Dwell-setting Procedure

[Henry J.]

After installing new points and when setting the dwell with a meter, should the vacuum line be disconnected from the VAC and plugged? Does the answer differ if the VAC sees manifold or ported vacuum?

I have seen conflicting instructions and would like to come to a definitive conclusion.

I do understand that, after the dwell has been set, the base timing should be checked and adjusted, as necessary, with the vacuum line disconnected and plugged.

Thank you.

[Peter H.]

Re: Dwell-setting Procedure

Hello Henry. Dwell Is A Mechanical Function And Vacuum Has No Bearing . When You Set Timing this Is The Time To Disconnect And Plug The Vacuum Line . Good Luck And Keep Safe . Peter

[Owen L.]

Re: Dwell-setting Procedure

Peter, would you mind clarifying a foggy part of my understanding on dwell?

When beginning from scratch with a distributor removed from the engine, does one set a rough dwell by feeler gauge-ing the points gap? Then after engine start, set it with a dwell meter? If so, what is the recommended feeler gauge gap? (454, stock ignition, plugs, wires, etc.)

My engine is also getting a rebuilt Q-Jet and I'm not sure how it will start from those settings either. So, if/when the engine fires up, what is your recommendation for order of setting timing, dwell, and carb mix/idle?

Thanks.

[Larry M.]

Re: Dwell-setting Procedure

Peter, would you mind clarifying a foggy part of my understanding on dwell?

When beginning from scratch with a distributor removed from the engine, does one set a rough dwell by feeler gauge-ing the points gap? Then after engine start, set it with a dwell meter? If so, what is the recommended feeler gauge gap? (454, stock ignition, plugs, wires, etc.)

My engine is also getting a rebuilt Q-Jet and I'm not sure how it will start from those settings either. So, if/when the engine fires up, what is your recommendation for order of setting timing, dwell, and carb mix/idle?

Thanks.

Not Peter, but will try to help answer your questions:

Point ignition systems have been set with feeler gages since the invention of the auto. Feeler gage opening is around 0.016 to 0.019 inches, with the distributor 8-lobe cam on the high point. Very easy to do with the adjustable points by GM. Most of us did not own a dwell meter when we were first working on cars. This gap will start the car very nicely. Some us us used to set this gap by just looking at it, without the feeler gage.

This gap is also shown in your Chevrolet Service Manual. Once the car is running you can set the dwell to 28-32 degrees. I like 30 degrees.

Add fuel to carb before first fire of the engine. Set initial timing by moving harmonic damper mark to your initial timing spec. Not sure for your specific car, but probably around 6 BTDC. BE CERTAIN this is for the #1 cylinder and not for the #6 cylinder. Hookup a 12 volt flashlight bulb with clip leads between the coil (-) terminal and engine ground. With the ignition at 12 volts and the key turned ON, rotate the distributor until the points just begin to open and the bulb just lights. Remember that there are TWO places on each distributor cam lobe where the points begin to change from close to open or open to closed, which will cause the bulb to change conditions. You want where points just begin to open. The distributor rotor normally rotates clockwise when the engine is running. When you achieve this, tighten the bolt on the distributor hold-down to lock in this position. Turn the ignition key OFF and remove the bulb and leads.

If you pre-filled the carb with gas and got the point gap and initial timing correct, the car should fire immediately. Just set the carb choke before trying to start.

Check timing advance with engine running to ensure it is close to the right value. Then set the dwell to 30 degrees, and after that set the initial timing, making sure the vacuum can hose is disconnected and plugged. Keep RPM very low at this point to avoid having centrifugal advance come into play. When these two items are done and the engine is warm, you can set the carb curb idle and mixture. For setting mixture, either vacuum gage or a sensitive RPM tach can be used. You want highest vacuum or highest RPM. It is preferred if you are on the LEAN side of maximum vacuum or highest RPM. This is where the mixture screws are "turned in" the most. There is a RICH side and a LEAN side to maximum vacuum or highest RPM..........you want the LEAN side.

Larry

[Owen L.]

[Leif A.]

Re: Dwell-setting Procedure

Wow, Larry, thank you for the detailed procedure. With so many variables going on I wasn't sure where to start. I'm saving your reply for reference.

My '72 service manual doesn't specify the 8-cylinder point gap - but does for a 6-cylinder. In the back of my head, .019 kept nagging at me... The light bulb "trick" is fantastic! Sure seems to take the guesswork out of cranking, cranking, rotate a bit, cranking, rotate a bit, etc. process.

I use to always set mine at 32* when new. As they wore in the gap, naturally, closed bit by bit. I would change mine, back in the day, once a year along with the spark plugs (12000 miles). The gap when I changed them was usually around 28*. Start on the high side (32*) and end they up on the low side (28*). My opinion and I'm sticking with it.

[Larry M.]

Re: Dwell-setting Procedure

I use to always set mine at 32* when new. As they wore in the gap, naturally, closed bit by bit. I would change mine, back in the day, once a year along with the spark plugs (12000 miles). The gap when I changed them was usually around 28*. Start on the high side (32*) and end they up on the low side (28*). My opinion and I'm sticking with it.

Leif:

By definition, dwell is the time or arc degrees that the points remain closed. More dwell, points closed longer. Less dwell, points open longer.

I have never seen points wear where they stay open more..........usually the other way.

More dwell will provide more time for the coil to generate a 'hotter" spark or provide more energy to the spark plug. That is why distributors with dual points (in series) are desired for the performance cars. Dual points will provide/yield around 34-36 total degrees of dwell.

TRIVIA: Mallory Rev-Pol high performance distributor from days gone by used two sets of points in parallel, and each point set serviced only four cylinders for a V8 engine.

Each one degree change in dwell will affect timing by one degree. So a change in dwell from say 28 to 32 degrees will reduce static or initial timing by 4 degrees. Lars and Critter1 and MikeM and a few others (self included) worked this out and tested it on a SUN Machine at the "other forum". 65tripleblack (rip) also was in this thread. Duke may have been there also.

But the beauty of using a dwell meter and setting new points the same each time, means that initial timing will never change from tune up to tune up. It used to be wthl feeler gages, that you would always have to adjust timing a bit with each point change.

Larry

[Larry M.]

Re: Dwell-setting Procedure

I learned to use the 12 volt test bulb when working on my motorcycles when I was (much) younger. That is the way Honda used to set timing. But I have been told that the military also used this technique ("trick") on planes to set timing.

The more dwell the more energy at the spark plug. But points rubbing blocks wear a bit with use, so dwell will change a bit over time. So you go for 30 degrees and let it wear to 32 degrees. That way you always were maximizing the energy to the spark plug, yet staying within accepted normal range.

Larry

[Frank D.]

Re: Dwell-setting Procedure

All good advice here, and I never owned a dwell meter for the first 20 years I owned cars.....

Back when I smoked and got stuck alongside the road with bad points, I'd swap 'em and set em close enough with a matchbook cover to get me home... Some members on another forum scoffed at this - guess they didn't have my same experiences in driving old cars.

Now that I DO own a dwell meter, I set dwell at 29*, when a range is given for something I try to get in the middle (or close)...

[John F.]

Re: Dwell-setting Procedure

All good advice here, and I never owned a dwell meter for the first 20 years I owned cars.....

Back when I smoked and got stuck alongside the road with bad points, I'd swap 'em and set em close enough with a matchbook cover to get me home... Some members on another forum scoffed at this - guess they didn't have my same experiences in driving old cars.

Now that I DO own a dwell meter, I set dwell at 29*, when a range is given for something I try to get in the middle (or close)...

I used a match book cover in the day as well. Mostly on GM products. As said above, if you did it enough times, you could just eye-ball it!

[Phillip M.]

Re: Dwell-setting Procedure

All good advice here, and I never owned a dwell meter for the first 20 years I owned cars.....

Back when I smoked and got stuck alongside the road with bad points, I'd swap 'em and set em close enough with a matchbook cover to get me home... Some members on another forum scoffed at this - guess they didn't have my same experiences in driving old cars.

Now that I DO own a dwell meter, I set dwell at 29*, when a range is given for something I try to get in the middle (or close)...

I was going to mention this, but you beat me to it. I would also bet that the matchbook covers 50 years ago were thicker than .015"!
Phil

[Leif A.]

Re: Dwell-setting Procedure

I learned to use the 12 volt test bulb when working on my motorcycles when I was (much) younger. That is the way Honda used to set timing. But I have been told that the military also used this technique ("trick") on planes to set timing.

The more dwell the more energy at the spark plug. But points rubbing blocks wear a bit with use, so dwell will change a bit over time. So you go for 30 degrees and let it wear to 32 degrees. That way you always were maximizing the energy to the spark plug, yet staying within accepted normal range.

Larry

Larry,
You are absolutely correct. I had my numbers bass ackwards...been a second since I tuned a car with points. Thanks for the correction and detailed explanation.

[Duke W.]

Re: Dwell-setting Procedure

After installing new points and when setting the dwell with a meter, should the vacuum line be disconnected from the VAC and plugged? Does the answer differ if the VAC sees manifold or ported vacuum?

I have seen conflicting instructions and would like to come to a definitive conclusion.

I do understand that, after the dwell has been set, the base timing should be checked and adjusted, as necessary, with the vacuum line disconnected and plugged.

Thank you.

The VAC has no effect on dwell angle. Dwell can be set with the VAC connected or the signal line disconnected and plugged. The nominal dwell angle setting is 30 degrees.

Once dwell is set the timing should be set. For this the VAC signal line MUST be disconnected and plugged. It may be difficult to get high overlap cam SHP/FI engines to idle below the centrifugal start point to set initial timing, so it may be easier to use the total WOT timing method by setting it at 36-40 degrees with a dial back timing light a few hundred revs above the point of maximum centrifugal advance, which is listed in the service manuals and is anywhere from 2350 to 5100 RPM depending on the engine.

For engines that don't achieve max centrifugal until 3500 or more, improved performance and fuel economy can usually be had by installing lighter centrifugal springs to bring it all in between 2500 and 3500. The quicker the better and the limiting factor is usually detonation.

Duke

[James G.]

Re: Dwell-setting Procedure

I used a match book cover in the day as well. Mostly on GM products. As said above, if you did it enough times, you could just eye-ball it!

I owned a 59 Plymouth with the 305hp COMMANDO big V-8 in college. Driving back to New Jersey from Detroit, the car stopped. I got out in the rain, and popped the distributor cap. A pig tail on one of the duel points was grounding itself, so I put a match book into the side of the distributor and all worked well. I drove it home over 175 miles.