Popping through the carb-4350

[seventynine]

My '77 Chief seems to be backfiring through the carb on occasion. It runs great other than that. It happens at random while driving. I thought it might be the timing which I had a bit advanced. Yesterday I reset it to spec but it didn't fix the problem.

Sometimes it feels like it just shuts off for a split second like its running out of gas....which makes me think vapor lock but other times I can hear it pop in the engine compartment....which would not seem to be a symptom of vapor lock. I'm assuming that it is a carb problem and not an ignition issue but I could be wrong.

I rebuilt the carb a few months back and like I said other wise it runs like a top. I also went through and replaced all of the vacuum lines at the time. The rebuild fixed some hesitation that I was having before.

It is completely stock with all original systems in place. 33k original miles.

Any suggestions as to what might be causing this?

Thanks,
Dean

[fulsizjeep]

Could be the carb but could be also ignition issue. The symptoms of the chewed cam gear seemed like bad Prestolite module on the 77 Waggy before the Mallory HEI was put in. It would miss and occasionally backfire. Then it got worse with the HEI.

[seventynine]

Intersting. Is that a common problem with the Prestolite ignition? It would seem unlikely with only 33k miles.

This is my first rig with the prestolite set up. Ill tell you one thing I've noticed though...the darn distributor caps don't stay on worth a crap!

[fulsizjeep]

Well, there are some crappy aftermarket ignition modules. Yours may be original with those low miles. It would have AMC markings on it if original. If no markings at all, it could be any after market unit. I have found the Borg Warner version costs twice as much but lasts much longer. Yeah, the caps are crappy too. Another thing I had happen once was the 2 little wires inside the dizzy gave up some insulation and was intermittently shorting to the dizzy housing. You might check there. Pretty sure you can still get new vacuum advance and pick up units for the Prestolite dizzys at NAPA. 33k miles is one thing but after 33 years, you never know what it might be. :cool:

[seventynine]

Almost 100% sure its the original ignition module and coil but I can check. I bought it from the original owner...a member here. Golddust....got your ears on?

Although my experience with ignition modules has always been that they either work or not. But my past experience has always been on the Motorcraft side of things.

Was your chewed cam gear problem on the distributor or the cam itself....or both?

[will e]

I had a similar problem with my old mustang. Usually on accelleration.
It was a spark plug that had a bit of stuff on it. I was getting some type of cross fire. Replaced all of the plugs and the problem went away.

[drlocke]

Fuel flow restriction (leaning out), ignition timing, valve timing.... Do the drill. On older systems the cam follower on the points in the ignition would wear down, and the point gap would close in, making ignition late, and causing this condition....

Check intake vacuum with a reliable gauge....Tests involve readings at idle and at various positions of throttle and resultant RPMs.

Check timing chain slop on the valve train assy....

If it's all stock then this should be pretty basic.

[seventynine]

Cool thanks for the tips guys. I have been thinking about changing the plugs and wires anyway so I'll start there. I think Ken told me that he changed them sometime back in the 80's:D

My '77 has electronic ignition so no points. I'll hook a gauge up to the manifold and check that too if the plugs and wires don't fix it.

Dean

[Golddust]

Yep, that should be the original ignition unit. It is probably covered with undercoating from Ziebart from December 1976. My guess on the problem would be in the wires. Can't remember when I might have changed them but perhaps I gave you that in with the reciepts. The plugs might be pretty old too, if not original. It always did that on start up while warming up so maybe the choke needs adjustment. I changed out the connecting rod as it broke once. I brazed it back together so maybe the original was put back in.

I saw something a few years back how there should have been a recall on the distributor clips for locking down the ditributor cap. I always had one clip coming loose and the cap only half held down. Upon reading that, I used pliers to bend it slightly and it never popped off again during the little I ran it.

Ken

[seventynine]

Thanks Ken.

Yep the clips on the cap won't stay on. I tried re-shaping them with pliers and ended up breaking one of them. I pulled the clips off of a spare prestolite distributor I had and they wouldn't keep the cap on either. now I have a zip tie wrapping around the cap and clips to keep them from falling off.

Definitely seems like a bad design.

I'll replace the wires and plugs to start. Anyone have a recommendation on a good set of wires?

Dean

[andy d]

genericly speaking, if it is backfiring through the carb, it is a timing or lean running issue. If it is popping in the exhaust ,it is running rich or is out of time :thumbsup: :thumbsup:

[seventynine]

I finally got around to replacing the plugs and wires. The existing wires had the AMC logo on them so they couldn't have been too much newer than the mid 80's. They were pretty stiff and brittle. I did look under the hood with engine running in the dark to see if I could see any arching but didn't see anything.

I was pleased that all of the spark plugs came out easily:thumbsup:. I was a little worried about that...being that they had probably been in there for 34 years:eek: They were Champion plugs...not sure what they used in the factory back in 1976.

I replaced them with a stock set of AC Delco R44xls. I bought a Universal set of Accel Superstock 8mm wires. Seem like nice wires but...what a PITA to install all the distributor boots and terminals! I'm not going to bother doing that again. I didn't really change the length of any of them anyway. They fit reasonbly well...one or two were a bit long and the coil wire was a bit short but it works.

I replaced the coil as well for good measure...original AMC logo on that too.

Anyway...the point is...It runs great!!:D I just took it for a spin and no popping or hesitating:thumbsup: I'll have to drive it some more to be sure but it normally would have acted up a bit in the length of drive I just took.
:dancingbanana:

Dean

[babywag]

Food for thought, in 1977 AMC used a plastic coated cam timing gear.

I would replace it before it decides to cause you some issues.

Had a low mileage '77 many many years ago, when the chain slipped it bent some valves.
When I took it apart I quickly found out what the problem was.
Chain was still good, but the plastic on the gear was cracked/broken allowing the chain to jump.

[seventynine]

Yeah...I had the front of the motor completely taken apart a few months back when I replaced the water pump and and reworked the AC system. I though about that...replacing the timing chain while I was so close to it...but I didn't.

I had already replaced a bunch of parts that I hadn't planned on...like the water pump/alternator/and fuel pump since I was in there. It was one of those "how far do you go" situations....I had to put a tourniquet on it at some point to stop the bleeding! Afterall it was a low mileage engine that ran great.:huh: Now though I wish I had taken a few more bolts out and replaced the timing chain.

Ahh well it will all come apart nice and easy next time:rolleyes:

Dean

[fulsizjeep]

Was your chewed cam gear problem on the distributor or the cam itself....or both?BOTH

FYI - Dunno if this helps but Prestolite modules from AutoZone are junk. We have had best luck with the Borg Warner brand.

[gsmikie]

flat cam

[fulsizjeep]

I finally got around to replacing the plugs and wires. The existing wires had the AMC logo on them so they couldn't have been too much newer than the mid 80's. They were pretty stiff and brittle. I did look under the hood with engine running in the dark to see if I could see any arching but didn't see anything.

I was pleased that all of the spark plugs came out easily:thumbsup:. I was a little worried about that...being that they had probably been in there for 34 years:eek: They were Champion plugs...not sure what they used in the factory back in 1976.

I replaced them with a stock set of AC Delco R44xls. I bought a Universal set of Accel Superstock 8mm wires. Seem like nice wires but...what a PITA to install all the distributor boots and terminals! I'm not going to bother doing that again. I didn't really change the length of any of them anyway. They fit reasonbly well...one or two were a bit long and the coil wire was a bit short but it works.

I replaced the coil as well for good measure...original AMC logo on that too.

Anyway...the point is...It runs great!!:D I just took it for a spin and no popping or hesitating:thumbsup: I'll have to drive it some more to be sure but it normally would have acted up a bit in the length of drive I just took.
:dancingbanana:

DeanCongrats! :thumbsup: I have had a few sets of plug wires over the years that would cross fire between the wires after they were a few years old.