Slant Six Forum :: Slant missing underload

The miss is steady and why I thought electrical too. I will pull the plugs and maybe just swap them for a new set. I pulled them back when the issue first started, cleaned them and checked the gap and also put new wires on.

The damper was replaced with a NOS one back years ago too after reading they fail on this site. I will double check that and also the rest of your list Reed. I did check to see if the vacuum advanced was holding vacuum before and it was ok. Maybe I will check the brake booster too. .

One question, if the ignition box was failing, could it cause this issue? It seems like the unit is an aftermarket replacement. I don't know what symptoms the boxes have when failing but it's the one item I haven't replaced since I bought the truck. The ignition is the standard OEM electronic ignition, no lean burn.

Author:	dmcphee07Si [ Thu Jan 05, 2017 3:16 pm ]
Post subject:	Slant missing underload
Using the information from this site years ago, I was able to get the negletected slant in my 78 D100 up and running great. At the time, I went all out with the tune up parts including all new lines, plugs, wires, cap and rotor, hoses and installed Remflex gaskets. I also did the valve lash, timing set, installed an aftermarket choke pull off and converted the truck to a super six. Truck ran awesome and would squawk the tires from a stop fairly good. Timing is set to 12btdc. Anyway , last summer after the truck was sitting it developed a miss under load that I can't seem to get rid of. It starts up and warms up coming off the choke as it should. It will then idle nice, and if I feather the throttle, it wil accerate smooth. When I do a hard off the line acceleration, passing pull, or drive up a hill, the engine pops and misses like crazy. I replaced the coil as I found it seemed to be leaking oil, and changed the plug wires. No change. I also cleaned out the carb good and replaced the gaskets in case something varnished up while sitting, no change. The truck really doesn't have many miles on it since I did the massive tune up and the valve lash, but I am wondering if I should double check it. The truck will still idle nicely and pull a steady 20 inches of vacuum while doing so. If anyone can give me some things to look for or suggestions It would be greatly appreciated.

Author:	drgonzo [ Thu Jan 05, 2017 4:07 pm ]
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Could it be a fuel delivery problem? Fuel filter stopped up? Dirty tank? Etc.

Author:	dmcphee07Si [ Thu Jan 05, 2017 5:03 pm ]
Post subject:
I don't think it's fuel, as the I replaced the accerator pump when I cleaned out the carb, and also did the fuel filter. I get a good shot of fuel now when I depress the carb linkage and the fuel pump is not very old either. I had to drop the tank years ago and clean it out because I did have a reoccurring dfuel problem. I could be wrong though! Hence why I am here for help lol

Author:	drgonzo [ Fri Jan 06, 2017 6:33 am ]
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Did you put in new plugs? I recently converted my slant to a recurved EI distributor and HEI. When I first installed the distributor I had it off one gear tooth and it started but ran very poorly. After getting the distributor installed correctly and my timing set, the engine would start quickly and idle smoothly and rev up in neutral, but when i drove it, it started breaking up and missing badly from about 3000rpms and up. New plugs and she is running great.

Author:	Jase [ Sat Jan 07, 2017 5:34 pm ]
Post subject:	Re: Slant missing underload
Quote: Anyway , last summer after the truck was sitting it developed a miss under load that I can't seem to get rid of. It starts up and warms up coming off the choke as it should. It will then idle nice, and if I feather the throttle, it wil accerate smooth. When I do a hard off the line acceleration, passing pull, or drive up a hill, the engine pops and misses like crazy. I replaced the coil as I found it seemed to be leaking oil, and changed the plug wires. No change. I also cleaned out the carb good and replaced the gaskets in case something varnished up while sitting, no change. If anyone can give me some things to look for or suggestions It would be greatly appreciated. So it misses under load... and you don't think it is fuel related... Have you looked at the plugs? are they clean or fowled with oil or gas?? Does it have a strong blue spark or a weak red-orange spark? Is this a points ignition, or electronic? Wait stupid question as it is a 78.. Don't think they had lean burn back in 78... Anyhow, checking out the quality of spark, and condition of the spark plugs may give some additional info, or rule them out so you can look elsewhere..

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Author:	coconuteater64 [ Mon Jan 09, 2017 10:42 am ]
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If the miss is regular and steady, I'd suspect it's electrical. If it's sporadic, probably fuel related. But it could be something random and off the wall too. My truck always missed and sputtered going uphill. I changed the fuel pickup "sock" in the tank, ran a new fuel line, new fuel filter, new fuel pump, and cleaned the carburetor. Still did it. Turns out I found the problem accidentally: my large key chain had just enough weight to turn the ignition switch between on and off! You just never know with a Mopar. I'd suspect all of the ignition system and systematically search each component until it stops.

Author:	Reed [ Mon Jan 09, 2017 11:13 am ]
Post subject:
(1) verify that the outer ring of the damper has not slipped and is giving a false reading of TDC. (2) verify that the diaphragm in the vacuum advance on the distributor still holds a vacuum and is connected to a ported vacuum source, not a constant manifold vacuum source. (3) Check for vacuum leaks/play at the throttle shaft of the carb. It could seal fine at closed idle position but leak once the throttle is opened. (4) does your truck have a catalytic converter? If so, it may have failed and bits of the converter matrix may have melted and broken apart and are clogging the exhaust.

Author:	dmcphee07Si [ Mon Jan 09, 2017 6:36 pm ]
Post subject:
The miss is steady and why I thought electrical too. I will pull the plugs and maybe just swap them for a new set. I pulled them back when the issue first started, cleaned them and checked the gap and also put new wires on. The damper was replaced with a NOS one back years ago too after reading they fail on this site. I will double check that and also the rest of your list Reed. I did check to see if the vacuum advanced was holding vacuum before and it was ok. Maybe I will check the brake booster too. . One question, if the ignition box was failing, could it cause this issue? It seems like the unit is an aftermarket replacement. I don't know what symptoms the boxes have when failing but it's the one item I haven't replaced since I bought the truck. The ignition is the standard OEM electronic ignition, no lean burn.

Author:	dmcphee07Si [ Mon Jan 09, 2017 6:45 pm ]
Post subject:
Oh and the truck has no converter to worry about, it goes straight from the manifolds to a muffler. I am assuming it did have one originally.

Author:	coconuteater64 [ Tue Jan 10, 2017 9:22 am ]
Post subject:
Quote: The miss is steady and why I thought electrical too. I will pull the plugs and maybe just swap them for a new set. I pulled them back when the issue first started, cleaned them and checked the gap and also put new wires on. The damper was replaced with a NOS one back years ago too after reading they fail on this site. I will double check that and also the rest of your list Reed. I did check to see if the vacuum advanced was holding vacuum before and it was ok. Maybe I will check the brake booster too. . One question, if the ignition box was failing, could it cause this issue? It seems like the unit is an aftermarket replacement. I don't know what symptoms the boxes have when failing but it's the one item I haven't replaced since I bought the truck. The ignition is the standard OEM electronic ignition, no lean burn. I've had those same symptoms from a failing ignition box, yes. Coil failing too. Try cranking it up in total darkness and look for an electrical storm under the hood at different RPM ranges.

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