Timing 390 and "dwell"

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Timing 390 and "dwell"

Postby R.Smith on Wed Dec 09, 2009 1:35 pm

Ok, I bought an old Heathkit dwell meter at the flea market this weekend and I've decided to give my 390 a 'by the book' timing check.

My book is a 1972 Chiltons Truck manual that covers all domestic trucks from 1961 to 1971. The info looks pretty good but I want to make sure.

It calls for a .17 point gap, 27-31 dwell and 10-12 BTDC timing on the harmonic balancing wheel.

I set the timing with the vaccum advance plugged ect, then checked the dwell and it was showing 8. Everything on the meter was set right.

I understand most of this but, what exactly is "dwell" and how do I change it without affecting the timing? I've asked around among jackleg mechanics I know and get the impression none of them know what dwell is.
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Re: Timing 390 and "dwell"

Postby R.Smith on Wed Dec 09, 2009 2:08 pm

OK. I'm an idiot.

I was reading the tach on the Heath meter. The actual dwell is reading 23-24...still off but much more gooder.

However, when I reattach the vac advance line, dwell drops to 15. is this normal? Plus the timing drops back to about 25 BTDC.
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Re: Timing 390 and "dwell"

Postby george worley on Wed Dec 09, 2009 5:27 pm

The numberof degrees that the points are closed is called the cam angle or dwell.On the average V8 the dwell runs 28-31 degrees. If the dwell is less than specified the points are not closed as long and can cause misfire at high speed or under hard load. It doesn'tgive the coil time to get fully saturated with magnetism and therefore the engine misfires. Small point gap= larger dwell, larger point gap =smaller dwell. After you get the dwell or point gap right check your timing because the gap or dwell will change the amount of timing degrees you have at idle. :2cents:
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Re: Timing 390 and "dwell"

Postby 70_F100 on Wed Dec 09, 2009 9:55 pm

R.Smith wrote:OK. I'm an idiot.

I was reading the tach on the Heath meter. The actual dwell is reading 23-24...still off but much more gooder.

However, when I reattach the vac advance line, dwell drops to 15. is this normal? Plus the timing drops back to about 25 BTDC.


Check your distributor shaft for any play, and check the breaker plate for looseness.

What's happening is when you attach the vacuum advance, the plate is moving (which is correct) but it's moving somewhere it shouldn't.

When the dwell changes, the timing changes, because the points are opening and closing at a different time.
Wise men talk because they have something to say; fools talk because they have to say something.--Plato
Why is it that there's seldom time to fix it right the first time, but there's always time to fix it right the second time???

That's not an oil leak :nono: That's SWEAT from all that HORSEPOWER!! :thup:
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Re: Timing 390 and "dwell"

Postby knightfire83 on Thu Dec 10, 2009 2:53 pm

I agree with 70_F100.  The dwell should not change by attaching the vacuum line.  The breaker plate or the distributor shaft bearings are worn out.

This would be a good time to upgrade to a duraspark ignition system since you will likely be replacing the dizzy.  :2cents:
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Re: Timing 390 and "dwell"

Postby R.Smith on Thu Dec 24, 2009 6:35 pm

Update. I bought and installed a new distributor, points, condensor, rotor, cap and vac advance.

Now the dwell is hovering even lower, around 21 as opposed to the 23 I was getting before.

The manual I'm using says to set the timing with the vac advance hose off and plugged. I did this and when I reattached the vac to hose to the vac advance it again advanced the timing beyong the scale. It's hovering about a half inch past the BTDC mark which is past 30 degrees.

If I adjust the timing with the vac hose on, then pull it off and plug it, the engine dies.

The cam in this engine was described by the prev owner as a "mild" aftermarket replacement.

I'm drawing vacumn off the bottom left side of the carb (facing it) The carb is a Holley 600cfm 4v, new.

The vaccumn guage reads 19 at idle.

I'm driving the truck now and it runs ok but seems sluggish on hills and when accelerating. I'm out of ideas. Any suggestions?

Thanks and Merry Christmas.
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Re: Timing 390 and "dwell"

Postby two-bit on Fri Dec 25, 2009 9:17 am

Many moons ago a flathead Ford taught me a very important lesson about Dwell and point gap.
Flat 6's LOVE to back fire when the dwell is wrong, much to the demise of the very old and worn out exhaust valves. Another time about that end of it.

Dwell and point gap.
The book says set the points @ .17, dwell angle @ 27-31degrees.
Most of the time i have found that the "recommended" point gap of .17 is a good place to start. I set up all of my point dizzy's by dwell. Not by point gap. Adjust your point gap to get the dwell reading of 27-31 degrees. Sometimes your point gap may end up being .15, sometimes .21, but what is the most important is your dwell angle.
I really don't use a timing light anymore, never did much in the past. I use a glass of water to set my engine timing.
Glass of water???? Hey Two-bit, are you loosing it??? No, i'm not loosing it.
I have seen a lot of balancer rings shift over the years, and your timing marks won't be quite right. So i use a glass of water to set my timing.

If you want to try it here's how it works:
Unplug your vacuum advance line like normal.
Get a clear glass container with a large base ( i prefer a mason jar), and find a good place to put it on the engine block where it will stand up right.
Start the engine, look very closely at the rings in the water in the glass. Are they originating from the center of the container, or from the sides?
Adjust your dizzy very slowly until the the rings in the water originate from the center of the container.
Recheck you dwell. Hook up your vacuum advance line, and take it for a spin around the block.
You might like it.

The water in the glass acts as a resonator so that you can physically see the harmonic balance of the engine. Once you have the Harmonic balance "tuned" to the correct "frequency" all eight cylinders in your engine will be firing in "balance" of each other. The better engine balance you have, the better performance you get.
I learned this trick from an old hot rodder who used to build flathead V-8's, and Buick OHV Straight 8's. He told me that once you change the cam angles and durations, the factory timing marks are no longer any good.
I have used this method for years on everything from air cooled 4-V engines to 3.0L Mitsu's. A friend of my sons even used this method to "tune" the engine in his Honda. He used the water glass on the air intake, and then a laptop computer to adjust his engine timing. He says reprogramming  his brain box according to this method has gained him .5 seconds in the 1/4 mile.
I don't know if this has helped you at all.
Hopefully it will some how.

Two-bit
Last edited by two-bit on Sat Dec 26, 2009 8:25 am, edited 1 time in total.
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Re: Timing 390 and "dwell"

Postby 70_F100 on Fri Dec 25, 2009 9:45 am

two-bit wrote: I have used this method for years on everything from air cooled 4-V engines to 3.0L Mitsu's to Detriot diesels.
Two-bit


I have to call BS on this one.   :nono:

Detroit Diesels are timed by the gear train and by adjusting the timing on the injectors.

No way is anyone going to go to the trouble of retiming the gear train.  In fact, if the gear train is out of time, it's probably going to destroy the engine.

By that same token, retiming the injectors is a very time-consuming task.

I've worked on Drip-Troits since the early 70's, so I know what I'm talking about.  The first one I built was in a 1957 GM Coach, and after that, everything up to  a Terex dozer and even a railcar engine.

Now, I can't speak with any knowledge as to whether this method works on gasoline engines   :dk: , never tried it.  It may work as described.  Just not on a Detroit engine.
Wise men talk because they have something to say; fools talk because they have to say something.--Plato
Why is it that there's seldom time to fix it right the first time, but there's always time to fix it right the second time???

That's not an oil leak :nono: That's SWEAT from all that HORSEPOWER!! :thup:
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Re: Timing 390 and "dwell"

Postby knightfire83 on Fri Dec 25, 2009 12:37 pm

R.Smith wrote:Update. I bought and installed a new distributor, points, condenser, rotor, cap and vac advance.


I can see buying a cap and rotor, but shouldn't a new dizzy come with all the other new pieces with it?  Strange  :hmm:

Now the dwell is hovering even lower, around 21 as opposed to the 23 I was getting before.


Then your point gap is set incorrectly.  Bigger gap= more dwell / Smaller Gap= less dwell.  The dwell angle will increase as the contacts on the points wear out over time, requiring the usual maintenance. The cleaning of the contacts and resetting of the gap every 7-8 months or so.

The manual I'm using says to set the timing with the vac advance hose off and plugged. I did this and when I reattached the vac to hose to the vac advance it again advanced the timing beyond the scale. It's hovering about a half inch past the BTDC mark which is past 30 degrees.


You have the vacuum hose connected to the wrong port on the carb., or your idle is set way to high.  The advance hose should only have vacuum when the throttle is opened beyond idle. (ported vacuum source)

If I adjust the timing with the vac hose on, then pull it off and plug it, the engine dies.


Because it will retard the timing to the point the engine wont run.  The hose is connected to the wrong port on the carb.

I'm driving the truck now and it runs ok but seems sluggish on hills and when accelerating. I'm out of ideas. Any suggestions?


This is because your base timing and vacuum advance is not set up correctly.  All you have to do is:

1. Set the point gap correctly to get the desired dwell. (Bigger gap= more dwell / Smaller Gap= less dwell)
2. Disconnect the hose to the dizzy, plug it, set the initial timing.
3. Connect the advance hose to the port on the side of the metering block, not the one on the base plate.
4. Reconnect the advance unit to the hose.
5. Adjust the carb. idle mixture screws for the best idle / highest manifold vacuum reading.

Once you get it running correctly to specs. you can also mess around with timing it to around 12-14* to get more performance.  You can also adjust how much total vacuum advance you have by adjusting the advance unit with an allen wrench, thru the vacuum port on the unit.  But only adjust the advance if you understand what you are doing, otherwise leave it alone.

You are soooo close.... You'll have it running like a top in no time, and Merry Christmas my friend.

:thup:
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Re: Timing 390 and "dwell"

Postby two-bit on Fri Dec 25, 2009 3:52 pm

I should have said i saw it done on a Detroit Diesel, that one i did not do, but watched it done. It was a 4-53 Detroit in a Cruz-air claw machine. He kept playing with the injector pump until he got the rings in the center.  But you are right, it does not work the same way with a diesel that it does with gas motor.

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Re: Timing 390 and "dwell"

Postby R.Smith on Fri Dec 25, 2009 5:46 pm

"But only adjust the advance if you understand what you are doing, otherwise leave it alone."

That's like setting a bottle of whikey in front of an alcoholic  and saying "don't drink this unless you think you can handle it !" :evil:

One post says less gap = more dwell and one says more gap = more dwell. I think that may be the problem dwellwise. I've got it set at a tight .017 now.

So in the morning I'm gonna open the points to get the correct dwell, swap the vaccum port on the carb and try the water glass idea. I'd do it now but it's Christmas and the assembled family might frown on me farting around with the bump during dinner.

Thanks and Merry Christmas.
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Re: Timing 390 and "dwell"

Postby 70_F100 on Fri Dec 25, 2009 8:30 pm

two-bit wrote:I should have said i saw it done on a Detroit Diesel, that one i did not do, but watched it done. It was a 4-53 Detroit in a Cruz-air claw machine. He kept playing with the injector pump until he got the rings in the center.  But you are right, it does not work the same way with a diesel that it does with gas motor.

Two-bit


A 4-53 Series Detroit Diesel does NOT have an injection pump, regardless of whether it is in a Cruz-Air Claw Machine, a Bucyrus Erie Dynahoe, a power plant or a truck (yes, GM did put them in trucks for a few years).

All 53 Series Detroits have what is known as "unit injectors", where all fuel system functions take place, i.e., timing, pressurization and injection.  These functions are all controlled by positioning the rocker arm correctly using a timing gauge, which, in the case of a Detroit, is basically a feeler gauge that measures the height of the top of the injector plunger from the injector body.  On a 53 Series, in most cases, the measurement was 1.460".  The rocker arm is always in contact with the injector plunger, so tightening the rocker arm decreases this measurement and loosening it increases this measurement.

External to the injectors, there is a transfer pump that supplies fuel from the fuel tank to the fuel lines on the engine at approximately 65 PSI.

So, I don't know what this guy was doing that he told you was "playing with the injection pump", but he was feeding you a line of crap.
Wise men talk because they have something to say; fools talk because they have to say something.--Plato
Why is it that there's seldom time to fix it right the first time, but there's always time to fix it right the second time???

That's not an oil leak :nono: That's SWEAT from all that HORSEPOWER!! :thup:
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Re: Timing 390 and "dwell"

Postby two-bit on Sat Dec 26, 2009 8:24 am

He must have been, i don't know what he was adjusting then. I was on top of the machine watching the water glass. He was down below wrenching on something. i asked him what, he said "injector pump". I'll go back and edit the post. no sense in having bad information out there.
Thanks for the correct information.

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Re: Timing 390 and "dwell"

Postby R.Smith on Tue Dec 29, 2009 11:24 am

Finally!!

I had to reduce the point gap to .010 to get a dwell of 29 degrees and switch the vac line from the throttle body to the metering block to fix the vac advance but, now she's running a lot better, especially from a dead stop.

Now my valves are pinging when I accelerate. I'm using 93 octane fuel religiously. Having said that, I do know that back when I was having problems with the new Holley, I had a friend analyse the fuel to see if it was fuel related. The gas was fine but only 89 octane, even though I always use the 93. I'm starting to think that Valero might be putting the wrong juice in the wrong underground tank.

Anybody ever use Marvel Mystery Oil in their gasoline? I put it in the '35 to lubricate the valves and it seems to work pretty good but, this is a 207cid with babbit bearings and mechanical lifters and it's a lot less picky about what it drinks.
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Re: Timing 390 and "dwell"

Postby knightfire83 on Tue Dec 29, 2009 12:47 pm

Typically valves tick, Detonation pings.....  

Detonation (timing to far advanced) will eventually damage your pistons / rings.   Wheres the base timing set at?  Did you mess with adjusting the vacuum advance with a allen wrench?

Might want to back the timing down a few degrees until it stops pinging when under load.  Once you get the proper octane fuel that would help with the pinging quite a bit.

If it's just the rocker arms ticking, that wouldn't be as bad.

Glad to see you got it running good  :thup:
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