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Official Chassis Saver Rust Preventive Truck & Auto UnRough Country Lift Kits and Parts!1999-2004 Jeep Grand Cherokee Limited Headlight Pair

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Unread 04-06-2013, 06:10 PM   #1
brtncstm160
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Recent rebuild with backfire issues

I have a 75 Cherokee with a 360 i did a full rebuild on very recently. Was running great for about 150 miles after the rebuild and now i am getting backfire out of the carb. The backfire started without any sort of notice. Ive checked all spark plugs and wires. Pulled valve covers and checked rockers those were tight and appeared fine. Checked ignition timing it appears like it may have wandered slightly off what it was set at during initial timing. When i check the timing it appears like the line bounces around a little bit and doesnt stay very solid. Im leaning towards a sticking valve or spring at this point but i want some input before i start taking things apart. This is my first jeep and my first engine rebuild so any advice or comments would be great.

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Unread 04-06-2013, 06:16 PM   #2
RockRollin
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If timing is wandering, may want to check everything from dizzy to crank. Internals of distributor, dizzy and cam gear, t-chain. You wouldn't be the first to overlook the dizzy hold down either.
Is the carb leaned too much? Is this a constant backfire or temp related?
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Unread 04-06-2013, 06:34 PM   #3
brtncstm160
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Distributor hold down is fine thats not moving anywhere, its a constant backfire and gets worse under load. The timing was set by my local shop once i got it running and they set the carb as well, used their exhaust reader to get it to a good mix. I replaced the timing chain and cam gear and dizzy with new ones when i rebuilt. Should i really be worried about a timing chain after 150 miles? i was also thinking maybe harmonic balancer being old and slipping causing my readings to be funky? possible? Thanks rockrollin for the quick reply
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Unread 04-06-2013, 06:37 PM   #4
revelc
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Quote:
Originally Posted by brtncstm160
Distributor hold down is fine thats not moving anywhere, its a constant backfire and gets worse under load. The timing was set by my local shop once i got it running and they set the carb as well, used their exhaust reader to get it to a good mix. I replaced the timing chain and cam gear and dizzy with new ones when i rebuilt. Should i really be worried about a timing chain after 150 miles? i was also thinking maybe harmonic balancer being old and slipping causing my readings to be funky? possible? Thanks rockrollin for the quick reply
The dizzy/cam gears might be toast. It's an issue with these engines.

Pull it out and give it a once over and look in the cover and see if the cam gear is okay.

Were the gears matched? Bulltear and MSD are the only ones I'd trust.
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Wag Rebuilt 360, 260H, 727, Locked 44, AMC 20, 208, 31" M/T's, 4" Rustys, 2.5" Duals. SOON:SOA SF, 496, T400, Doubler, D60's, 37" M/T's <-TOY
Cherokee Rebuilt 360, 2" Duals, mostly stock. SOON: Hot Street 401, 4'' Rustys, 33" BFG A/T's maybe a T-case to get rid of Q-Track <-TOW RIG
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Unread 04-06-2013, 06:48 PM   #5
olliehopnoodle
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Hi,
Welcome.
Please give some specifics on your rebuild.

It was running fine and then started to back fire? Backfire under load is typically a crossfire.
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Unread 04-06-2013, 07:12 PM   #6
RockRollin
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Quote:
Originally Posted by olliehopnoodle View Post
Hi,
Welcome.
Backfire under load is typically a crossfire.
That makes sense, could explain the wandering also
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Clawed: 90GW, TFI, Streetfire, 3"lift all spring, working RKE,
Edel2131, Holly 1850, mild cam (105) 13mpg for now.
"Its a Jeep thing, and I still don't understand"
05 Renegade OME 927, 947, top plate, JBA 4.5s ,ST/MAXX 245/75/16
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Unread 04-06-2013, 07:28 PM   #7
revelc
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Quote:
Originally Posted by RockRollin

That makes sense, could explain the wandering also
Caused by moisture in the cap?
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Wag Rebuilt 360, 260H, 727, Locked 44, AMC 20, 208, 31" M/T's, 4" Rustys, 2.5" Duals. SOON:SOA SF, 496, T400, Doubler, D60's, 37" M/T's <-TOY
Cherokee Rebuilt 360, 2" Duals, mostly stock. SOON: Hot Street 401, 4'' Rustys, 33" BFG A/T's maybe a T-case to get rid of Q-Track <-TOW RIG
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Unread 04-06-2013, 07:49 PM   #8
RockRollin
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Quote:
Originally Posted by revelc View Post
Caused by moisture in the cap?
Could be moisture. Could be loose vacuum advance linkage, broken or missing mechanical advance springs, chewed dizzy or cam gear. Or as Brtncstm suspected sticky valve.
If full rebuild has been done (this including heads?) I would start from scratch like I never seen the thing before. Firing order, tdc on #1cylinder, time it. If problem persist (with AMC anyway) and fluctuation on the timing light, cap & rotor, visual inspection of interior of distributor, dizzy gear and cam gear. Then onto compression check.
The harmonic balancer is possible, but if it failed I dont think it would cause the sudden problem. It would be hard to time though.
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"I like fun" Edd
Clawed: 90GW, TFI, Streetfire, 3"lift all spring, working RKE,
Edel2131, Holly 1850, mild cam (105) 13mpg for now.
"Its a Jeep thing, and I still don't understand"
05 Renegade OME 927, 947, top plate, JBA 4.5s ,ST/MAXX 245/75/16
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Unread 04-06-2013, 08:05 PM   #9
revelc
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Quote:
Originally Posted by RockRollin

Could be moisture. Could be loose vacuum advance linkage, broken or missing mechanical advance springs, chewed dizzy or cam gear. Or as Brtncstm suspected sticky valve.
If full rebuild has been done (this including heads?) I would start from scratch like I never seen the thing before. Firing order, tdc on #1cylinder, time it. If problem persist (with AMC anyway) and fluctuation on the timing light, cap & rotor, visual inspection of interior of distributor, dizzy gear and cam gear. Then onto compression check.
The harmonic balancer is possible, but if it failed I dont think it would cause the sudden problem. It would be hard to time though.
My money is on cheap dizzy gears.
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Wag Rebuilt 360, 260H, 727, Locked 44, AMC 20, 208, 31" M/T's, 4" Rustys, 2.5" Duals. SOON:SOA SF, 496, T400, Doubler, D60's, 37" M/T's <-TOY
Cherokee Rebuilt 360, 2" Duals, mostly stock. SOON: Hot Street 401, 4'' Rustys, 33" BFG A/T's maybe a T-case to get rid of Q-Track <-TOW RIG
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Unread 04-06-2013, 08:58 PM   #10
RockRollin
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Quote:
Originally Posted by revelc View Post
My money is on cheap dizzy gears.
Solid bet
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"I like fun" Edd
Clawed: 90GW, TFI, Streetfire, 3"lift all spring, working RKE,
Edel2131, Holly 1850, mild cam (105) 13mpg for now.
"Its a Jeep thing, and I still don't understand"
05 Renegade OME 927, 947, top plate, JBA 4.5s ,ST/MAXX 245/75/16
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Unread 04-06-2013, 10:00 PM   #11
brtncstm160
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Thanks ollie, as far as specifics the reason i rebuilt it was because of a timing chain failure, had 15 out of 16 push rods shaped like S's when i took it apart, along with that the vehicle sat for who knows how long before i bought it, grease and oil everywhere it was dying for some TLC. Replaced all bearings, rods, lifters, piston rings, cylinders honed, upgraded the ignition with jeephammers thread, replaced radiator, heater core, new water pump, oil pump rebuild, had egr but i got rid of that with the edelbrock 2131 intake and a edelbrock 1406 carb since its a 75 i dont need to smog. has no vaccuum lines just one to the vaccuum advance which i have plugged at the moment cause im checking the timing. This was my first time spent with an engine so i just followed the haynes manual and sought advice from online sources and my local shop.

The heads i took to my local shop when i took them off, they cleaned up the heads magnafluxed the valves and replaced the seals.

Been through TDC and firing order, everything checked out.

It backfires even when its idling and the vacuum advance is plugged so my misfires shouldnt have anything to do with advance correct? As far as the rotor and cap everything appears fine, im not sure if its normal for these taller ford style caps to have some play in them since they are just held down with those clamps but it doesnt seem to affect the engine misfires when i jiggle it.

Tomorrow morning ill pull the distributor and see what the gears look like inside. I know they werent aftermarket just whatever came with it.

As far as inspecting the interior of the distributor what should i be looking for? dont see any loose connections or anything rattling around.

Thanks a lot for your guys replies, really appreciate it.
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Unread 04-06-2013, 10:30 PM   #12
revelc
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Quote:
Originally Posted by brtncstm160
Thanks ollie, as far as specifics the reason i rebuilt it was because of a timing chain failure, had 15 out of 16 push rods shaped like S's when i took it apart, along with that the vehicle sat for who knows how long before i bought it, grease and oil everywhere it was dying for some TLC. Replaced all bearings, rods, lifters, piston rings, cylinders honed, upgraded the ignition with jeephammers thread, replaced radiator, heater core, new water pump, oil pump rebuild, had egr but i got rid of that with the edelbrock 2131 intake and a edelbrock 1406 carb since its a 75 i dont need to smog. has no vaccuum lines just one to the vaccuum advance which i have plugged at the moment cause im checking the timing. This was my first time spent with an engine so i just followed the haynes manual and sought advice from online sources and my local shop.

The heads i took to my local shop when i took them off, they cleaned up the heads magnafluxed the valves and replaced the seals.

Been through TDC and firing order, everything checked out.

It backfires even when its idling and the vacuum advance is plugged so my misfires shouldnt have anything to do with advance correct? As far as the rotor and cap everything appears fine, im not sure if its normal for these taller ford style caps to have some play in them since they are just held down with those clamps but it doesnt seem to affect the engine misfires when i jiggle it.

Tomorrow morning ill pull the distributor and see what the gears look like inside. I know they werent aftermarket just whatever came with it.

As far as inspecting the interior of the distributor what should i be looking for? dont see any loose connections or anything rattling around.

Thanks a lot for your guys replies, really appreciate it.
Always make sure the gears are a matched set with AMC V8's

I used MSD matched gears from my 360. The new 401 had some but I didn't trust them.

http://www.bulltear.com/catalog/prod...products_id=33
Bulltear set = $129.99

http://www.summitracing.com/parts/msd-8007 $49.99
http://www.summitracing.com/parts/ms...merican-motors
$52.99
MSD set = $102.98

My MSD dizzy came with it's own gear so I only needed the MSD cam gear. I put around 1,200 miles on mine and they hardly had any wear when I put them in the 401. I can personally vouch for them.

Both solutions aren't cheap but they will give you peace of mind.


The dizzy interior.... Loose rotor, moisture, broken/disconnected springs or weights, idk what else.

Was it raining when this all started or did you wash your engine compartment recently?
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Wag Rebuilt 360, 260H, 727, Locked 44, AMC 20, 208, 31" M/T's, 4" Rustys, 2.5" Duals. SOON:SOA SF, 496, T400, Doubler, D60's, 37" M/T's <-TOY
Cherokee Rebuilt 360, 2" Duals, mostly stock. SOON: Hot Street 401, 4'' Rustys, 33" BFG A/T's maybe a T-case to get rid of Q-Track <-TOW RIG
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Unread 04-07-2013, 05:50 AM   #13
RockRollin
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After reading your #11 post, I'm leaning towards broken valve spring like you suspected. I don't see that you replaced cam, lifters, or valve springs. Were those parts replaced? What about rocker arms , inspected for damage?
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"I like fun" Edd
Clawed: 90GW, TFI, Streetfire, 3"lift all spring, working RKE,
Edel2131, Holly 1850, mild cam (105) 13mpg for now.
"Its a Jeep thing, and I still don't understand"
05 Renegade OME 927, 947, top plate, JBA 4.5s ,ST/MAXX 245/75/16
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Unread 04-07-2013, 09:14 AM   #14
revelc
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Quote:
Originally Posted by RockRollin
After reading your #11 post, I'm leaning towards broken valve spring like you suspected. I don't see that you replaced cam, lifters, or valve springs. Were those parts replaced? What about rocker arms , inspected for damage?
I hope that's not the case for his sake. Praying for an easy fix for you sir.
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Wag Rebuilt 360, 260H, 727, Locked 44, AMC 20, 208, 31" M/T's, 4" Rustys, 2.5" Duals. SOON:SOA SF, 496, T400, Doubler, D60's, 37" M/T's <-TOY
Cherokee Rebuilt 360, 2" Duals, mostly stock. SOON: Hot Street 401, 4'' Rustys, 33" BFG A/T's maybe a T-case to get rid of Q-Track <-TOW RIG
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Unread 04-07-2013, 02:40 PM   #15
olliehopnoodle
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Valve spring would be an easy fix.

Bent rocker arm would have caused issues right away.

I am starting to lean towards dist gears.

If that all looks good. A compression check is in order.
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