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I need help diagnosing what's wrong with my engine
Someone mentioned earlier if you checked for any vacuum leaks. Worn thru or cracked vacuum hose. You may already tried these....Spray carb cleaner around the base of the carb and listen for the engine idle to pick up, if it does you have a vacuum leak at the carb base. Remove the spark plug wires one at a time see if the engine idle drops, if it does plug it back in and try the next one. If it doesn't, check the wire for spark. When cold take the radiator cap off, run the engine and check for gas bubbles coming up in the antifreeze?
I'm betting on the distributor is the cause of the problem. Since the problem gradually gets worse the longer the engine runs, it is probably the control module inside the dizzy.

73Pony Wrote:When you pulled and set gap on the plugs what did the #8 plug look like? Were any of them black and oily?

These are the plugs from 1 & 2, but I don't remember any of the additional plugs looking much different than these. I can pull 8 again tonight to be sure:

[Image: oVpXlJVl.jpg]
[Image: pqIZ6ZAl.jpg]
[Image: MODXMh9l.jpg]

73Pony Wrote:Where in Iowa are you located?

I'm SW of Adel

Fabrice - I'm still planning on digging into the carb once I have a chance to watch a little youtube so I have some idea of what i'm looking for / at.

Rackerm - All of the hoses and vacuum lines were installed brand new about 2 years ago, but I'll go through them more carefully. I can also try the carb cleaner trick. I have checked all of the spark plug wires, tested them for continuity, and visually witnessed spark occurring from the distributor to the wire (not from the wire to the spark plug though). I also pulled wires with the car running, and the only suspect cylinder is 8.

Hemikiller - Is there a control module in these HEI style distributors? Is that something repairable, or am i better of just getting a different electronic distributor?
Thought you had the vacuum lines checked by now, tho I would expect a constant prob if that was the case, higher idle rpms and hesitation when you'd pressed the pedal.
Even if if the dist may be involved, which would be an easy fix, you will need tune that carb and ensure your air filter is clean/sufficient for the air volume it's supposed to let thru. The plugs are black, as there's rust on the thread, it shows a rich condition over a long period of time. Should be brownish/ dull grey.

[Since the problem gradually gets worse the longer the engine runs]
Could you develop a bit on this? Not familiar with these, but eager to learn, why this module would progressively fail and not right away?

73 modified Grandé 351C. Almost done. 
71 429CJ. In progress
[+] 1 user Likes Fabrice's post
The GM HEI system has an internal control module that functions much like the Duraspark control box on Fords. The module is mounted to the distributor base with a thermal compound to transfer excess heat away. One of the failure modes I personally experienced was a misfire on warmer days at sustained operating temperature. Since I usually had a bunch of parts lying about, I simply started swapping one component at a time until the problem was fixed, in this particular case it was the module. To be honest, I have no love for the HEI, but it works well when it's working.

Since the OP's distributor is a Summit, most likely of Chinese origin, every component is suspect. My first thought is the module, as it's the most complex component in the unit. It very well could be the coil or the pickup, or none of the above.

Something to consider before you do anything is you need to double check that you have a solid ground path from Battery (-) -> fender apron -> block. Many aftermarket ground cables do not have the factory lug for the fender apron and it can cause some seriously weird issues. I had a 71 sportsroof that I did an auto to 4 speed swap on, and apparently the tranny cooler lines were the ground path for the engine to body. Some seriously odd issues popped up, lots of head scratching but I finally figured it out. 

You can see the lug crimped in the middle of the cable on the ground cable in this set.


+1 on that. I have 2 extra braided lines, you can never have enough grounds. Make sure no rust is at the bolts.

thx for the module tech.

73 modified Grandé 351C. Almost done. 
71 429CJ. In progress
Regardless if the HEI is the issue or not, it's something that I'd probably like to replace at some point.  Do you have a recommendation on what to replace it with?
The two most recommended I have seen are the ford Duraspark upgrade or a stock style distributor with a Pertronix 2 unit. DO NOT USE A PERTRONIX 3!!!!! THEY ARE JUNK.

73 Grandé H Code. Headman long tube headers, T-5 Transmission, 3.70 Traclok, Lowered 1" all around, Aussie 2v heads w/ 2.19 intake, 1.71 exhaust, screw in studs, full roller cam 608/612 lift 280/281 duration LSA 112, Quick Fuel 750 CFM double pumper, AirGap intake.

- Jason

[Image: 082-hot-rod-power-tour-2017-1970-1970s.jpg]
(04-28-2019, 07:01 PM)aasukisuki Wrote: Thanks for all of the suggestions.  I haven't had time to dive into the carb yet, so no updates there.  I did re-gap the plugs to .050, and the car seems to idle better when cold.

Here are a couple of videos I was able to take.  I can re-take them if they aren't helpful

Here is right after I gapped the plugs to .050 and running cold:  https://youtu.be/nZtlHOZzfZ0

I then took the car out for a short drive.  You can hear a noticeable difference in how the car is running around the 2:20 mark.  Shortly after the video cuts out, the car started running so poorly, I was barely able to get it back to my driveway before the engine cut out completely.  I should note that i'm a total jackass, and didn't realize that I had the car in 2 instead of drive, so this is in one gear the whole time.  https://youtu.be/jOO1QKcQIUA

73Pony - There is no play in the distributor.  I don't own a timing light (and have never timed a car), so maybe it's time to invest in one.  The gas was fresh, from within the last couple of weeks.  I also have never checked the compression on a cylinder, so I'll need to buy that equipment too.

Carolina - I checked and re-checked that all plug wires were firmly attached, and snapping onto the top of the plugs.  I also used a multimeter to check for continuity on all of the wires, and they are seem to be fine.  I did pull each wire as the car was running, and it seems like cylinder 8 is the one missing.

TommyK - As far as I can tell, all lines are connected or capped (should the distributer be capped or actually connected to something via hose?)  I tried to snag a new fuel filter this morning at AutoZone, but of course the one closest to me was the only one without in stock.

Thanks for all of the help and ideas so far.

It sounds to me like the ignition is breaking down possibly due to a failing distributor pick-up. What brand is the HEI unit that you have? I have seen practically new over seas no name distributors fail more than once   Ron
I agree with the above posts, it sounds like a distributor/ignition problem. I had a bad MSD distributor and the moron mechanic I used to go to couldn't find the issue. The engine was missing and idling rough, and the car bucked when I gave it gas. Eventually it took 8 cranks to fire it up then it wouldn't start at all. I took it upon myself to learn the design of these engines and have never used a mechanic since (this car doesn't deserve to be experimented on by novice mechanics). I replaced my MSD with a cheap Summit Racing HEI distributor for diagnostic purposes and the engine ran normally. If you want you can have my old MSD Pro Billet distributor, but you would need to send it in to have a new control module put in it which costs around $100.
Edit: Oops, I don't think my old distributor would work for your 302 though, its for 351C/351W
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