6 cyl backfire through exhaust and carb question

[rrent]

I recently acquired 1982 with a 250 6 cyl. integrated head 3 speed standard. Starts, runs, drives, licensed, inspected. Had a big backfire through the exhaust when I depressed the clutch (at speed coasting or just when shifting) and was running so rich it would make your eyes water. I adjusted the idle mixture screws (plural) - got the exhaust smell acceptable. Went from big backfire to popping when clutch is depressed. When starting cold - it backfires after I let off of the throttle, when even slightly “revving” it.

Not sure of the make/model of the carb. It seems to be mounted to an aluminum plate - possibly a homemade carb adapter of some sort.

From a search here and of the inter’webs - my list:
Check timing
Check/inspect exhaust for leaks
Inspect spark plugs, wires, distributor.

I’m sure it could be a much bigger (read internal) issue. I have no knowledge of this motor’s history. Odometer shows 55k, my guess is 155k - probably original motor, but (again) no knowledge.

Question:
Does anything in the picture(s) stand out as a possible concern?
Any idea about that strange carb spacer setup?
Any other idea if something about that strange carb setup might be causing backfire city?
What carb should I have? And is there a source on the correct carb?

I’ve got a bored and stroked 454, but was trying to keep the truck “roadworthy” with the 250, until I rebuilt the front suspension.

Thanks in advance
R

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[Rusty Nail]

Wild! One can see any number of crazy things that people do to these trucks, on this site and that is part of the reason i'm here. It's damned entertaining!

That LOOKS LIKE a vacuum leak to me but hay, i'm no expert. Wonder what that carb came off of..I don't know, but I can't stop staring at the way the spring attachment point appears to have been bent to make it fit! WILD!
A Dodge CAR?

But seriously folks, if it were ME, I would remove the carburetor and start over.
A base plate vacuum leak would certainly cause a run to "backfire city".

See one (1) attached photo.

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You will never find that gasket for sale, you'll have to make it. O'Reilly's sells cork gasket maker in a roll. Be ready to trace that neat adaptor and cut it out with an Exact-o. That's likely the only way to get one (a gasket).

When you see something like that , the answer is almost always "vacuum leak".
I bet there is no gasket at all. Wanna bet?

 

[DBarre]

I've got what I think is the original Rochester Varajet from my '81 250 if you want it. That engine has it's share of problems, but the carb is fine. I just swapped to a '78 engine without the integral head and don't have any use for the old carb.

I think Rusty is on the right track with the vacuum leak. That homemade adapter is most likely the source of your issues.

 

[chengny]

I recently acquired 1982 with a 250 6 cyl. integrated head 3 speed standard. Starts, runs, drives, licensed, inspected. Had a big backfire through the exhaust when I depressed the clutch (at speed coasting or just when shifting) and was running so rich it would make your eyes water. I adjusted the idle mixture screws (plural) - got the exhaust smell acceptable. Went from big backfire to popping when clutch is depressed. When starting cold - it backfires after I let off of the throttle, when even slightly “revving” it.

Not sure of the make/model of the carb. It seems to be mounted to an aluminum plate - possibly a homemade carb adapter of some sort.

From a search here and of the inter’webs - my list:
Check timing
Check/inspect exhaust for leaks
Inspect spark plugs, wires, distributor.

I’m sure it could be a much bigger (read internal) issue. I have no knowledge of this motor’s history. Odometer shows 55k, my guess is 155k - probably original motor, but (again) no knowledge.

Question:
Does anything in the picture(s) stand out as a possible concern?
Any idea about that strange carb spacer setup?
Any other idea if something about that strange carb setup might be causing backfire city?
What carb should I have? And is there a source on the correct carb?

I’ve got a bored and stroked 454, but was trying to keep the truck “roadworthy” with the 250, until I rebuilt the front suspension.

Thanks in advance
R

It is difficult to say for sure, but from what I can make out of the images provided, it appears that your Decel valve has been effectively disabled.

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It doesn't have a high vacuum hose attached to the actuating chamber and the associated plumbing doesn't look to be completely intact. This is relevant because the function of a decel valve is to prevent backfiring upon a reduction in engine load. From the emissions control section of the 82 service manual :

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I can't read the part number, but it should be 14003916.

Here is what the system should look like if all the hoses are connected - and connected to the right components:

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See if you can pull a part number off that carburetor and I will see if it is correct for your application - it actually looks to be in decent condition.

 

[Rusty Nail]

Lol omg /pwn.

So yur tellin me it don't got no vacuum leak?

 

[rrent]

Chengny- thanks I’ve attached a better picture indicating the missing manifold vacuum line. I’ll get a vacuum line hooked up, thanks for the info.

DBarre - yes I’ll take the carb, let me know how much you need for it and/or shipping to central Texas (Belton, Tx 76513)

Rusty Nail - I’ll let you know my gasket situation when I pull the carb.

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[DBarre]

About $40 and I can get that carb to you in a few days. I've got the whole engine just sitting in my garage if you want anything else off it. Maybe the air cleaner?

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[rrent]

About $40 and I can get that carb to you in a few days. I've got the whole engine just sitting in my garage if you want anything else off it. Maybe the air cleaner?

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Let me get a vacuum line on the decelerator valve and see if that helps. Next step will be pulling the carb and checking to see what mayhem has been done to the manifold. If at that point it looks like your carb will bolt up I’ll figure the best way to get $40 in your pocket.
Thanks

 

[DBarre]

Sounds good to me. Good luck on that decelerator valve! You might want to make sure you don't have any cracks or splits in any of the other vacuum lines as well.

 

[rrent]

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Got a vacuum line to the decelerator valve. I started it, but didn’t run it long enough to warm up - didn’t get it out of the garage. Carb way way rich, still some backfire.
I should have some quality time with the truck this coming week.
I’ll replace/check the plugs/wires/distributor, check for vacuum leaks - I’ll vacuum test the decelerator valve.
Been thinking about pulling all the smog goodies off and see if I can get it running. If my memory is correct 1/4” or so pipe plugs will plug the air tube homes in the exhaust manifold. Is the first picture an EGR valve that should have a line running to it?