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Author Topic: Ignition timing and rough running at 4000 rpm  (Read 1430 times)

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Offline LongTimeFordMan

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Ignition timing and rough running at 4000 rpm
« on: January 26, 2018, 07:40:09 PM »
Hi.. I know some of you racers have had experience with running 2.0 3ngines and need some advice. 

I have noticed that my engine, 1973 2.0 factory stock, about 4 degrees cam advance, rebuilt factory FoMoCo bosch distributor dual advance cams, pertronix ignitor i, stock oem 1.5 ohm coil resistor wire from ignition to coil. stock rebuilt holly weber progressive 2 bbl 4 speed manual trans seems to have a rough spot at 4000 rpm.

The engine pulls strong from about 2000 rpm to about 4000 rpm and then from 4000 to 5000 runs rough.

I have tried connecting the vacuum advance to the carb port and the manifold port and even run it without the vacuum.  Without the vacuum the centrifugal advance seems to have about 20 degrees.

The best low end torque seems to be with initial timing set at 12 degrees, at 1000 rpm, no vacuum advance for total advance of about 33 degrees but runs rough at 4000, with initial timing set at 10 degrees, total at about 28-30 it runs smooth at 4000 but bogs at 2300 rpm and lacks low end torque.

Any vacuum connection seems to exaggurate the problem. 

I am considerating a couple ot solutions

1. Replacing oem coil with 3 ohm pertronix coil and wiring the coil to 12 v around the resistor.

2. Recurving the distributor by adjusting the limit tabs in the distributor to limit the centrifugal advance to about 16 degrees so i can set initial at 12-14 and have the total limited to about 28-30.

Also when using a timing light, the light flashes as expected up to 4000 rpm th3n tends to flicker as if either it cant keep up or the coul cant keep up. I am pretty sure the problem is with the ignition rather than a vibration.

I understand these engines had some vibration problems at this range...is this normal?

Has anyone had any experience or suggestions.
Red 1973 pinto wagon DD, SoCal desert car, Factory 4 speed, 3.40 gears, Stock engine, 14" rims and tires, 60 K original miles

Offline Wittsend

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Re: Ignition timing and rough running at 4000 rpm
« Reply #1 on: January 26, 2018, 10:49:33 PM »
 The timing light at 4K seems more a ignition problem than a carburation issue. I'm not a long time racer but in some ways I'd suspect the coil (resistor possible too). Or, how the Pertronix is wired.


  I run a Pertronix in my SBC powered Studebaker and have investigated the hook up extensively.  The #1 problem people make is to wire the power (red wire) to the Pertronix at the resistor (on the resistive side). The Pertronix is designed to run on 12 volts and if wired through the resistor it is NOT getting that. I have my Pertronix wired directly to the ignition switch.  BTW, be careful with the Original (#1) version of the Pertronix. Supposedly left on but the engine not running can fry one in as little as 30 seconds.


Another Pertronix mistake is that people assume since the Pertronix is 12 volts they can run that directly to the coil too. No, the coil is still dependent upon the proper resistor being used.    Yet so many people think because it is 12 volts and "electronic" it sends 12 volts to the positive side of the coil - No! All the Pertronix does is momentarily complete the coils negative terminal to ground just like points do.


You may have it all correct so I thank you for bearing with me it is just that I have seen a LOT of misunderstandi ng about the Pertronix. The other option is your coil may be breaking down. The resistors are typically open to the elements and can cause corrosion problems too. I grasp the frustration. I have a Turbo Coupe 2.3 and had wacky ignition issues too.  I eventually found a cure, but not the cause.  If I ran 12 volts directly from the battery to the coil (this DOES use 12 volts at the coil) the problem went away. But that took about two years. Hopefully yours will be faster.

Offline LongTimeFordMan

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Re: Ignition timing and rough running at 4000 rpm
« Reply #2 on: January 26, 2018, 11:50:49 PM »
Hi. Thanks for the input.. i sort of suspected that either the supply voltage or the coil was the issue.. i plan to buy the 3 ohm pertronix coil and run the ignition on 12 volts. My problem is that the 73 wiring harness utilizes a 1.5 ohm resistor wire from the igntion switch rather than a separate resistor near the coil, and there are no other sources of switched 12v, other than the accessory  so its not readily possible to connect the pertronix ahead of the resistor.   I am considering wiring a wire from the batttery positive thru a relay to supply power to the coil.and pertronix. The relay can be activated by power from the existing resistor wire normally wired to the coil so it switches on with the ignition switch. Will probably order the 3 ohm pertronix coil in a week or so and see what happens.

BTW..  I had a chat with the tech guy at pertronix and apparently with breaker point ignitions, the coils are designed to work on 4 amps, so the total resistance in the coil circuit needs to be 3 ohms.  There seem to be two types of coils, 1.5 ohm used with a 1.5 ohm resistor wire or external resistor and 3 ohm coils used with no resistor. The advantage of the 1.5 ohm coil is that when the starter is engaged, the resistor can be bypassed to provide more voltage at the coil for starting.
Red 1973 pinto wagon DD, SoCal desert car, Factory 4 speed, 3.40 gears, Stock engine, 14" rims and tires, 60 K original miles

Offline LongTimeFordMan

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Re: Ignition timing and rough running at 4000 rpm
« Reply #3 on: January 27, 2018, 12:02:59 AM »
Also.. i was wondering if the 2.0 engine has some sort of characteristic roughness at 4000-5000 rpm and or what the total spark advance should be.

And how well the factory oem FoMoCo bosch distributors do at high revs. Are there other distributor chouces for the 2.0 engine
Red 1973 pinto wagon DD, SoCal desert car, Factory 4 speed, 3.40 gears, Stock engine, 14" rims and tires, 60 K original miles

Offline dick1172762

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Re: Ignition timing and rough running at 4000 rpm
« Reply #4 on: January 27, 2018, 10:09:28 AM »
In one of my 100+ tech tips(FAQ) I posted about how you can use a V6 distributors in a 2.0L to have electronic ignition. It's posted in the tech tips in the Capri Club of Chicago. web site.
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Offline nnn0wqk

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Re: Ignition timing and rough running at 4000 rpm
« Reply #5 on: January 27, 2018, 01:45:47 PM »
Back in the day when I worked as a tune-up tech it seems to me that most FOMOCO products ran around 40-45 degrees total advance and that usually hit at around 2500 rpm. Of course that would be a no load condition on the motor. Most everything in the day would have used ported vacuum to the dist. Base timing on a lot of FOMOCO engines was 6 degrees. Centrifugal advance ball park would be probably 22-26 degrees. Which would give you around 32 degrees; base plus centrifugal. The rest was made with the vacuum advance system and that system usually was 8-10 degrees.  What is your dwell doing as the rpm goes up? If that is going out of range it will certainly mess with the timing curve. You mention the timing light acting up over 4000. It just may be that your getting a lot of points bounce if the tension is not correct or the dwell is way out of range and not allowing a good build up of voltage in the coil. Personally I would also rethink your total advance as it seems your shooting for a low number. Remember under load the vacuum advance drops off anyway and under WOT conditions probably is not much more than 2-3 degrees if even that. Try to find the original advance curve for that dist. Motor or Chilton manual often have that information in them. I would start out with factory specs and then tweak it from there. The numbers I mentioned are as I remember them but 35 or so years of time do tend to cloud ones memory so please do not hold be to them.

Offline nnn0wqk

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Re: Ignition timing and rough running at 4000 rpm
« Reply #6 on: January 27, 2018, 07:11:16 PM »
These specs are from a 75 Motor manual. Not sure if your using the OEM dist. but this should at least give you some ideas what the timing curves were in 73.

Offline LongTimeFordMan

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Re: Ignition timing and rough running at 4000 rpm
« Reply #7 on: January 28, 2018, 10:38:13 AM »
Im using a pertroni, ignitor I module, so no point bounce and dwell issues. Oem style 1.5 ohm aftermarket coil. Ordered a pertronix flamethrower 3 ohm coil, it arrives today. I am gonna wire up a 12v non resistor supply to the coil and pertronix as per pertronix tech suggested and see what happens.  Going to use a relay activated by existing resistor wire in harness to activate the relay and switch on power to coil and pertronix when ign switch is on.. will let you know result..
Red 1973 pinto wagon DD, SoCal desert car, Factory 4 speed, 3.40 gears, Stock engine, 14" rims and tires, 60 K original miles

Offline LongTimeFordMan

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Re: Ignition timing and rough running at 4000 rpm
« Reply #8 on: January 28, 2018, 06:48:16 PM »
Problem partially solved.. I replaced the aftermarket oem spec 1.5 ohm coil with a 3 ohm pertronix flamethrower and the engine pulls better. While rewiring to c9nnect the 3 ohm coil I noticed that the fa tory wiring harness had push on connectors for the coil and the  connector for the negative post to distributor was extremely loose and probably not making good contact.

I still need to work on the mechanical advance curve. With initial set at 12 degrees at 1000 rpm, mechanical with no vacuum advance connected.. at 3000 rpm the timing is 34 degrees. So the mechanical is about 22 degrees. I understand that the vacuum only works at part throttle and at load is minimal, however as i reduce throttle at top of power curve before shifting the vacuum is at or near max and is too much.

It seems that I need more advance at bottom of curve and less at top.

I will adjust the limiting pegs in the distributor by putting some rubber tubing over them to limit max mechanical to about 16-18 and try with and without vacuum.

Will post how this works..

Also.. when I got the car the factory distributor had been replaced with a "rebuilt" one so I have no idea if this distributor has the proper curve.
Red 1973 pinto wagon DD, SoCal desert car, Factory 4 speed, 3.40 gears, Stock engine, 14" rims and tires, 60 K original miles

Offline LongTimeFordMan

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Re: Ignition timing and rough running at 4000 rpm
« Reply #9 on: January 29, 2018, 03:49:44 PM »
Update on rough spot at 4000 rpm..  well i finally solved the problem.. it was the mechanical advance in The distributor. As I mentioned, my car seems to like about 12-14 degrees at 1100 rpm and about 28 total.  My distributor was advancing to about 20 degrees to a total of 35 at 3000 rpm.

I solved the problem by opening up the distributor and removing the secondary spring, the strong one and replacing it with a loop of wire to limit the mechanical advance to about 10-12 degrees . See photo.  Also i can adjust the total mechanical by bending the wire loop.

This with inital set at 13 degrees at 1100 rpm provides about 26 total with no vacuum advance.

As i mentioned i need more advance at low rpm and less at high.
3ngine now pulls strong and smooth from about 2000 to 5000, no roughness and lots of torque at 3000..

I did advance the cam timing about 4 degrees to get more low end torque since i have a 4 speed trans and 14 inch tires, so at 60 mph the engine runs at about 2700 rpm.

One reason i wanted to get this sorted out is that is that i am building a new engine with SU carbs and custom manifold so there would only be manifold vacuum.

As for total advance, i tend to feel that running a little retarded at the top of the power curve would be better than to advanced..

Anyway for my engine 13 degrees at 1100 and 28 -30 at 3000 seems to work.

Red 1973 pinto wagon DD, SoCal desert car, Factory 4 speed, 3.40 gears, Stock engine, 14" rims and tires, 60 K original miles

Offline Wittsend

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Re: Ignition timing and rough running at 4000 rpm
« Reply #10 on: January 29, 2018, 04:08:07 PM »
Great that you found your solution.  Looking forward to seeing the SU set up.  I have them on my Datsun 510, had them on my 240Z and Volvo 544.  Back in the mid 70's I even converted a stock 2 bbl. Datsun down draft intake to take a side draft single SU setup that I made an adapter out of sheet metal (see illustration).