Wee bit confused

[obxtrainman]

I am most difinetly a novice here on the internal combustion engine. I can can swap parts with the best of 'em, but don't know much about the technical issues.

I bought a dwell meter and timing light in hopes of learning to use them on my Z. I do have a couple of issues though.

First, should I be testing dwell at a specific RPM? I can only get it up to about 33 on the meter, and that is with almost closing the points. I think my car is idling around 700 to 800. The meter says it is about 500.

Secondly, There is the plate with the timing marks on front of engine. I have been loosening the 5/16, or is it 8mm bolt on the bottom of the dizzy to adjust time. What is the plate on fan side of dizzy with the marks and the A & D on it for? I'm assuming it the letters stand for retard and advance,, speaking of retards:stupid: I feel like one these days. I'm also assuming the more larger nut/#3 phillips screw head is for adjusting it.

And lastly, should the advance be plugged off or not. There seemed to be some confusion on it. I read many, many pages in the search over the weekend. I'm more confused than ever now.

I got her running again friday night,,best shes ran in the year I've had her. There was some of the rumble and backfire on deaccelaration though. I tried to pump out my tank yesterday, My rig would get the gas up to where I could see it,,then drop it back down. I'm really sure I have crud problem also. It had not ran for 12 days,,then ran like the ****ens,,then stir up the bottom with a hose,,now dying again.

[Carl Beck]

Hi Kenneth:

Most dwell meters have settings for 4, 6 and 8 cylinders - make sure yours is set to 6 cylinders.

You set the point gap (0.0177 to 0.0217 inch or 0.45 to 0.55 mm) and measure the dwell angle at 700-800 RPM (35 to 41 degrees). Then rev the engine slightly and check to see that it stays the same. If you see your dwell angle changing, it means you didn't tighten down the point base..and your points are bouncing.

You use the larger nut/ phillips screw head to make final adjustments in the timing - yes "A" is for advance and "R" is for retard. You use the smaller nut at the base to get the initial timing within the adjustment range on the top.

The vacuum advance hose should be disconnected from the distributor and plugged - because you adjust the carb's and set the idle speed with it hooked up to the carb. When you disconnect the vacuum hose, to disable the vacuum advance at the distributor - you throw off the tune on the carb's if the hose isn't plugged... as the #1 SU now has a leak at the intake port - so to speak.

FWIW,

Carl B.

[obxtrainman]

Thanx for the info Carl,, it is a great help. If you don't mind though, would you clarify one last thing? The 10 degrees BTDC everyone has talked about. Is that on the intial timing from below on the engine, or from the final adjustment from the top? The Haynes manual shows the Mark lining up on the large O on the plate,, it seems like it was the second point on the plate if I remember correctly.

Would the reason I couldn't quite get to 35 on the dwell meter be from not being timed right, or maybe even bad tuning on the carbs? I had the points set at .019 then kept closing them down to get a higher dwell reading. I was reading around 25 before I closed them a little.

I didn't mess with it yesterday. I had timed it once with the malfunctiong advance mech hooked up,,then I plugged it from wrong end.

I have a reman dizzy setting on the work bench now. This Z has had the smog stuff removed also. Is it really nessassary to start at DTC if I'm careful to mark the placement of the rotor? The dizzy can only be put in one way, correct? So therefore, if the rotor doesn't turn, I don't see how it could be put in 180 degrees out.

[obxtrainman]

Told you I was a retard:stupid: I was looking at the 8 cylinder curv on my cheap actron dwell meter. You would think I could read an analog meter correctly since I'm an electrician. It is reminding me of when I was a novice elec using an old analog. I use an expensive digital these last several years.

Anyway, I've got the dwell at about 38, and the timing set at the 10 just moments ago. That would be the ten mark on the plate at the bottom near the pulley. It ran good up to about 2500 rpm then backfired, or missed, stumbled with a fair crack out the muffler.

I'm having fun with my new diagnostic tools:rambo: Still wide open for suggestions on putting in my new reman dizzy.

[Carl Beck]

Hi Kenneth:

"Initial" engine timing is marked or set when the engine is being put together. That is the Top Dead Center of the Number 1 cylinder, on the compression stroke. The initial MARK on the crankshaft is "0".

The 10 degrees BTDC is the initial "spark" timing. Meaning you want the spark plug to fire 10 degrees before the piston reaches Top Dead Center - at the RPM the engine is set to idle at. So you dial that in by turning the distributor while using a timing light to watch the marks on the crankshaft pulley. It's important to have the vacuum advance disabled, by removing the vacuum line from the distributor and the line plugged off, when you do this.

Generally speaking - when people talk about "initial timing" - they really mean the initial timing of the spark at engine idle speed.

After that - you reconnect the Vacuum Advance. As engine RPM increases the timing curves built into the distributor by both centrifugal weights spinning outward, and vacuum advancing the points come into play. If you are running a stock distributor - you don't have to mess with this aspect - except to quickly open (blip) the throttle - and see if the timing at the crankshaft advances quickly - then falls back to 10 degrees BTDC.

If your car backfired at 2000 RPM - it is possible that you sat the initial spark timing with the vacuum advance connected, or it is possible that you forgot to reconnect it. If it isn't one of the two -then it is possible that your vacuum advance isn't working properly.

FWIW,

Carl B.

[obxtrainman]

Carl,

Once again, thanx for the help.

I'm saying at this point in time, I have the vacuum advance disconnected. Should I set the time at the 10 degrees point on the plate? Don't assume I know what you are talking about.

Yes, I believe the advance is stuck/ sticky/ ball bearings rusty. I bought a remain dizzy for this reason. Courtesy Nissan shows zero of the Vacuum advance assemblies across the board at this time.

I believe I have gummy carbs, slightly bad timing, rusty gas tank, blocked off ECU parts incorrectly, oh yeah, and woman problems:smoke:

[Carl Beck]

Hi Kenneth:

"10 degrees on the plate"...... If you mean the plate on the distributor itself - NO.

When you hold a timing light on the crankshaft pulley - with the engine at idle... you see the timing marks on the crankshaft pulley - and a "Pointer" that is bolted to the engine. The "0" timing mark is always the largest/deepest notch in the crankshaft pulley.

You'll be standing on the passenger side of the car with the Timing Light in hand at that point.

If you turn the distributor clockwise / counter clockwise very little and very slowly - while carefully observing the timing marks on the crankshaft pulley - you will see the the marks advance or retard (move ahead or behind the "0" mark). You want to set the initial spark timing on the 10 degree mark.

Your Public Profile gives me no indication of what year Z you own - so I can't tell you exactly what the marks/notches on the crankshaft pulley should look like - but in general they are marked in 5 degree increments. So the Pointer bolted to the block - should line up with the second mark past the larger "0" mark.

If you can't see the timing marks/notches clearly on the crankshaft pulley - you have to clean the pulley and put some white paint in the marks/notches...

Install the rebuilt distributor - and start over.

FWIW,

Carl

[MikeW]

Alternatively you can use an "advance" timing light. This allows you to read the degrees from the light instead of the scale which allows for more precision. For instance, you can set the timing light for 35 degrees and then twist the distributor until the pointer points to the biggest notch. You can also adjust the light until the pointer shows zero and then read your current timing from the light.

[obxtrainman]

At this point, I have changed the old distributor for the reman. The new one from BD auto is marked 612-53. The dwell is at about 38, the upside down V on the pulley is directly inverted at the 10 mark on the plate at the bottom of the engine. This is all within the RPM range suggested for a 1973 240Z with a manual tranny. It sounds great, and runs somewhat smoothly. However, as I hard accelerate going up through the gears, she misses a bit. Kind of a cough, or fart. I'm thinking at this point,,I need to tinker with the top adjustment.????????????/ Keeping in mind, the original carbs were swapped out for round top 4 screw carbs at some point. There is an actual plumbing cap on one of the old ECU ports. The old smog pump is gone, and most of the other components are removed, except for a dashpot on the top of the balance tube.:lick: There is no hose hooked to it. Ah, life is goodLOL

[obxtrainman]

I've had the timing all over the place, anywhere from 5btdc to 25tdc,,I am still getting a miss or backfire:bunny: It seems like it was best closer to 10btdc. No matter how minute of adjustment, I still get a problem:mad:

[Carl Beck]

If the timing is set correctly, the vacuum advance is working - and the points are set correctly - - - then perhaps your hesitation/misfire is caused by something else.

Carb's not adjusted correctly, sticky vacuum slides, wrong weight damper oil, float levels not correct etc etc.

or

Perhaps your plug wires are breaking down, loose wiring connections at the plugs or in the distributor .

or

Perhaps your fuel supply isn't consistent at a constant pressure.. Lots of stuff can cause the symptoms your talking about....

FWIW,

Carl B.