360 backfiring through carb

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   Hello again everyone!

Apologies for how long it's taken me to get back to everyone; like I said, nutty schedule this summer 

Car buddy stopped by to have a look and we were all scratching our heads by the end of the evening. We double checked timing and firing order, everything seemed to be in line. I pulled the valve covers and checked over the valve train after he left, everything is moving and seems to be in working order. I might do a second check, as it's very possible that I missed something, but I'm not sure. Did a compression check the following day and all 8 cylinders seem to be cycling and making good compression.

Replaced the spark plugs, again, double checked timing and tried again today, but still no luck. Got the car to run for about 10 seconds or so before the carb puked fuel and refused to run after that. No amount of fiddling seemed to work.

My thoughts so far...

-Would the engine show good compression if a cam lobe was actually flat? And could this be detected with a simple oil change and looking for metal on the magnetic drain plug? Again, I'm fairly certain all the valves were moving the way they should have, but I could have overlooked something in my haste that night. Might go back and double check to be sure.
-Continuing from first thought, would the engine be showing good compression if the timing of the cam was off by a tooth?
-Could it simply be a fuel or spark problem? I'm running a 12v line straight to the dizzy on it's own independent fuse block and switch, didn't know if that might be the culprit there. Likewise, though the carburetor is new out of the box, I've heard countless tales from other friends of Edelbrock carbs malfunctioning shortly after install (my one other buddy swears by Holley's and refuses to buy Edelbrock after a few too many bad carbs on his cars).

Still haven't acquired a vac gauge from Summit (I've been offline for a while dealing with personal issues and didn't see the suggestion, but am definitely gonna grab one here soon). Mostly hoping that my schedule will go a little easier on me here soon and that I might be able to spend some more time in the garage.

Appreciate all the ideas and help again everyone! Fingers crossed we can solve it here soon!

Cheers!
-Brad L.

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FreeMoCo2009 Members Profile Send Private Message Find Members Posts Add to Buddy List AMC Apprentice Joined: Feb/12/2020 Location: Pennsylvania Status: Offline Points: 37	Post Options Post Reply Quote FreeMoCo2009 Report Post Thanks(0) Quote Reply Posted: Sep/05/2022 at 11:40am
	Hello again everyone! Apologies for how long it's taken me to get back to everyone; like I said, nutty schedule this summer Car buddy stopped by to have a look and we were all scratching our heads by the end of the evening. We double checked timing and firing order, everything seemed to be in line. I pulled the valve covers and checked over the valve train after he left, everything is moving and seems to be in working order. I might do a second check, as it's very possible that I missed something, but I'm not sure. Did a compression check the following day and all 8 cylinders seem to be cycling and making good compression. Replaced the spark plugs, again, double checked timing and tried again today, but still no luck. Got the car to run for about 10 seconds or so before the carb puked fuel and refused to run after that. No amount of fiddling seemed to work. My thoughts so far... -Would the engine show good compression if a cam lobe was actually flat? And could this be detected with a simple oil change and looking for metal on the magnetic drain plug? Again, I'm fairly certain all the valves were moving the way they should have, but I could have overlooked something in my haste that night. Might go back and double check to be sure. -Continuing from first thought, would the engine be showing good compression if the timing of the cam was off by a tooth? -Could it simply be a fuel or spark problem? I'm running a 12v line straight to the dizzy on it's own independent fuse block and switch, didn't know if that might be the culprit there. Likewise, though the carburetor is new out of the box, I've heard countless tales from other friends of Edelbrock carbs malfunctioning shortly after install (my one other buddy swears by Holley's and refuses to buy Edelbrock after a few too many bad carbs on his cars). Still haven't acquired a vac gauge from Summit (I've been offline for a while dealing with personal issues and didn't see the suggestion, but am definitely gonna grab one here soon). Mostly hoping that my schedule will go a little easier on me here soon and that I might be able to spend some more time in the garage. Appreciate all the ideas and help again everyone! Fingers crossed we can solve it here soon! Cheers! -Brad L.

farna Members Profile Send Private Message Find Members Posts Add to Buddy List Supporter of TheAMCForum Moderator Lost Dealership Project Joined: Jul/08/2007 Location: South Carolina Status: Offline Points: 19677	Post Options Post Reply Quote farna Report Post Thanks(0) Quote Reply Posted: Sep/05/2022 at 1:54pm
	Backfiring through the carb is usually the timing off by 180 degrees. I'd check that first. Make sure #1 cylinder is at TDC and the rotor button in the dizzy is pointing at the #1 spark plug wire. That happens TWICE in an engine's four stroke cycle, once at TDC on compression stroke, and once at TDC on exhaust stroke. If it fires on the exhaust stroke it can backfire through the carb. So make sure it's on compression stroke by putting your finger over spark plug hole while turning the engine (by hand, either by fan ir ratchet on the balancer bolt) until you feel it "puff" out on your finger. Then you can stick a long wire down the hole and rock the crank back and forth a bit until you can tell the piston is at the top. Another way is to take the valve cover off and see that both valves are shut, then use the wire and rock crank until piston is at top. The timing marks should be at the pointer at TDC, but you can't tell which stroke it's on.
	Frank Swygert

FreeMoCo2009 Members Profile Send Private Message Find Members Posts Add to Buddy List AMC Apprentice Joined: Feb/12/2020 Location: Pennsylvania Status: Offline Points: 37	Post Options Post Reply Quote FreeMoCo2009 Report Post Thanks(0) Quote Reply Posted: Sep/29/2022 at 5:05pm
	Hello again! Not to bump the thread too much, but still struggling to figure out what's going on. I did verify that the valves are all moving the way they should, spark plugs are all in working order, and fuel is getting to the carb. Flushed out the fuel system and put fresh gas in it just to be certain, and also swapped to a Holley carburetor to be sure. Verified engine is on TDC on number one and not 180 degrees off. Managed to get the engine running for a few seconds, but once again, it puffed out through the carb and refused to fire. I've been trying to get it running since but have been hitting the same issues as before. Biggest question I have so far, how easy is it for the engine to slip a tooth on the timing chain? I've been reading up on other forums and this appears to be a potential problem at hand. The engine makes good compression, but I'm wondering if it still would, even with only one tooth being off. I know the timing gear set is brand new, but in the words of other hotrodders, "'new' doesn't mean anything in the car world." As much as I'd like to avoid ripping the front of the engine off to verify this, I'm beginning to think it's going to be the next logical step to see what might have happened. Any thoughts here? Appreciate all the help and patience folks! Looking forward to finally figuring this out and getting the Jav on the road! Cheers! -Brad L.

Heavy 488 Members Profile Send Private Message Find Members Posts Add to Buddy List AMC Addicted Joined: Apr/27/2019 Location: In the Status: Offline Points: 3553	Post Options Post Reply Quote Heavy 488 Report Post Thanks(0) Quote Reply Posted: Sep/29/2022 at 5:24pm
	So while you had the left valve cover off, did you confirm #1 compression position and where the rotor is pointing?

FreeMoCo2009 Members Profile Send Private Message Find Members Posts Add to Buddy List AMC Apprentice Joined: Feb/12/2020 Location: Pennsylvania Status: Offline Points: 37	Post Options Post Reply Quote FreeMoCo2009 Report Post Thanks(0) Quote Reply Posted: Sep/29/2022 at 5:57pm
	Heavy488, can confirm, set the dizzy on TDC when the valve cover was off. Both valves were shut, harmonic balancer was at TDC, and verified with a insulated wire through the spark plug hole. Cheers! -Brad L.

troutwilly Members Profile Send Private Message Find Members Posts Add to Buddy List AMC Addicted Joined: Sep/14/2007 Location: Carriere, MS Status: Offline Points: 992	Post Options Post Reply Quote troutwilly Report Post Thanks(0) Quote Reply Posted: Sep/29/2022 at 10:53pm
	If you're sure you've checked all the timing stuff and have the distributor at TDC, compression, #1 cylinder (front driver side), wires in correct order, advanced to at least 6 degrees,...then it sounds like an electrical problem. Not being familiar with DUIs, I Googled "DUI electrical requirements" and came across this site: https://www.enginelabs.com/engine-tech/we-examine-the-top-five-calls-to-performance-distributors-tech-line/ It seems to cover several potential problems, like a good ground and at least 12 volts constant (no resistor). There are several others which might be good to read. It may have already been mentioned, but something I've had problems with in the past is the condition of the distributor cap, especially the center electrode, and how well it is secured. Does it look like there's been arcing between electrodes under the cap? If you can keep it running long enough, move the timing light pickup to each wire/plug and verify a constant/steady spark at each one. With the number of times the wires have been removed and installed, it might be good the check for continuity and resistance on each one. Also, verify 12 volts to coil at ignition "on", at "start" and with engine running. And if you want to check the timing chain, maybe use one of those inspection cameras through the distributor hole. Has anyone ever tried that? Just throwing some ideas out there. Edited by troutwilly - Sep/29/2022 at 10:56pm
	Bill O. 70 AMX

Boris Badanov Members Profile Send Private Message Find Members Posts Add to Buddy List AMC Addicted Joined: Dec/14/2013 Location: NH USA Status: Offline Points: 4209	Post Options Post Reply Quote Boris Badanov Report Post Thanks(0) Quote Reply Posted: Sep/30/2022 at 4:49am
	Do you have electronic ignition?
	Gremlin Dreams

jcisworthy Members Profile Send Private Message Find Members Posts Add to Buddy List AMC Addicted Joined: Jul/23/2009 Location: North Carolina Status: Offline Points: 2805	Post Options Post Reply Quote jcisworthy Report Post Thanks(1) Quote Reply Posted: Sep/30/2022 at 6:17am
	Are you sure the distributor and or cam gear is not wiped out? If you are not pull the distributor and look at both gears
	Specializing in dyno services, engine building, and cylinder head porting rbjracing.com Phone Number 518-915-3203

farna Members Profile Send Private Message Find Members Posts Add to Buddy List Supporter of TheAMCForum Moderator Lost Dealership Project Joined: Jul/08/2007 Location: South Carolina Status: Offline Points: 19677	Post Options Post Reply Quote farna Report Post Thanks(0) Quote Reply Posted: Sep/30/2022 at 7:20am
	If the rotor is rotating properly the gears aren't wiped -- no need to pull the distributor. Assuming you have a DUI since troutwilly mentioned it, try running a wire directly from the battery to the +12V power lead of the DUI. You don't want to leave it like that, but that will verify that you're getting a full 12V and the power lead you're using is the issue. 70s cars have a resistance wire in the harness, so the coil wire will have 12V to it when cold but drop to under 9V (I think it drops to 6V, but going from memory not sure) as it warms up. The DUI won't operate properly under 9V... may not before voltage gets that low. If it runs right with 12V from the battery you just need to find another switched 12V lead.
	Frank Swygert

jcisworthy Members Profile Send Private Message Find Members Posts Add to Buddy List AMC Addicted Joined: Jul/23/2009 Location: North Carolina Status: Offline Points: 2805	Post Options Post Reply Quote jcisworthy Report Post Thanks(0) Quote Reply Posted: Sep/30/2022 at 7:33am
	They do not wear enough to stop the rotor from turning but enough to move the timing and make the car not start, pop if it does start etc
	Specializing in dyno services, engine building, and cylinder head porting rbjracing.com Phone Number 518-915-3203