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Need help understanding timing


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So I want to get the timing correct on my 1972 Mustang before further tuning the carburetor. I had a friend help me a while back with it last time, but I still don't really understand how to figure out what timing is best. I am running a 351 Cleveland with 2 valve heads, an Edelbrock 4 barrel 650cfm carburetor, and pertronix ignition. I've been trying to figure out why my engine backfires when i step on the throttle hard. If I do a steady acceleration it is typically fine but I notice a lack of power when i accelerate hard. I don't have a tachometer unfortunately but I do have a timing light. Is anyone able to help walk me through the process? 

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My experience is only with my 71 Mach 1 351C 4V 4 speed. This is an M code motor, but with 13 cc dished pistons to give about 10:1 comp ratio. I have a Holley 670 Street Avenger carb, slightly modified. At the start after it was rebuilt, I too had a lot of issues with timing, which is something like a "black art". Much has been posted on this over the years, so a search might help you. 

Before I or others can go further, can you tell us what distributor you have. You did say a Pertronix, but which model. Do you have the matching coil. What plugs and plug wires are you using. All this matter a great deal, so if you can give us everything you can, I or someone else will get back to you.

Backfiring could be a lean condition also, but I have zero knowledge of Edelbrock carbs.

Geoff.

 I learn something new every day!

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+1 with Geoff.  A lot of factors come into play when  setting your timing.  Here is a good article that explains the difference types of timing and how timing works.  It is well worth the read for someone without much experience. 

 FORDMUSCLE webmagazine: Timing is Everything - Distributor Curving for Maximum Power

Kilgon

 

 

"The only dumb question is the one not asked"

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Plug wires are Autolite 7mm Professional Series and Spark plugs are Motorcraft ASF42CA Copper Core. The distributer cap isn’t anything other than just a stock one I got at the parts store. Distributer itself is original as far as I know. And it’s just the Pertronix igniter inside the distributer cap. 

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Backfire on quick throttle change is a lean condition. You need to increase the accelerator pump shot by moving the linkage. You may need to go to larger discharge nozzles to get more fuel into the intake quicker. 

 

 

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See I was wondering if it was running rich initially because a friend driving behind me said that when I’d accelerate there would be black smoke coming from the passenger side exhaust pipe. However my spark plugs don’t seem blackened by carbon. I was hesitant to install larger jets because of that. I did install the silver 8 inch HG step up springs in the carburetor to see if that helped my acceleration issue.

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Rule # 1 with the timing light - mind the fan and belts.  They will mess up a timing light and remove fingers in an instant.  Before you start get a good look with a flashlight and plan where you are going to place the timing light, wires, hands etc.

 

Do you need a guide on how to use the light / adjust timing?  
 

Also what distributor and camshaft do you have?

 

Got $300 sitting around to buy a wide band Air fuel ratio gauge and pay an exhaust shop to weld a bung in?  Tuning is way easier if you know what your AFR is in real time.  Best $300 you will spend

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Bigfoot72,

For anyone to be able to help you, we are going to need to know these specifics.

1 what pump slot is your accelerator pump in ?

2 what is your base timing

3 what is your idle speed (with vacuum advance disconnected and plugged)

4 how many (distributor degrees) is your mechanical advance your distributor should have 2 slots 180 degrees and only one of these two slots will be occupied (might say 10L or 13L.

you can also tell how much mechanical advance you have by subtracting your TOTAL timing (where it quits advancing with timing light and subtracting that from your base timing (divided by 2)

 

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You can get an automotive multimeter with tachometer at Harbor Freight without breaking the bank.

LCD Automotive Multimeter with Tachometer Kit (harborfreight.com)

Knowing your RPM is essential for timing your engine and adjusting the carburetor.

Start with setting your initial timing (with vacuum advance hose disconnected and plugged) at 12° before top dead center (BTDC), and idle speed adjusted to 650 RPM (in drive if you have an automatic transmission). That will give you a good starting point for getting your carburetor adjusted.

As others have said, backfiring on hard acceleration is indicative of a lean condition.

Black smoke can be other things besides a rich carburetor, such as oil burning or blowing carbon out of the engine and exhaust. As you noted, your spark plugs aren't showing rich.

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“If you can't explain it simply, you don't understand it well enough.”

--Albert Einstein

 

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Thanks for the update on what you have. Again, I'm not at all familiar with 73's, but a few more things come to mind.

1) is this motor stock or modified in any way? (other than the carb/intake) 

2) Pertronix module, is it a black ignitor I or a red ignitor II. I hope it's not an ignitor III which is known to be problematic.

3) Your plugs are the equivalent to Autolite 25's, which were the plug spec'd for the 71 Boss motor. I use these in my motor as it proved to need a hotter plug with 10:1 compression ratio. I'm not sure what the spec'd plug is for a stock 73, might be worth checking as your plugs might be a bit too hot.

4) the distributor is an original Motorcraft/Autolite. What condition is it in overall, can you feel any sideways play on the shaft? Excessive movement might cause the timing to "wander", but not the cause of the backfire situation I'm thinking. If the dizzy is in ok condition, I would want to know what the timing slot number is. For this, you would need to pull the points breaker plate out. I might think it will be a 15L which equals 30 degrees of crank timing. This would likely have had only 6 degrees of initial timing for a total of 36 mechanical, no vacuum applied. For some reason, this number seems to be the max, BUT other than for emissions, 6 degrees is not enough for Cleveland's. That is why 12-16 degrees is desirable initially and why the slot number needs to be reduced. If your dizzy has a 13L slot, you could set the initial to 10 degrees and try that. Now here's a quick trick, place a piece of NYLON 1/8" tubing over the little post. That will get you close to a 10L slot. A 10L slot I found to be perfect on mine with 14 degrees of initial timing, 34 total, 20+14.

This can and will get overwhelming I know. It took me a looong time to get it straight in my head, but then it's simple..... sort of! Maybe I'm getting a bit too far ahead at this time, but bottom line is you need to know what you're dealing with. Here's a couple of pics of the slot plates that might clear things.

 

 

DSC00253_LI (4).jpg

IMG_0432.1.JPG

IMG_0435_L2.jpg

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Geoff.

 I learn something new every day!

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The pump slot I’m in is the middle, however I tried the highest one and made no difference. I’m going to try increasing the jet size and then see if that helps. Should I only increase the primaries or also do the secondaries too? 

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bigfoot72,

Go back and read your very first sentence of your thread, you state you want to get your timing correct BEFORE adjusting your carb!.

The people in the know here are going to tell you it is Not the main jetting and SURELY not the secondary jetting.

Stanglover has shown you what to look for in the distributor, you NEEDto do that first.

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@Bigfoot72,

Not wishing to upset you, but please, can you tell us exactly what your motor is and what's been done to it. I stress, I'm not an engine expert by a long shot, but over the last 13 years, I've come to find out how to get the best out of MY engine, which is pretty much stock. 

There are still other possibilities to consider, timing chain and gears, cam timing, incorrect cam, valves not seating correctly etc. and I'm sure I'm missing other items. What I'm asking here is for a starting point. As Boilermaster said, the carb is NOT what you should be playing with just yet. I know squat about Edelbrock carbs, not a carb I would chose, but from what I have read about them, they should be pretty close right out of the box and like Holley's, may need minor adjustments. A 650 cfm ought to run so-so on your motor.

Please get back to us.

Geoff.

 I learn something new every day!

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The engine is a 351-2v Cleveland. Originally a 2bbl carburetor. The internals are stock to the best of my knowledge. The only notable aftermarket parts are an Edelbrock 650cfm carb, RPM air gap intake manifold, Edelbrock valve covers, air cleaner and pertronix ignitor. The cap/rotor, plugs, relay, and fuel pump haven’t been modified, only routine replacements with identical parts. I took out the factory air conditioning a while ago since it wasn’t working and I wanted to have more space to work on the engine. The factory heads were also refurbished within the last 3 years due to hairline cracks in both. Aside from all that, I can’t think of anything else. 

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I apologize I haven’t had the chance to read everything yet, been fairly busy the last 2 days and the catalytic converters got stolen out of my other car 😕 I also want to note that I do have a tachometer now. It’s an old engine analyzer tool that I just found and got working again. I also have a timing light and vacuum gauge. I dropped the vacuum gauge by accident the other day and I don’t know if it’s reading accurately. Im showing 19hg at 710rpms idle. Seems a bit high in my opinion but wanted to confirm if that sounds right to others.  

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That's about right for an altitude near sea level and a mildly modified or stock engine.

As long as it works is what is important, you'll be looking at variations in the vacuum as you tune it.

 

 

“If you can't explain it simply, you don't understand it well enough.”

--Albert Einstein

 

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I’d do an as-found timing test and report.  Measure the initial and total timing.

Disconnect the vacuum advance and plug the hose.  You may have to turn your idle screw up a bit to get it to run.  Measure the timing at idle- that is the initial.  


Rev the engine and record the peak advance- that is your total.  With a stock distributor you may have to rev it upwards of 4,000 RPM.

Be mindful of fingers and cables, the belts and fan are closer than you think when the engine is running.

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Good luck with it. Don't get frustrated if you don't get the results you want first time. It took me several "tries" to get to where I am with it now. One item I don't recall being mentioned was the distributor weight springs. These control the rate of advance and can be a bit tricky to get just right. If you look at my pic with the markings, you will note I have 1 of the original heavy springs and one Mr.Gasket D925 (I think that's the number). The heavy spring is set just slightly loose at rest. The lighter D925 is set with slight tension. Tension is set by adjusting the outer post. 

Other than that, I think you'll figure it out, so let us know eh! 

Geoff.

 I learn something new every day!

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 If you have recurve your distributor and modified the width of the advance slot then one of the best ways to take some of the guest work out of what you have as initial and mechanical advance is to set your timing at the highest "all in" rpm's.  Using your timing light have someone slowly accelerated the rpms until you no longer see any advance in the timing and  then set your timing at the desire number - usually at 36 degrees for most engine.  Depending on the springs you used the "all in" should be somewhere between the lows 2 to low 3000 rpm range.

Once you have this number let the engine come back to idle and take a reading.  Subtract the idle timing from the total and the difference is what you have as advance.  You can then make adjustments as needed to either the initial or if needed the amount provided by the advance slot.  Remember that  your total combine should not exceed your engines recommend amount.   This might call for additional adjustment to the advance slot to get the final numbers you want as far as initial and advance. You can also bend the spring tabs to change spring tension to adjust how quick the advance comes in.   

Remember not to exceed the amount of total timing that is recommended for your engine.  

Edited by Kilgon
Added "all In'" for clarification.
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Kilgon

 

 

"The only dumb question is the one not asked"

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2 hours ago, Kilgon said:

 

 If you have recurve your distributor and modified the width of the advance slot then one of the best ways to take some of the guest work out of what you have as initial and mechanical advance is to set your timing at the highest "all in" rpm's.  Using your timing light have someone slowly accelerated the rpms until you no longer see any advance in the timing and  then set your timing as the desire number - usually at 36 degrees for most engine.  Depending on the springs you used this should be somewhere between the lows 2 to low 3000 rpm range.

Once you have this number let the engine come back to idle and take a reading.  Subtract the idle timing from the total and the difference is what you have as advance.  You can then make adjustments as needed to either the initial or if needed the amount provided by the advance slot.  Remember that  your total combine should not exceed your engines recommend amount.   This might call for additional adjustment to the advance slot to get the final numbers you want as far as initial and advance. You can also bend the spring tabs to change spring tension to adjust how quick the advance comes in.   

Remember not to exceed the amount of total timing that is recommended for your engine.  

A far better description. 

Edit: I should mention that I had to tig weld a wider slot on the outer side end, then file it to a dimension of .410". I don't remember where I found that number, but it is correct. This may be tricky for someone without access to a tig welder. 

Edited by Stanglover

Geoff.

 I learn something new every day!

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4 hours ago, Stanglover said:

A far better description. 

Edit: I should mention that I had to tig weld a wider slot on the outer side end, then file it to a dimension of .410". I don't remember where I found that number, but it is correct. This may be tricky for someone without access to a tig welder. 

Here are the degrees and slot widths.

8L slot = 16 degrees centrifugal advance = .358”
9L slot = 18 degrees centrifugal advance = .384”
10L slot = 20 degrees centrifugal advance = .410”
11L slot = 22 degrees centrifugal advance = .436”
12L slot = 24 degrees centrifugal advance = .462”
13L slot = 26 degrees centrifugal advance = .488”
14L slot = 28 degrees centrifugal advance = .514”
 

 

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Kilgon

 

 

"The only dumb question is the one not asked"

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On 9/18/2021 at 7:38 PM, Kilgon said:

Here are the degrees and slot widths.

8L slot = 16 degrees centrifugal advance = .358”
9L slot = 18 degrees centrifugal advance = .384”
10L slot = 20 degrees centrifugal advance = .410”
11L slot = 22 degrees centrifugal advance = .436”
12L slot = 24 degrees centrifugal advance = .462”
13L slot = 26 degrees centrifugal advance = .488”
14L slot = 28 degrees centrifugal advance = .514”
 

 

 Do you have 15L? That is a very commonly used slot width. 30 degrees c/a + 6 initial = 36 total mechanical.

Geoff.

 I learn something new every day!

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