340 hard/no start

[340_6pak]

I am restoring a 73 challenger with a 340 engine and 6-pak carburation.  I have replaced the voltage regulator, the ECU and have the proper gap for the distributor reluctor. The plugs and plug wires and the resistor are new. When cranking all of the cylinders appear to fire, (individual header pipes get warm), and when I let off of the starter, it dies. When the starter is cranking, there is about 9.5 volts to the coil. At times it will backfire up the center carburator. If you ever want to see the neighbors kids eyes get really big, let them see that! Any ideas and thanks much!

[MEK-Dangerfield]

Welcome aboard here. 

Is gas getting to the carb? A backfire thru the carb could be a weak spark, or the distributor is installed 180* out of phase(backwards). Are you saying it will turn over but only stay running while turning the key?


  Mike

[340_6pak]

Plenty of gas in the carb, as a matter of fact, maybe too much. Distributor is in properly, not 180 out. It tries to start, but always dies when the key is returned to the run position, so I am thinking 1) power is dying to the coil in this position or 2) someone suggested the timing chain may be off a little. I will pull the cover and look at it. I also have very little room now to adjust the distributor because the vacuum advance on the distributor is now hitting up against the firewall and I am unable to move the distributor any farther in that direction. All ideas welcome!

[ted]

Classic bad ballast resistor.

[340_6pak]

That part is new as well, but I can put the old one back on for a test......

[MEK-Dangerfield]

That part is new as well, but I can put the old one back on for a test......

With the key in the "on" position, check what voltages you have going into and out of the ballast resistor. You mentioned 9.5 going to the coil, so I suspect the ballast might be OK. I'm thinking ignition switch here. With the key in the "on" position, if you have around 9 volts on the "+" side of the coil, pull the plug off the ECU. One of those pins on the connector should have 12 volts on it. Also, it wouldn't hurt to check that one of the pins on the ECU module is connected to ground.


  Mike

[Chryco Psycho]

you could have a contact problem either at the firewall plug or at the plug at the bottom of the column for the 12v feed in the run position , clean the terminals & coat them in di-electric grease

[340_6pak]

Update - I read somewhere that some folks had moved plug wires on the distributor one position counter-clockwise and had good results. Since my distributor would not move any further due to the vacuum advance hitting the firewall and I did not want to cut a groove in the firewall for the advance, I thought I would move the plug wires. After marking the number one plug on the distributor cap, I moved all of the wires one position counter clockwise and the engine came to life, well, for about 5 seconds. No more stuttering and missing, but stopping after 4 - 6 seconds. This tells me a few things. 1 - the plug wires are in the correct sequence on the distributor and are conected to the correct plugs. 2 - Timing may be off, but it is much closer than before. 3 - The coil, voltage regulator, ECU are ok, and maybe even the resistor. 4 -If it was not for boards like this, and the helpfull folks on it, I wouldn't be as close as I am the getting the 340 to come to life. But even running for 3 - 5 seconds, all of the dogs in a 5 block redius began to bark. You don't think that might have something to do with running open headers, do you?

[MEK-Dangerfield]

Now that you have spark, maybe the carb is flooding the engine out.  If it fires up again right away after it stalls, then I'm barking up the wrong tree.


  Mike

[hemiken]

You mentioned you had a back fire in the carb.

You may have to swap out the power valve as they are prone to break with engine backfires.   

Turn the idle screw up to see if it runs and while it is running look down the center carb and see if it is flodding, if it is it may just be a sticky needle & seat or a simple float adjustment.   That is if it is a gas problem.