems On Older Cars
382 messages, Last post on Jul 01, 2009 at 1:16 AM
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Replying to: zaken1 (Jun 15, 2009 10:46 am) |
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1979 Trans am w/ 403 and stock Rochester 4 barrel, no emissions, and a very mild cam. Hello guys, I need your help again; since my last post I received the electric choke, and plugged a few vacuum leaks I found. First of all how do I adjust the choke? Do I turn it until it barely opens the butterfly on top of the carburetor? Lastly what are the normal starting positions of both idle screws, hot and cold? Thanks! -Red
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Replying to: red79phoenix (Jun 18, 2009 9:40 am) Glad to see you back. The choke should be adjusted when the engine is cold and the air temperature around the car is less than 70 degrees F. Turn the adjustment until the choke butterfly is fully closed (but no further). The choke pull off (vacuum break) diaphragm on the carb should then open the butterfly slightly as soon as the engine starts. If your carb does not have a choke vacuum break diaphragm on it; it will be impossible to properly adjust the choke. The idle mixture screws should both be initially set to the same number of turns out from closed. That position will normally be somewhere between 2 turns and 4 1/2 turns out. But the idle screw setting is highly dependent on the ignition timing, and on the volume of air which enters the carb from external sources; such as the evaporation cannister purge and the PCV valve. The carb was originally calibrated to work with those two items connected. If the PCV valve has been plugged off; the idle screws will probably have to be set nearly completely closed; and the engine will tend to foul plugs and run too rich all the time. I would use 3 turns out on the mixture screws as a starting point; and then drive the car until it is fully warmed up, before trying to make a final mixture adjustment. A vacuum gauge connected to intake manifold vacuum can be a great help in setting the mixture screws. Just set them as lean (as far closed) as possible without losing any vacuum from the highest level it reaches when the screws are turned. If your spark plugs are not gapped to .060" or if the plug wires have too much resistance; it will not be possible to properly adjust the carb. |
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I finally got my engine running excellent but now when I hammer down on the accelerator at anywhere between 40-60 mph I hear a pinging noise like the valves are being forced shut. The noise is less at the lower speeds and louder at the higher speeds. I don't have the get up and go that I used to have. What could cause this to happen.
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Replying to: crampton1998 (Jun 22, 2009 7:02 pm) If the timing is set too far advanced; the pressure wave will strike the piston while it is still moving upward, and has not yet reached the top of the compression stroke. When this happens; the pressure will push back against the piston, which reduces the amount of power the engine produces. The impact of the pressure wave striking the upward moving piston also creates a pinging or knocking noise; which is what you are hearing. And the further before the top of the stroke the piston is when the wave hits; the louder the pinging becomes, the more shock is transferred to the piston, and the more power is lost. If the pinging continues for prolonged periods, it can cause damage to the pistons or rings. Since this did not happen before; it sounds like you either 1> advanced the timing further than it previously was, or 2> you are using regular or a lower octane fuel than you previously did (which burns faster than premium, and which will send a faster pressure wave toward the piston) or 3> you have changed to smaller diameter metering rods; which richened the fuel mixture (and a rich mixture burns faster than a lean mixture; so the pressure wave will strike the piston earlier in the stroke) or 4> the engine has been running consistently better than it previously did; which gradually caused the quality of the sealing of the piston rings against the cylinder wall to improve, which in turn increased the compression pressure in the cylinders, causing the pressure wave to move faster. The solution for any of those four possibilities is the same: The timing must be retarded to the point where the pinging stops. But there is another issue involved here. You observed that the pinging becomes louder at higher speeds. What this means is that the distributor's advance curve (the rate at which the timing advances as the engine speed increases) is set to advance the timing relatively further at high speeds than it does at lower speeds. But the desired advance curve setting is dependent on the fuel mixture strength at different throttle openings. When you change the metering rods; the advance curve will usually have to be reset. So I would not recommend resetting the advance curve unless you are totally happy with the current set of metering rods that are being used in the carb. The simplest thing to do is just to retard the idle timing. If you still want to experiment further with the metering rods, then that is how I would deal with the timing at this stage. If you have to retard the idle timing to 6 degrees BTDC or lower; the engine may then run better with the vacuum advance driven from manifold vacuum again. And of course the idle mixture and speed would have to be reset. There are two adjustments that can be made on the advance curve: The sensitivity of the vacuum advance can be adjusted on some (but not all) brands of vacuum advance units. This is done by inserting the long end of an allen wrench of the largest size that will fit in the opening, into the vacuum spigot for the advance unit. If you can insert the allen wrench all the way into the vacuum spigot, and feel it engage with an allen screw; then turning the wrench clockwise will advance the timing at part throttle, compared to the timing at idle and at heavy throttle. If you turn the wrench clockwise until it stops; that is the most sensitive (most advanced) you can make the vacuum advance. If you are using the right size wrench, and cannot feel it engage with an allen screw inside the advance unit; then that unit is not adjustable. But you can buy an adjustable vacuum advance for that distributor if you so desire. The vacuum advance only works from just off idle to about 3/4 throttle. It does not work above 3/4 throttle or when the throttle is floored. The other advance adjusment is for the centrifugal advance. This mechanism is composed of two springs and two metal weights; which can be seen when you remove the two rotor screws and lift off the rotor. You can buy an advance curve kit for Delco Remy distributors, which contains three different sets of springs and several different weights. But I don't want to deal with that at this time. For now; just see what you can do by retarding the idle timing and maybe changing the vacuum spigot and possibly adjusting the vacuum advance. If you want to make further advance adjustments; please update me on all the other carb and ignition changes you have made.
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Replying to: zaken1 (Jun 22, 2009 11:31 pm) |
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Replying to: crampton1998 (Jun 23, 2009 5:50 am) Here are links to three articles on "reading" spark plugs. The first photo in the following article is of a non projected tip plug. This plug design keeps the insulator tip out of the combustion swirl, to prevent it from either overheating or loading up with deposits: http://www.strappe.com/plugs.html The next article is a good general discussion about how to interpret spark plug appearance: http://www.ngksparkplugs.com/tech_support/spark_plugs/faqs/faqread.asp The third article contains a good photo of what a projected tip plug looks like when it is in an engine where a non projected tip plug would work better. The picture I'm referring to is under the heading "splash deposits." Do not pay too much attention to the explanation they have there. In this condition; there will always be a dark area near the tip of the insulator; which does not extend into the plug body below the end of the shell. http://www.nightrider.com/biketech/spkplghnbook.htm So, before doing anything else to the timing, at a time when the truck was last driven normally on the road, rather than being run in the garage or idled while it was being adjusted. I would like you to pull the plugs from cylinders # 1, 5, 4, and 6; and inspect them closely in sunlight under a magnifying glass. I am interested in finding out four things: One is whether the insulators are blistered or glazed. The second is whether the insulator are generally cleaner and lighter in color in the section that is inside the plug shell; or in the section that extends beyond the end of the shell; or whether those two areas are pretty much the same color. The third is whether the general color of the insulators is pretty much white, tan or yellow; or whether there are black areas anywhere on the insulator. The fourth is whether these 4 plugs are all pretty much the same in appearance; or whether they very considerably from each other. |
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Replying to: crampton1998 (Jun 23, 2009 5:50 am) I wanted to add another perspective to your truck's tuning project, before you start turning wrenches on it: At this point, there are 4 factors which are up in the air and need to be juggled; metering rods, springs, timing, and spark plug selection. Since these four things ALL INTERACT WITH AND DEPEND ON EACH OTHER; there is a logical sequence in which they must be addressed. If that sequence is not followed; it can lead to a series of wrong conclusions and inappropriate choices. The first step is to find the metering rod that runs best with the engine tuned as it now is; and it sounds like you have already done this. The second step is to find the springs which work best with that particular metering rod; when the motor is in this state of tune. It seems to me that you have gotten a wrong impression that springs and rods must BOTH be changed at the same time. I may be partly responsible for this; in that I once suggested changing rods and springs at the same time. But that particular recommendation was made for a certain specific situation; and was not intended as a general rule. Right now, the thing to do is to stay with the rods you now have; and try different springs until you find the set that runs best with these rods. Since you are now experiencing pinging which increases with engine speed, and are also reporting that the motor is less responsive than it previously was, I suspect that the springs are too strong, and are making the mixture too rich at low and medium speeds, while it becomes better at higher speeds. If this is the case; a weaker set of springs may be better overall. But I might be wrong about which way the springs need to go; so you'll need to check this out yourself. The difference between the effect of the rods and the effect of the springs is that the rods establish the basic mixture through the full range of engine speeds; while the springs determine how soon or how late in the speed and load range that effect comes in. A stronger spring will make the richness come in at lighter throttle and lower speeds. A weaker spring will delay the richness so that it only comes in at heavier throttle and higher speeds. The selection of rods and springs should be made strictly by evaluating how well it responds and how strongly it accelerates. If they make it ping more, or make it ping less, that should not be a concern at this point. The pinging will be addressed in the later steps. The third step is to find the best ignition timing setting. That will require some experimenting. The goal is to find the timing point that gives the best performance at high speeds; along with the least amount of pinging. At first, I would suggest changing the timing in 4 degree steps. When you find the best setting that way; them try moving it 2 degrees in either direction from that point. And when you find that point, then try moving it 1 degree in either direction from there. This will show you the best base timing setting. And always recheck the idle mixture and speed adjustment when you have changed the timing. But you might find that the timing that gives the best performance at high speeds does not give the best performance at low speeds. If that happens; use the setting that runs best at high speeds, and see if the low speed perfomance improves if you move the vacuum advance back to manifold vacuum, and reset the idle screws for that. Please let me know how what you ended up changing and much you changed it in these steps. I'll then make some suggestions about the advance curve and spark plugs. Thanks, Joel
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Replying to: zaken1 (Jun 27, 2009 9:45 am)
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Replying to: crampton1998 (Jun 30, 2009 6:02 pm) Because of this; and because of the outstanding fuel economy you are now getting, I expect that the spark plug configuration and the springs will not benefit from further changes. I think this is as good as it gets (or can get). So I would leave well enough alone!!! |
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