cherokee backfire problem

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   I have a 1974 jeep j20 that is all factory with an amc 360 the only mod is an Elelbrock 1805 with the same problem. I have changed the fuel pump  and changed the points, dist. cap, and the rotor. I did not check the timing chain or the valves yet but I will. there is also a slight tapping that might be a piston or a valve.

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chickenjeeper Members Profile Send Private Message Find Members Posts Add to Buddy List AMC Fan Joined: Feb/25/2017 Location: toledo ohio Status: Offline Points: 1	Post Options Post Reply Quote chickenjeeper Report Post Thanks(0) Quote Reply Posted: Feb/25/2017 at 9:47pm
	I have a 1974 jeep j20 that is all factory with an amc 360 the only mod is an Elelbrock 1805 with the same problem. I have changed the fuel pump and changed the points, dist. cap, and the rotor. I did not check the timing chain or the valves yet but I will. there is also a slight tapping that might be a piston or a valve.

sweatlock Members Profile Send Private Message Find Members Posts Add to Buddy List AMC Addicted Joined: Apr/28/2014 Location: Largo, FL Status: Offline Points: 3314	Post Options Post Reply Quote sweatlock Report Post Thanks(0) Quote Reply Posted: Feb/26/2017 at 2:22pm
	I'm not sure what '8 lbs. low' means with regards to a vacuum check, but on my stock 360 V8 engine at idle vacuum is 18-19 in.Hg. You can do a lot with a vacuum gauge and they're relatively inexpensive to buy a good one - I'd suggest getting one and reading up on how to use it. You can diagnose a host of engine issues and even tune it up using one. In your case, if you truly have low vacuum, you might have a vacuum leak somewhere, which is causing a lean condition and may be creating the conditions for the backfire. Although retarded timing (stretched chain, slipped harmonic balancer) may be causing a low vacuum reading as well. Edited by sweatlock - Feb/26/2017 at 6:42pm

Lyle Members Profile Send Private Message Find Members Posts Add to Buddy List AMC Addicted Joined: Jul/17/2014 Location: None Status: Offline Points: 772	Post Options Post Reply Quote Lyle Report Post Thanks(0) Quote Reply Posted: Feb/26/2017 at 9:50pm
	You have not stated the miles on the engine but if that is the original timing set in the engine I would be pulling the distributor to check for any slack on the timing chain. The nylon gear does not age well and the "silent" chain is know to stretch. At xxxxx miles on a Jeep or near 40 years that may have seen towing or other abuse it's not worth the risk of that chain failing. I'd check it regardless.

dbomb Members Profile Send Private Message Find Members Posts Add to Buddy List AMC Addicted Joined: Jul/22/2012 Location: new jersey Status: Offline Points: 705	Post Options Post Reply Quote dbomb Report Post Thanks(0) Quote Reply Posted: Mar/24/2017 at 8:34pm
	Unhook the vacuum advance on distributor maybe the the pickup plate is twisting changing the dwell or distributor bushing could be bad Take a peek at the heat riser if it's stuck it could be causing troubles. If it has an egr valve a stuck heat riser could make it push open leaning out the charge
	rebuiling 73 amx need parts

Lyle Members Profile Send Private Message Find Members Posts Add to Buddy List AMC Addicted Joined: Jul/17/2014 Location: None Status: Offline Points: 772	Post Options Post Reply Quote Lyle Report Post Thanks(0) Quote Reply Posted: Mar/24/2017 at 9:03pm
	Simple check, take off the distributor cap and rotate the crank back and forth. If the crank moves and the arm does not then you should be taking a good look at the timing chain. Bought and scrapped running 360, hole through #1 piston, missing ring lands on 5, 7, 4 and melt marks on 2 and 6. If the chain is original and you have done regular maintenance now may be the time for a lower cost refresh. Cam, chain, hone, rings, bearings ... Better then waiting for a new long block. It's surprising what these engines can live through!