The ignition timing is very important to the carb adjustments. Both are integrally tied to each other. That is why I told you to go back and start with the initial timing setting, then tune the carb. 90% of so called "carb problems" are actually caused by changes in the ignition. When you set a carb, it stays set. The only moving parts are the accellerator pump linkage and the throttle shaft(s) plus the float, but the float moves so very slowly that it isn't a factor in the settings as far as wear goes. The choke moves too, but that's the first thing I remove from a carb, it usually causes more problems than the benefit it provides, and unless you are in a cold climate and drive the vehicle when it is cold frequently, the choke is pretty much worthless in my opinion. I'd rather baby the throttle a few minutes til the engine will idle on it's own. Jory, you're near Dallas, you do not need a choke this time of year, it's virtually worthless sitting there on your carb in 80-100* heat. Wire the damned thing open, reset your initial timing and then go back and start fresh with the carb settings and see how it runs then. If you're running a points ignition, the point setting also has everything to do with the tune. If you do have points, use a dwell meter to set the gap, unless you use a dwell meter, you're shooting in the dark. The point gap setting (.017) is only a baseline setting when installing a set of points, you HAVE to use a dwell meter to get the gap set where it's supposed to be. The dwell controls the length of time the coil has to build up the proper voltage for the next spark. Poor spark = poor running engine. If you don;t like setting points or don't have a dwell meter, then replace the points with a Pertronix (or any other electronic ignition) to eliminate the ignition as a source of tune problems.
This problem has persisted using both a Pertronix Ignitor III and a new GM style HEI with a stock GM module in it. My car runs great at 12º initial and 36º total timing, it's just this one area that is the problem. I get what you're saying about the choke, but that doesn't seem to be the problem considering I'm not having a starting problem. When you said reset my initial timing, did you mean back to the stock 6º or just back to 12º?
the choke can still affect the A/F ratio at other throttle angles too if it's not completely open.. or wired to prevent movement. Which is why he/we rec'd getting rid of it and eliminating it from the equation altogether. Basically, one less variable to tune around. And I'm pretty sure he meant to use the 12 degrees he originally mentioned earlier. Are you sure that the mechanical advance plate is moving freely by verifying with a timing light that timing advances smoothly as rpm increases?
My car seems to run okay until I'm on the freeway. When I'm cruising at 55 mph I can hear it popping through the exhaust. I'm dumfounded. Can't figure out if it's carb related or needs tune up.... Other than that, it runs awesome... Any ideas?
Could be the ignition, or could be a lean condition. When was it last tuned up ? What carb is on it ? What ignition ? (you might want to start a new thread for your problem too)
My problem exists when the choke plate is both partly closed when cold and also when the motor is at operating temps and the choke is fully open. I do all adjustments when the choke is open and the car is warm. I have tried many times starting over with resetting timing and adjusting the carb and I always get back to this same problem. I've watched the timing advance as I increase rpms and it appears smooth when using my timing light. Does the vacuum advance stop adding timing around 2000 rpms usually as the mechanical advance comes in?
You're running an Autolite, right ? The dry bowl after sitting seems to be a problem with them, I've seem em do the same thing, that's one reason I run Holleys now. Have you rebuilt the carb lately ? Now, although you say the choke is open, they can and will shut while the engine is under full load from the vacuum. That's why I suggested to wire it open or better yet, remove it altogether to eliminate it as a cause of any problems.
vac advance can be hooked up 2 ways. Ported and full manifold vacuum. Niether one will "stop" at that rpm range under lighter loads. To effectively.. and properly.. tune an engine.. you always tune without the vac advance pot up to what the motor will allow before pinging or loss of power results.. THEN you add the pot to the equation with a lighter setting and work up to more aggressive settings. If you already have lean condition?.. adding the vac advance will only compound the issue even further. In severe cases of lean out.. you can actually crack/collapse ring lands and melt holes. And sometimes.. the motor will make more power than usual right up to the point of failure so be very careful here. Did you have this same exact carb on this particular motor to begin with? and was it jetted in identical fashion as it is now?
I'm running an Autolite 2100 (original) that i just rebuilt hoping to get rid of this problem. I'm currently running off of ported vacuum and I do all timing settings with the vac adv disconnected (hole plugged). Does it need to be disconnected for the carb settings too? At my current timing and how I always set it up, it never pings under hard acceleration. I usually set up my timing based on the highest vacuum I can get and then turn it down a couple degrees. I'm just lost why it's running lean at only that particular rpm when everywhere else seems fine.
If you're running it with ported vacuum, you can leave it hooked up when you set the initial timing. Make sure it's working too, you can do this by removing the distributor cap, then pull the vacuum hose off the carb port, then while sucking on the hose, watch the breaker plate to see if it moves as you pull a vacuum on the hose.
Ok, this is how I do it. When I have my idle screws out 1-1/2 turns, it runs like crap until I get them far out like I do now (4 to 4-1/2). It's always been like that. And my vacuum advance works fine now, just like my old one did. BTW: Baddad, GRoberts, all you guys....thanks for all your advice and patience...I'm still learning, I forget things, some things I totally misunderstand, some things I just don't think about....so thanks for all your help in trying to explain things to help me get my car working properly!!!!
POST EDITED: it was late and I was exhausted but still wholeheartedly trying to help you out.. my post was redundant as hell(even moreso than usual).. and some points about port functions were obviously misunderstood. This threads 1st post will explain more about advance curves than anyone on here would probably ever take the time to.. even if they actually knew how to do it. I highly suggest that you read it and learn some things about recurving distributors so you can enjoy some added benefits and fuel savings. Plus.. once you know.. you can apply it to just about any non-computerized engine you'll ever own from here on out. http://www.hotrodders.com/forum/distributor-tuning-theory-part-1-a-59033.html A final piece of advice here. If the engine is running rich at idle(as evidenced by the current "4 1/2 turns out" mixture screw adjustment).. running ported vacuum does absolutely nothing to improve the situation. Full manifold vacuum will improve it considerably and it's extremely simple to test by merely swapping vacuum sources. Just be sure that the total figure for no-load advance(initial, mechanical, AND the added vacuum advance) seen by the timing light does not exceed around 50 degrees at higher rpm's(around 3,000-4,000rpm when all the mechanical has come in) or efficiency will be negated and start becoming counterproductive. If the engine makes more light throttle power.. and almost all do.. but starts to surge as yours seems to be doing here?.. then you need more main jet to compensate for the lean out. Good luck with it all.
I remember hooking up a tee in the hose to my distributor advance line, and I connected a manifold pressure gauge to it inside the car. My vacuum advance is connected to ported vacuum on my car. I do know that on my particular carb ( rbs Carter ) that the vacuum increases up to a point of throttle opening and then drops off as the throttle is opened further. I can't give you an exact percentage of throttle opening, but I will say that the vacuum reading is just about the highest at highway speeds. If you open the throttle more ( say passing another vehicle or going up a hill ) the vacuum drops off. I'm in no way saying that all carbs are designed this way, I just know mine works this way. I also remember earlier cars that only had a mechanical advance! I believe that the vacuum advance was developed for emission control. I may be totally wrong on this, but I do know emission controls weren't to make your car run better!!! I still don't understand how a vehicle can be made to use more fuel and create less pollution? When ( years ago ) I got rid The egr and the air injector, the car ran twice as good and the gas mileage was way better!
To answer your question,(simple man) many so called "performance improvements" (i.e. hot rod tricks) are really designed to maximize fuel efficiency, they're not necesssarily meant to make the engine burn more fuel. Some emmisions equipment systems are meant to do the same thing, although not all do. Vacuum advance was meant to maximize the spark curve to do just this. I've never run mine on full manifold vacuum, only ported, and I've never felt the need to do otherwise. Many of those so called engine builders use full advance when setting up their engines due to the fact that most are building "racing only" engines where it's more desireable to set it up to run "locked In full advance". Your test (simple man) shows that a ported vacuum source does the opposite of what groberts suggested, the timing is added and dropped with engine load in some instances.
