BETTER but -New theory - DJET–Worn Distributor /trigger points

[meltedpanda]

So ol Pearl has been through every DJET test I know to perform, the electrical system has been vetted but ( and it is better) the misfire returns. Seems only when hot now. Pick up is good, no lag, no ping, plug tips look good, fuel pressure normal, no vac leaks. Pulling 17 on the vac guage. No leak down of pressure after shutting her off . EVERYTHING IS WITHIN SPECS and that is why this is driving me crazy

I have not adjusted CO with meter , but will, I hope, soon as I have a line on a CO meter ( old retired gent in town ) .
I am having hard time thinking that is the issue though, this seems electrical
So going through my components that could be failing
Coil- doubt this – probably would not run or run very poorly
Points /condenser – fairly new , Dwell - good ( see note below)
Distributor – worn bushings? I have dwell that kinda bounces around a few degrees when running
Trigger points – I have two sets, both act the same , but could be worn equally.??

OH master GRAHAM or others
here are the questions for the day .
1. what are symptoms of a worn distributor shaft/bushings and could this be the cause
2. How do I confirm – wobble ?? measure something??
3. What can be done for a “loose” or worn distributor?
4. trigger points – is the simple on off ohm test when distributor rotates a good enough test?
5. Is it possible that this is all CO related - ???

Open to ideas and suggestions , but really focused on this dizzy – anyone have a known good one or where I could find one??

[230/8]

1. Firing point varies as you watch it with the timing light. Check it on other cylinders as well to confirm. Also, check the timing for the companion cylinder, that would be the fifth one in the firing order, to see if you have the same timing value as for cylinder #1. Your varying dwell is a clue.
2. Give the shaft a wiggle side to side and up/down. If it seems loose then and test #1 is positive...
3. Replace or rebuild to get it in specification. Too much end-play or side-to-side wobble can cause your symptom.
4. Usually, but they need to be clean and kept clean.
5. Yes, a too lean or too rich mixture can duplicate your symptoms at idle, but usually it is the too lean condition that causes the mis-fire.

Have you adjusted your timing to allow a more realistic spark advance? Your car is limited to roughly 18-22 degrees advance for smog reasons. European and rest of the world cars have 30-32 degrees of advance. Makes a difference in overall running efficiency. Also, do not run the vacuum line directly to the distributor, this causes a permanent spark retard condition. It is only supposed to be retarding at idle speed then it kicks out above idle, when the AC is on and when temperature exceeds 100C.

[meltedpanda]

230/8 - you gave me an idea , I have been futzing around with the dial on the ECU, the exhaust smells rich at idle so I been rotating the knob to lean. SO I think , you know it is running worse since I have been doing that , so I rotate back to rich , even more so than before, and it smooths out, a little stumble but not much, I stopped while I was ahead, lets see what happens on cold start in morning

[winecountryone]

I originally used the tried and true trick of adjusting to get highest idle vacuum on my d-jet(1974) When I was able to get a meter on in I was at 4+% CO,4X the specification!
If it smells rich it probably is... Very small adjustments on the MAP make HUGE differences.
If you are only adjusting the knob on the ECU and you can actually smell the richness I doubt you will be able to "fix" it with the limited adjustment available there.
My car is at about 0.8-0.9% CO.It puts out less than 100PPM hydrocarbons at idle without having a cat! Don't let anyone convince you these engines won't run clean! Your 72 probably doesn't have an air pump so double my numbers as a baseline as the air injection will reduce emissions by 50%.
I was amazed at how much better my car runs when properly adjusted.

[meltedpanda]

I was amazed that the technology in the old ECU is that percise -but sound like I need a MAP adjustment procedure as well - intersting. Do you have that process?
so at idle 1.6-1.8 % Is it good enough to just get it set at idle?

[Graham]

Originally Posted by meltedpanda

I was amazed that the technology in the old ECU is that percise -but sound like I need a MAP adjustment procedure as well - intersting. Do you have that process?
so at idle 1.6-1.8 % Is it good enough to just get it set at idle?

Ron,
If you suspect distributor wear (and even if you don't), consider installing a Pertronix 1885 in place of the points. It is not sensitive to distr shaft wobble and eliminates ever having to set points or measure dwell again! Cost is under $100. No need to do anything more than take the points out, slip the Pertronix in and attach two wires down near resistors.

I have a write up on how to adjust the MPS, but you have to have a GOOD CO meter or a wide band AFR meter before you start. Don't bother with one of those old wheatstone bridge type units. A Gunson or a pro type 5-gas meter is needed, or an AFR meter like the ones I referred you to earlier.

In the meantime, have you pulled your plugs? They can tell you a lot about the way your engine is tuned. This document shows my plugs during a time when my trigger points were worn. To check the trigger points, you need to remove the distributor, then connect a meter to common and then to each other contact in turn. While rotating the rotor by hand, note the angle that each of the 4 contacts closes for. It should be something in the 100deg range. In my case one was barely closing, but showed up as OK using the MB test. Typical plug after correcting trigger point problem.

Finally, an easy job - locate the ground connection on top/rear of engine where the injection system is grounded - clean this connection as well as the chassis ground near the battery. These can cause intermittent faults.

Aren't these old D-Jets fun

[meltedpanda]

Thanks Graham, since last post I haev eliminated trigger points per some of your other posts. MY plugs look very good now . I am intrigued by the pertronix, and may give that a go. I think I am narrowed to two items, dist.shaft and CO adjustment. IS your MPS write up on line somewhere

[Graham]

Ron,
I had to do a search to find the write up on BW:

The emission specs for the 117 engine according to the MB Technical Data Manual are:

Full load shifter position S 3,000 rpm = 2% - 5%
Upper partial load position S, 2,500 rpm 300 mm Hg vacuum 0.1% - 0.5%
Lower partial load 3rd gear shifter position S, 1,500 rpm 300 mm Hg vacuum = 0.2% - 0.5%
Idle neutral, idle speed, oil at operating temp = 0.5% - 2.0%

All % CO and would apply, I guess, to either dyno or road testing. I just used the idle and the full load for setting MPS & ECU.

Those are the North American specs. Euro are slightly different.

What type of meter do you have? Some of the old Wheatstone Bridge ones are apparently susceptible to moisture and kind of useless. If it is not one you can take in the car with, then you could try this:

First set timing so you have 5-8deg BTDC without vac and with A/C turned OFF. Check that you have good vacuum at idle (16-17" is normal). Also make sure your Throttle Position idle switch is functioning properly. Method is in engine manual (also HERE )

- Warm the car by driving it for at least five miles.
- Disconnect the throttle position switch plug (so as to disable the idle circuit) and set the ECU knob in the middle of it's range which is 11 clicks from right or left.
- Adjust the 4mm MPS screw in VERY SMALL increments so that you get about 3.5 percent CO at 2500-3000rpm. (at least as a starting point)
- Shut engine down and reconnect throttle plug.
- Start up again and set idle speed to about 750-800 rpm's.
- Use the ECU knob to set the idle co to about 1.5- 2.5 %CO. (rev enginel in between adjustments and taking new reading)
-Take car for a drive and then recheck CO at same conditions.

On my own car, I used this procedure. At first, I had a loan of an older CO meter - But it did not give consistent results. Since then, I have purchased a wideband Air/Fuel ratio gauge. (There are a number of manufacturers - mine is an AEM and is a permanent mount. Innovation makes portable units. Cost $200-$350 and well worth it.) This allows me to monitor AFR at all times. Better than watching TV

I learned above from a long time MB mechanic, but in this link, Steve Brotherton uses a similar technique, but seems to adjust the MPS at idle rather than at 3000rpm.

If you decide on using Pertronix, I can post a wiring diagram - the one that comes with the unit is not applicable to our cars when only doing a points replacement. But you don't really need a diagram - just connect the red to the 12v side of one of the resistors and the black to same place the green distr wire now connects. Also an idea to run a separate ground wire from distributor body to the battery or a good ground. A search on the Vintage forum should bring up many posts on this subject. (including THIS ONE)

[meltedpanda]

the adjustments make sense but so many thoughts on timing - 5-8deg BTDC without vac ?? I thought we wanted more like 28 ??
SO JUST WHAT is the definitive numbers with and without vacuum , at idle and at 3000RPMs

[Graham]

Originally Posted by meltedpanda

the adjustments make sense but so many thoughts on timing - 5-8deg BTDC without vac ?? I thought we wanted more like 28 ??
SO JUST WHAT is the definitive numbers with and without vacuum , at idle and at 3000RPMs

Ron,
If you set tining to say 5 BTDC at idle with no vac (and vac line plugged with golf tee!), then check at 3000rpm, you should see the centrifugal advance move the timing to about 28 BTDC. I don't think there is a definitive number. I check vac at MPS while doing timing and try to get good vac (17" or so) without overdoing the advance. I think I ended up at about 27-28, but I hate revving car like that for long!

PS: I once found I had no centrifugal advance - That's when I learned how to take the distr apart!

[meltedpanda]

Ok got it, SO the "T" on the fly wheel I assume is TDC so facing engine and looking down to the left of this T ATDC and to the R of this T is BTDC . I hope so cause that what seems to be the case on mine
Graham , I think you need to make a trip to Kentucky and stop over to walk me through all these adjustments (lol)

[RadioTek]

I had a problem with losing the dwell adjustment on mine after only a couple of months. I suspected that I had shaft wobble that was causing this, so I converted to a Crane / S&S breakerless system. Pertronix doesn't cover the 1975 on their application guide. The optoelectronic or magnetic retrofit kits are pretty much immune to the shaft wobble problem. My problems cleared right up.

The electronic systems are comprised of a shutter that fits on the distributor shaft, and a module that replaces the points to look for the openings in the shutter. This is usually a simple 'drop in' type of conversion, and thorough reading and understanding of the instructions will help.

[meltedpanda]

thanks, leaning to the Pertronix, simply because it would eliminate one of my factors, wobbly or worn shaft??
I wonder if I would need a new coil as well, and then there are posts about needing a shut off switch??
Too bad I have to work or I could play with cars all day , the list of things to do is endless

[Graham]

Originally Posted by meltedpanda

thanks, leaning to the Pertronix, simply because it would eliminate one of my factors, wobbly or worn shaft??
I wonder if I would need a new coil as well, and then there are posts about needing a shut off switch??
Too bad I have to work or I could play with cars all day , the list of things to do is endless

You don't need a new coil unless you plan on bypassing the MB "switchgear", which is not necessary and can lead to tach problems. I would suggest one step at a time . When switchgear fails, is when I will go to step 2

You don't really need a shutoff switch. But it would be handy. Alternative may be to put in an in-line fuse or one of those bayonet connectors on the 12V wire to allow a quick disconnect when you are messing with the car and have to leave the ignition switch turned on for more than a minute or so. I just disconnect where the 12v connects to the resistor.

[meltedpanda]

that is a good idea, I like that