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Old 04-26-2007, 08:09 PM   #1
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Amazing what a tune up will do

I pulled my car up to the garage to poke around and stick in a 195 degree thermostat. Ended up also adjusting the points in my distributor, I didn't realize how bad they had become. I wonder if they were ever right.

My engine idles really fast now, I am guessing 1500-2000 rpm (don't have an rpm meter). My carb is next in line for a tune. It hasn't been tuned since I pulled it off a junkyard rack more than a year ago .

I am hoping to hit 25mpg on this next tank of gas considering how much a difference those points made. And hopefully higher when I can get my car in for a carb tune. Unless someone here has a good resource on how to tune a carb for FE, I would love to do it myself just don't have any idea how. O'reilly didn't even carry the rochester carb book! lol

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Old 04-27-2007, 01:52 AM   #2
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I think i have that book. I rebuilt mine several months ago. For 20 bucks or so you can get a bucket of Berriman's B12 carb dip. It makes cleaning parts easy! What kind of carburator? Mine is a dualjet.

85 Chevrolet. 30 MPG or bust!
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Old 04-27-2007, 06:34 AM   #3
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If you adjusted the points you probably changed the ignition timing at the same time i.e. if the gap opening is larger now then you advanced the timing and that would make the idle increase since the timing at idle is usually retarded to make it run smooth, stall less and easier to start.
As far as the carb adjustments lean it out until is starts skipping then bring is back a little to run smooth - used to be that you adjusted for max rpm but leaner is burning a bit less fuel for a little lower rpm. High speed jets could probably be reduced a few sizes so that you don't get into the rich A/F mixture at all but still have enough power to go fast - this will allow more efficient fuel use with less max power. Don't know how much work that would be however or the size change needed - maybe a % of the jet size for the % change in A/F ratio would be my first guess.
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Old 04-27-2007, 07:55 AM   #4
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Ahh the good old days of carbs and distributors. Now that is wrench turning!

Some other fun things to do during your tune-up...
1)Advance the timing until you get some ping during normal driving in warm weather, then retard three or four degrees. This will be really close to your best ignition timing setting.
2)Blueprint your points. Make sure they close in the middle and not on the edge of the points. Use a magnifying glass to really see it well. You want maximum contact surface area. Carefully bend the contact arm with needlenose pliers, do not touch the contact surfaces!
3)Blueprint your cap and rotor. Check clearance of your rotor to the cap contacts. Use a dab of dielectric grease on the rotor, spin the motor by hand and see how much grease is left on each. If possible, carefully bend the cap contacts (some you do not want to bend as they'll break the cap!) for minimum clearance. If you cant bend the cap contacts, some manufacturers make rotors a little larger than others, use the largest ones that will fit.
4)Use dielectric grease on your plug wires. Prevents electrical leakage and makes it easier to remove wires in the future.
5)Open your plug gap in .005" increments to where you get some miss under load, then close the gap .010" this is pretty close to your optimum plug gap (make sure your other ignition parts are in excellent shape as this will reveal the next weakest point in your ignition system).
6)Index your plugs.
7)You can build a pretty good fuel mixture analyzer for about 60 bucks, less if you find good deals. If you don't have an o2 sensor regulated carb, get a universal o2 sensor, o2 sensor bung from an exhaust shop, and an inexpensive air/fuel ratio gauge. Weld in the bung close to the manifold, wire it according to the instructions and it will give you a good indication of your mixture. This way you won't be guessing, reading plugs with accuraccy is really hard with todays fuels. An expensive wide band sensor and gauge will give a much greater range of mixture and will be more accurate but will take a lot longer to pay for itsself in fuel savings.

Other people will have even more ideas on how to optimize a tune-up.
Happy wrenching!
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