383 running like s#%t.

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74dusterman

74dusterman

74dusterman
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May 4, 2011
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Yesterday I drove my 67 D200 truck to my shop in the morning and it was running great. I fired it up to go home and noticed it was running rough. It sounds like it is not firing on all cylinders. It is a fresh rebuild to stock specs. The motor has about 100 miles on it now. It is a 67 383, points ignition, edelbrock 4bbl, everything is basically stock. I checked the points gap, checked all of the plugs and wires and all of them are firing. I even pulled the valve covers and checked to make sure the valves were opening, thought maybe the cam went flat. Everything seems to be OK. Anyone have any ideas? It isn't back firing, just not firing on all 8. I am going to do a compression test tomorrow just to double check.
 
Do the compression test and then use an IR temperature sensor to measure exhaust temperatures at each cylinder to find the non-firing culprit. You could also do the temperature test first too!
 
Hard to tell without knowing if the misfire is random multiple or if it is dead cylinders. Hell it could be a vacuum leak on the carb throttle shaft for all we know. They are notorious for wearing out quick.
 
Just off the top it sounds like a fuel issue to me.
A hundred miles is plenty long for an engine to run and just suddenly go from running fine to not without making some tell tail noises of some kind.

My feeling is that it is something simple.

How long did it sit before the new 383 went in?
 
You didn't buy gas shortly before this started to occur did you? Simple but who knows?
 
Hey guys, sorry I haven't checked in for a couple days. I didn't get a chance to work on it yet, it has been raining since Saturday. The truck has been sitting for 2 years. I would start it occasionally to keep the oil flowing in the motor. When I finally got it on the road, a couple weeks ago, the gas tank was empty so I filled up with fresh gas. There is a filter between the fuel pump and carb. I changed the filter but it is still doing it. At idle it sounds like I can hear the dead cylinder(s) kick in and out through the exhaust. I have an IR heat gun to check each cylinder and a compression gauge. As soon as it stops raining so I can work on it I will post back. Thanks guys.
 
By the way, the carb I have now is an edelbrock 1405 model which is only 600cfm. I plan on getting a 750 but wanted to work some bugs out first. It has been running good with the 600 so far.
 
Recheck the fuel filter and see if its clean? If thats not pluged make sure the line is clear from the tank to the pump. If you ran it in on some old gas? it might be that the Plugs are just pissed off? might throw another set at it just to be sure?

:)
 
I finally got time to dig into it today. After running on high idle for 3-4 minutes I checked headder temps with IR heat gun right at the head. #1 259, #2 248, #3 290, #4 220, #5 165, #6 415, #7 95, #8 301. So #7 is for sure dead, I swapped plugs with #5 and no change. I swapped wires with #5 and no change. Fuel filter looks clean yet. I am wondering if it is fuel or ignition related? Maybe the old points aren't firing correctly on all cylinders? I have to see if I have a gasket for the carb so I can tear it apart and clean it. Any other ideas?
 
I am going to borrow a compression gauge tomorrow. I traced all of the wires to make sure none of them were swapped.
 
I too vote fuel.
It's not uncommon for the IR gun to give erroneous readings;selecting the target is often critical.But your temps do seem to have a pretty big spread.
I'd like to think it's something simple like random water slugs moving on thru. But there's just so many things it could be. If it seems to always be the same cylinder, that would make it way easier to diagnose.
But you know what, I have seen that kindof thing happen when the engine is given a bit too much timing as well.
 
Sorry guys, I have been busy and haven't had time to monkey with it lately. I am going to check compression and tear the carb apart fora good cleaning when I get time. Thanks again for the suggestions so far.
 
Finally got around to doing a compression test and leak down test. Compression is good, checked a few cylinders. Leak down test not so good. It is way off the chart on #7 cylinder. Didn't bother checking other cylinders. Next move is to pull the heads I guess. I didn't do any work to the heads when I rebuilt the motor except lap the valves and clean everything. I was hoping to get by for a while until I could get different heads. I have a set of 452 heads off a 440, they should be good.
 
What is "good"? and
What is "not so good"? and
Why did you abandon the test?
Your methodology is strange. Especially since the problem popped up like a light switch, as per post #1

With a "good" compression test, it is rare to see a "not so good" LD test. To abandon the test after just one hole, is to assume your LD tester never makes a mistake.
Why are you so gung-ho to take it apart. It could just be a piece of carbon stuck on a valve.Or a pumped up lifter.
I suppose it could be a bad head gasket. But again; Finish the LD test to know.
 
Last edited:
AJ/FormS said:
What is "good"? and
What is "not so good"? and
Why did you abandon the test?
Your methodology is strange. Especially since the problem popped up like a light switch, as per post #1
With a "good" compression test, it is rare to see a "not so good" LD test. To abandon the test after just one hole, is to assume your LD tester never makes a mistake.
Why are you so gung-ho to take it apart. It could just be a piece of carbon stuck on a valve.Or a pumped up lifter.
I suppose it could be a bad head gasket. But again; Finish the LD test to know.
Click to expand...
I guess I didn't think of checking the rest of the cylinders since #7 is the dead cylinder. I did the compression test and leak down test to confirm that it wasn't an ignition problem. Even if I finish the tests I will have to pull the heads regardless right? Is there any way to remedy it without pulling the heads off?
 
I did pull the valve covers and spin it over to make sure the valve train was working. Everything looks like it is moving correctly. I didn't measure anything with a dial indicator. Guess I will have to finish the leak down test one of these days.
 
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