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Attn engine builders need help troubleshooting 383
Posted By: repad
Attn engine builders need help troubleshooting 383 - 07/14/10 12:52 AM
I recently fired up a fresh rebuilt 69 383 and have a problem with the #3 cylinder not combusting. Valve train looks good from under the cover, compression looks good at 160 and have spark. The only problem during initial break-in was a loud backfire through the carb which shot fuel almost to the cieling. Other than getting my hands on a leakdown tester, what else should I be looking at?
Posted By: buildanother
Re: Attn engine builders need help troubleshooting 383 - 07/14/10 12:58 AM
160 is good, should fire well enough to keep up with the other 7 cyls. If it only misses at idle, I'd check for a vacuum leak.
Posted By: repad
Re: Attn engine builders need help troubleshooting 383 - 07/14/10 01:12 AM
It not only misses at idle, but does seem to decreasd around 2500 rpm. Would a vaccuum leak cause the cylinder to not combust?
Posted By: buildanother
Re: Attn engine builders need help troubleshooting 383 - 07/14/10 01:19 AM
It can if there is a decent sized vac leak nearest to that cyl, so it is pulling in mostly air and not fuel. How are you determining which cyl is dead?
Posted By: repad
Re: Attn engine builders need help troubleshooting 383 - 07/14/10 01:27 AM
I began by pulling one wire at a time. Starting with #1, when I got to #3 there was no change the way the engine ran. Then I removed the plug and it looks like new, never been fired.
Posted By: buildanother
Re: Attn engine builders need help troubleshooting 383 - 07/14/10 01:34 AM
Well, if you verified the valves on that cyl are indeed opening a fair amount and closing, I would try swapping that wire and/or plug with another cyl for a quick test, to see if #3 still stays dead.
Posted By: repad
Re: Attn engine builders need help troubleshooting 383 - 07/14/10 01:37 AM
The rockers are moving well and I have tried two other wires and a new plug with same results.
Posted By: buildanother
Re: Attn engine builders need help troubleshooting 383 - 07/14/10 01:41 AM
Did you find the source of the killer initial backfire? And does it run reasonably well now without backfire?
Posted By: repad
Re: Attn engine builders need help troubleshooting 383 - 07/14/10 01:47 AM
I assume that the original backfire was caused by too much timing advance or the original rebult AVS carb. The engine does run well, with a slight backfire now and again.
Posted By: 66Dodge
Re: Attn engine builders need help troubleshooting 383 - 07/14/10 03:11 AM
Quote:
I began by pulling one wire at a time. Starting with #1, when I got to #3 there was no change the way the engine ran. Then I removed the plug and it looks like new, never been fired.
Check under the dist. cap. Maybe contact for #3 is bad ?
Posted By: repad
Re: Attn engine builders need help troubleshooting 383 - 07/14/10 03:19 AM
Any advice on how to verify if the #3 contact on the cap is at fault?
Posted By: GregCon
Re: Attn engine builders need help troubleshooting 383 - 07/14/10 03:38 AM
You could buy one of those Lisle spark plug lights to see if you are getting spark at the plug. It goes between the plug and the plug wire and has a window that lets you see the spark.
Posted By: kzinge1
Re: Attn engine builders need help troubleshooting 383 - 07/14/10 03:40 AM
Might be a bad ignition box. I had a brand new MSD box that wouldn't fire #1 and #4.
Posted By: repad
Re: Attn engine builders need help troubleshooting 383 - 07/14/10 04:10 AM
Quote:
You could buy one of those Lisle spark plug lights to see if you are getting spark at the plug. It goes between the plug and the plug wire and has a window that lets you see the spark.
I checked the plug removed from the cylinder and an additional wire run from the plug to ground and have spark.
Posted By: repad
Re: Attn engine builders need help troubleshooting 383 - 07/14/10 04:12 AM
Quote:
Might be a bad ignition box. I had a brand new MSD box that wouldn't fire #1 and #4.
Sorry, no ignition box, running on points.
Posted By: 383man
383man - 07/14/10 04:26 AM
You said the plug looked like new ?? If it was not firing it would be wet with fuel. Sounds like a vacum leak effecting that intake runner causing a very lean condition in that cyl. Ron
Posted By: 66Dodge
Re: Attn engine builders need help troubleshooting 383 - 07/14/10 04:32 AM
Quote:
Any advice on how to verify if the #3 contact on the cap is at fault?
Just suggesting a general visual inspection to see if it looks any different from the other 7 contacts.
Posted By: GregCon
Re: 383man - 07/14/10 04:50 AM
A leakdown test won't tell you anything regarding this problem. If you have 160lbs on a regular gauge, that's all you need to know.
You could have a dud plug that only fires at atmospheric pressure but I doubt it.
How about this - have you verified the spark is occurring at the time of compression? My next guess is you have two wires swapped so #3 is sparking on the exhaust stroke somewhere and the other cylinder happens to be close enough that it fires even if it is not exactly right.
I'd pull the plug, put my finger over the hole, and crank the motor while watching the spark - does it fire as your finger is shoved off the hole?
The engine is not real smart...if it has compression, fuel, and spark all more or less at the right time it has no choice but to fire.
Posted By: repad
Re: 383man - 07/14/10 04:59 AM
Quote:
You said the plug looked like new ?? If it was not firing it would be wet with fuel. Sounds like a vacum leak effecting that intake runner causing a very lean condition in that cyl. Ron
I concur, the plug is not wet with fuel which leads me to now believe that the backfire may have caused a leak to develope in the valley pan around the intake runner for #3. I had not considered this because there is no visual evidence of a leak but thanks to your and the first responders insight, that is where I will concentrate my efforts tomorrow and post an update.
Posted By: repad
Re: 383man - 07/14/10 05:06 AM
Quote:
A leakdown test won't tell you anything regarding this problem. If you have 160lbs on a regular gauge, that's all you need to know.
You could have a dud plug that only fires at atmospheric pressure but I doubt it.
How about this - have you verified the spark is occurring at the time of compression? My next guess is you have two wires swapped so #3 is sparking on the exhaust stroke somewhere and the other cylinder happens to be close enough that it fires even if it is not exactly right.
I'd pull the plug, put my finger over the hole, and crank the motor while watching the spark - does it fire as your finger is shoved off the hole?
The engine is not real smart...if it has compression, fuel, and spark all more or less at the right time it has no choice but to fire.
The possibility of crossed wires was one of my initial thoughts. I did check the wire routing and they are all correct.
Posted By: GregCon
Re: 383man - 07/14/10 10:37 AM
I just doubt an intake leak could cause that level of misfire - you'd need a big leak to entirely prevent the cylinder from hitting. And, how would you blow out a metal gasket? Or are you using those thin paper gaskets too?
Posted By: 383'D
Re: Attn engine builders need help troubleshooting 383 - 07/14/10 11:52 AM
Look at the the springs on that cylinder real good for a broken coil.It may look like a hairline crack. Does it smell rich when it runs?Mine ended up being a exhaust spring on #4.Even though almost new.It acted just like that.
Posted By: repad
Re: 383man - 07/14/10 12:38 PM
Quote:
I just doubt an intake leak could cause that level of misfire - you'd need a big leak to entirely prevent the cylinder from hitting. And, how would you blow out a metal gasket? Or are you using those thin paper gaskets too?
What else other than the items that I checked could cause the dead cylinder? I'm not using anything but the metal valley pan with unblocked heat crossover and silicone sealant.
Posted By: repad
Re: Attn engine builders need help troubleshooting 383 - 07/14/10 12:42 PM
Quote:
Look at the the springs on that cylinder real good for a broken coil.It may look like a hairline crack. Does it smell rich when it runs?Mine ended up being a exhaust spring on #4.Even though almost new.It acted just like that.
I have inspected the springs but will take a closer look. Wouldn't the good compression test rule out a broken spring?
Posted By: Challenger 1
Re: Attn engine builders need help troubleshooting 383 - 07/14/10 12:43 PM
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Might be a bad ignition box. I had a brand new MSD box that wouldn't fire #1 and #4.
Sorry, no ignition box, running on points.
It sounds like you need to adjust the dwell. Have you checked that?
Since you said it still back fires sometimes I bet your points are off, what's the dwell?
Posted By: repad
Re: Attn engine builders need help troubleshooting 383 - 07/14/10 12:47 PM
Quote:
Quote:
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Might be a bad ignition box. I had a brand new MSD box that wouldn't fire #1 and #4.
Sorry, no ignition box, running on points.
It sounds like you need to adjust the dwell. Have you checked that?
Since you said it still back fires sometimes I bet your points are off, what's the dwell?
I set the dwell at 29.
Posted By: GregCon
Re: Attn engine builders need help troubleshooting 383 - 07/14/10 12:54 PM
If there's good spark it doesn't matter what the dwell is.
Does the plug look bone dry or does it look like it is getting washed with some gas?
Again, if you have some gas, air, compression and spark in time it is going to fire. Which is missing?
Just for kick what firing order are you using?
Posted By: repad
Re: Attn engine builders need help troubleshooting 383 - 07/14/10 01:02 PM
Quote:
If there's good spark it doesn't matter what the dwell is.
Does the plug look bone dry or does it look like it is getting washed with some gas?
Again, if you have some gas, air, compression and spark in time it is going to fire. Which is missing?
Just for kick what firing order are you using?
The plug looks very dry with a silght gas odor which tells me that it is not getting a proper shot of fuel. Firing order 18436572
Posted By: JohnRR
Re: Attn engine builders need help troubleshooting 383 - 07/14/10 01:06 PM
Did you check the compression of other cylinders as a comparison ?
Posted By: 66Dodge
Re: Attn engine builders need help troubleshooting 383 - 07/14/10 01:36 PM
Quote:
Quote:
If there's good spark it doesn't matter what the dwell is.
Does the plug look bone dry or does it look like it is getting washed with some gas?
Again, if you have some gas, air, compression and spark in time it is going to fire. Which is missing?
Just for kick what firing order are you using?
The plug looks very dry with a silght gas odor which tells me that it is not getting a proper shot of fuel. Firing order 18436572
1). Check the valve train to see what would cause #3 intake valve to not open properly. (compression good because it stays closed okay, but will not open properly)
2). Just for kicks try a diffrerent dist. cap. (bad contact on terminal #3 could be missing link in spark path)
Posted By: 66Dodge
Re: Attn engine builders need help troubleshooting 383 - 07/14/10 02:18 PM
Quote:
Quote:
Quote:
If there's good spark it doesn't matter what the dwell is.
Does the plug look bone dry or does it look like it is getting washed with some gas?
Again, if you have some gas, air, compression and spark in time it is going to fire. Which is missing?
Just for kick what firing order are you using?
The plug looks very dry with a silght gas odor which tells me that it is not getting a proper shot of fuel. Firing order 18436572
1). Check the valve train to see what would cause #3 intake valve to not open properly. (compression good because it stays closed okay, but will not open properly)
2). Just for kicks try a diffrerent dist. cap. (bad contact on terminal #3 could be missing link in spark path)
Okay, we've covered all the basic things that a cylinder needs to fire/combust. Time think outside the box and to look at the weird things that could have happened.
3). Inspect cam lobes in the dist. (worn cam lobe will not close the points all the way, thus no charging of the coil before the points open again)
4). Check for some sort of obstruction in the intake path/runner to cyl.#3. (maybe the ball of aluminum foil was stuffed into intake runner #3 instead of the heat crossover opening ? LOL) (that's why the carb backfired big time ?)
5). Verify you actually saw a spark jump when #3 boot is held close to ground while cranking the engine ?
Posted By: epaul
Re: Attn engine builders need help troubleshooting 383 - 07/14/10 03:06 PM
"3). Inspect cam lobes in the dist. (worn cam lobe will not close the points all the way, thus no charging of the coil before the points open again)
ha, ha
I had a 71 Dodge station wagon given to me that I chased a miss on one cylinder for a few days. I had another 318 I stole the distributer off of and it ran perfect. Funny thing with the old dist. I could re-set the points and it would miss on just one cylinder. If I then re-checked and re-set the points it would miss on a different cylinder. On cam lobe was worn down, nearly impossible to see with the dist. in the engine.
Posted By: 66Dodge
Re: Attn engine builders need help troubleshooting 383 - 07/14/10 03:25 PM
6). Spray carb cleaner around the driver's side intake area with engine running to check for vacuum leak ?