suspected erractic misfire/weak ignition

[jwgarvey]

My set up 240z 1972 with stock ignition single point, L28 n42 block and n47 head, headers, stock SU's three bolt (rebuilt and all adjusted included float levels)

car presently runs well but I suspect a weak ingition due to fouling plug 3, changes in plug presention with and without ballast

(see below), noticable power increase with ballast bypassed (see below).

1. My consern is that I have an eratic misfire or eractic weak spark resulting in loss power.

2. manifold vacuum 12 hg inches consistantly at idle (too low for spec) (all valves have been adjusted cold and hot). no white smoke, barely noticeable

black smoke at wot on freeway.

3. cylinders 6 and 5 presently show little if any change in rpm when spark plug wire is pulled off the spark plug.

a. spark can be heard firing when spark plug wire is separted from spark plug six and five and held close to plug.

during this separation process it is difficult at best to determine if a change in rpm is noted. In addition, an external rpm gauge is used to see rpm changes, none are noted during this separation process.

4. A timing light is used to verify if each cylinder is firing by way of using clamp (pick up) on each plug wire to see if it lights. at present all except cylinder number one lights consistanly. number one shows an erratic light during idle and shows consistant lighting with increased throttle somewhere around 1200 rpm and up thereafter. this method has shown multiple results/inconsistancy throughout my search of this suspected problem. current timing is set at 20 btd (I have tried 15 and 17 as well without success). anything below 15 the car runs noticeably poor and thereafter

exponentially.

5. spark plug analysis (present gap 32, have tried 35 without success)

a. with ballast in place all plugs have carbon build up.

b. without ballast 6 good, 5 carbon, 4 carbon, 3 carbon, 2 good, 1 good.

what I have done-

1. checked all ignition wiring (resistance, voltage, all normal spec) including to ballast, from ballast, to coil, from coil, to dizzy, in dizzy

and to condenser.

2. plug wire resistance within spec, cap resistance good, new rotor, new points (dwell angle set, dwell angle rock solid at any rpm so no slop in dizzy), mechanical advanced cleaned lubed and verified in working order, vacuum

advance capped atm( this should cause no problem), new condenser, coil wire resistance good, coil good (primary and secondary

windings withing spec), ballast resistor withing spec.

3. no vacuum leaks noted spraying with brake fluid at intake. no vacuum leak noted with applying brake (ie. brake booster disconneted and capped at connection at present).

4. cleaned all contacts at battery. battery within voltage spec as well.

what I have not done-

1. have not changed cap, cap does not currently show any crackes or carbon build up or any real sign of arcing, all 6 contacts

have been cleaned. plug wires have been securely placed in cap!

2. dizzy cam has not been replaced, it was a bit rough so I cleaned it up with some thousand grit (just a bit, I did not grind it down!).

3. I have not added a motor to chassis ground wire. currently there is one ground from battery neg post to starter contacting block.

there is an additional approx 10 gauge wire from neg post on battery conected to firewall (this has been cleaned and now has good contact and zero resistance.

4. have not done a compression check or leak down. Let it be noted that all symptoms are subtle and erratic and not isolated to any one cylinder at

any given time.

man I am tired atm, tired of working on her for about two weeks without a solid answer and tired because it is late and I need sleep. I

hope I did not miss anything.

fyi, for those of you who are going to recomend tonix I will be doing so only after I have found the problem as I do not want to cover my issue with a bandage. thanks

Arne? Carl? Jimmy? any thoughts? forgive me if I have left you out

thanks

[akorna]

Are you sure your carbs are not running too rich? You may also have a sticking nozzle that does not move properly up into the carb after releasing the choke. After the car has warmed up, release the choke and then push up on the nozzle under the carb for each carb. If it moves at all the nozzle/choke mechanism is sticking.

Why is your vacuum advance disconnected?

[jwgarvey]

Are you sure your carbs are not running too rich? You may also have a sticking nozzle that does not move properly up into the carb after releasing the choke. After the car has warmed up, release the choke and then push up on the nozzle under the carb for each carb. If it moves at all the nozzle/choke mechanism is sticking.

Why is your vacuum advance disconnected?

thanks for the response akorna .

I am convinced the carbs are working fine, I rebuilt and tuned them myself. float levels set, throttle valves set (air flow), linkage not binding, pistons not sticking, etc..

1. choke cables are disconnected atm (I did have an old none function set connected but I just purchased a new set of cables and will be installing them soon). nozzles are moving freely so this is rulled out. (btw, the car starts without difficulty in the morning)

2.vacuum advance is disconnected for one primary reason atm (soon to be fixed). the vacuum port/ported vacuum I believe its called (nipple at front carbi) is not on there. it came this way when I bought the car. it is sealed with silicon atm with no sign of vacuum leak. fyi, I no longer use the manifold vacuum after discovering through research, hands on vacuum pump work and recommendation from my reputable mechanic friend that this vacuum works differently and should not been used. I will be slightly drilling out this port at its carbi surface to replace the otherwise missing port/nipple with a piece of brass tubing (recommended by bruce at ztherapy). The advance mechanism itself (ie. breaker plate and spring activated diaphram that pulls the breaker plate) works when vacuum pump is applied (activates at approx 8 to 10 hg in vac and holds vacuum and breaker plate moves freely) and deactivates when vacuum is released. all internal dizzy parts have been been completely cleaned and lubed (including dizzy breaker plate).

My brain is tired akorna ugh!

1. I am debating on buying a cap atm. maybe there is a hairline fracture in the cap that I am unable to seen. this could cause an unstable arc within the cap. I am scratching my head.

2. I am also wondering if there is a way to deffinatively determine if the plug cables are arcing at the block. I am running out of options. ugh!

keep the ideas coming akorna

[SteveJ]

Do you have any play in the distributor shaft that could keep the rotor from touch the contacts on the distributor cap? Have you thought of converting to a pertronix or electronic ignition to replace the points?

[jwgarvey]

Do you have any play in the distributor shaft that could keep the rotor from touch the contacts on the distributor cap? Have you thought of converting to a pertronix or electronic ignition to replace the points?

Thanks for the response stevej

dizzy is rock solid, no play back or forth, bearings present good. my mechanic friend looked and verified there solid as well.

yes pertronix will happen as soon as I resolve this issue. I want to make sure not to cover a up a problem first.

keep the ideas coming stevej

[jwgarvey]

, what the heck were u doing up at 3:21 am for akorna . are you reviewing your threads frequently as well

[akorna]

2.vacuum advance is disconnected for one primary reason atm (soon to be fixed). the vacuum port/ported vacuum I believe its called (nipple at front carbi) is not on there. it came this way when I bought the car. it is sealed with silicon atm with no sign of vacuum leak. fyi, I no longer use the manifold vacuum after discovering through research, hands on vacuum pump work and recommendation from my reputable mechanic friend that this vacuum works differently and should not been used. I will be slightly drilling out this port at its carbi surface to replace the otherwise missing port/nipple with a piece of brass tubing (recommended by bruce at ztherapy). The advance mechanism itself (ie. breaker plate and spring activated diaphram that pulls the breaker plate) works when vacuum pump is applied (activates at approx 8 to 10 hg in vac and holds vacuum and breaker plate moves freely) and deactivates when vacuum is released. all internal dizzy parts have been been completely cleaned and lubed (including dizzy breaker plate).

Sorry for being ignorant but what is "atm"

Also - I just solved a problem w/my distributor - I thought the breaker plate was ok - but in fact it was sticking/catching. If you have a hand vacuum pump, recheck your vacuum advance operation - it should start operating at 4-5 inches Hg - not 8-10 (see the FSM) If your vacuum advance is not even connected, not sure that would be an issue - but I would proceed w/the installation of the port and connect the vacuum.

Another diagnostic thought - have you checked compression?

[jwgarvey]

Sorry for being ignorant but what is "atm"

Also - I just solved a problem w/my distributor - I thought the breaker plate was ok - but in fact it was sticking/catching. If you have a hand vacuum pump, recheck your vacuum advance operation - it should start operating at 4-5 inches Hg - not 8-10 (see the FSM) If your vacuum advance is not even connected, not sure that would be an issue - but I would proceed w/the installation of the port and connect the vacuum.

Another diagnostic thought - have you checked compression?

grats on your resolution akorna.

yes u are correct 4-5 inches I have fsm but stated wrong. my vacuum advance still kicks in at about 8 as a result of a worn lever connection (where the spring loaded diaphram connects to the breaker plate. it should be round at the connnection but is oval from wear and tear. the breaker plate however has been tested and otherwise works and does not stick).

Based on what I have read and my knowledge base, this should not cause the problem I am having. also, a new port for the vacuum is coming soon.

update. I purchased a new cap and placed a new ground from the chassis to block and recleaned all other grounds. problem still not solved!!! lastly I connect my mallory hyfire ignition box as a test to see if this would solve the problem. result, symptoms still occuring.

The only thing left to fix/change at this point is a new cam on the dizzy. my thinking is the following. 1. how significant must a cam's lobes be worn in order to produce a symptomatic problem? 2 What kind of symptoms would develop? I welcome your input as this is the last of my ignition diagnostic search.

my first mod at this point will be a pertronix and yes the mallory hyfire is coming of as it was just used as a test.

compression check will probably come before pertronix.

[jwgarvey]

hmm, noticed something different last night. when using the timing light (motor now set at 17 degrees), at idle around 700rpm, the timing mark on the balancer would retard inconsistantly back and forth approx 3-5 degrees from 17 degrees to approx 12 degrees. keep in mind the vacuum advance is plugged atm. and the mechanical advance has checked out fine. the dizzy is absolutly rock solid with not sign of play. NOW I AM THINKING TIMING CHAIN IS STRETCHED! AM I CORRECT IN SUPPECTING THIS?

[tlorber]

Garvey,

I wouldn't start suspecting more complicated problems at this point. Idle tends to wander a bit with carbs even in good circumstances. Even if the chain is stretched (which is what tensioners are for) it would cause a consistent lag and should not cause wandering idle/timing variance.

I understand what you are saying about not wanting to introduce more variables until you have solved a problem. However, I just picked up an electronic dizzy (from an '80zx) off Craigs List. I have not yet installed it, but it only cost me $60.00. You could try that and see if the problem improves. I've read and heard that it yields a much stronger and consistent spark.

[alhbln]

jwgarey,

the symptoms you describe sound like either a defective ignition (in your setup the coil is the only thing left not yet fully tested on a workbench with different RPMs while measuring primary current and secondary voltage. Measuring the primary/secondary resistance with a multimeter only diagnoses broken wires or severe shorts due to wire isolation issues in the winding, but does not show issues with the inductivity or thermal issues) or timing/phase irregularities induced by the distributor.

With a good distributor, in general the timing should not wander more than 1-2 degrees while idling. 3-5 degrees are a lot and point to a worn out distributor/distributor drive. A worn out chain could create similar effects.

If the issues are only apparent on specific plugs (you last posted that this is not the case), the distributor, spark cables and plugs need to be checked. The most common issue here is a defect in one of the spark wires, cap issues, or a partially worn down or dirty cam. If your issues are random (spread to all plugs), the full ignition setup must be checked, with a focus on the points, capacitor, coil, ground connection and voltage supply to the ignition.

Assuming that air and fuel have been checked and are fully functional, you might want to do the following checks to ensure the ignition components are ok:

1. Measure the resistance from the batteries negative terminal to the engine block to ensure the ground connection is good. The resistance should be not higher than 0,5 ohm.

2. Use a simple variable spark tester to test the available spark energy at the end of each spark cable going to the plugs. The spark should be strong enough to at least jump a gap of 1 cm/0.4 inch. A healthy ignition can manage 1,5 cm/0.6 inch, a CDI such as the Mallory Hyfire or MSD 6A easily does 2 cm/0.8 inch.

(example: http://www.thetoolwarehouse.net/p-3156-lisle-50850.aspx)

3. Remove the distributor and do a thorough run on a distributor tester. Even if the distributor axle seems to have no play, a combination of a worn out drive, axle and cam can create critical timing and phase errors. A worn cam can create irregularities in the dwell, ignition timing and phase errors.

4. Test the running engine with a motor tester (ignition oscilloscope, as build by Sun, Bosch, Pico and others) and compare the spark discharge pattern by each cylinder to check for timing and phase errors and correct ignition and burn phase. If used by an experienced person (your local workshop might help), using a motor tester with an oscilloscope is the quickest and easiest way to diagnose ignition and timing problems.

I would recommend leaving the vacuum disconnected for testing, add this later on if the core issues have been fixed. Vacuum advance is only relevant for more economic fuel usage in idle (no load condition). Close the vacuum tube coming from the engine while testing without vacuum.

As a first quick test, you might want to change the coil against a new one (Bosch red coil with a resistor should be fine for your setup).

Check the engine/ground resistance as in step 1 first. You might also want to change the plugs against a set of new ones to be sure. NGK BPR6EIX are a very good choice for the stock points and coil ignition.

Let us know how you get on.

Good luck,

Adrian

[mlc240z]

Had that same "wandering" timing on, if you can believe it, a 1973 Ford Pinto.

Turned out to be a worn distributor.

Replaced it with rebuilt and problem went away.