QuestionHi again Dick.
Two questions please?
When I cold start my 64 Pontiac 230 6 cyl. satandard,it shoots out carbon from the exhaust that is noticeable on my driveway.
Is this something I can fix with a fuel additive,or does it need a carb adjustment?
Speaking of carbs,on the one barrell carb,I have found two adjustment screws.One is for the curb idle and is connected to the throttle.What is the one on the other side for?,and how do you adjust the choke?
Also is the idle too high if,after starting,the vehicle moves ahead by itself after shifting into first and letting the clutch out?
On other standards I have driven,I always gave the engine some throttle while I let out the clutch in first.Thanks!
AnswerBlack water and water vapor shooting out the tail pipe is normal for all cars on a cold start, as water is a product of combustion, and the black stuff is unburned carbon, a normal result of the excessivly rich mixture that the carburetor provides as a needed enrichment that is required to start a cold engine. Modern cars with fuel injection controlled by the computer and oxygen sensor do not have as much of this, but they have some. Older cars used a more crude enrichment system and usually produce a noticeable amount of such emissions for a few minutes as the engine warms up.
The first adjustment screw is for the "curb idle" speed, which is the one you see which affects the accelerator linkage. I think you understand that already, but just for clarity, it moves the stop which prevents total closure of the butterfly valve inside the carburetor - closing that valve prevents any fuel/air mixuture from getting into the engine, so it will stop the engine if it allows total closure of the valve.
The second screw adjusts the idle mixture - another word for the ratio of fuel and air which the engine requires for a smooth idle. The correct ratio is about 13 to one (weight of air mass to fuel mass), and can only be set with precision by using a fuel/air ratio meter - a complicated and expensive instrument. In normal practice, this screw is adjusted only when you experience a poor idle, such as a rocking back and forth of the engine at idle, rough enough to disturb the passengers. The proper way to adjust this screw is: with the engine turned off, turn it clockwise until you feel a bit of resistance to turning (Note that it is very important that you not tighten it any further than this, as it will be damaged if you do so!). Then, reverse the direction and back it out about 1 1/2 total revolutions. Then start the engine, and move the screw in or out in 1/4 turn increments, waiting for the engine idle to stablilze at the new setting. Search for the smoothest, fastest idle you can achieve, then adjust the idle speed screw to reset the idle speed to a slow idle (550 RPM or so), and if you have made any significant change in the idle speed, re-do the 1/4 turn at a time fine tuning steps until you can no longer improve the idle smoothness or increase the idle speed. Now you have it at the optimum point. This whole procedure must be done with the engine at normal operating conditions.
This adjustment makes no difference in cold starting, or in road performance - it is only effective in improving the idle.
To adjust the choke, you rotate a theromstatic spring that is attached to the upper butterfly flap at the top of the carburetor. Your carburetor will have a black plastic cover over the spring, held on with three small screws around its edge. The cap may be marked with an arrow, and indicator marks for "lean" and "rich" around its edge. To make the car easier to start in winter, you may need to move this cap toward "rich" a small amount, but if the car is starting OK, just leave it alone. In summer, you can move it toward "lean" a small amount, if the rapid cold idle gets on your nerves. This adjustment affects only the cold starting and idling for the first few minutes - it has no affect on road performance or warm idle.
If the car starts well when cold, but you want to reduce the amount of engine racing, you have to bend a wire link that connects the choke flap to a cam which moves under the accelerator stop when the choke is in operation. This cam is stepped so that it increases the cold idle speed in steps depening on how far closed the choke flap is. This linkage has no affect unless you manually operate the accelerator as if to race the engine (with the engine off, of course) as the cam cannot move until the warm idle stop is swung out of its way.
It is normal for any car with standard transmission to move as soon as the clutch is engaged (pedal released) - either the car must move or the engine must stall - there is no provision for slippage as there is in a car with an automatic transmission. It is also pretty normal to have to feed in some gas to keep the engine from stalling, especially when starting out on an uphill slope. If you feel the car is too energetic about starting off without you feeding extra gas to it, perhaps your curb idle speed is too high for your liking. Be sure you adjust this only for a warm engine - the high idle speed when it it cold is needed and normal - if you lower the idle speed too far when the engine is cold, it will not be able to keep running.
Normal idling speeds for your engine are about 550 RPM warm, and up as high as 1200 RPM if it is quite cold, until the engine warms up. At 1200 RPM the car will move off quickly, even in first gear, without you touching the gas pedal.
If your engine has recently been rebuilt, the mechanic probably set the idle speed higher than normal - this is a standard practice for newly built engines. The idle speed should be left high until the engine has been run for at least 500 miles.
To measure RPM, and other useful tune-up parameters, you should consider buying an instrument from your local auto-parts store called a "Point Dwell and Tachometer" - these are not expensive, and very useful for maintenance of any older vehicle (Pre 1990s or so).
Dick
