'77 J20 - AMC 360 - Poor starting - intake backfire - no idle

[SPK_J20]

Hey all,

Just acquired an old j20!

When I picked it up the previous owner had thrown an Edelbrock 1407 (too big... I know) on it that morning and said it wouldn't idle. I got it home with my foot on the gas, seemed to run just fine once she got going, but would die the moment you came off the pedal.

Since it's been home I haven't been able to get it started for more then a few moments before it dies.

It's backfiring through the carb at seems to be flooding, smells of gas and the carb even burns for awhile after trying to get started.

- I've pulled the carb apart, everything looked nice, appeared to be a fresh rebuild.
- I've replaced most of the vacuum lines and chased as many vacuum leaks as I can find. Still not sure if vacuum routing is all perfect, struggling to get a definitive diagram for the '77.
- New spark plugs (they were seriously carbon fouled).
- Checked the firing order (all good).
- Checked for vacuum leak in the brake booster (clamped the line).

Interesting tid bits I've noticed:

- It's got a points ignition on it, apparently converted.
- I noticed the fuel pump is leaking, but I can see it's getting fuel through the filter (although it's kind of a sputter, don't know if that's normal), based on the gas smell and carb burning I feel like it's not a fuel delivery thing... could be wrong.

Appreciate ya'lls help on this one, I'm kind of at the end of my rope!!

Thanks!!

[will e]

Welcome to the club!
J20's are cool.

First thing. If the fuel pump is leaking you need to check the oil. Does it smell like gas? A bad fuel pump can leak fuel into the crank case. This is really bad for the oil.

I don't know how to 'set up' and edelbrock carb, so I can't help there. Chasing down vacuum leaks and checking plugs and firing order are all good ideas.

When does it backfire? At idle or ??

[SPK_J20]

will e, thanks! Glad to be here.

I did check the oil, at first sniff it didn't smell like gas, the plugs sure did though! I'll give it a double check next time I'm out there.

It backfires when it's attempting to start. Kind of a put, put, put, boom!

[BCRAWLER]

Timing?Timing chain?

[Stuka]

Sounds like it is way too rich. I am not sure you can even tune down a 1407 enough for a stock 360. But you could certainly get it a lot closer.

Being way too rich can cause backfires out the intake. But it could also be the timing is out. Unlikely you will be able to time it while its running the way it is. But you could try retarding it a bit manually (Turn the distributer a little clockwise). But I am thinking you are going to need to address your fuel issue.

Oh, and converting 74-77 to points is a semi-common thing. The factory electronic ignition for those years is garbage. 78+ has duraspark, and is super reliable. But the prestolite used in 74-77 is very prone to failure. The other common switch is to HEI.

[SPK_J20]

Thanks for all the great information. Love learning about this old truck.

I think ya'll are on the right track with a timing issue. I'm afraid it jumped. I'm not sure what the best way to diagnose that issue would be when it's not running... I'll try retarding the timing a bit, by the way, what's a bit? Like a 5 deg CW turn on the distributor?

Have a feeling I'm gonna end up with a new timing chain and HEI distributor by the end of this saga!

Thanks again ya'll!

[will e]

Before you move the distributor use a marker or some paint to mark it's current position, this will allow you to get it back to where it was before you started (in case moving it makes it worse).

It's pretty unlikely that the timing chain jumped. Do you have another carb you can toss on?

[SPK_J20]

Will do!

I don't have another carb. Would love to do a quick swap, but it's tricky to get ahold of used parts in the days of COVID-19!!

[Icethrasher]

You have received solid advice. The symptoms sound very similar to ones I have had when there is a good size vacuum leak. Check twice, and plug all open vacuum ports.

Could reference check the timing by marking the distributor for #1 plug wire, remove distributor cap, and rotate the crank until the distributor rotor is pointing to where you marked #1 plug wire location and then check the timing marks on the crank pulley...

I would definitely get the right carb on there and eliminate that as a variable. New is best, but if the 1407 is in good shape, maybe find someone who is interested in a trade... PM me if you are interested in a 1405.

[SPK_J20]

You have received solid advice. The symptoms sound very similar to ones I have had when there is a good size vacuum leak. Check twice, and plug all open vacuum ports.

Could reference check the timing by marking the distributor for #1 plug wire, remove distributor cap, and rotate the crank until the distributor rotor is pointing to where you marked #1 plug wire location and then check the timing marks on the crank pulley...

Good call on both. It's amusing, every time I look I find another vacuum leak, it's a game of wack-a-mole. Don't know how it ever ran at all.

I'll check timing as suggested this evening. Good plan! Thanks!

[SPK_J20]

Just made a bit of break through! Did as suggested, marked number one on the cap, pulled it off, cranked to TDC. The rotor was pretty far off of the mark. I spun it back onto the mark, and sure a BEEP, she lit right up and idled nicely. That is until I touched the throttle, she pooped out and wont restart. Kind of bogs and sputters and back fires, takes a lot of throttle feathering to get started, fuel burns in the carb like Jimmy Hendrix' guitar.

Wonder if the points/condenser are worn out?

[BCRAWLER]

My money is on jumped chain. Qiuck check on chain strech. Rotate engine in one direction with socket on crankshaft bolt. Reverse direction with socket and see how much movement before distributor rotor moves. Not sure on acceptable limits but sure somebody on here will provide direction. Maybe before tearing it all down confirm chain has jumped. Set #1 piston at TDC and see where your valves wind up.

[will e]

That's great news. Do you have a dwell meter?

[BCRAWLER]

Good point, forgot he had points.

[will e]

Yeah, at this point (no pun intended) the best bet is to pick up a new set of points and condenser. Bad or burnt points can cause all kinds of weird problems. They are super cheap so even if it is just to get it running again and then will be replaced with a electronic distributor, it's worth the small cost.

[itselliott]

RE the timing ...you turned the dist. to get it close to the correct timing,...you probably should have pulled the dist. and moved it 1 tooth on the gear, to get timing back to "close".
You wont have good vacuum advance the way you did it.

[will e]

. The rotor was pretty far off of the mark. I spun it back onto the mark

What does this mean?

[Stuka]

. The rotor was pretty far off of the mark. I spun it back onto the mark

What does this mean?

From the sounds of it, he put #1 at TDC, pulled the cap to see where it was in relation to the #1 spark plug. It was off, so he rotated the distributer so that it lined up. This got it close enough to run.

[SPK_J20]

Thanks for everyone's input.

I ordered an HEI distributor, cables, fuel pump. Not going to mess around with a points ignition I don't have any interest in keeping!

Get back to ya'll soon!

[letank]

Recheck for vacuum leak at the base of the carb at the thick fiber gasket, the edelbrock might be different in term of seal with a brake cleaner spray, carb cleaner to make sure that there are no base gasket issues. If you have an edelbrock carb you may have either a different intake manifold or an adapter. Pict are always good.
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