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

[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.

[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.

[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.

[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.

[Mike C]

90% of carb issues are ignition is the old hot rodders adage. But somewhere above it says 4 different carbs produced the same result?

Backfiring out the carb is usuall due to a lean condition. If all 4 did the same thing this suggests something other than the carb. And on my Jimmy it was a broken valve guide which allowed the valve to hang open on occasion really upsetting the combustion process to say the least.

But the poster above me is correct in that very little info was provided to make a diagnosis.

[TBONE1964]

Originally Posted by BigCountryx

Install an LS, that'll fix it

[Derek.Matirne]

I had a similar problem, it was driving me nuts, I just could not figure out what the problem was, I adjusted the carb, and timing...then it totally cramped out on me, leaving me stranded... I thought the coil was bad because I lost spark, that wasn't it, so I figured the module gave out... When replacing the module, I noticed the plug from the distributor was in really bad shape, but I plugged the module in anyways, still nothing... I broke down and bought a complete Accel distributor, hooked it all up, and bam, it was running better than it had ever before, no back firing, no spitting and sputtering, tons of power, I didn't know that motor had that much power... That being said, I'm not saying that is your problem, just happens to be mine.... It could be a lot of things, look at everything carefully, and check it twice.... I would not have guessed that the wire from the distributor to the module would have been the problem, until it went completely out.... Hope this helps....
BTW, put a timing light on each wire, watch it for a few seconds, if it misses a beat, on any of them, you have an ignition problem....

[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...

[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.

[toolboxchev]

Originally Posted by Mike C

90% of carb issues are ignition is the old hot rodders adage. But somewhere above it says 4 different carbs produced the same result?

Backfiring out the carb is usuall due to a lean condition. If all 4 did the same thing this suggests something other than the carb. And on my Jimmy it was a broken valve guide which allowed the valve to hang open on occasion really upsetting the combustion process to say the least.

But the poster above me is correct in that very little info was provided to make a diagnosis.

I agree with checking all ignition issues out first. Mainly the HEI module if you have one.

Been my problem many times now.

[benbloom]

just replaced my cap, wires and plugs. ive had my distributor a couple years, maybe time to replace it?

[Derek.Matirne]

Doesn't necessarily mean you need to replace anything, put a timing light on each wire, make sure it has a constant fire on each revolution.... Eliminate one thing before moving on to the next.... Even new parts could be bad....

[leftybass209]

Distributors aren't really a 2 year replacement item. it's possible it has issues, but instead of replacing things hoping you find the fault, it's better to properly diagnose it and save yourself some money.

I'm going to assume, and forgive me if i'm wrong, you don't know much about engines. I gather this from you taking it to someone else to time your engine and replace the things you've had done already.

At this point, you need to ask yourself if owning an older vehicle is worth trying to hunt down faults you don't understand, or if paying to have someone else do it for you is worth it. I don't want to discourage you, but the internet and forum sites can only get you so far because we can't physically be at your truck doing the diagnosis.

You said you just replaced the cap and plug wires, but why? Did the terminals look burnt or corroded on the cap? Did the plug wires check for continuity, and did you check the resistance of the wires before replacing? Did you have a cylinder not firing that would warrant needing to check a spark plug, wire, or cap terminal?

Let me give you a for instance that simulates understanding WHY you're replacing things: Fuel soaked spark plugs can trail the incoming spark along the soaked portion out through the metal threads (path of least resistance), severely reducing the spark produced in the cylinder. You think you have a dead cylinder caused by a bad wire, cap, or plug, because you aren't checking it properly. The whole time, it's because the carb isn't adjusted properly so you have an ongoing issue.

[benbloom]

you assume right, my knowledge is limited, but I'm young and i figure the best way to learn is to go through the headaches myself, and hopefully get to a point where i can competently work on my own truck 100%.
i took it to the mechanic in the first place to replace part of my clutch linkage, the screw that connects to the engine block had broken off and i couldn't remove it for the life of me. it was also backfiring worse than usual so i figured the timing could use an adjustment, and since i don't own a timing light i asked them to just go ahead and replace the distributor cap (which i had had for about two years, and definitely needed to be replaced) and check the timing. the wires were a mistake, i thought they were telling me that for two hours i could have them nicely installed in brackets running under the ram horn manifolds, but they ended up doing a worse job than i had done, i regretted not being more specific/deciding to pay someone else for that job.
so, how would you diagnose the backfiring problem in this situation? are there certain tests you would conduct?
thanks!
ben

[leftybass209]

I always start with the cheapest tests to do first.

If it were a motor that looked well used, or I assumed it had some miles there's certain things to do to verify if it's even worth throwing money at. I would start with the cheapest diagnostic check, and work my way up from there.
Pull the plugs and look at them- Free, and can tell you if you have oil in your cylinders, if it's lean or rich, or if a cylinder is missing.

Compression check- it doesn't cost much and can tell you a lot about your motor without tearing it apart, but it's vague as to what may be wrong if you've lost compression somewhere, ie: piston rings, valves.

Ignition- Find out what condition the components are in. A worn distributor can cause a lot of problems. Every 0.010 of shaft play, is around 1 degree the timing will be off.

The most time consuming and costly thing to check is the timing chain. If it's stretched your timing won't be accurate. It can be done in the vehicle, but it's a pain with everything that has to come out, and once you've disassembled the engine that far, you'd have to weigh the longevity of the motor versus a rebuild, versus your time spent. Something like the camshaft could also be inspected when the timing chain comes off.

There could also be a bend valve that is hung open, causing the backfire. It's hard to tell without being there, i'm just generalizing issues that can cause the symptoms.

What I mean is, replacing a timing chain just to get the engine running for another 5000 miles (assuming it's well worn and compression wasn't great), isn't worth the amount of work that goes into it (at least to me). It's far cheaper and less of a hassle to pick up a crate motor and be done with it for $1xxx bucks.

I think it's awesome you want to learn, but you have to start with the basics. Understand what all the issues regarding backfire could be, so you understand WHY you are doing checks and diagnosis people are suggesting.

[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