I am about at my wits end here. I have a 77 D100 survivor with a Slant 6 I purchased recently. At first it would not fire at all, and I finally installed a GM HEI ignition control module, which works very well. Next, the engine was very difficult to start. I started looking at the carburetor. All of the small passages were blocked. After clearing the passages, and finding a wrong gasket inside of the carburetor, I was able to adjust the mixture screws until the vaccum guage indicates about 17" of mercury at idle. when the throttle is depressed without the engine running there is a strong flow of fuel into each barrel. Now, when I am moving the truck around the yard all I have to do is touch the starter and the engine is running and idling smoothly. BUT, when I step on the throttle the engine does not rev up. It stumbles and bucks. If too much throttle is added it backfires. The spark plugs are coated with black powder when I check them. The cylinder compression was measured at 120 Lbs on all 6 cylinders. The head was rebuilt at a cost of $750 a couple of years ago. The timing is set at the correct 0 degrees TDC. Other timing settings do not allow the engine to rev above idle. Has anyone seen this, or have an idea of where I should be looking now? I am about out of ideas.

For a stock engine in good shape 17 on the vacuum gauge is a little bit on the low side. And you say "0" on timing is "correct". By what measure? The underhood sticker? Unless you're somewhere like California that is just a good starting point. Many will take more advance. Your outer ring on the damper may have slipped position too meaning your timing won't even be where you think it is.
Black plugs sounds way rich. Vacuum leak?
I'd also look real close at the ignition, like the condition of the distributor cap. I just had a real similar issue yesterday with my Dakota with the other Mopar 6......a 3.9. Running similar to how you say when getting on the gas. Found carbon tracks starting from 2 terminals and cap itself was all "ate out" around all 6 plug terminals on t underside, some worse than others. And don't fall for "it's not that old". The bad cap was brand new Napa Echlin installed right when winter starter to break earlier this year. Maybe 6k miles on it.

I have not checked the outer ring of the damper yet. That is something I am probably going to look at next. I have had the distributor out and checked it. The cap appears to be new, without any arcing on the contacts. No arcing on the rotor either. I have replaced the wires and plugs, and also clean them when I swap the two sets I have. The plugs that came with the truck seem to be fairly new, but who knows. The seller had replace the Chrysler ignition control module, and when I found it was not reading the correct voltage on the low side of the coil I replaced it with no better luck. Then I put the GM module in and electronically it is correct now. The carburetor is the original Carter carb and still has the copper tag with it. There is a vacuum post on the rear of the intake manifold with 5 or 6 posts on it. All are capped off. I have not looked at the valve adjustment yet, and now that it idles very nicely I might check the rocker gaps. One thing I have noted is when I put the timing light on any of the wires the light flash is not steady. It will flash 4 or 5 times and then skip a flash. I thought it might be the carbon on the plugs, but a new set of plugs did no better. On the coil it appears to be steady, but it may be too fast to see a change. I replaced the rotor with a better design that has stonger contact with the coil connection in the cap, but that made no difference.

I just checked the damper ring and to the best of my ability to feel the piston it appears to be in the correct position. There is one other detail I thought I noticed the other day, and confirmed today. The rear 3 spark plugs seem to be getting fouled while the front 3 do not seem fouled at the moment. Not sure what that means, but it is another detail to the puzzle.

Sounds like you are making good progress.. some thoughts that come to mind:

It is possible to have a rich idle circuit, and a lean accelerator pump circuit, or the engine is going lean for some reason on acceleration or when the main jet is supposed to come in. Black plugs could also be due to idling.... I would not worry about that aspect right this moment, it is good info, but I would try to figure out the stumble and backfire condition first.

Backfires (in my mind) often suggest a lean condition.

IF you look down the throat (engine running) choke open and work the throttle, you should see the accelerator pump expressing a strong shot of fuel when you first open the throttle and for the duration that the throttle blades are being opened. Sounds like you have that. Keep looking down the carb at 15-1800RPM and open the throttle slowly you should see the main jet begins dribbling fuel out the booster..

Too much air, and or not enough fuel will cause the backfire towards the carb. Backfire out the exhaust is unburnt fuel for some reason making it to the tail pipe and lighting off.

Be carefull doing this . Don't put your face directly over the carb. If it backfires, you might need new eyebrows, or worse.

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Charrlie_S
65 Valiant 100 2dr post 170 turbo
66 Valiant Signet 170 nitrous
64 Valiant Signet
64 Valiant 4dr 170
64 Valiant 4dr 225

What is "correct voltage" in your mind? If you have the GM HEI module did you also install an HEI coil, or is it still using the factory "external resistor required" coil? Running an externally resisted coil on 12v all the time will barbeque it and I have seen it cause this exact issue on multiple vehicles.

~THOR~

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1984 D100 Shorty Custom
Certified Auto Appraiser - RevItUp Classic Appraisals
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Part of Tyrde-Browne Racing

The accellerator pump is pushing fuel into the carberator correctly. When I installed the HEI system I replaced the coil with a recommended high output coil as well, so that should not be a problem. The "correct voltage" I refered to is the voltage on the low side of the coil. A bad control module will show 2-3 volts. A good one should show 7-10 volts or so. I have noticed that the vacuum seems to deteriorate over time. The last time I installed the carburetor the engine reved normally for about 2 seconds and then went back to stumbling with an idle that gets rougher over time. The vacuum guage shows less and less vacuum as well. Right now it won't start and no vacuum is shown when cranking the engine. I am wondering if there is a leak around the base of the carb and will try adding handmade gaskets on either side of the spacer to confirm or eliminate that possibility, and follow up with the caps around the vacuum tap for the power brake booster to see if one of those is deteriorating. If all else fails I will replace the manifold gasket, which should only be about 3 years old.

vacuum decreasing... plugged exhaust?

The tailpipe and muffler appear to be fairly recent. Brakes, tires, fuel, water pump, and radiator were all replaced not long before I purchased the truck, which was running when I purchased it. I am not sure what the damper in the exhaust manifold is for, but it is bouncing, which to me seems right. The vacuum when idling is bouncing between 12 & 14 inches of mercury. Ihave seen a steady 18. Looking for vacuum leaks I sprayed everything with a thick liquid. No change at all in the idle. All of this started after the Chrysler ignition failed, and a second Chrysler control module did not correct the ignition problem, so I installed the GM HEI control module per many YouTube articles. Since then the ignition has been working, but the light is flashing irregularly on each of the plug wires, there is a miss at idle, and the engine will not rev above idle. I reset the gap to .045" with no change.

One thing mentioned in reading about understanding the vacuum guage is valve timing. The crank damper seems to be showing me the correct timing mark position, and it is set to about 5 degrees BTDC. I do not hear the valves clicking at all, so I am going to adjust the valves to be sure they are correct or a hair on the loose side. I have a receipt showing the head was rebuilt for $750 about 3 years ago.

Another thing I have seen, but not since rebuilding the carb, was while the idle circuits were plugged I could try to rev the engine, and it would stumble and try to rev up. As I let the throttle down slowly the engine would catch, rev up, and do whatever I wanted it to do until it was returned to idle. After I cleared the carburetor idle passages this never happened again. Just backfire and maybe fire through the carb. To be honest I am running out of ideas.

Since the note above I have adjusted the valves. As I suspected they were a little tight, but all springs and valves appear to be new. Thinking about where I firest noticed the inability to rev above idle, and noticing the irregular flashing of each spark plug wire, except for one that is fairly regular. Is it possible that the gap is too small and effectively shorting out the spark? I have some spare plugs and will set some for .050, .055, and .060, install them and see if there is any difference.

I'm sorry, but opening the spark plug gap is not going to achieve anything. You really need to figure out where the excess fuel is coming from so that you don't keep fouling spark plugs.

Have you looked down the carburetor with the engine idling? The excess fuel could be due to a bad/dirty needle and seat allowing the carburetor to overflow and drip from the booster venturi. One possible cause of the idle deteriorating as the engine runs for a few minutes is far too much fuel. You should be able to close the idle mixture screw and kill the engine. If you cannot kill the engine with the idle mixture screw then there's fuel getting in some way it should not.

The distributor pickup is polarity sensitive. If it's connected backwards the timing will change with RPM. Another thing with HEI is that GM HEI modules are good, but many Chinese ones are not. I gave a friend an old, original GM module and it cured his idle misfire.

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Joshua

I'm favoring the clogged exhaust suggestion because of the vacuum signals you stated. However, you might also want to check timing chain slop. Remove the distributor cap, rotate the engine until the dampener mark lines up with TDC. Next rotate the engine in the opposite direction until you see the distributor rotor just start to move. Check how many crank degrees it took to move the rotor. On a slant, you don't want more than 8-10 degrees, as it uses a rather short chain (unlike small block Mopars).

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Recognized by US Federal Courts as a Fuel Economy Expert!

After listening to Tony on YouTube's Uncle Tony's Garage discussing the poor quality of many new parts today I decided the suggestion that the Ignition Module might be part of my issues and ordered a new module. After I installed it I found the engine was back in the I'm not gonna run mode. But unlike the old module this time the engine would try to fire a couple of times each time I pumped the throttle and turned the engine over. I tried unsuccessfully to post a video of this starting effort to show what happened when the engine sent a puff of smoke back up through the carburetor. The upload did not work, but you can see the video at: http://tahsales.com/Dodge%20Carb%20Smoke-edited.mp4 . When you look at the video, watch the left bottom of the carburetor on the third attempt at starting the engine. I am wondering if the puff of smoke you see there is telling me the carb is too worn to work properly, and is this a big part of my problem? When starting the engine, the vacuum guage does not move at all. I would think it might at least bounce a little bit. Where you would expect to see the original Chrysler Ignition Control Module you will see a 4" square heat sink mounted to the original mounting holes. That is where I have mounted the new control module. I don't expect to have any heat problems with this heat sink, and there is provision to carry a spare module if I decide to add one.

0

There is a gap when the choke is closed, I think it is a little wider than 5/32 for now. For now the choke is not needed to start the engine because it is often above 80 degrees, up to almost 100 here this summer. Once I get the engine running correctly I will pay closeer attention to the minor adjustments and rebuild the fuel line between the fuel pump and the carburetor. It rests agains a radiator hose and on the water pump. After about 30 minutes the fuel line develops air bubbles from the heat in the line. I wish that were my problem right now. It would be an easy fix to make a new hard line and route the fuel another way.