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Old 07-16-2004, 11:08 AM
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What Came first Chicken or the Egg, or Carb Adj vs Timing

I have been told to adjust my carb using vacuum, which is easy enough, but my timing is way out. The intial timing is at about 20 deg to get it to run right. Should I set up the carb for max vacuum and then re time it or should I time it and than set the carb up?

This is what I have done in the last three months to my 1984 GMC 4x4 with a 305(Stock), 700 r4, 4ME Quadrajet, HEI Dist.

I had the tranny rebuilt, I then pulled it in the garage to rebuild the carb as it seemed to be a little sluggish at low speed when I got on it(it just sputtered slightly and than took off, the carb was rebuilt by me and my uncle who is a 30 yr GM Mechanic). When I put the carb back on I was revving it up under the hood when it died. I thought thecam lobe that drove the fuel pump was NFG but the timing chain had slipped off. So I replaced the timing chain and the cam and crank gears. I set it up so that it was TDC to #! piston at #1 plug firing. Then when I started it up it blew a little white/blue smoke, actually lots during startups(I was told that since it sat a couple of months my valve seats probably dries up). SO I checked it with a timing light and it ran best way of my chart of 12 deg, I would estimate 20 deg. I than had a plug wire shorting on the manifold so I replaced the plug wires with new ones, what is the significance of crossing plug wires? I heard you just don't cross 5 +7??? What should I do first????????????

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Old 07-16-2004, 11:24 AM
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Does your truck have eletronic spark control [esc]??? It should have an emblem on the tailgate if it does. This makes a computer control your timing so adjusting it only makes it worse. The fan shroud usually has a sticker that tells what wire to undo to check timing and at what degree it should be at. Get it timed correctly then set the carb at highest vacuum at idle with a vacuum gauge.
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Old 07-16-2004, 11:31 AM
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Always remember that the timming can affect the carb but the carb cannot affect the timming.
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Old 07-16-2004, 12:35 PM
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Set both the carb and ignition timing to max vacuum and you won't go wrong. Timing may need to be backed down a degree or two from max vacuum if it tends to run-on after key is shut off but near max vacuum is correct, regardless of what the degree wheel says.
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Old 07-16-2004, 01:01 PM
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Ignition timing to manifold vacuum, and carb vacuum at carb. Correct? No esc on this unit.
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Old 07-16-2004, 01:52 PM
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No, maximize manifold vacuum @ idle for both carb and timing. Takes a little iteration - idle jets, idle throttle setting, distributor timing, then back to throttle setting then back to idle jets, etc. until the thing is optimized. Sounds harder than it is. On engines like my hemi that are so modified that there aren't any books to tell me the proper settings, vacuum, is the ticket. In fact, vacuum optimization should be the over-riding parameter on any engine. Except for maybe the modern computer engines but I wouldn't even consider fooling with one of those so it isn't an issue!
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Old 07-16-2004, 07:02 PM
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Unrelated, but interesting.

"what is the significance of crossing plug wires? I heard you just don't cross 5 +7???"

I just bought some new wires for my truck, and on the installation instructions for the wires, it said to avoid running them paralell to each other. To make sure that they criss cross or something to avoid one wire from causing a spark in another wire. I couldn't figure out how I should do that, so I ignored those instructions.

I've never seen or heard of such a thing before, but it did say this on the instructions.

Bizar.
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Old 07-16-2004, 07:39 PM
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basically they just want you to keep the two wires apart from each other.
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Old 07-17-2004, 07:47 AM
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If I understand your question correctly, you want to know whether you should attempt to adjust the carb or the timing first.

For non-computer controlled distributor only, I would set the 'Total Mechanical' timing first as this is done at a higher than idle RPM (usually 2500-3000 RPM) and is not dependent on the carbs idle circuit or the vacuum to the vacuum advance. The vacuum advance must be disconnected and plugged off when setting the timing. Hook the vacuum advance back up after you have set the timing.

After you have done that; set your curb idle speed to what you wish (usually 650-750 RPM in gear for an automatic trans). Next, hook up a vacuum gauge to a 'FULL' manifold vacuum source and slowly adjust the air/idle screws to achieve the highest vacuum at idle that you can get on the gauge. You will probably have to re-adjust the curb idle speed during or after these air/idle screw adjustments.

After you have adjusted the carb for the highest vacuum at idle and adjusted the curb idle speed; check the timing once again. You may need to re-adjust the timing slightly. If you do have to adjust the timing, go back and re-check the vacuum readings at idle and re-adjust as needed.

The idea is to get the timing set where you want it to be and have the highest vacuum reading at idle at the same time.

Take your time as it is initially a back and forth (timing, carb adjust, re-check timing, etc.) process.

After you are satisfied with all that a road test is next. Hesitation or bog when going to WOT usually is caused by a temporary lean condition and can be eliminated by increasing the accelerator pump shot. This may be done by a linkage adjustment, accelerator cam change or a larger squirter (depending on the carb you are using).

Pinging when going up a slight grade in high gear at a modest speed is caused by too much advance and can be eliminated by retarding the timing slightly. Trial and error will solve this condition. Make small adjustments and road test until you get what you want.
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Old 07-17-2004, 03:01 PM
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I adjusted the timing from where I had it at idle 9 deg(15 in HG vacuum), to highest vacuum (20 in HG)which put me at aprox 24 deg. I then adjusted the carb at which I got it to 21.5 in hg, this was at 6.5 turns out of the screw. I then went for a test drive at which it ran good but at a coasting speed I can hear a slight noise like something was loose? and when I came home the idle was sitting way up at approx 1500 rpm but then I shut it off got a wrench turned it back on and it was back to where I set it around 600? Why can I not set my timing to factory spec of 4 deg??????
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Old 07-17-2004, 03:46 PM
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Quote:
Originally posted by willys36@aol.com
Set both the carb and ignition timing to max vacuum and you won't go wrong. Timing may need to be backed down a degree or two from max vacuum if it tends to run-on after key is shut off but near max vacuum is correct, regardless of what the degree wheel says.
Never ever set your timing to maximum vacuum. First of all, advancing the timing more will increase your idle. Increased idle will raise your vacuum. So, that is not a good baseline.

The real way to set your timing is to figure out what you total timing is using a timing light or timing tape. Your total timing is your initial plus your distributor timing.

Since your truck is smog year, I would set the timing pretty close to the recommended curve. You can add a little more initial if you want a little more performance. I wouldn't go more than 4o if you don't also decrease your distributor advance to compensate. Again, make sure you know exactly what your timing curve is doing using the correct tools.

Quote:
Originally posted by riesman20
I adjusted the timing from where I had it at idle 9 deg(15 in HG vacuum), to highest vacuum (20 in HG)which put me at aprox 24 deg. I then adjusted the carb at which I got it to 21.5 in hg, this was at 6.5 turns out of the screw. I then went for a test drive at which it ran good but at a coasting speed I can hear a slight noise like something was loose? and when I came home the idle was sitting way up at approx 1500 rpm but then I shut it off got a wrench turned it back on and it was back to where I set it around 600? Why can I not set my timing to factory spec of 4 deg??????
That sound you said that was like something was loose was pinging. That is what happens when you adjust your timing to the highest vacuum reading.

Your distributor springs and advance limiters may be worn or not working properly.

Quote:
Originally posted by Terje
"what is the significance of crossing plug wires? I heard you just don't cross 5 +7???"

I just bought some new wires for my truck, and on the installation instructions for the wires, it said to avoid running them paralell to each other. To make sure that they criss cross or something to avoid one wire from causing a spark in another wire. I couldn't figure out how I should do that, so I ignored those instructions.

I've never seen or heard of such a thing before, but it did say this on the instructions.

Bizar.
When you cross wires in an x they have less surface contact. That way the induce less voltage. Inductance is resistive in nature and will reduce the current to the spark plugs. It is also very possible to induce enough current between the plugs to actually cross fire the spark plugs.

For your stock engine I would just recommend going with the stock timing curve. Verify that the curve is correct, and then adjust the carb to the highest manifold vacuum.
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Old 07-17-2004, 06:26 PM
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Quote:
Originally posted by lluciano77
First of all, advancing the timing more will increase your idle. Increased idle will raise your vacuum.
That's why you need to some iterations on tuning; set idle screws, then set timing which as lluciano points out increases idle speed, reduce idle speed, reset idle screws, reset timing and so on all to max vacuum. Two or three iterations will get all three perfect. Only further adjustment necessary is back down timing a degree or two if it pings or runs on when the key is turned off.
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Old 07-17-2004, 06:31 PM
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Still, the overall optimum timing will not come at maximum vacuum. The more initial advance you can run without detonation or the starter kicking back on start up, the better. If you have to pass emissions, you will have to go with factory specs.
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Old 07-17-2004, 07:05 PM
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I probably have no idea what I'm talking about here, but it seems to me that maybe for any given airflow the timing and fuel delivery that maximizes the manifold vacuum will probably maximize horsepower and torque output for that airflow.

But, since there is a centrifugal and vacuum advance on there that has been tuned by the engineers who designed the vehicle, and you have no control over them, you have to follow the instructions when adjusting the timing and mixture at idle in order to make it so that the timing and mixture at different throttle positions and under different load conditions are correct, otherwise you will be adjusting your engine for maximum efficiency or power at idle completely ignoring it's performance under load.

But in my opinion, the power and efficiency of the engine at idle really doesn't matter as much as the power and efficiency of the engine under load.

Maybe you have some awesome distributor that allows you complete control over the advance under every conceivable load conditions, so you can optimize the performance of your engine for power without regard for efficiency at every conceivable loading and throttle position, but I think that such an awesome distributor would be difficult to adjust and tune correctly.

As a matter of fact, if the engineers who design the engines could design such a distributor and carburetor for engines, there would be absolutely no reason why they would start using computers to control things such as the timing and fuel delivery to your engine based on all sorts of sensors distributed all over your engine.

I believe that what you should do is make sure that the timing at idle is exactly what is indicated on the smog sticker on the engine, and make sure that the advance mechanisms on the distributor are functioning as they were designed, then go back and forth between adjusting the air fuel mixture and idle speed until they are perfect.
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Old 07-17-2004, 07:45 PM
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Just think about all of the qualified technicians setting at the dyno, changing fuel and timing curves. Or the guy at the track that changes his timing between runs. If all it took was to get maximum manifold vacuum, and your timing would be optimal, wouldn't the experts do it? Why are they wasting their time?

Manifold vacuum doesn't take into account gearing, stall, engine output, vehicle weight, ambient temperature, etc. A motorhome will not tolerate the same advance curve as a Falcon Sprint with 4:56s.

Like I said earlier, go with the stock curve. If you want more performance, you can try adding up to 4o more to your total.

The maximum vacuum method of timing changes will destroy your engine.
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