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Fuel or spark, scratching my head on tuning out a stumble.

Smokinnjokin

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So, bottom line up front I just did a lot of work to my 360 and now it has a bad stumble/backfire/hesitation when I rev it up past cruise throttle positions. I am trying to figure out if it is a carb setting, or ignition problem. Car starts and idles beautifully, easy hot start after it has been sitting awhile, it just has a bad reaction when you stomp on the big pedal.

Here's the dirty details.

I have a smog 360 in my '67 sat that i just installed new lifters, timing chain set, edelbrock intake manifold, and swapped from points to HEI.

I installed the timing set exactly as old one came off, dots lined up both at 12" and used a tensioner. New eddy manifold has heat riser blocked. The carb, an edelbrock 1403, ran well with the previous combination of parts and had no issues. So the only thing that has changed the fuel/air demand on this engine is more accurate timing (stretched old chain) and an Edelbrock RPM intake with no heat crossover vs the old cast iron.

For the HEI system, I used a new OEM-style vac advance electronic dist. from R. Ehrenberg, he states it has a performance curve. I have the LX-301 ignition module mounted under distributor on the designed2drive plate, and I am using a standard ignition E-core coil. Ballast is bypassed. All the wiring is solder connections, heat shrinked. The ignition module is grounded to the block. My work is very clean, I am a former helicopter mechanic so I obsess over details. I checked continuity on all connections, all looks good. The cap is brass terminals, the wires are new 8mm and plugs look good and are gapped to 0.045". I double and triple-checked plug firing order.
Again, the car starts instantly and idles perfectly which seems to indicate all is well with ignition, but I don't want to rule it out just yet.

The carb is a 500 cfm eddy, again it worked perfect before this project started (about a month ago) and it sat on the workbench covered with a rag during that time. I have the correct pump seat/seal installed for mopars (blue? I forget which one but its good). Float height was set to factory settings a couple months ago, everything inside is clean and in good working order. Fuel in tank was seasoned with sta-bil before sitting for the month. Car has been in a garage the whole time. Clear fuel filter, I can see flow to engine.

Timing is set to 12deg BTDC with the vac advance unhooked and carb plugged. Car may want more advance but im not going to mess with that until I get the issue figured out. A couple degrees of fine tuning should not cause this bad of a throttle response.

When I apply throttle at lower settings, response is great and power is good. When i stomp on it, or get up to higher throttle settings slowly, it runs like crap and backfires.

So here's my troubleshooting theories:

1) somehow I did not install the chain perfectly lined up, mechanical timing is off. I put new chain on without turning anything in the motor, and the timing dots lined up exactly with a ruler. So this seems unlikely.

2) new intake manifold, ignition and chain were a radical enough difference to air/fuel demand that the carb needs jetting changes. Again, seems really unlikely.

3) carb has a plugged jet or passage or something? Has 2 fuel filters leading to it, and it sat in a clean garage covered up. Worked perfectly when removed. Seems unlikely.

4) Something is wrong with the ignition. High-speed miss or something. Arcing, grounding out? Again, my wiring job is solder connections, heat shrink, double and triple checked. And starting is super reliable. I ran it for over an hour today flushing the radiator too, engine was at operating temp for a long time (idling) with no issues whatsoever. So does not seem to be heat related.
I am going to run it tonight when it is pitch black and look for arcing, just to rule it out.

What do you guys think?
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Nope, you got me there. I really don't want to hit my fresh paint with carb cleaner. But you are right, I should rule that out after putting a new manifold on. I was a little worried about how thick the cork end gaskets are, causing potential sealing issues.
 
I would check the metering rods are free. And pull the top of the carb and make sure the secondary jets are clear. Is the carb hard piped or a rubber hose? A small piece of rubber can cause problems. Double check that the carb studs are tight. Again no vacuum leaks. Accelerator pump shot is good? Another possibility is rotor phasing.
 
Don’t have any carb cleaner, but i did check tq on intake bolts and carb studs, all was good. Ran car in pitch dark, revved up to where it ran crappy, no arcing or anything. Drove around the block. Feels strong at cruising speeds. Stomp on the pedal, anything over 1/2 throttle and it stumbles, backfires and dies. I’m done for tonight.
 
Set plug gap to .035” and try it(?)
 
If your confident your intake's sealed then I would look at your vacuum and mechanical advance curves. Could be the vacuum advance is coming in too quick for your combo. If the vacuum can is adjustable then you might try playing around with that.
 
You could try checking for vacuum leaks with a small propane torch, not lit of course. Saves getting carb cleaner everywhere.
 
You could try checking for vacuum leaks with a small propane torch, not lit of course. Saves getting carb cleaner everywhere.
Damn that's a smart way of doing it. That's the next check on my list.

As for fuel starvation, that seems unlikely, its a new pump and everything ran great before I took it apart. I'm thinking I can rule that out by revv'ing it up till it runs crappy, shut the car off immediately and then pop the top on the carb to look how full the bowls are. Have to open the carb anyway to see if jets are blocked.

I'm also in contact with Nathan with designed2drive to get his input, see if it sounds like I have an HEI install issue.
 
Sounds like lean missfire.
Takes more voltage to fire lean mixture,which
can cause your missfire.
Richin it up
 
How do the plugs look ?
 
So I ran through a bunch of troubleshooting steps and still have no clue what the hell is wrong.

Torque checked the manifold again, fired the car up and warmed up the engine. Checked for vacuum/intake leaks. Set timing to 10 btdc. The weird thing is that the timing is erratic-the mark bounces around a couple degrees at idle. New chain with tensioner. The dampener was in good shape, rubber portion intact. So it makes no sense that the timing would not be accurate, unless my cheap timing light is to blame.

Revved it up till it started backfiring and immediately cut the ignition to check float bowl level in the carbs. Steady 5psi on my fuel pressure from idle thru the bad running condition. Opened the carb up, half full on both bowls and float height correct. Jets clear. Needles free. So It does not appear to be a fuel issue.

Put everything back together and ran it again, still idles beautifully, crisp throttle response it just gets really erratic at higher rpm and backfires. Sometimes it would rev crisply right up to redline, other time a quick blip off idle would cause the backfiring. Doesn't really matter if it is a quick hard stomp or slow smooth application, would start to run poorly either way. Seemed to happen around 2.5k rpm.

I also noticed that the timing light would cut out randomly while the engine was backfiring/sputtering - AHA! Apparently the problem is ignition related. New standard ignition products (not the cheapo "T" parts but the blue flame ones) coil and module. Rick E. Electronic distributor. So something is not working right.

The coil is an E-core, solid state and seems really unlikely that it is 'bad'. A resistance check between terminals showed .04 ohms and it doesn't appear damaged in any way. It was only warm to the touch while running.

Same thing with the ign module. Brand new, mounted to a heat sink with thermal paste, wasn't even hot. Good grounds everywhere, from module to block and from coil frame to block.

There is something a little concerning with the distributor, the reluctor pickup with the 8 contact points is poorly manufactured, and the gaps are different between all 8 points. I don't know if this could be causing the issue. Seems like the poor running would be persistent throughout the rev range if so.

I'm in the process of putting my factory points back on (done actually, just have to scrounge up a male-male ignition coil wire) to triple-check that the ignition system was indeed the cause of this problem.
 
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Try advancing the timing to around 14 btdc. See if that helps. My old Belvedere I set the timing according to the manual a 5 btdc and it would move the car but anything more than an idle it ran like crap and had no power. I think playing with the timing 12,14,16 etc etc may show you some improvements.
 
I have advanced it all the way to like 20+, same issue. I don’t think it has anything to do with timing.
 
Well crap. Been cruising FABO and it looks like i may have made a serious mistake buying this distributor. Apparently the "340 timing curve" it is advertised as having is not even close. I should have done my homework, I really don't know much about timing and trusted the ad.

https://www.forabodiesonly.com/mopa...istributors-from-rick-ehrenberg.386164/page-1

I'm not 100% sure the distributor is the issue, but everything else has checked out...
 
Not saying that your distributor is the issue, but I will say that not all of the stuff Ehrenburg sells is the best out there. He's kind of a dick imo. All about $ and making sure in his mind he is better than the rest of us.
 
I’d have to refer you to post #8 again.
 
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