Vacuum reading on 318 @ 62 MPH cruise = 19 inches - is that normal / expected?

[StreetSleeper]

I have a car that will ping on regular. If I mix 50/50 with super, no ping. If I buy the middle grade it pings just like straight regular. You are better off mixing it yourself.

Turns out the procedure is the drivers put their leftovers on the mid grade tank, and since they sell way more regular then the mid grade tank gets more regular put in! Another bad deal for consumers.

I worked several gas stations in my youth.

The owners only had integrity to their bottom dollar.

They would allow tankers with excess 87 to dump the rest into their midgrade.

They never got caught, I never trusted them or midgrade after that.

This was in south texas (poor), before turbo chargers and super chargers were prominent. 1990’s.

 

[racerdan80]

I know gas can be different in different regions, but I got my 68 barracuda 318, 904 with 3000 stall(tight) converter with 3.23 gears and 26” tall tires to run happy on 87 octane. 10 btc at 800 rpm idle, 34 degrees with 2 medium springs and about 18” vacuum at 60 mph. Total timing and how fast you bring it in is critical and combo dependent. Weight of car, gears, stall and tire height are all a factor. Maybe put heavy springs in first as a base line and increase/decrease total in small increments while reading your plugs. Take notes and good pictures. Then work on vacuum advance. Lots of smart people on this site can help you dial in

 

[67CBodyGuy]

I won't get a chance to get a total timing reading before I hit the road tommorrow for Carlisle. The car has been there before - in '97. Same engine, same distributor. Gas was different back then though. The only variable would have been the idle timing setting, but I'm sure it was 10 BTDC then like it is now. And I like to keep the idle RPM low - no need to waste gas and generate heat at stop lights. I dial the rpm down to the point just before the carb starts to whistle.

 

[67CBodyGuy]

Ok, here's the deal.

My drive to Carlisle was 433 miles (one way). So that's 866 miles round trip. A pretty good guess of my fuel use is 47 gallons, MPG = 18.5

Spark plug photos below. Number 6 seems different than the rest. RN14YC Champ.

Confirmed that my idle timing is 10 BTDC with a dial-back timing gun. If I use the dial to get timing mark to zero when I rev the engine, the dial is set to 50 degrees. That seems to be the max all-in advance (vacuum + centrif). That's the correct way to measure it yes?

Driving through PA is tough because you're always going up or down, but what I noticed during the flat parts is that when maintaining a speed like 50, 60 or 65, the vac needle would gravitate to 18.5 inches and I'd hear a faint to moderate tick. If I decelerate, tick goes away. If I push the pedal and get the vacuum to maybe 16 or 15, tick goes away. Tick is very sensitive to pedal position. This is a tick, not marbles.

I ran the whole trip with speedo cable disconnected, and removed the center wire. I thought that was causing my tick during testing before the trip. I think I do have a tick in the speedo, it was much louder when it was connected.

I think this tick is related to the distributor / ignition, and could possibly be spark knock. The plug colors look good, maybe too white? Mixture too lean?

Engine temp was well controlled, my stock gauge works well and is responsive (like if I put it into neutral down a long hill, I will see temp go down). Sometimes I'd have to put it into 2'nd gear for several minutes at a time going up hill to maintain 50 mph without putting the vacuum below 10. If I lived in PA I'd probably have a 383 instead of a 318.

Stock ignition (new points and capacitor, new rotor). Plug wires are low resistance (500 ohms/ft like original). Voltage readout (cigarette lighter USB power port with voltage readout) shows steady 14.2 - 14.4 volts. 93 octane gas (10% ethanol) all the way. I wanted to buy 0% ethanol (or octane higher than 93) but could not find any.

I think this carb is tuned for slant-6 (ie - super-6 knock-off). Is 50 degrees total advance too much for highway cruise?

 

[Woody500]

Hmm..I always thought that the pump did the mixing. You learn something new every day.

That was a Sunoco Advertising Line from years ago. I'd bet there's someone out there that knows if it was just advertising or if pumps actually mixed grades.
Any Pump Repairmen out there?

 

[MopaR&D]

Ok, here's the deal.

My drive to Carlisle was 433 miles (one way). So that's 866 miles round trip. A pretty good guess of my fuel use is 47 gallons, MPG = 18.5

Spark plug photos below. Number 6 seems different than the rest. RN14YC Champ.


View attachment 1716430122

View attachment 1716430123


View attachment 1716430124

Confirmed that my idle timing is 10 BTDC with a dial-back timing gun. If I use the dial to get timing mark to zero when I rev the engine, the dial is set to 50 degrees. That seems to be the max all-in advance (vacuum + centrif). That's the correct way to measure it yes?

Driving through PA is tough because you're always going up or down, but what I noticed during the flat parts is that when maintaining a speed like 50, 60 or 65, the vac needle would gravitate to 18.5 inches and I'd hear a faint to moderate tick. If I decelerate, tick goes away. If I push the pedal and get the vacuum to maybe 16 or 15, tick goes away. Tick is very sensitive to pedal position. This is a tick, not marbles.

I ran the whole trip with speedo cable disconnected, and removed the center wire. I thought that was causing my tick during testing before the trip. I think I do have a tick in the speedo, it was much louder when it was connected.

I think this tick is related to the distributor / ignition, and could possibly be spark knock. The plug colors look good, maybe too white? Mixture too lean?

Engine temp was well controlled, my stock gauge works well and is responsive (like if I put it into neutral down a long hill, I will see temp go down). Sometimes I'd have to put it into 2'nd gear for several minutes at a time going up hill to maintain 50 mph without putting the vacuum below 10. If I lived in PA I'd probably have a 383 instead of a 318.

Stock ignition (new points and capacitor, new rotor). Plug wires are low resistance (500 ohms/ft like original). Voltage readout (cigarette lighter USB power port with voltage readout) shows steady 14.2 - 14.4 volts. 93 octane gas (10% ethanol) all the way. I wanted to buy 0% ethanol (or octane higher than 93) but could not find any.

I think this carb is tuned for slant-6 (ie - super-6 knock-off). Is 50 degrees total advance too much for highway cruise?

50 deg total with vacuum advance isn't too much. If the vacuum advance canister is adjustable I'd first try turning that about 1/2 turn (can never remember which direction, can someone let us know?) to reduce the "sensitivity" (degrees of advance vs inches of vacuum). It sounds like it's just pulling in too much advance under light load. Then again your mixture does look a little lean judging from the plugs which can also cause knock/pinging.

Since you have a dial-back timing light I'd like to see what your mechanical advance curve looks like. Disconnect vacuum advance and plot total advance at various engine RPMs (1000, 1500, 2000, 2500 etc).

A stock rebuilt 318 should be able to run on 87-octane with no issues.

 

[Bewy]

Yup, 50* at cruise is not excessive. #6 plug could be from a/f distribution, # 6 running a little rich. I wouldn't worry about it if car runs ok.

Adj vac adv units: turning the Allen Key CW softens the spring tension. I would start with fully CW. If you get det on light load, turn 2 turns CCW & re-test.

 

[67CBodyGuy]

Does this help to ID the distributor advance curve?

The last 6 digits on the weights being 889264. Or maybe it's the springs that need to be known?

My vacuum advance diaphram is not adjustable. Is it a "thing" to block the slot to reduce the pin movement? I need to get a good tach but I do intend to measure the curve.

 

[crackedback]

Measure the slot length. There is a table with the respective advance for slot length posted here. I know I put one up a long time ago.

That is a SLOW curve with the factory style garage door spring (top one)

Allen key goes in the advance diaphram nipple to adjust the vacuum level the advance starts to come in. It does nothing to change the advance amount in the can.

 

[Bewy]

Nope, the Allen key [ AK ] can change the amount of advance. The vacuum of this engine in the title seems low-ish to me; could be many things; worn engine, low CR, retarded ign timing, big-ger cam etc.
Depending on the vac can model & other variables, even with the softest spring setting, the VA unit actuator may not fully activate, resulting in timing variation, which is about the worst condition you would want. As the actuator moves, the spring tension becomes greater & the engine vacuum may not be high enough to overcome it.

For ported vac adv, the proper way to dial in VA is to control the total amount of VA with the actuator arm. If you need less timing, fit a stop to the actuator arm. Start with the AK fully CW, softest setting. If no detonation, leave it there. Only move to CCW if you have det.
Manifold vac adv is different.

 

[67CBodyGuy]

The vacuum of this engine in the title seems low-ish to me; could be many things; worn engine, low CR, retarded ign timing, big-ger cam etc.

From what I read, 18 - 22 inches is what to expect at idle or economy cruise. My home altitude is 250 meters (820 feet) ASL, and the PA trip I was just on ranged up to 3100 ft ASL. For a good part of the trip I was on I-219 which follows the Allegheny mountain ridge.

They say that every 1000 ft increase in elevation will knock 1 inch of mercury off your manifold vacuum.

A compression test I did a couple years ago gave numbers of 150 - 155 PSI.

 

[Mattax]

Does this help to ID the distributor advance curve?

View attachment 1716431646

The last 6 digits on the weights being 889264. Or maybe it's the springs that need to be known?

My vacuum advance diaphram is not adjustable. Is it a "thing" to block the slot to reduce the pin movement? I need to get a good tach but I do intend to measure the curve.

That's the advance for a factory points distributor that looks to be in good shape.
The question is what was the application. I'll assume the side tag is gone and this is a 'rebuild' from a parts store. So we really don't know. Those wieghts were used on the majority of distributors so no they are not a clue. The governor should be stamped top and bottom with the number of distributor degrees. Since its not, it's probably a rebuild.
No big deal. You're going to get a tach and measure the timing and plot it. Excellent! You have to do this anyway if you want to really learn what it has (or bring it to someone with a distributor machine).

The arm on the vacuum advance also should have a number stamped on it - so again probably a rebuild. No big deal. That you're not finding it to be adjustable may not be an issue. I wouldn't sweat it. Everything you have posted indicates the timing and fueling is pretty close to perfect for economy and power - with the possible exception of loosing rpm in drive on some long hills. '67 autotrans doesn't have the part throttle kickdown and there may be other factors but we will keep this in mind.

 

[Mattax]

rebuilt 318 circa...'67 Monaco

Large pipe coming off the box above the bowl is open.

There should be an opening from above the bowl into the air cleaner. That's the bowl vent. Some carbs run it into the airstream to increase pressure in the bowl slightly with increasing airflow. Others run the vent into a stable atmosphere in the air cleaner so the pressure above the bowl fuel is atmospheric. That pipe probably is for venting to a vapor system when the engine is off. Confirm that and if so then it should be capped, especially if it has a bowl vent into the airstream.

I think this carb is tuned for slant-6 (ie - super-6 knock-off)

Carb's don't care too much what they are installed but if you are correct, then yes some adjustment could be needed. This would be more so if the carb was hyper -specific to an intake and engine. Sometimes boosters will have notches or spivies to direct fuel.

After this run yesterday 50-60 miles on the highway, I pulled a couple today and looked at them. Very clean and white again. RN14YC.

Considering it was mixed but mostly interstate and lots of hills, does looks to be on the lean side except for 6. And I would think we're mostly looking at the effects of those hills and 65+ mph driving.

idle timing 10 btdc

The only variable would have been the idle timing setting, but I'm sure it was 10 BTDC then like it is now. And I like to keep the idle RPM low - no need to waste gas and generate heat at stop lights. I dial the rpm down to the point just before the carb starts to whistle.

Depending on the distributor, keeping 10 degrees and changing the rpm may have shifted the timing curve.

Is 50 degrees total advance too much for highway cruise?

If you mean initial timing plus centrifical and vacuum advance at the rpm the engine is cruising, that's very close to factory spec. Let's see if I have graphed that.

 

[Mattax]

Let's see if I have graphed that.

Assuming a US built 49 state car, the mechanical timing for an LA 318 according to the 67 Dodge Chassis manual falls in the ranges shown.

Initial is set at 500 rpm and advance doesn't begin until 650 rpm so should be easier to set than later smoggers.

When manifold vacuum is above 15", the vacuum advance will add 21 to 27 crank degrees. (This info is in the 67 Plymouth and Dodge Shop Manuals)

If at 65 mph the engine is turning 2800 rpm, the mechanical timing is

20.5 to 23.5 degrees.
Add in the vacuum advance and the timing on the interstate at light throttle should be 41.5 to 50.5 degrees.

Notice the mechanical advance does not slow down much at 1800 rpm rpm and does not stop until 4800 rpm. This is typical for standard 2 bbl engines.

There's only a couple of years with non-smogged 49 state 318 engines for passenger cars. So definately measure the timing at various rpm and see what you have. The aftermarket loves one size fits all.

For comparison here's the specs for '68 318 with and without Clean Air Package.

If we use that distributor intended to be set with 5* inital and set it at 10 degrees, then the timing will be too advanced once the engine is above 1400 rpm.

So go figure what you have for timing before making any changes. You know from experience its not wildly out of acceptable range. Dialback light and tach will be accurate enough up to 3000 which is enough to know whats going on. if you're not comfortable at 3000 rpm, go to 2500 or so. That's enough to answer the questions.

 

[67CBodyGuy]

I'm building a tach from scratch. First I need a signal. I'm getting it off the ballast resistor. The voltage drop across the resistor is 3 to 3.5 volts when the points open and close. There are spikes on this signal, on the rising and falling edges, that can be 50+ volts. So first I put some .47 uf caps on the resistor. From each leg to ground, and one cap across it. That tamed the spikes.

Then I fed that into an opto-isolator. Here's what that looks like on the other side (1 millisecond per div):

From left to right, the coil is fully charged, then the points open, and there are some pulses before current goes to zero, then the points close and a much slower ramp-up on the coil. But the fast pulses are going to be a problem. What's causing that? Are the points bouncing? Or is that flyback noise from the coil?

Here are those pulses in more detail. 50 micro-seconds per div. Ok, so how to get rid of them?

Put a diode across the points. They say doing that is good for the points, prevents sparking and pitting. I have some 1N5408, they seem to do the job. No more ringing on this signal. Should also put a diode across the starter solenoid for the same reason.

Prototype is built on perf board, mounted under the ballast resistor. This gives me a clean and safe signal to feed into an arduino or ESP32.

I still don't like that there is so much variability (time-wise) in this signal:

2.5 ms/div. Like I said, idle is rough. I think my 2-bbl intake is dual plane, so that makes carb idle mixture screw adjustments so important I guess. I do have electronic distributor from my '73 Satellite, I think I will eventually use it.

I made another highway run, I backed the timing off from 10 (or 12) to 7.5 BDTC. I still got this ticking sound at cruise, very dependent on throttle position, I can make it go away if I slow down a little or speed up a little, but it always comes back at the new cruise speed. Vacuum was good - like 20 on the dot, 65 mph, but I had a slight tail wind.

I'm wondering if the ticking is a lifter? I don't hear it at all during city driving, only highway. It's not loud, but it bugs me.

 

[Bewy]

Sounds like an exh leak if it comes & goes with load.

 

[67CBodyGuy]

Yesterday - 100 mile highway drive from A to B. Tick is moderately loud at cruise when pedal is held constant, speed is constant (55 to 65 mph). Any slight movement of the pedal down or up and tick instantly goes away. This pedal movement will also instantly change manifold vacuum by a few inches (down or up).

Remove distributor vac advance line and plug it. 100 mile drive from B to A. Tick is gone. Vac reading while in cruise seems to be 1 or 2 inches higher vs when line was connected (19 inches vs 17 - 18 inches). Timing at idle was set to 10 BTDC.

Extremely flat terrain between A and B.

There's a lot of talk and video's posted about recurving distributors, many of them talk about welding / shortening the centrifical slot - ie reducing the advance. Don't see anyone talking about increasing it.

So reducing centrifical advance is a thing. Why? Why is that? Was it a thing 20 years ago? Is it because of today's gasoline? Remember, I'm using 93 octane (10% ethanol) on what I think is a stock 318 (commercial rebuilder engine, circa 1986) the last time I did a compression test was getting 150 - 155.

I couldn't easily reduce centrif advance for this test, so I did the next best thing - pull the vac advance.

 

[mbaird]

Modification of the slot is because different cars came with different length VA slots .
Big heavy low compression cars might require more VA than a light 340 car. And of course various engines required different amounts of advance .
As mentioned already there is a number on the arms that indicate how much distributor advance that arm will provide .

 

[mbaird]

If you are only getting 1-2 degrees of advance either you diaphragm is shot or you have the can adjust too tight .

Halifaxhops would be your best contact for new components/ help

 

[mbaird]

I got that mixed up … the Mechanical advance arms have the number stamped on them .
Sorry . But again… diffferent car/engine combos got different distributor mechanical and vacuum curves. There is a chart in most FSM that break out the differences by part number . You might have a distributor from a totally different car.

 

[TT5.9mag]

So you pulled the vacuum advance line off and engine vacuum went UP during cruise AND your tick went away? You need serious distributor tuning if that’s the case. The reason for welding/filing slots to limit mechanical advance is simple, we run more aggressive camshafts now and they like more initial timing. To increase initial and NOT increase total timing, you have to limit the mechanical timing.

 

[AJ/FormS]

Just so you know, my combo, before overdrive with 65= 2870
1 asked for up to 63 degrees of Cruise timing.
2) 50 degrees was never enough

After overdrive, with 65=2240 now, it's a different story.

 

[Mattax]

Timing Terms and Purpose of Timing