Looking for a little advice on the timing numbers I just pulled from my 73’ Duster (backfire issue)

[tonyp25]

Very new to timing and just bought a digital light to try it out.

At idle (after it’s warmed up) my RPMs were roughly 780 and drops about 100 when putting it into drive. The timing was at 0 degrees. At around 2500rpm the timing is at 18 degrees. This is all with vacuum advance disconnected.

0 degrees doesn’t seem right for a 318, automatic, 4bbl Holley, 318/360 edlebrock manifold (I’m not sure it has a Cam) from what I’ve read here. Though if it did have a cam I’d imagine it would be much higher than 0 degrees. The air pressure coming out of the exhaust is pretty strong. No headers but it does have thrush dual exhaust. Running non-ethanol gas. It has a non working AC unit too but doubt that impacts timing.

Other than that small plume of smoke that happens when I start the car the engine runs perfect (see pictures for carbon build up on back of air cleaner lid). One thing to note is that before I bought the car a local mopar shop had to put a new ignition cylinder in…but not sure if they would’ve messed with the timing at all to do that.

From what I’ve read on this site normally timing would be at least 8 degrees at idle? Is this a possible cause of my backfire out the carb? Also not sure if this might indicate anything but the only other problem I’m having is my speedometer is off by about 5-7mph (over). So when I’m doing 30mph it reads at 37mph. But I’d imagine that is unrelated

 

[67Dart273]

You likely have a bone stock "smogger" distributor with a very long advance curve. What you need to do, to start with is to determine the curve vs RPM and then set out to to make it a more performance curve

There are a number of ways
1...But a performance distributor, preferably with an adjustable advance
2...Recurve your old one by finding and changing springs, and by brazing and filing the advance slots to limit advance
3...Buy the FBO advance limit plate kit

There are a number of opinions on just what that advance should be, but with a stock cam, 8BTDC is a start. Make sure that it is not OVER advancing however

Be careful of fancy timing lights. They are known to be trouble with stuff like MSD, and in my years i"ve seen a few that just plain were not accurate.

Also be aware that the traditional manuals list the dist. curve in DISTRIBUTOR degrees which are double at the crank. AKA 10 in the distributor is 20 at the crank

 

[Mattax]

The timing was at 0 degrees. At around 2500rpm the timing is at 18 degrees. This is all with vacuum advance disconnected.

roughly 780 and drops about 100 when putting it into drive.

Good start. Having the rpms with the timing is very helpful.

Zero degrees at idle was for the smog distributor curve.
lets looks at the '73 shop manual and see how your two measurements compare with the timing for a '73 318 (2 bbl of course)

That looks like this when usin g TDC at 750 rpm. If the engine was set at 2.5BTDC at 750, then shift everything up 2.5 degrees.

Lets add your measurements to the graph.

Sure looks like that distributor does not have the original advance curve.

I would try 5* at 750 rpm, then get a few more timing measurements.
Early 318s used 5* initial, so that's certainly safe.

With the 4 bbl, it should be Ok with more advance early as illustrated by the 273 4bbl timing limit

 

[RustyRatRod]

That's way retarded. But you've got two of the best guys on the forum already replying, so follow their lead!

 

[tonyp25]

You likely have a bone stock "smogger" distributor with a very long advance curve. What you need to do, to start with is to determine the curve vs RPM and then set out to to make it a more performance curve

There are a number of ways
1...But a performance distributor, preferably with an adjustable advance
2...Recurve your old one by finding and changing springs, and by brazing and filing the advance slots to limit advance
3...Buy the FBO advance limit plate kit

There are a number of opinions on just what that advance should be, but with a stock cam, 8BTDC is a start. Make sure that it is not OVER advancing however

Be careful of fancy timing lights. They are known to be trouble with stuff like MSD, and in my years i"ve seen a few that just plain were not accurate.

Also be aware that the traditional manuals list the dist. curve in DISTRIBUTOR degrees which are double at the crank. AKA 10 in the distributor is 20 at the crank

Thank you for all the good info. I think I like either your 1st or 3rd option the best. Would you happen to recommend a specific performance distributor? In the meantime I’ll try advancing it slowly to see if that helps the small amount of backfire happening through the carb.

 

[tonyp25]

Good start. Having the rpms with the timing is very helpful.

Zero degrees at idle was for the smog distributor curve.
lets looks at the '73 shop manual and see how your two measurements compare with the timing for a '73 318 (2 bbl of course)

View attachment 1716114540

That looks like this when usin g TDC at 750 rpm. If the engine was set at 2.5BTDC at 750, then shift everything up 2.5 degrees.
View attachment 1716114542

Lets add your measurements to the graph.
View attachment 1716114548

Sure looks like that distributor does not have the original advance curve.

I would try 5* at 750 rpm, then get a few more timing measurements.
Early 318s used 5* initial, so that's certainly safe.

With the 4 bbl, it should be Ok with more advance early as illustrated by the 273 4bbl timing limit
View attachment 1716114554

Wow! Thanks so much for all that information. It really helps me to better understand what happening with my timing and RPMs.

I’m going to try advancing it a bit today. As a newbie to distributors as well is the below circled bolt the one I need to loosen in order to turn the distributor?

 

[Demonracer]

Yes, loosen it ever so slightly so there is resistance when you try to turn it. Don't forget to tighten it when you are done. LOL

 

[67Dart273]

Should have mentioned, there are other causes of backfire. Lean carb/ no or poorly working accelerator pump, something wrong in the valve train AKA burned or stuck valve, etc. Flat exhaust lobe

 

[Mattax]

Wow! Thanks so much for all that information. It really helps me to better understand what happening with my timing and RPMs.

I’m going to try advancing it a bit today. As a newbie to distributors as well is the below circled bolt the one I need to loosen in order to turn the distributor?

View attachment 1716114732

View attachment 1716114733

Yes and also its best to remove the hose going to the vacuum advance and stick a golf tee in it. Or remove the hose at the carb and cap the port.

When you readjust the idle mix, turn each screw in until there is a slight drop in rpm or vacuum. Then turn them back 1/8 to 1/4 turn richer.


From Chrysler

 

[tonyp25]

Yes and also its best to remove the hose going to the vacuum advance and stick a golf tee in it. Or remove the hose at the carb and cap the port.

When you readjust the idle mix, turn each screw in until there is a slight drop in rpm or vacuum. Then turn them back 1/8 to 1/4 turn richer.


From Chrysler



View attachment 1716114740



View attachment 1716114741

View attachment 1716114742

Thanks for this. Super helpful. Now was I supposed to cap the vacuum advance when I was getting my intial timing? Or is that only if my car is stalling out when the hose is disconnected? I just disconnected the hose and both the hole on the carb and vacuum advance were open.

 

[tonyp25]

Should have mentioned, there are other causes of backfire. Lean carb/ no or poorly working accelerator pump, something wrong in the valve train AKA burned or stuck valve, etc. Flat exhaust lobe

Thanks! All things I’m going to eventually check. Figured I’d start with what I was most comfortable with like plugs and cleaning the carb. Timing seemed the next easiest to me.

 

[RustyRatRod]

It's also important to differentiate between a "backfire" and a "pop back". A backfire is through the exhaust. A pop back is through the intake and carburetor. Those two are caused by different things most times.

 

[Mattax]

Thanks for this. Super helpful. Now was I supposed to cap the vacuum advance when I was getting my intial timing? Or is that only if my car is stalling out when the hose is disconnected? I just disconnected the hose and both the hole on the carb and vacuum advance were open.

it won't effect timing if you had the rpm correct. It could effect idle mix and rpm if the throttle was open too far. More of an issue with later cars like yours with higher idle speeds. But if there was no vacuum on the post, then its OK.

 

[tonyp25]

It's also important to differentiate between a "backfire" and a "pop back". A backfire is through the exhaust. A pop back is through the intake and carburetor. Those two are caused by different things most times.

Can both manifest themselves as a small plume of smoke that comes out of the carb right when I start the car? Sounds an awful lot like a *pop* noise…more so than when a car backfires. If you’re not right over the engine you wouldn’t even hear it or notice it.

 

[RustyRatRod]

Can both manifest themselves as a small plume of smoke that comes out of the carb right when I start the car? Sounds an awful lot like a *pop* noise…more so than when a car backfires. If you’re not right over the engine you wouldn’t even hear it or notice it.

That's a pop back through the intake and carburetor. Several causes. Incorrect firing order. Distributor out 180. Lean mixture. Incorrect timing. The problem is, all of those can also cause backfires, too! You just have to pay attention to "which end" it's coming from. lol

 

[tonyp25]

I think I got lucky all! I advanced the timing to 5 degrees from 0, tuned the idle to 750 with advance disconnected, hooked the advance backup and it now starts better and no more smoke or popping sound coming out of carb. Thank you everyone for your wisdom

 

[tonyp25]

it won't effect timing if you had the rpm correct. It could affect idle mix and rpm if the throttle was open too far. More of an issue with later cars like yours with higher idle speeds. But if there was no vacuum on the post, then it’s OK.

Thanks again! Looks like 5 degrees made all the difference. One last question I had is that with the vacuum advance connected and the car in Park the idle is right in the 860-900 range. Is that a little too high? Sounds good and runs well. Especially when in drive and my foot on the brake.

 

[RustyRatRod]

I'm sure it can take more than 5 degrees. That's a very low timing setting. If it was mine, I would experiment around with it. Maybe bump 2 degrees in it at the time and see how it runs. Rinse and repeat.

 

[Mattax]

Thanks again! Looks like 5 degrees made all the difference. One last question I had is that with the vacuum advance connected and the car in Park the idle is right in the 860-900 range. Is that a little too high? Sounds good and runs well. Especially when in drive and my foot on the brake.

The best answer will require knowing the timing vs. rpm. For example, you set timing at 5* at 750 rpm What was timing at 900 or 1000 rpm, 1400 rpm etc.?

My guess is the throttle was too far open,, so when the vacuum advance was reconnected, it added timing and the idle rpm floated up.
(Just as Chrysler explained it.)

Further guessing the engine is not stock.
I would reduce the throttle stop position (slow idle) and increase the initial a bit more so the engine again is idling at 750 or or slower if it will do so.

Plot the mechanical timing vs rpm out on the chart. (no vacuum advance connected, plug the port)
As long as the timing is between green lines and blue lines it should be fine.

 

[Bewy]

Manufacturers set ign timing conservatively because they have to allow for the worst case scenario....which might be carrying a full load in summer heat up a long incline.
Hot rodders who baby their cars therefore have some latitude when setting timing.

The first line of the quote in post #9 is incorrect. Combustion is never complete at idle because of exh gas dilution ie, residual exh gas remaining in the chamber & contaminating the incoming fresh charge. Chrys DID use vac adv connected to manifold vacuum in the 70s to cool the engine at idle.

 

[pt70]

Thanks again! Looks like 5 degrees made all the difference. One last question I had is that with the vacuum advance connected and the car in Park the idle is right in the 860-900 range. Is that a little too high? Sounds good and runs well. Especially when in drive and my foot on the brake.

Drop the idle to around 750 rpm. Since someone went through the trouble of adding a 4bbl and manifold, they might have changed the distributor too. See if you can find a tag or a number stamped on the outside of the dist. housing.

 

[Mattax]

And of course someone has to muddy the waters.
Chrysler was correct and once again your 'fact' that Chrysler used manifold vacuum connection lacks the key points as to why. In particular, it was due to more heat being put into cylinder walls to produce a more complete burning of left over hydrocarbons and to reduce CO. This was done for improved emissions purposes. The drawbacks were more energy is converted to heat and increased engine temperatures. On the rare occasions the coolant temperature becomes excessive the temperature controlled override connects comes into action.

When Chrysler wrote 'complete combustion' it was essential true for the purposes of explaining why and how to check timing. They wisely did not spend the time going over the differences of chemically perfect combustion versus most mechanically efficient combustion, or flow bench stuff, shapes of combustion chambers, quench, pulsing and reversion, harmonics, entry shape, fuel composition, or any of the other factors that go into 'complete combustions.

Complete combustion in that paragraph of the 1969 Ignition MTSC was in reference to the previous sections explaining the purpose of centrifical advance and vacuum advance.
I'll post them here so the context clear, and makes a complete picture (pun intended)

 

[Bewy]

Mattax,
You can carry on until the end of time but what I said in post #20 is correct.
Combustion at idle is NEVER complete. This is due to contamination of residual exh gas in the com chamber; worse on low comp engines because the chamber is bigger & more exh gas left over. Anybody can prove the point by taking their stock Chrys engine & increasing the initial timing of 0 ~12*. Rpm increases. The rpm increases because with the lower timing, combustion was poor, but combustion is now improved [ but not perfect or complete ] with more timing & the engine makes more HP [ rpm went up ].

Chrys used MVA like other companies did in the 70s to reduce engine temps. You can blame emissions, hydrocarbons etc but the answer is far more simple. Same as the paragraph above: more timing [ added via MVA ] increased idle rpm, which increased water pump & fan speed & brought down engine temps.

 

[Mattax]

So you really don't understand that the time of the burn, relative to the piston position and crank angle is what determines how much energy goes into the cylinder walls.

 

[Bewy]

Yes, I do understand. Do you? I will explain it for you.
Testing & research has shown that the optimum crank angle for maximum mechanical advantage [ MA ] of the force on the piston needs to occur with the crank angle 14-17* after TDC. Obviously, if the crank angle was 0, the crank would not rotate.
'Force on the piston' refers to the maximum expansion force from the burning mixture. At idle & low speeds, the toggling of the amount of ign advance that produces the highest engine rpm means that the above situation has occurred. At idle & low speed, the throttle is....throttled. Less dense, more distance between molecules, takes more time for the mixture to burn; hence the need for more timing [ time ] for the mixture to produce the optimum MA at 14-17* ATDC.

With retarded ign, the exh temp goes up. Also, the engine has to turn more rpm to make the same hp because it is now less efficient. Both factors produce additional heat which has to be dissipated by the engine's cooling system.