Backfiring through carb, what could be causing this??

[benbloom]

hey guys, since i got my truck ive had this problem of occasional to not so occasional backfires through the carb. at first i blamed the carb, but 4 carbs of doing the exact thing i am sure its something else. i got my timing adjusted by a mechanic recently, and ignition system is supposedly running fine. could it be the distributor? or something in the engine? i am assuming its not how my carbs were adjusted because they have been adjusted by different people and different ways. anyone have this problem? let me know, this one has left me scratching my head. thanks!

[Baker72chevy]

You could have a vacuum leak

[BigCountryx]

Install an LS, that'll fix it

[cooperhw]

Mine was backfiring and I adjusted the accelerator pump arm so it wasn't squirting until the throttle arm moved. It was squirting all the time so it couldn't shut off.

[Robznob11]

Originally Posted by cooperhw

Mine was backfiring and I adjusted the accelerator pump arm so it wasn't squirting until the throttle arm moved. It was squirting all the time so it couldn't shut off.

I always thought that backfire through carb indicated a lean condition . If carb is adjusted correct and timing is adjusted , could it be that some one installed light spring s in the hei and your timing advance is coming in before your fuel, also could it be an advance problem du to vacuum/mechanical advance on the hei. Just thoughts it has to be in the timing some how.

[Fitz]

Originally Posted by BigCountryx

Install an LS, that'll fix it

Yes, it will! What a great idea.

[Mike C]

On my Jimmy it was a broken valve guide. Ran fine most of the time, but ever once in awhile it would spit back out the carb under load.

[leftybass209]

I don't know how anyone could start diagnosing this issue before they understood when the symptoms occur.

Does it backfire while idling, does it backfire when accelerating, or does it backfire when you let off the gas? A mechanic set your timing, but to what? What condition is the distributor in? What carb is currently on it? What port is your vacuum advance hooked to, timed or ported? Do you even have a vacuum advance?

There's not much information given, so there's not much diagnosing that can be done.

[benbloom]

sorry, here goes:
i just had a quadrajett rebuilt professionally, by guys with a great reputation. its sitting on a small block 350. the backfires occur often when getting warmed up, in the mornings, or when i really step on the gas. i only payed 1 mechanic to adjust it, other wise it was friends who have had these trucks for years, but basically there was variation so i can't imagine thats the problem. what more info is needed? not sure in what kind of shape th distributor is in, ive had it for a couple years, but changed the wires and cap and plugs recently.

[TBONE1964]

Originally Posted by BigCountryx

Install an LS, that'll fix it

[TBONE1964]

Has anyone done a compression test on your engine?

Find out what the condition of the inside is (compression test) before even thinking about the other 16 things that could cause your issue.

Read the plugs while they are out. Spark plugs reading is like a drug test on an engine , it tells a story...

[Fitz]

Originally Posted by benbloom

hey guys, since i got my truck ive had this problem of occasional to not so occasional backfires through the carb. at first i blamed the carb, but 4 carbs of doing the exact thing i am sure its something else. i got my timing adjusted by a mechanic recently, and ignition system is supposedly running fine. could it be the distributor? or something in the engine? i am assuming its not how my carbs were adjusted because they have been adjusted by different people and different ways. anyone have this problem? let me know, this one has left me scratching my head. thanks!

Any time someone else has adjusted anything on your truck and you still have a problem check what they did before spending more money.

[my67c20]

You need to provide more information. What size engine, type of carb, when this issues occurs, etc? If the information you provided this board, is the same as was provided to the mechanic, about all you are getting out of flat, is a lighter wallet. Not trying to sound mean, but can't help without more information.

[piecesparts]

A bad valve spring is coming to mind.
However you did say a quadrajet is on your engine---was it built by Rochester or Carter? The body of the carter quadrajets do not have a brass port from the bowl where the needle goes through it, where the Rochester does. This allows for the potmetal in the carb body to errode away and thus can cause a larger than normal hole around the needle. This will let your engine bog or even back fire when pushing on the accelerator.

Have you looked at your balancer. It may be the cause of your problem. Your distributor advance may be hanging up. To me a backfire problem is usually from inside the engine and the outside items are only the process points that many look at.

[GASoline71]

Timing... If you have a worn out stretched timing chain, it can cause an intake valve to still be briefly open during the firing of the fuel/air charge. Or if the distributor is installed wrong.

Gary

[Derek.Matirne]

Some suggestions I would like to make... Since you are new to working on these old gems, I would watch videos on YouTube, but use a little common sense, just because someone post a video, doesn't mean they know what they are doing... I have found videos very helpful to figure out how to do things that I don't know much about, but I am able to tell if they know what they are doing or just making videos, to make videos... Another thing I would consider is, which I would assume you might have done, is check around with friends and neighbors, to see if there is anyone around that plays around with the older vehicles, that could show you how to work on your own vehicle.... I was fortunate enough, as a kid, to have a great neighborhood mechanic that was always willing to pass on the knowledge that he has on to me, it's been over 30 years now, and I still find myself, on occasion, calling him and asking questions.... Also, you don't always have to buy tools to work on your vehicles, Autozone usually will loan out tools to help a guy out...

[62beater]

Knowing how to read a spark plug is the first step before anything else. Could be a worn chain or dist. shaft but, as has been mentioned, the only way to get fire coming back through the carb is from an open intake valve during combustion. A leak down test will definitely tell you condition of engine and if a valve is open or burnt. A chunk of the valve where it seats will be missing if it's burnt and compression will be very low. You can also find out if a head gasket is bad or the rings are worn by doing this. Get a cylinder leak down kit. Less than $25 or so. Summit sells a good one. It's very easy. You don't even need one with a guage on it. You just want the hose that one end threads into the spark plug hole and the other end is a male fitting that plugs into an air hose. Compressing the cylinder with air and rotating the engine by hand around till both valves are fully closed. Listen for air leaking out of carb to check intake valve and air from tailpipe to check exhaust valve. Check each cylinder this way. Open radiator cap and look for bubbles to check head gaskets and open oil fill cap on valve covers and listen and feel for air to check for worn rings. Take all the plugs out to make it easy to rotate. Do this one yourself. It will tell you alot and if you haven't done this before it's cool to learn something new

[Gromit]

Great advice above; I'd start with all those; but one that I don't see mentioned and that might be a cause would be cross-firing plug wires.

Cheap way to check for this - wait until dark and then go out and start your engine put it in park and set the emergency brake; then get out and pop the hood. Watch the engine running and look for flashes of light around the plug wires.

I believe this is a possibility for your symptom, though I admit less so than the comments above - but easy to check for - I see a lot of folks just drape all their plug wires all around the engine bay potentially shorting against the block and sheet metal - the factory even took precautions against this..

One of our list members (whom I think all of us would agree obviously knows what he is doing) - He sets a positive example and I'll attach a photo of his - he was my role model for end-of-story plug wire routing.

Also a please see below photo of a cheap zip-tie method for tying up plug wires to prevent cross-firing.