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Carburetor Problems On Older Cars - READ ONLY

399 messages,  Last post on Jan 26, 2011 at 12:39 AM

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Messages Page 38 of 40

#370 of 399 Re: Carb still bogging [Mr_Shiftright] by zaken1

Jun 14, 2009 (7:56 pm)

Replying to: Mr_Shiftright (Jun 14, 2009 5:28 pm)
Joe, There were many different problems. For one thing; the ignition system was not getting adequate voltage from the battery. This was caused by 1> a defective ignition switch; 2> the use of TWO ballast resistances in series (one being the original resistance wire that was stock on the Ford pickup that the Chevy V-8 engine was in; and the second being a GM ballast that had been mounted on the firewall), so one of them had to be removed. 3> the use of a battery ground cable that was connected to a painted accessory bracket, rather than being grounded to the motor.
 
Secondly; the plug wires needed replacement, and the plugs were fouled.
 
Thirdly; the distributor vacuum advance was being driven by ported vacuum; rather than the manifold vacuum which is normally used on Chevys of that era.
 
Fourth; the base ignition timing was set to the stock figure; which was about 6 degrees too retarded for the cam that was in this motor.
 
Fifth; the distributor point dwell was not being set properly; until he got a dwell meter.
 
Sixth; because the ignition was so weak; whenever an attempt was made to adjust the idle mixture; it couldn't be set properly.
 
But it was replacing the plugs and wires (after the ignition switch, timing, vacuum advance source and the ballast resistor had been straightened out) which led to the spectacular improvement in performance.
 
But he just posted a new message; which suggests that it is now flooding (or the idle mixture is set too rich; or there is insufficient air bypassing the throttle, because the PCV system is still plugged up). So there is still some more work to be done. I just hope the plugs don't foul before the mixture is corrected.

#371 of 399 Re: Carb still bogging [crampton1998] by zaken1

Jun 14, 2009 (8:39 pm)

Replying to: crampton1998 (Jun 14, 2009 5:36 pm)
What was your wife doing when it died???
 
Seriously, whenever a major change (like replacing plug wires and plugs) takes place; it will often be followed by the idle mixture changing, and then needing to be readjusted. It sounds to me like the idle mixture is now way too rich (screws set too far out). So I recommend first turning both mixture screws in to 2 1/2 turns from seated, driving it to clear it out, and then adjusting the screws to the furthest closed setting that still gives best vacuum and best idle quality. If the screws end up at less than 1 turn out; this means that the carb is now not getting enough air; so you're going to have to bite the bullet and either install the breather on the PCV spigot; or connect the PCV valve in the stock location. But the PCV valve will not work properly if there is no place for fresh air to enter the engine.
 
There are two sections to the PCV system; the PCV valve and hose draw air from the engine crankcase into the carb, but there also has to be a top half of the system which allows fresh air to flow into the engine, so there can be a cross draft of air which replenishes the air drawn through the PCV valve. Fresh air normally flows into the top half of the PCV system through a vent opening in the top of the valve cover on the passenger side. This valve cover vent is normally connected to a metal tube which plugs into the side of the air filter housing. But if your carb has a custom air cleaner; there may not be a place in the air cleaner for a crankcase air supply hose (which is called a KV hose). If there is an opening in the top of the passenger side valve cover; you might be able to attach a filtered breather to that opening. Otherwise; if there is an oil filler pipe which sticks up in the middle of the front of the engine, you could install a filtered type oil filler cap on that pipe. But if the oil filler cap is only on the valve cover; you'll either have to get a vented passenger side valve cover from a wrecking yard; or make do with the breather connected to the PCV spigot. I expect that the motor will now run better with the breather than it did before. But you may have to set the mixture screws very differently when the breather is connected than how they are set without the breather. The vacuum gauge will tell you the best setting for the mixture screws; but bear in mind that the adjustment may change after you drive the truck a while.

#372 of 399 Re: Carb still bogging [crampton1998] by zaken1

Jun 15, 2009 (11:46 am)

Replying to: crampton1998 (Jun 14, 2009 5:36 pm)
One other thing you ought to know about; when all the critical adjustments on an engine are dialed in at the same time, something miraculous happens; the engine suddenly reaches a level of performance which was never attainable before. But there is a consequence of such a change; the engine's fuel octane requirement typically increases substantially. Small block Chevys inherently tend to have a higher fuel octane requirement than just about any other V-8 out there; and when they are massaged, that octane requirement increases even further!!!
 
I have seen small block Chevys that were tuned perfectly; and as a result became barely able to run on regular gas. They would become extremely hard to start, and stall, stumble, and miss; just as if the tuning was all messed up. But simply switching to the highest octane premium fuel available would completely clear up the problem.
 
I am painfully aware of the outrageous cost of using premium fuel today. But if the idle mixture adjustment and PCV volume is not the cause of your problem; the only other option besides premium fuel is to retard the ignition timing to about 6 degrees BTDC, and then reset the idle mixture and speed. And detuning it that way will reduce the gas mileage and power substantially.

#373 of 399 Re: Carb still bogging [zaken1] by crampton1998

Jun 15, 2009 (4:18 pm)

Replying to: zaken1 (Jun 15, 2009 11:46 am)
I switched the vacuum advance to the port intake and adjusted the fuel mixture screws, the screws didn't make a difference when adjusted on the manifold intake. The engine is running great with no problems whatsoever. My wife was clutching anything that she could get her hands on, she had never experienced an engine staling before. Thanks again for all of your help.

#374 of 399 Carburetor help tuning by red79phoenix

Jun 18, 2009 (10:40 am)

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

#375 of 399 Re: Carburetor help tuning [red79phoenix] by zaken1

Jun 18, 2009 (12:54 pm)

Replying to: red79phoenix (Jun 18, 2009 10:40 am)
Hi again Red,
 
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.

#376 of 399 Pinging noise when accelerating ? by crampton1998

Jun 22, 2009 (8:02 pm)

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.

#377 of 399 Re: Pinging noise when accelerating ? [crampton1998] by zaken1

Jun 23, 2009 (12:31 am)

Replying to: crampton1998 (Jun 22, 2009 8:02 pm)
The ignition timing in an engine is supposed to be set so that the pressure wave from the burning mixture in the cylinders strikes the piston as close as possible to the top of the compression stroke, when it is just starting to move downward on the power stroke. The closer to TDC the wave hits the piston; the more power will be produced; so the goal in setting the timing is to advance it as far as possible; BUT NOT SO FAR ADVANCED THAT THE WAVE STRIKES THE PISTON BEFORE IT REACHES THE TOP.
 
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.

#378 of 399 Re: Pinging noise when accelerating ? [zaken1] by crampton1998

Jun 23, 2009 (6:50 am)

Replying to: zaken1 (Jun 23, 2009 12:31 am)
Thanks again Zaken, I will re-adjust the timing on my next day off and see what happens. Once again you are correct, I did swap out metering rods in an attempt to "fine tune" the engine for optimal efficiency. Thanks again and I will post the results as soon as possible.

#379 of 399 Re: Pinging noise when accelerating ? [crampton1998] by zaken1

Jun 23, 2009 (11:05 am)

Replying to: crampton1998 (Jun 23, 2009 6:50 am)
There is one other issue that we have not yet addressed: As the engine's state of tune evolves, the spark plug heat range and gap style may need to be re-evaluated. Your current spark plugs have a projected tip, but some Chevy engines run better with non-projected tip spark plugs (in which the center porcelain insulator does not extend beyond the end of the metal shell). This is not a common design these days; but it frequently runs better in older model or modified motors. If the spark plugs are unsuitable, the timing will not be able to be adjusted satisfactorally.
 
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.

Messages Page 38 of 40

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