Backfiring thru the carb, Weber 32/36 AMC 258

[Grand_Wag_85]

I have an AMC 258 with a Weber 32/36 and HEI. Recently the Jeep started to have issues starting up, also acting cold natured and will backfire thru the carb and stall out if the Jeep hasn't been warmed up and you give it throttle. The issues started after I took it out of winter storage, unfortunately I didn't put fuel stabilizer in the fuel before it got put away for the winter but the Jeep was started and run every few weeks all winter. It was sitting for 3 or 4 months tops.

The truck ran perfect last year, almost as if it had fuel injection but again after it came out of storage for the winter it never really ran the same. The other day I ran a can of Sea Foam and Lucas upper cylinder lube thru the fuel tank then gave the carb a good thorough wash out with Gumout spray and it helped somewhat but I'm still having trouble starting it and getting it to run right until it's up to operating temp.

Any ideas on what I can do to make it run better? The carb looks pretty clean FWIW.

[Stuka]

Have you checked your timing? It shouldn't have changed, but its possible.

Bad fuel can cause pre-ignition as its octane drops, but you would suffer from bad power and general poor running most likely as well.

But, from what it sounds like, it runs fine once warmed up, and its only an issue when it cold? If thats the case, is the choke opening like it should? And do you still have any of the factory CTO's hooked up? I have mine all ripped out, so I cannot compare my setup directly to yours.

[jsinajeep]

X2 on the choke. Check the choke stove line to and from the manifold and also the electrical wire to the choke cover.

[Stuka]

X2 on the choke. Check the choke stove line to and from the manifold and also the electrical wire to the choke cover.

Since this is a 258 with a weber, there is no choke stove. Its purely an electric choke.

[carnuck]

Sounds like the choke is stuck open when cold. Maybe the timing got retarded somehow too? (loose dist? Stuck advance mechanism?)

[Grand_Wag_85]

I'll check the choke. No CTO stuff or vacuum emission stuff left on this Jeep.

Another thing I just thought of...Occasionally from time to time while downshifting it will backfire once real quick thru the carb even if the Jeep is warm so I don't think it's entirely a choke issue?

[Stuka]

I'll check the choke. No CTO stuff or vacuum emission stuff left on this Jeep.

Another thing I just thought of...Occasionally from time to time while downshifting it will backfire once real quick thru the carb even if the Jeep is warm so I don't think it's entirely a choke issue?

Make sure your vacuum advance is working properly. Wouldnt hurt to check the mechanical advance as well. But if the vacuum advance didnt work right, the timing would not change as it should as throttle is applied and let off.

[Grand_Wag_85]

I'll check the choke. No CTO stuff or vacuum emission stuff left on this Jeep.

Another thing I just thought of...Occasionally from time to time while downshifting it will backfire once real quick thru the carb even if the Jeep is warm so I don't think it's entirely a choke issue?

Make sure your vacuum advance is working properly. Wouldnt hurt to check the mechanical advance as well. But if the vacuum advance didnt work right, the timing would not change as it should as throttle is applied and let off.

Vacuum advance is working properly as is the mechanical advance. While I was in there I spotted a weird issue that I think is the problem. With the rotor off of the distributor I held the mechanical advance closed and rotated the guts of the distributor, IIRC there's supposed to be little or no movement but there was atleast 20*-45* of play when I did this plus there was a 'crack' that came from the distributor that didn't sound quite right. I put the rotor on and rotated as far to the right as it would let me then put everything back together, the Jeep fired right up and idled just as good as it used to then after I gave it some gas it proceeded to run like crap. It took nearly half the skinny pedal to hold it at 500rpm's then I was able to get it up to 2Krpm's but for seemingly no reason it backfired thru the carb several times and tried to die but held a really low 300ish rpm idle.

Any ideas as to what the problem is? I'm thinking that the distributor's gotta come out, maybe the roll pin at the dizzy gear failed? What else should I be checking for? I got video footage of the problem that I'll try to upload shortly.

[Grand_Wag_85]

Here's the video. At first I applied vacuum to the vacuum advance then I cycled the mechanical advance and after that I held the mechanical advance closed and rotated the shaft showing the shaft play. The video is a bit shakey.

[Stuka]

How does the upper bushing feel? To test that, grab the shaft and try to move it side to side. Not rotate, but shake. If that is bad, it can cause your timing to fluctuate, which could result in what you are seeing.

And technically a very worn timing chain can also cause what you are seeing. Before I replaced the chain in my J10 holding a timing light on it showed the timing bouncing around because of how much slop was in the chain. When I did change it, I was almost able to pull the chain off without pulling the gears From the factory, 80's 258's had Teflon gears. Not sure on 70's 258's.

The rotational play you show is a bit much. But its hard to tell what is causing it. It could be the cotter pin that holds the distributer gear on, or it could be the gears themselves.

[Grand_Wag_85]

Not sure on that upper bushing, will give that a check.

How bad of a job is doing the timing chain on the 258? I've only swapped them on the V8's, each time due to nylon tooth timing sets...Can't put into words how much I HATE those things! I guess I better check the play. AFAIK this engine came from an '86ish CJ7 and is reported to only have 90K on the clock but who knows.

After shooting that video I tried to take the J10 for a drive and only made it about 2 miles before having to turn around, ended up barely making it home. There was a major loss of power, LOTS of backfiring thru the carb on acceleration and the Jeep was running HOT. Until I get this sorted out it's parked.

[Grand_Wag_85]

OK, did the upper bushing test and there was no play whatsoever. While doing that test I realized that I was able to spin the rotor about 100* with the mechanical advance maxxed out. Yanked the distributor thinking that the roll pin was AWOL which it wasn't. Locked the oil pump drive flange tang thing down and found out that I could rotate the rotor 360* an unlimited amount of times...There be the problem. Shot a video of it, will throw it up shortly.

[Grand_Wag_85]

Here's the video. Any idea on what it is that broke?



BTW, thanks for the help!

[Stuka]

Pull the pin on the gear and slide the shaft out.

I did the timing chain on my J10 in about 4 hours. I still have photos of the complete job, step by step. Its a lot less work than on an AMC 360. You dont have to pull the distributer, you dont have to pull the water pump, non of that. I did pull the radiator. But otherwise the cover it by itself.

[Grand_Wag_85]

Pull the pin on the gear and slide the shaft out.

I did the timing chain on my J10 in about 4 hours. I still have photos of the complete job, step by step. Its a lot less work than on an AMC 360. You dont have to pull the distributer, you dont have to pull the water pump, non of that. I did pull the radiator. But otherwise the cover it by itself.

After pulling the shaft what exactly am I looking for? Is there another pin at the top of the distributor or a key way that holds everything in alignment or is it just pressed into place? It's been forever since I've torn down a distributor.

As for the timing chain I think I'll be doing it if nothing else for piece of mind. What do you recommend as far as a good timing set? I'm glad it's not set up like the V8 where the water pump and distributor, fuel pump, alt, PS pump and dizzy drive gears have to come out...If that was the case I think my Jeep ADD would take over and I'd be tempted to replace everything I touch whether it needs it or not then not be able to drive it for who knows how long lol.

[Stuka]

As I recall there is another key way at the top. You dont have to pull the shaft, would just let you inspect it.

As I recall (its been a while) there is not a pin or anything at the top. There is a slot cut in the shaft that the upper part slides into (mech advance, etc). Its possible this is either work, or the mech advance key that slides into that is worn.

For a timing set, I got mine from napa. Got the best one they offered. All steel, etc. Its not a double roller or anything, but it has worked fine since 2005 when I did it.

Here is a photo of a 258 distributer that shows the slot:

EDIT: This appears to be a points distributer, but the shaft should be similar.

[Stuka]

Oh, just looked through my photos, I did have to rotate back the alternator bracket, but that wasn't a big deal. Also, be aware the crank shaft is not keyed, so take extra care in making sure the gear is lined up properly. Otherwise you can change your cam timing without realizing it.

[Grand_Wag_85]

Just tried to pull the distributor gear off but I could not get that roll pin out for the life of me. Got it to budge slightly but not enough to get it with the vice grip.

Alternator bracket is not a real big deal. Didn't realize the crank is not keyed, sounds like that diesel I did. I will give that extra attention when it comes down to that time.

Are there alignment marks on the 258 timing gears like the V8's have? Also, since the cam on the 258 if offset do the timing marks point right at each other like the V8's or is there another method?

[Stuka]

I pointed my timing marks at each other. As I recall (would have to look it up) thats how my TSM said to line them up.

Watching your video again, the top part of the shaft turns fine. Its almost like the shaft split or something, which is extremely unlikely. But its very weight.

Here is an image (just searched) which is the same as what my TSM has. The crank gear has a key in it, but its to slide over the keyway for the main pully. The key stops short of where the gear goes, as you can kind of see in my photo.



Here is mine with the old and then new timing set. And yes, I changed the water pump not long after I did this

[Grand_Wag_85]

Thanks for the reference photos! That old timing chain looks like it was on borrowed time. On a side note, did your Jeep have serpentine belts from the factory or did you swap them on aftermarket? Any real advantage to it?

As far as the distributor goes it seems as if the shaft is broken but when I pull up on the rotor or down on the shaft it stays put, might try to tear into it again tomorrow but I'm calling it a night for now. I will be giving the distributor folks a call on Monday and see if they'll make good on the lifetime warranty.