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Have gotten rid of the series of pop pop pop for the most part. Loud bang if I let off the gas quickly.
Distributor Cap not moving Timing on 78 CJ7 304 v8
Hello,
Always gotten good help so hoping to get it again! 78 CJ7 with a 304. Has developed a backfire when shifting/letting off the gas. Have done new plugs, wires, distributor cap and coil, so trying to set the timing.
When rotating the distributor cap, the timing shows around 20 deg below TDC with a timing light no matter what position the cap is in. Have rotated far enough to kill the engine and the timing indicator doesn’t move. Any idea what could be causing this? Or what I am missing?
Thanks in advance!
Always gotten good help so hoping to get it again! 78 CJ7 with a 304. Has developed a backfire when shifting/letting off the gas. Have done new plugs, wires, distributor cap and coil, so trying to set the timing.
When rotating the distributor cap, the timing shows around 20 deg below TDC with a timing light no matter what position the cap is in. Have rotated far enough to kill the engine and the timing indicator doesn’t move. Any idea what could be causing this? Or what I am missing?
Thanks in advance!
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2,348 Posts
HI,
In post #3 you said
good luck take care be safe
tim
In post #3 you said
This is an indication that the timing chain is shot .. If the mark does not stay in one spot when idling the timing chain is bad..
good luck take care be safe
tim
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24,193 Posts
Popping upon deceleration can also be caused by loose exhaust manifold bolts, or manifold gasket leaks. It took me forever to figure that out on my 258. I don't think it's as common on the V8s, tho.
Matt
Matt
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20 Posts
Check for vacuum leaks around the carb. I had a CJ that the carb would loosen up by itself over time. As soon as I heard the backfires when I let off the gas going down a hill I knew it was time to tighten it down again.
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2,042 Posts
OP - This is probably a long shot but - Does your 304 still have the factory exhaust manifolds?
If so check to see if the heat valve flapper to help warm the engine with exhaust gases is still installed on
the passenger exhaust manifold. Page 1K-8 in the TSM. Click link below. This may cause problems.
Can't hurt to check.
If so check to see if the heat valve flapper to help warm the engine with exhaust gases is still installed on
the passenger exhaust manifold. Page 1K-8 in the TSM. Click link below. This may cause problems.
Can't hurt to check.
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2,348 Posts
HI,
have you rechecked the timing mark for movement??? IF IT DOES NOT STAY IN ONE PLACE OR THE LIGHT DOES NOT SEAM TO FLASH AT IDLE THE CHAIN IS BAD....... Been there...
good luck take care be safe
tim
Hunter, update on this please.
If youve tried checking your timing again, moving the whole dist., is th timing light still skippinga cylinder? are you able to advance/retard the timing?
If so, what did you set it at and what are your current results?
working through a symptom/cause tree, it seems that the most likely culprits for back fire only on decelleration are timing and running rich , or a combo of both...can you tell if the backfire is through the carb or the exhaust?
If youve tried checking your timing again, moving the whole dist., is th timing light still skippinga cylinder? are you able to advance/retard the timing?
If so, what did you set it at and what are your current results?
working through a symptom/cause tree, it seems that the most likely culprits for back fire only on decelleration are timing and running rich , or a combo of both...can you tell if the backfire is through the carb or the exhaust?
Mine used to backfire all the time when I first got it. Replaced the exhaust manifold gasket and it doesn’t backfire anymore. What a pain in the *** though, the manifold seemed to lose its shaped once unbolted and was incredibly hard to get back on.
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Thank you all for the help! Have not had a chance to get back to it since last posting but will try to take a look when I get done with work and will provide updates. Thank you all for checking in!
To answer a few of the questions- PO put headers on the jeep so nothing factory there. But I do know that the front most bolt, passenger side on the exhaust manifold is loose since the threading in the block is stripped (I believe in front of cylinder 2 if I have numbering right?). Will need to drill and tap to get a new bolt to hold. The backfire occurs in the exhaust currently, will adjust timing (the right way) and see if I can move idle exhaust to run more lean. Will also make sure carb is bolted down tight.
Thanks again for all the help, seem to definitely be in the right place!
To answer a few of the questions- PO put headers on the jeep so nothing factory there. But I do know that the front most bolt, passenger side on the exhaust manifold is loose since the threading in the block is stripped (I believe in front of cylinder 2 if I have numbering right?). Will need to drill and tap to get a new bolt to hold. The backfire occurs in the exhaust currently, will adjust timing (the right way) and see if I can move idle exhaust to run more lean. Will also make sure carb is bolted down tight.
Thanks again for all the help, seem to definitely be in the right place!
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2,348 Posts
HI, Remember if the timing marks moves around at idle or sometimes the light dose not flash the chain is bad
good luck take care be safe
tim
good luck take care be safe
tim
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14 Posts
I guess a little background on the backfire might help as well…
Was not happening before I took it to a shop to get the rear pinion seal replaced and do some alignment on the drivetrain. I know it mostly sat for 3-4 weeks before they got to it. While they had it, the only thing they did with the engine was relocate the throttle spring (I didn’t realize it had been rigged up by PO since it had been that way since I had it).
After picking it up, the backfire started happening almost immediately so I started down the new plugs/wires/distributor cap/ignition coil path and didn’t get anywhere.
One question I have- knowing the exhaust manifold is loose on that one bolt, would this cause a backfire through the exhaust? Whether that was tight or not, wouldn’t a backfire through the exhaust mean too much gas is getting through to burn off and firing in the exhaust? Or is it letting enough air in to allow it to combust? Always happy to learn and again, appreciate all the help!
Was not happening before I took it to a shop to get the rear pinion seal replaced and do some alignment on the drivetrain. I know it mostly sat for 3-4 weeks before they got to it. While they had it, the only thing they did with the engine was relocate the throttle spring (I didn’t realize it had been rigged up by PO since it had been that way since I had it).
After picking it up, the backfire started happening almost immediately so I started down the new plugs/wires/distributor cap/ignition coil path and didn’t get anywhere.
One question I have- knowing the exhaust manifold is loose on that one bolt, would this cause a backfire through the exhaust? Whether that was tight or not, wouldn’t a backfire through the exhaust mean too much gas is getting through to burn off and firing in the exhaust? Or is it letting enough air in to allow it to combust? Always happy to learn and again, appreciate all the help!
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Will do, appreciate the heads up Tim and will keep you posted. I guess I am mostly hoping that isn’t the problem… hoping it is just that I am using a cheap gun or it was user error when I noticed it last time!
Thanks,
Hunter
What they said is correct. Exhaust (especially if rich), and you add air (oxygen) can burn in the exhaust. If it accumulates (you've heard of mufflers blowing apart), it can ignite. The diverter valve and air pump do the same thing in a controlled way. Puts air into the exhaust to help burn the excess fuel to meet emissions (a half a** way, used in the beginning of crazy emission control).
It certainly could
This is going back 30 yrs. Its on the pass side. along the engine about a third of the way back from the front of the engine. The air pump is I remember correctly is the lower front of engine/ pass side. They connect to metal pipes that have a check valve in the pipe (stops the exhaust from getting the the diverter valve). If they have headers they probably removed the diverter valve (may have left the air pump since it's operated by a belt) The web site I listed up above shows a valve.
Sounds like your bolt holes got distorted and the bolt holes lose their alignment. If there’s a next time, buy some manifold spreaders. They help keep your bolt holes aligned by minimizing manifold distortion.
Good to know, thank you
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