I forgot to mention: I've replaced new points, dizzy, rotor, condensor, plugs and plug wires. Plugs gapped properly and dwell is at 28...have not had a chance to set timing but when I do will try 10 dbtdc.

 

dwell is at 28...have not had a chance to set timing but when I do will try 10 dbtdc.

Try timing first, maybe 10°-12° BTDC.

 

You meantion "carb backfire"
For it to ignite back thru the carb u need a valve open (not sealed, misaddjusted) something like that, or the timing to be so far off a valve is open.

A back fire thru the exhuast is a different kettle of fish, this usully indicates a lean condition in the carb....
So which is it, carb or exhaust?

If u dont belive it is a lean condition...and timing curves are good, change the spark plugs...

 

I have usually considered a backfire from the carb could be casued by a lean condition.

This condition can be created by:

vacuum leak
bad acellerator pump squirt
float level to low


Other issues can be:
Timing
Vad wires or plugs
Bad cap
mis adjusted or sticky valves


Afterburn or backfire from the tailpipe can be caused by an excessivly rich condition.This can be created by a misfire.

 

Some possibilities:

1) Cracked distributor cap
1a) Water in the cap
2) Worn plug wires
3) Broken valve spring(s)
4) Wrong firing order

 

It definitly is a carb backfire. Plugs, plug wires, and dizzy cap are all new. How do I change the "lean" condition of the carb?

 

"How do I change the "lean" condition of the carb?"

First i would look for the obvious.

Vacuum leaks
Float to low
Bad eccelerotor pump discharge
secondaries open slightly at idle
incorectly installed vacc hoses

Is you dist vac advance?

is it connected?

Then check timing and all related to this function

 

I would think something like a valve open would spit or backfire every time that cyl fires.

You would have a definate miss.This can be checked with a erratic vacc gauge reading.

The answer is in the clues everybody has given you. You only need to elliminate the possibilities one at a time.

 

You might have a loose dizzy and as the block warms up the timing changes. As Everett suggests check the timing first before you start chasing your tail all over the place. The bolt may just need to be tightened and call it done. Then if its is the float or tune of the carb which are easy to adjust, you are not compounding the situation with a loose dizzy.

 

Lean condition no doubt you described hesitation too that is accelerator pump. Engines run leaner until they warm up. What carb.??

 

It is lean condition that causes exhaust backfire, not rich.
Real common on vintage cars with very worn butterfly shaft bushes.

And a crab should be rich on fire up, cold engines will only fire on a slightly rich condition, not lean....hence why the invention of a choke...which orginaly only limited air flow and didnt increase rpms...a later advanced mod.

 

My carb is an Edelbrock 1406 600cfm and up to now it's been a great carb. As mentioined earlier...Im not even close to being a mechanic so if there was some kind of PM the carb needed over the years...it's never been done and I did neglect to mention that the carb is about 10 years old. A rebuild is out of the question as I would not have the first clue where to start much less what I was looking at so that would have to be done by a real mechanic.

I do spend a lot of time using the search feature on this board and must say I am most impressed with the knowledge most of you have. Of what I've read...a common theme seems to be a bad/weak coil, accelerator pump or timing and I realize there could be dozens of issues causing my problem.

Changing the coil is not problme and I think I'll do that first since the thing is quite old. Could someone recommend a good coil from Jegs, Summit..etc? I've also noticed that some coils have more Voltage than others such as the MSD Blaster 2 lists 45,000 V's then Pertronix has one with 40,000 V's. Is there really that much difference in these two and is "More" better in this application?

Can the accelerator pump be replaced without an entire rebuild?

Thanks again for the direction.

 

if you want reliable performance just go with a stock HEI...i've used MSD coils and caps and all that garbage in the past...MSD coil burned up so i put my old OEM coil back in and no problems whatsoever

 

Dean, with engine off, look into the carb and stroke the linkage to full throttle a few times. The accel pump will squirt fuel into the front two bowls. If it looks like a good, strong stream then the pump is good.

As far as changing the pump. It's more intimidating than hard. I believe on a 1406 you need to strip the linkage and take the screws holding the top plate on. You then (slow and careful) pick straight up on the top plate, floats should be hanging right there, so again slow and careful. The accel pump is now right in front of you (drivers side, front). It is easy to drop a screw into the carb so be careful or pull the carb off the car to do.

 

Islanddean -

My $.02 as the rookie in the forum.

Start of with the cheap stuff first. I just spent two weeks rebuilding a carb, checking timing, plugs, etc. Mine ended up being a bad tank of gas. It would backfire through the carb and diesel quite a bit. After checking everything else, I determined the cause to be bad gas from a local station. The actual firing timing was off with gas that was not up to posted octane rating. When the octane rating is off, the compression and firing happens at the wrong time making it look like bad timing. Due to the incorrect firing time, not all gas is burned properly and the backfire may occur through the carb.

Try the following in order:
1. Run a can of Sea Foam through the motor at operating temp, 1/3 through the carb at about 2000 RPM and the rest in the gas tank, let it sit for 20 minutes, then run it around town for about 15 minutes to clean it out. Should smoke a lot!!
2. Then add some octane booster to the gas tank and fill it with the highest octane gas for the next couple of tanks.
3. Run some carb cleaner into the carb to clear out any residual left in the carb.
4. Recheck air/fuel mix and curb idle. Confirm timing if you have a light.

Everything should be running pretty tight at this point. This should cost you about $20 in SeaFoam and a little time. If it doesn't resolve your issue, the motor will be clean and you can move on to more expensive troubleshooting.

Good luck,
69DFW

 

My carb is an Edelbrock 1406 600cfm and up to now it's been a great carb. As mentioined earlier...Im not even close to being a mechanic so if there was some kind of PM the carb needed over the years...it's never been done and I did neglect to mention that the carb is about 10 years old.

1st..A carb will not cause a backfire back thru the carb
2nd Carbs are reliable once setup and last for yrs,s are used amd maintained....A quick check...see if acess play in the butterfly shart bushes...this is where carbs start to give up the ghost and cause lean conditions.

Regardless it is not the carb causing backfire thru the carb
The ONLY way to back fire thru the carb is to have a piont of ignition to explode the fuel...
AND the ONLY way to get an ingn piont to expolde the fuel is it a cylinder fires and a inlet valve has not closed.....This is not caused by dizzy cap pionts leads what ever UNLESS the ign intion system is in effect turned off...as in turning off the ign switch and turning back on again....backfire.
BUT if that was the case the engine would be missing badly..real bad, and it is not.

So we are back to timing so far out or the cent curve in the dizzy is stuffed...broke spring what ever...but that would also throw idling alsorts of other stuff out that u would have mentioned by now
A valve not closing, baddly adjusted lifters.
And since it is in the carb and NOT the exhaust...u are sure 100% on this? its not the carb leaning out.

 

The "Lean" issue is certainly one I'm going to address but resolving that process is above my pay-grade as I have absolutely NO experience with carbs. I know to look for vacuum leaks around the base of the carb..etc...but as far as tearing the carb apart...that won't happen.

Now that I'm thinking about this...I moved the vacuum line to the "full time" port on the carb and the car never backfired until I changed ports. I'll move the line back and see what happens.

 

If it is popping badly thru the carb, check valve adjustment. Could have come loose or you could have broken a lifter or wiped a lobe on the cam. Ask me how I know

 

AintQik....how do you know? Also...what position should the accelerator pump be set at? On the adjustment lever there are 3 homes...currently the pump is set in the middle hole.

 

I just went through this with the 400 mile 350 in my 1950 Chevy truck. I say this because the car was running. While lots of things are possible, I doubt all of a sudden the pump is bad. If it worked before, it should work now. However, anything is possible.

Can you adjust the valves? I didn't read what type of cam you are running. On mine, all of a sudden it was backfiring through the carb. I adjusted the valves and it went away. Then it came back. The lifters were eating away on the cam. Then it finally got so bad I started looking into the motor.

As others have said start with easy. You say you are not a mech so I doubt you have a vacuum gauge to hook to the carb. But, rule out all the easy free stuff before you go throwing parts at it. As others have said, backfiring means timing most of the time. Think what affects, or is it effects, timing. What could cause the motor to all of a sudden be out of time? Ask yourself these questions after you have ruled out the obvious. If you have a buddy and can swap carbs, try that to rule out something going bad in the carb. Then look at timing. Something is causing a valve to be open (or closed) when its not supposed to be. I'd like to hear the backfire. Cam issues are pretty easy to id. Maybe describe the backfire a little. Does it start and run but when you open the throttle it bangs loudly? Does it gurgle and pop when you open it up? Does it bang and not start at all? Good luck!

 

I am no engineer but I do know a lean issue will cause a backfire through the carb. Read every troubleshooting guide for holley or demon and they say the same thing. A steady pop through the carb is a valve or distributor cap misfiring. But a backfire which the thread is titled can be lean condition. I realize steptoe is a senior tech and can run circles around me as far as knowledge but I don't understand how you can say
1st..A carb will not cause a backfire back thru the carb
Not trying to be a jerk just don't understand. thanks

 

On a four stroke engine, please explain how a 'lean condition' would cause a backfire through the carb?

I'll save you some time. It can't. A lean condition CAN cause backfiring through the exhaust.

As somebody above said, backfiring through the carb is happening due to igntion in the chamber with an intake valve open, at least partially.

1) Either a valve is sticking, poorly adjusted, or a valve spring(s) are broken, etc.

2) Or, (most likely) something is amiss in the ignition system; firing order, moisture in the distributor cap (creates cross-firing), cracked cap (creates cross-firing), bad wires, etc. Again, on a four stroke engine, the intake valve is open primarily on the intake stroke. That chamber's spark plug won't light unless something in the ignition system is misfiring.

Quick tip. On a very dark street, pop the hood with the engine idling. Sometimes you can see the misfiring if it's happening outside the distributor.

Good luck!

 

http://www.nzmustang.com/TechData/carbfixes.htm
Please read #3
http://www.championparts.com/carb_troubleshooting_guide.pdf
look at the 3rd page
http://www.recarbco.com/technical/newtrouble.html
These sights are not my opinion and they all have carburetor cures to fix a backfire through the carb.
I am not making this up to piss someone off it is the truth I had a lean problem with mine a bigger squirter solved it I found out through various forums also I was just trying to help another hotrodder that's all.

 

From the first link above:

Problem: "Backfires through carburetor"

Solution: "Screw out mixture screws, increase squirter size, raise float levels, reset butterflies, increase main jet size, check engine timing".

My comments: What and where is the ignition source for the backfiring? The only way, without a spark from the plug, for a fuel charge (lean, rich or otherwise) to ignite, is if the combustion chamber was so hot that it ignited spontaneously with the intake valve open (at least partially open). I suppose a lean condition (very, very lean) could contribute to an over-heated combustion chamber causing ill-timed combustion events, but put this one in the very unlikely category in this case. And if this were the problem, I'd be diagnosing something more nefarious than what a jet or squirter change could fix.

The second and third links (which are the same) are useless. It states a 6 or 8 different problems and solves them with 10-12 different solutions. Here's the info from the links:

Problems:

A) Hesitation
B) Stalling
C) Stumbling
D) Flatspot, or deadspot during acceleration
E) Backfiring or spitback up through carb.

Solutions:
1) Vacuum leak.
1) Check for vacuum leak & fix it.

2) Ignition timing retarded too far.
2) Reset timing.

3) Accelerator pump nozzle has dirt in it.
3) Clean out the nozzle tip.

4) Accelerator pump cup swollen up from contact with bad gas or chemicals.
4) Replace the pump cup.

5) Economizer jet too small or partly blocked.
5) Clean out economizer jet & check the size.

6) Choke pull-off open too far.
6) Adjust the pull-off tighter.

7) Secondary throttle plates not closing all the way.
7) Fix it.

8) Vacuum hoses hooked up wrong.
8) Connect up right. Be especially careful of the EGR & Dist. connections: sometimes the pipe locations are reversed on Rochesters.

9) Idle jet partly blocked with dirt.
9) Clean out the jet & any other dirt that is in there.

10) Distributor timing not advancing properly. (worn breaker plate, worn shaft, pin hole in diaphram, crack in hose, etc.)
10) Check distributor and all related systems carefully. Replace defective parts

 

A back fire is an explosion right?
And explosion requires an idnition piont right?
A lean condition will explode in the exhaust becuse of very hot (hotter than normal because it is lean) bits of carbon and burrs in the pipe...

No to the back fire in the carb, there are no 'hot spots' so it needs a little trol sitting inside the inle manifold playing with matches OR and ignition from a cyclinder that is firing.

Now those who think a back fire can be caused by ANYTHING else please explain....to the over hundred yrs of hands on experiance of the guys above repeatly tryin to explain something that is not rocket science.

 

A back fire is an explosion right?
And explosion requires an idnition piont right?
A lean condition will explode in the exhaust becuse of very hot (hotter than normal because it is lean) bits of carbon and burrs in the pipe...

No to the back fire in the carb, there are no 'hot spots' so it needs a little trol sitting inside the inle manifold playing with matches OR and ignition from a cyclinder that is firing.

Now those who think a back fire can be caused by ANYTHING else please explain....to the over hundred yrs of hands on experiance of the guys above repeatly tryin to explain something that is not rocket science .

I think the technical term "Back-fire" is defined as any time fuel ignites in the intake or the exhaust systems.

In my experience if the back-fire occurs in the intake systems, it is caused by timing issues where the intake valves are still open as the ignition cycle of that cylinder is started. Timing is advanced in that case. Most of the time you see visible flame in the carb area. The back-fire will burn off the atomized fuel expelled from the cylinder, maybe burn off your eyebrows or set fire to your air filter but the flash of the burn off is quick.

If the back-fire occurs in the exhaust system it is more then likely caused by a lean fuel/air mixture not rich enough to ignite in the chamber but once exhausted the mixture combines and ignites in the headers or tailpipe. These are much louder and are rarely visible to the eye.

Agreed Step I have never seen one caused by anything else and the two are very easy to solve based on what you see or don't see. Timing if through the carb, mixture if through the exhaust.

 

Whoa, am I in Alice in Wonderland or did I get some wacky weed?:wacko:
Islanddean, You changed ignition components. That's not a tune-up until those parts are properly adjusted. (Guy's, last time I set dwell was 35 years ago. If memory serves, and it rarely does, shouldn't dwell be 19, not 28?) Anyway Islanddean, you're chasing your tail. First set your dwell and THEN set your timing. Did I miss it or did someone tell you to also do a compression check? If the change you made, replacing ignition parts, came before the symptoms that you have now,,,,, you gotta quit talking and finish your tuneup. Then we'll be happy to help, but as of now, as I see it, we're all waisting our time. Oh, IMHO, of course.:yes:

 

Fred, I have not done a compression check because I don't know how to do those. THe backfire issue did start after I changed the points, rotor, plugs & wires. I did set the dwell and for a few days the car seemed to run pretty good. The timing has not been set as of today...just have not had the time. (Running a multi-million dollar, international, business takes up most of my day) The backfire started after I installed a new Edelbrock fuel pump because I thought the old pump was not delivering the correct psi to the Edelbrock carb. It appears this topic has touch a few nerves and that was not my intent and I realize this particular issue is almost childs play to most of you but to someone like me it's confusing because I've never had any type of formal training.

I have no idea how to trouble shoot issues like this because I've never done it before and I have no idea how one thing can effect something else. I give all the info I can in the best possible way I know how and I certainly don't mean to waste your time.

 

Fred, I have not done a compression check because I don't know how to do those. THe backfire issue did start after I changed the points, rotor, plugs & wires. I did set the dwell and for a few days the car seemed to run pretty good. The timing has not been set as of today...just have not had the time. (Running a multi-million dollar, international, business takes up most of my day) The backfire started after I installed a new Edelbrock fuel pump because I thought the old pump was not delivering the correct psi to the Edelbrock carb. It appears this topic has touch a few nerves and that was not my intent and I realize this particular issue is almost childs play to most of you but to someone like me it's confusing because I've never had any type of formal training.

I have no idea how to trouble shoot issues like this because I've never done it before and I have no idea how one thing can effect something else. I give all the info I can in the best possible way I know how and I certainly don't mean to waste your time.

Cool profession Dean. It was obvious you're equipped with brain power. I don't know your circle of friends but hopefully one of them won't be a geek and you can pick their brain. We have suggested that to others and also to drag said gearhead over to your garage to give it a look. I can be there in no time. Well, maybe not.