consistant misfire out of nowhere

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Diaddigo

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I'm having trouble with my slant six in my 72 duster. while I was out driving on saturday, it was running fine and normally. I stopped at a stop sign and when i accelerated again it started bogging, misfiring, and the car stopped moving. Also the RPMs would not change even if I floored the gas. it finally died and after many attempts to restart it to no avail, I consented to have it towed back to my house (less than 1 mile back). once it cooled off in the garage, my first inclination was to check to make sure it was getting enough and clean fuel, I checked the fuel pump, and replaced the filter and with that I could start it as long as I held the gas pedal to the floor. I checked the distributer rotor, that seems good. My next consensus was bad spark, seeing that the plugs only had 100-150 miles on them I replaced the ignition coil (which was the original one) with one from Accell (the yellow one) and now it starts without holding the gas, runs and I can rev it a little but still the bad misfire. Has anyone had this issue before? and ideas?

thanks,
Jonathan
 
When is the last time you set the air gap in the distributor?

Had a similar issue.

.008 with a brass feeler gauge.

What's the timing look like? What carb?
 
I had someone rebuild the engine for me and got it back 3 months ago (it was sitting for 20 years prior) and I'm slowly discovering that he didn't do that well. So probably not anytime soon on the distributor. Not sure about timing as I do not have a way to check that in my garage yet. its a Holley 1920. Sorry I don't have much info as I am pretty new to working on my own car.
 
Cool, I'd go to Harbor Freight and get one of those dial back timing lights, and a brass feeler can be procured at the autoparts store.

1534Mopar_dist_3763.JPG


The procedure is called 'pick-up to reluctor air gap' just loosen that flat head screwon the top there.

Here's a guide to timing:

http://www.forabodiesonly.com/mopar/showthread.php?t=160642

For fun:

TIMING AND VACUUM ADVANCE 101

The most important concept to understand is that lean mixtures, such as at idle and steady highway cruise, take longer to burn than rich mixtures; idle in particular, as idle mixture is affected by exhaust gas dilution. This requires that lean mixtures have "the fire lit" earlier in the compression cycle (spark timing advanced), allowing more burn time so that peak cylinder pressure is reached just after TDC for peak efficiency and reduced exhaust gas temperature (wasted combustion energy). Rich mixtures, on the other hand, burn faster than lean mixtures, so they need to have "the fire lit" later in the compression cycle (spark timing retarded slightly) so maximum cylinder pressure is still achieved at the same point after TDC as with the lean mixture, for maximum efficiency.

The centrifugal advance system in a distributor advances spark timing purely as a function of engine rpm (irrespective of engine load or operating conditions), with the amount of advance and the rate at which it comes in determined by the weights and springs on top of the autocam mechanism. The amount of advance added by the distributor, combined with initial static timing, is "total timing" (i.e., the 34-36 degrees at high rpm that most SBC's like). Vacuum advance has absolutely nothing to do with total timing or performance, as when the throttle is opened, manifold vacuum drops essentially to zero, and the vacuum advance drops out entirely; it has no part in the "total timing" equation.

At idle, the engine needs additional spark advance in order to fire that lean, diluted mixture earlier in order to develop maximum cylinder pressure at the proper point, so the vacuum advance can (connected to manifold vacuum, not "ported" vacuum - more on that aberration later) is activated by the high manifold vacuum, and adds about 15 degrees of spark advance, on top of the initial static timing setting (i.e., if your static timing is at 10 degrees, at idle it's actually around 25 degrees with the vacuum advance connected). The same thing occurs at steady-state highway cruise; the mixture is lean, takes longer to burn, the load on the engine is low, the manifold vacuum is high, so the vacuum advance is again deployed, and if you had a timing light set up so you could see the balancer as you were going down the highway, you'd see about 50 degrees advance (10 degrees initial, 20-25 degrees from the centrifugal advance, and 15 degrees from the vacuum advance) at steady-state cruise (it only takes about 40 horsepower to cruise at 50mph).

When you accelerate, the mixture is instantly enriched (by the accelerator pump, power valve, etc.), burns faster, doesn't need the additional spark advance, and when the throttle plates open, manifold vacuum drops, and the vacuum advance can returns to zero, retarding the spark timing back to what is provided by the initial static timing plus the centrifugal advance provided by the distributor at that engine rpm; the vacuum advance doesn't come back into play until you back off the gas and manifold vacuum increases again as you return to steady-state cruise, when the mixture again becomes lean.

The key difference is that centrifugal advance (in the distributor autocam via weights and springs) is purely rpm-sensitive; nothing changes it except changes in rpm. Vacuum advance, on the other hand, responds to engine load and rapidly-changing operating conditions, providing the correct degree of spark advance at any point in time based on engine load, to deal with both lean and rich mixture conditions. By today's terms, this was a relatively crude mechanical system, but it did a good job of optimizing engine efficiency, throttle response, fuel economy, and idle cooling, with absolutely ZERO effect on wide-open throttle performance, as vacuum advance is inoperative under wide-open throttle conditions. In modern cars with computerized engine controllers, all those sensors and the controller change both mixture and spark timing 50 to 100 times per second, and we don't even HAVE a distributor any more - it's all electronic.

Now, to the widely-misunderstood manifold-vs.-ported vacuum aberration. After 30-40 years of controlling vacuum advance with full manifold vacuum, along came emissions requirements, years before catalytic converter technology had been developed, and all manner of crude band-aid systems were developed to try and reduce hydrocarbons and oxides of nitrogen in the exhaust stream. One of these band-aids was "ported spark", which moved the vacuum pickup orifice in the carburetor venturi from below the throttle plate (where it was exposed to full manifold vacuum at idle) to above the throttle plate, where it saw no manifold vacuum at all at idle. This meant the vacuum advance was inoperative at idle (retarding spark timing from its optimum value), and these applications also had VERY low initial static timing (usually 4 degrees or less, and some actually were set at 2 degrees AFTER TDC). This was done in order to increase exhaust gas temperature (due to "lighting the fire late") to improve the effectiveness of the "afterburning" of hydrocarbons by the air injected into the exhaust manifolds by the A.I.R. system; as a result, these engines ran like crap, and an enormous amount of wasted heat energy was transferred through the exhaust port walls into the coolant, causing them to run hot at idle - cylinder pressure fell off, engine temperatures went up, combustion efficiency went down the drain, and fuel economy went down with it.

If you look at the centrifugal advance calibrations for these "ported spark, late-timed" engines, you'll see that instead of having 20 degrees of advance, they had up to 34 degrees of advance in the distributor, in order to get back to the 34-36 degrees "total timing" at high rpm wide-open throttle to get some of the performance back. The vacuum advance still worked at steady-state highway cruise (lean mixture = low emissions), but it was inoperative at idle, which caused all manner of problems - "ported vacuum" was strictly an early, pre-converter crude emissions strategy, and nothing more.

What about the Harry high-school non-vacuum advance polished billet "whizbang" distributors you see in the Summit and Jeg's catalogs? They're JUNK on a street-driven car, but some people keep buying them because they're "race car" parts, so they must be "good for my car" - they're NOT. "Race cars" run at wide-open throttle, rich mixture, full load, and high rpm all the time, so they don't need a system (vacuum advance) to deal with the full range of driving conditions encountered in street operation. Anyone driving a street-driven car without manifold-connected vacuum advance is sacrificing idle cooling, throttle response, engine efficiency, and fuel economy, probably because they don't understand what vacuum advance is, how it works, and what it's for - there are lots of long-time experienced "mechanics" who don't understand the principles and operation of vacuum advance either, so they're not alone.

Vacuum advance calibrations are different between stock engines and modified engines, especially if you have a lot of cam and have relatively low manifold vacuum at idle. Most stock vacuum advance cans aren’t fully-deployed until they see about 15” Hg. Manifold vacuum, so those cans don’t work very well on a modified engine; with less than 15” Hg. at a rough idle, the stock can will “dither” in and out in response to the rapidly-changing manifold vacuum, constantly varying the amount of vacuum advance, which creates an unstable idle. Modified engines with more cam that generate less than 15” Hg. of vacuum at idle need a vacuum advance can that’s fully-deployed at least 1”, preferably 2” of vacuum less than idle vacuum level so idle advance is solid and stable; the Echlin #VC-1810 advance can (about $10 at NAPA) provides the same amount of advance as the stock can (15 degrees), but is fully-deployed at only 8” of vacuum, so there is no variation in idle timing even with a stout cam.

For peak engine performance, driveability, idle cooling and efficiency in a street-driven car, you need vacuum advance, connected to full manifold vacuum. Absolutely. Positively. Don't ask Summit or Jeg's about it – they don’t understand it, they're on commission, and they want to sell "race car" parts.
 
make sure your float isnt sticking in the carb maybe tap it with a hammer .
 
Thanks guys, I'll have to try the brass feelers and setting the timing. here is a link to a video of my poor slant six if this helps any further diagnosis. [ame="https://youtu.be/1Y0zILA6HD0"]https://youtu.be/1Y0zILA6HD0[/ame]
 
was there a new timing chain used when rebuilt ? maybe pull the valve cover an d check to see if an y valves are sticking .
 
I say carburetor myself .

My 70 duster that I bought last year had been sitting a long time and I replaced the tank.

The main jet in the carb clogged twice after first getting it on the road and has been fine since and it acted just like yours is doing.

Very simple fix take off the float bowl and unscrew the jet and clean out the hole , I was care full and was able to reuse the gasket both times. Blow out every thing in the bowl with carb cleaner and reassemble
 
You can clean out the Holley 6 needle and seat on the 1920 model by taking the needle valve out from the outside; no need to remove the bowl.

For the OP:

- Remove the distributor cap and verify for all of us that the inside looks like the picture above, with a 6 pointed 'star' under the spark rotor button.
- Putting an an Accel cail may be good or not; the Mopar system works with a specific resistance of coil. Look up your coil model and see if it around 1.5 ohms primary coil resistance.
- As simple, no-equipment tests of spark to see if that is basically good:

1. Remove the distributor end of the spark wire from the coil to the distributor cap center post, and place that end 1/4" to 3/8" from metal, crank the car, and see if you get a steady series of good blue sparks from that wire. If so, your coil and ballast and ECU and distributor electronic pickup ignition power feed are good.
2. If good in 1., then repeat the test by putting the spark wire from the coil back into the distributor, and then take an individual spark plug wire, pull back the boot, and set that 1/4" to 3/8" from metal and repeat; you should get a good blue spark at 1/6 of the rate as before.
3. If bad in test 1., then the problem is localized to the coil, ballast, ECU, distributor pickup, or ignition feed.
4. If 1. and 3. are good, then it is time to look at the fuel situation in more detail.

Let us know what you find from these tests.
 
Some seriously wrong there. Time to remove the valve cover and see what the relationship between TDC on the balancer and the valves and distributor rotor look like.

Don't mess with the carb yet or go buy buy more new plugs or wires or anything else. These threads have a way of leading people around in big circles.

Check or HAVE checked the mechanical condition of the engine first.
 
Classic fuel system symptoms. The FIRST thing I always do when I get an older car is automatically remove the gas tank, drain it and clean it. You cannot expect a thirty plus year old tank not to have sediment and rust buildup. Unless you fix the SOURCE, you will be forever having the same issue.

The pump sucks trash out of the tank, puts it through the filter and into the carburetor. The filter eventually stops up. You replace the filter to get it running again, only to find the carburetor is messed up from the trash from the tank. Clean the tank, replace the filter again, remove disassemble, clean and put a kit in the carburetor and you will fix the problem. Anything less and you are pissin in a fan.
 
wow thanks guys, I haven't been out there lately due to work but tomorrow I will do some of the tests you guys suggested. It is a new tank and the old filter was not clogged yet, just a little junk. Is there a way to check the timing chain without removing the radiator, belt etc? I'll let you guys know how it goes.
 
Yes and no.
You can do a crank-rock test. This is rocking the crank backwards and forwards,just enough to take up the slack in the chain. The valve gear will tend to keep the cam from turning.
The results are interpretive. The best result is if the there is near zero crank-rock.This is only achievable with a new chain, and even then,its only near zero. An old ready-to-jump chain might have 10* slop, or a bit more. So between 2 and 10 is subjective. A double roller chain on steel sprockets will run pretty good, and reliably nearly forever. One of those "silent" chains and nylon toothed sprockets, not so much. They often expired on teeners at well before 100,000 miles.
There are two ways to tell if the chain has jumped.
1) if you have not yet disturbed the timing, You can check the rotor to #1 tower alignment, with the #1 piston at TDC/compression. This is somewhat difficult to do, on account of the dizzy being down in a dark crowded hole. And it is somewhat interpretive, as the rotor can be quite a ways off, even in a normally running engine.
2) Split overlap. The point in the cam cycle when both the valves for a certain cylinder are open about the same amount, is called split overlap. The exhaust will be closing and very nearly so, and the intake will be just opening.This happens at the end of the exhaust stroke, and very near to TDC.Usually within about 5 degrees or less depending on the cam, the install, and chain-stretch.
So off comes the valve cover. Set the balancer to TDC/exhaust stroke. Then rock the crank back and forth while observing the rocker arms for #1 cylinder. This assumes the balancer marks are accurate, and the valves are correctly lashed. If in doubt, lash the valves to near zero and both the same. Dont forget to reset them after the tests are in.
 
Re post 6, my 1969 Dart 225 had the "timed port" vacuum advance in the carburetor (I think) and the book spec'ed a really low base advance (almost TDC I recall). So, that sounds like the "late timing" emissions setup. But, it did not have air injection into the exhaust. I don't think that started until ~1982 (my Chevy S-10 and Dodge Aries had). My 69 Dart idled much better w/ ~20 deg advance, but I never set that because it was so far off the FSM value. I think the problem was a lean mixture, since it ran like a charm on the 4th Holley 1920 after decades of rough idle.

The OP should first check the ground of the spark ECU box. That is a common problem and often erratic from shaking. Don't rely on a rusty sheet-metal screw, run a dedicated ground to the case.
 
Before ANY of the above do a simple voltage check at the coil pos with the ignition in the run position.
If you have a ballast resistor ignition it should be about 8-9 volts.

(You did say it changed with a different coil) :)


It happened suddenly, so I wouldn't suspect a lack of fuel right away. (would probably be popping and stuff anyway)
It was running fine till you tried to leave the stop (timing jumps usually happen after a shutdown)

I say it's a weak spark issue.

While it's running you can jumper right from the battery positive to the coil pos temporarily, and if it acts good again, then you know you have a low spark/voltage problem.
If it doesn't change you can move on to harder things.

I always like to start with the simple and easy stuff first.
 
Piece of wire with an alligator clip on each end.

That would be perfect of course, but you can also put the bare end of a wire under the nut on the coil and the other end stuck in the gap of the battery terminal where it can contact.

We don't want to turn it into it's own project, just a quick test. :D
 
That would be perfect of course, but you can also put the bare end of a wire under the nut on the coil and the other end stuck in the gap of the battery terminal where it can contact.

We don't want to turn it into it's own project, just a quick test. :D

Everyone should have one in the toolbox anyway.
 
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