My daily driver 1972 390 with Motor Craft 2 barrel wont accelerate from a stop without me feathering the gas pedal. This FLAT SPOT on accel has been an issue for as long as I can remember.
More importantly and why Im posting for help now is that a few days ago a random brief stumble at idle developed that sometimes leads to complete SUDDEN shut down at idle (like no spark). Engine shut down also started to happen when ever I reduce the rpms suddenly like deciding to exit off of a freeway. The engine restarts easily and quickly with a turn of the key and I can proceed. BUT Every 10th or so time it does not want to fire back up easily it takes a few attempts to restart and then the very next time it dies it restarts easily again.

This is mainly what I need to fix and am seeking your help with. Also the idle had become a little rough also. About a year ago this truck became my daily driver. There is about 10k miles on a new set of ngk plugs, taylor ignition wires, a summit racing HEI spaced distributor cap and new breaker points and condenser.

Yesterday I rebuilt the carburetor with a Mike's carb kit I ordered and received last year. I did so even though the carb was found clean inside like it was freshly rebuilt still. I found the pcv valve gummy and cleaned it. Now my idle is smooth again but the stumble that happens sporadically is still there and the sudden shut down happens still after a few minutes of idling. It restarts each time easily with no random no catch episodes but then again I only ran it for about 45 minutes last night after doing the carb/pcv work that I did...The lag on initial acceleration is also STILL THERE. A new accelerator pump did nothing to solve this flat spot.

More pertinent info:
The vacuum advance has a bad diaphragm and the hose has been plugged since Jan 2020 when I first checked vacuum, timing and idle speed and found it did not hold vacuum. It remains plugged off while I procrastinate over if I should put a complete distributor in or just a new vacuum advance. The engine is a very low mileage example especially for a 50 year old engine....my truck just turned its FIRST 78,000 miles and I have no evidence that its had its distributor replaced ever. I lament over solely replacing the vacuum advance because the cost to just replace a vacuum advance is almost as much as doing a whole rebuilt distributor and why would I want to take apart a distributor to do a vacuum advance when for a few bucks more I can just do a complete rebuilt distributor?...I am blocked from proceeding because I see that so many rebuilt parts today are DOA right out of the box that I am leery to pull my original one that looks so clean under the distributor cap and replace it with a rebuild that might not be as good as what I have.

When I first got the truck in Oct 2018 it had been sitting for a long time. I had to replace the gas tank. I had the carburetor rebuilt by Rick Stevens the local 75 year old guru that did all the local race guys stuff. Old Rick who happened to also be my sons next door neighbor has very recently retired and moved out of California as part of the great "Im sick of California Exodus" that has claimed so many of my "go to" folks. . I replaced the fuel pump and its canister type gas filter. And all of the rubber fuel hoses back then also.

Back then, after his work and my work this truck was very hard to start each morning then would start easily the rest of the day after that morning hard start stuff. It also would flat spot bog down on accel from a stop if I did not "feather" the gas pedal hot or cold. I asked Old Rick about all of this and also told him the engine was far from impressive for power. Rick asked me what the timing was set at and how much vacuum it had and what Idle speed was....I had never checked any of that so I just let it go .

In April 2021 it became REALLY hard to start on each first start of the day and was also now requiring a long time to warm up or it would die. I went exploring. I found i had broken the little plastic piece off of the vacuum choke pull off valve that is there to connect to the lever that operates the butterfly. I probably broke this plastic piece off when I put the difficult to install air cleaner housing back on in a look see at the carb.. I replaced that carb choke vacuum pull off valve which sits atop the carb and the excessive warm up stuff and REALLY hard to start each morning thing stopped. The move out from a stop FLAT SPOT Stuff needing a feathering of the gas pedal still remained but i let that be since I hadnt still checked timing etc..

The engine was really a dog for performance. The truck came to me from the deceased only prior owner to me with journal records of every thing he had ever done including things like "blowing out the air cleaner hand written in 4 spiral type note books. A journal entry showed that a Pertronix electronic ignition had done some 25 years earlier. That was one of the few things he did to upgrade the original design.

I not liking how much of a dog this truck was in Jan 2020, I decided to check the timing, vacuum pressure and idle. I found the timing off and the idle speed a bit low. I also found the vacuum advance leaking so I plugged it off. I adjusted the idle speed , I replaced the spark plugs, distributor cap, rotor and all the ignition wires. I also replaced and repositioned the coil so it stood up instead of laying flat on the intake manifold.

Somehow while doing all of that tune up related stuff I left the key in the ON position with out the engine running after checking all of this timing etc while I ate lunch. When I next tried to start it after that 45 minute lunch break, The truck would no longer fire up to start. The pertronix unit had failed. Since it wasnt obviously burnt and the Pertronix people were baffled some, as a diagnostic test, I removed the Pertronix and instead put a brand new set of points and a condenser in in place of the partonic unit. I was surprised to see that the truck had much better power and started easily with the points installed. So I have just left the points and condenser installed. They have been in there since Jan 0f 2021..

Additional info I am not anti Pertronix. I installed a brand new one in my 1955 C600 Y block powered tool truck and that Pertronix unit stopped all the many crank to start things etc going on with what was a breaker points driven system in THAT truck. So one truck favorable Pertronix results with a fresh Pertronix and this one with more questions than answers for sure with its 25 year old Pertronix. The reason I went with the HEI type distributor cap found on 1976 trucks was to help the Pertronix not possibly cross fire since the electrodes are placed farther apart than the smaller points type of cap. That HEI type distributor cap remains in play even though I pulled the Pertronix and installed breaker points.

I have a brand new mechanical fuel pump waiting here for my install. I also have a brand new set of breaker points and condenser here also.
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My truck sat for 20yrs when I bought it. Previous owner tried to make it run by changing the 2v carb twice, adding Pertronics, tune-ups all the normal stuff. Still unreliable so he sold it. I fiddled with that carb until I threw it across the yard. I found a Holley street Dominator 4v manifold and a Edelbrock 1406 carb. It started right up and idled like a champ. Some minor adjustments and that was 7yrs ago. The only thing I had to do was add a 2" 4 hole spacer to help with the open plenum and heat soak. Most of the carb issues I hear about on here are from the 2V Autolite/Motorcraft guys. A 2v Holley just might be the ticket for you.

 

BTW get rid of those NGK plugs and get the Autolite copper cores opened up to .042-.045. The Pertronics will help in the long run. Get the 1 or 2 not the 3 and a matching coil.

 

Question....for you OR ANYONE ELSE....what Holley 2V model will work with my original linkages AND my FACTORY ORIGINAL AIR CLEANER HOUSING? A Motorcraft 2100 is what is on the truck right now...so I am looking for the equivalent sized Holley

 

What trans are you running? Picture of your motor would help.

 

x2 on the transmission and is there linkage from it to the factory carb?
Holley 2300 in a 500 cfm should bolt on to your intake manifold.
Air filter should also fit and if no trans linkage the throttle should not be an issue.
Even if you do have trans linkage you should be able to get it hooked up.
I say this as I have that carb on a 75 AMC 304 motor with auto trans and factory air filter.
It replaced the Motorcraft carb I could not get to run right.

If I read the post right you are running points / condenser, been 10K miles and a year since a tune up and them NGK plugs.

I think I would go back with a Pertronx kit and coil again and if you dont want to file the points and adjust / set by dwell.
Replace the vacuum advance as this can help on the stumble.
Check that the mechanical advance is working as I also dont think it is. You can check with a timing light and bring up the RPM the timing should advance.

Never use any fancy plugs in older motors with older ign. systems as the ign. system was never made for them.
Good old copper plugs with proper gap.
What is this HEI spacing thing?
What is your base timing set to?

I believe once you get the dist. taken care of, it will run better and with out the bog.
Then if you have run / start issues come back and we can fine tune it as I got a feeling the carb / choke will need adjusting.
Dave ----

BTW the points & condensers you get today are junk.
Seen posted the condensers dont last.
If you stay with points add a little grease to the shaft lobe the points rub onor the rub block can ware and the point gap / dwell changes.

 

Tomorrow the Pertronix Flame Thrower II complete distributor with male electrode cap and its matching Coil will arrive from Summit Racing. I am still experiencing some misfire here and there. Most of the disheartening performance stuff has gone away with the carburetor rebuild, the new pcv valve and moving the connecting rod for the accelerator pump to the top most hole on the throttle lever of the carburetor. There is still a SLIGHT annoying unwillingness to perform if I hold very light pressure on the gas pedal. I will see what the new distributor with its vacuum advance will do before questioning that slight flat spot holding steady light accelerator pressure.

Concerning the NGK spark plugs. I was very disappointed in the Mexico built Autolites when I compared fit and finish between the two brands....Besides...Ive had such good luck with the NGK in all my other cars including the 1973 Dodge RV with 440. I was having an issue with that rig because the original Champion plugs were just "that much shorter" than anything else so clearing the RV chassis built stuff in the way for cylinder one was not fun for installing the spark plug wire for that cylinder...The NGK has the same length as that original Champion so the NGK solved that issue for that rig. I wont touch Champions since I had issues with them 20 years ago.....NGK is a copper core spark plug also.

Concerning "HEI spacing" When I was chasing the problems back in April 2021, I wanted to go with better ignition wires. Summit sells a cap that was used by Ford for its first HEI distributor in 1976. Ford being Ford didnt change the distributor it just made an adapter to enable the larger cap that moves the electrodes both to male configuration and also spaces them apart more to limit any cross fire under the cap...I went with this to get both handled without having to buy a distributor then.....I didnt change the cap back to the old style of cap when I removed my failed Pertronix back then in favor of the points and condenser. I had brand new stuff and the truck was running again back then...It had annoying issues like the bog I have been chasing forever but it Idled and ran again....

to answer you also....I have a C6 transmission so the transmission kick down lever will also need addressing with the advent of a different brand and type of carburetor.....

HEI style of Distributor Cap that fits right on the OE distributor to enable the use of bigger HEI type ignition wires. Also it is thought there is an advantage in spacing the electrodes slightly farther apart will also prevent cross fire when using a Pertronix and the original distributor

 

Well thanks to Fuzzface to chiming in what I was preaching. Still standing my ground on the Autolite 45's and the fact Motorcraft/ Autolite carbs are garbage.

 

UPDATE::::As per Chrismas' and Fuzzface's suggestion(s)....I have started by installing a Pertronix Flame Thrower ii distributor and its matching Coil. This has eliminated almost every drivability failure I still had AFTER rebuilding my 2100 Motor Craft 2 barrel carburetor and then on another attempt to solve all the misbehavior then replaced the mechanical fuel pump. Each time I did something some of the misbehavior was gone but now I am really happy,

I still need to mess with timing it,------ a momentary ping on light accel at slow speed and a very very VERY slight momentary lag on accel from a stop still is there. The big lag and all the other misfire, that sometimes would suddenly shut off the engine are thankfully gone. I am thrilled with how this truck drives now. It has the power I was always expecting. It starts right up it idles smooth.

If the off the line slight delay thing happens after timing the ping out I will be pulling the 2100 Motorcraft 2 barrel off in favor of a 4 barrel Eddlebrock 1406 and a Dominator intake also a Christmas suggestion.

With all of the benefit I am realizing........Im more than a little miffed that my deffinition of "plug and play" is so different to what Pertronix \'s is...... I dont understand why Pertronix didnt build this distributor a few millimeter shorter so the ignition wires dont jam up against the factory original air cleaner housing . I dislike the aftermarket air cleaner look and also dont like an open air filter. I intend to massage my factory original air cleaner housing in that contact area to see if I can bend in some kind of a gap...

This new Pertronix Flame Thrower II Distributor is just a little taller than the Original Distributor and because it is the Factory OE Air Cleaner Housing jams up against the ignition wires keeping the cleaner from sitting in place properly

New Distributor is just a few millimeters taller than the original distributor and now the OE air cleaner housing doesnt fit flush. Modification to the housing will be needed

The new distributor misses its mark by "Just THAT Much" Come on Pertronix you coulda been much cooler than this!

The Original Distributor fit nicely tucked right under the air cleaner. I should have just replaced the failed vacuum advance, and installed a Pertronix Ignitor II under that pictured distributor cap...what B.S. that Pertronix didnt engineer this new distributor to fit.

 

I believe that they make spacers that fit between the carb and air filter housing so you don't have to modify your air filter housing.

 

Thank you for the suggestion...I have considered a spacer to elevate the air cleaner housing so it clears the distributor cap area. If I raise the air cleaner up then the rest of the attached cold start tubing stuff wont line up and fit...Those Pertronix folks missed the mark with this element in designing this distributor. They claim(the written description on the box states) this is a "plug and play" design...well it ALMOST IS.