i just dont get it

car started backfiring today. wasnt doing it earlier. it has always had just a bit of part throttle stumble but today it started backfiring every time i get on it.

here is the setup.

72 454.
carb is a holley 4160 with the secondary metering block conversion.
it has a proform flat throttle body instead of the standard holley center with choke tower.

front jets are 74
rear jets are 80
pump shooter is 31
pump cam is orange
vacuum secondary spring is the silver one

i have tried both holes on the pump cam
i have tried the yellow and black secondary springs

timing is 10 degrees at idle with the vacuum line unplugged
at 2500 rpms timing is 36 degrees

car idles nice and smooth at idle with right at 19 on the vacuum gauge.

if i ease into the throttle it will stumble a little but no backfire




i took it to a mechanic here in san antonio. he had about 30 old classic cars in his shop so i thought he was good to go as far as getting these things working.
apparently all he does is install fuel injection in them so no luck there.
i am not ready for that expense yet.

i am thinking go up to a 35 shooter.
any opinions.

 

From what i have read the Holly TBI is prety good and its simpler to install and stuff. from what i can read its like the old Crossfire but a heck of a lot more reliable and ezer to fix

Holly TBI Link 4 barrel

Holly TBI Link 2 barrel

Ryan

 

i have actually been reading a lot about the powerjection III system from professional products.

a little more expensive and more self contained

 

Do you have an link for it?

 

OK so when does the backfire occur. Just off idle? at cruise? WOT? Really, if it's just a carb issue there is NO reason to run off and get some EFI thing (unless that's what your end goal is!) And if it's not a carb issue it'll still backfire with EFI.

 

its one of 3 things

1. Carb tuning ishue

2. Spark Timing

3. herd about thing not shure if its true timing Chane is loose from ware i dont know if that can even happen

i put more faith in it being 1 or 2

but with anything it could be problem X yah problem X the one you find about after you have replaed all kinds of other stuff best thing is start simple and go from there (Our schools Autos teachers words)

Ryan

 

it is more than likely a carb issue
the backfire only ocurrs when i am hard on the throttle.
a light gentle press and it goes no problem

idle to WOT it sounds like popcorn popping
1/4 throttle to 3/4 throttle bang
1/4 to WOT bang

i am hoping something simple like pump cam or shooter.

3 local mechanics today told me that the timing is way too high.
i read here over and over again that 36 degrees by 2500 rpms is perfect.
today i am told different so which is it in reality.
one guy was totally convinced that if i backed the timing down to around 28 degrees by 2500 rpms the backfire would go away.


could it possible be the proform throttle body i am using. it used to have a holley throttle body that had about half of the choke tower cut off and it actually ran pretty decent.

my next goal is to put the holley one back on and see what happens.

i even wired the secondaries shut and still got some popping btw.

 

check manifiold bolts are still tight and carb stud bolts while your at it.

 

Does your distributor still have the points in it? If it does I would check the gap and dwell.

 

From that, here's my guess(es)/ thoughts.

1 The guy that told you your timing was off "IS" off. You want to end up with 36* TOTAL at 3000 RPM. Forget where the initial s for now. Get that 36*, see where your initial is and then figure out what your distributor is giving you. My MSD with light springs and the red stop bushing sets me right at 14* initial and 41* total- but remember, I'm running 12.5:1 compression and VP 110 fuel.

2. Check your plug wires- resistance, visually inspect for damage. They go all stoopid when a plug wire is open, has high resistance or for whatever reason is not getting full voltage to the plugs.

3. Make sure your accelerator pump shot is starting as SOON as the throttle starts to move- too late and it'll lean out, too soon and it'll run out. Go back to Holleys website and figure out what the original shooter is for that carb. Begin your tuning from there. Establish a firm known baseline and adjust as needed. You have verified all the passages in the carb body are clear?

4 Since it just all of a sudden started the backfire/popping thing, I'd really have to lean toward an ignition problem. If I burn a plug wire I get all kinds of weird things happening- First thing I notice is the idle gets a wacked out. are you still running a points iginition? A condenser on it's way out will cause it to pop too.

I'm running demon 850 DP- Lots of guys posted about quality problems from BG- I bought mine right in the middle of all that. Opened it up just to check and it was perfect.

 

timing check 36 degrees total. it hits 36 at 2500 and wont go any higher when further revved to 3000. like i said i read that part here so many times i knew the guy was messed up at that point.

new points, new condensor, new rotor, new cap, new wires with heat shielding on them.

so many times these past few days i thought i had it fixed.

carb was just rebuilt a few months ago. for clearance issues with the BB hood, i used a milled top carb. is it possible that is the problem.

check one
the pump cam screw had backed itself out so the pump cam was wiggling. tightened, still backfired.

check two timing
somehow the hold down bolt on the distributer had came loose (or i forgot to tighten it all the way) so the timing was way retarded. i thought this has to be it. nope

can a bad plug wire still idle smoothly but start missing at anything over idle.

it seems that 2500 is right about where the backfiring starts.

i have went through an entire can of carb cleaner trying to find a vacuum leak.

carb was just rebuilt a few months ago. for clearance issues with the BB hood, i used a milled top carb from proform. is it possible that is the problem.

 

Ok stupid question to consider here could the Cam Chane be loose enough to mess it up like that or is there no grounds on this theory

the carb thing Humm to it very well could be your problem (best thing ot do is to see if an junk yard, shop or Friend has an carb like yours to test this theory out.

Ryan

 

there is that possibilty of a timing chain issue.
other than pulling the timing cover off is there a way to check that.

 

OK, this is out of my area of expertise, but the situation looks dire enough that I'll post a few thoughts. First, what is a backfire? If I understand that correctly, it is the result of one or more exhaust valves being open when gas is still burning in that cylinder/portion of the head. Given that, it seems like this could not happen if there wasn't a timing issue of some kind, correct? Either the spark timing or the valve timing, or both, would have to be off to be burning gas with the exhaust valve open. Given that, the first thing I would check would be point dwell and gap (as suggested before). Too much dwell and you could be making backfires, inusfficient gap and you could be getting arcing that could also cause a backfire. Your ignition timing sounds fine, from memory mine ('70 LS5) is supposed to be 6 degrees with the vacuum plugged and 12 degrees with it unplugged, so you are right in the range.

I'm not smelling a carboretor problem here, but I may be proven wrong in the end. If you are sure the points are clean, new, and properly adjusted, can you give us more of the history of the engine? How many miles are on it? How long have you owned it? Has it had this problem all along, or did it come up suddenly? What was going on with the car when the problem developed? That kind of thing.

 

Does it start up OK? Does it turn over slow, like a timing issue, or does it start up with no problem? Was the carb the last item worked on prior to the stumble/backfire/popping/banging issue? I realize the base timing is not a concern once the full advance is dialed in; however, I would start at square one with the timing, and make sure the base timing is set correctly/can be set correctly, and then go from there. You could turn your crankshaft back and forth to see how much play you have in the timing chain. I don't think play in the chain will cause your problem, unless it has jumped a tooth. Then you would have other problems. See if your rotor is correctly positioned with the engine at TDC. Cheers

TommyJ

 

i have had the car for right at 1 year
odo has 75K miles on it
the engine is not the original.


starts perfectly.
without a choke
i press the pedal once and the engine is running and idling before one full revolution of crankshaft.

from what i have read backfires can also be caused by a sudden lean condition as well. this is the only thing that makes me think it is a carb issue. too much air going in. not enough fuel.


when i initially bought the proform center it did say that it allowed 25% more airflow so when i get time again i am going to install the plain jane holley that is sitting on my old mustang and see what i get. if it runs and no backfires it means that i need to figure out a way to clear the hood with a choke tower installed.


just to make sure i read it right though. at TDC rotor at #1 given. if the chain jumped or otherwise crapped out it will be different.

i also read that if there is too much slack in the chain the timing will walk at idle. using my light the timing mark is rock steady at all steady rpms while sitting in park.


the only work that was done to the carb from the time it ran great until now was a float bowl adjustement and get this. they were too low. with the car running on a level surface you couldnt see the fuel in the bowl. i adjusted it to just below the hole like in the holley manual and that is when it started.

i hate carbs. i am going FI as soon as i can pony up the dough.

 

Are you buying fuel at a different place than before ? My car was running fine and I got some fuel with a higher Ethanol % and it caused two backfires at the first traffic light I came to after re-fueling. The backfires blew the power valve and it was all down hill after that. Changed the Power Valve (same 6.5 as before) and richened up the idle mixture a tad and haven't had a problem since. Check your points too !!

 

Vacuum leak.

 

ignition wires or chafed and grounding.....water in the fuel or a contaminant....wiped cam....head gasket.....vacuum leak.....lean "pop" problem....base gasket....arcing or bad coil.....acceleraror pump.....

 

Not having a choke (or the tower the choke mounts in) will NOT be a problem. Even the Pro-Form claim of 25% more airflow won't be an issue. The Demon carb I'm running has none of that.

I'd still be looking at an ignition problem- mostly the secondary side. Get a spray bottle full of water and spray the plug wires while it's running. Soak them down good. I add a few drops of dish soap so the water dosen't evaporate so fast and sticks to the wires. With all the things that you've checked, I'm really starting to lean toward a plug wire problem. Even new wires can be bad- have high resistance or just leak. Take a good look at your distributor cap- even though it's new it could have a crack. Double check the firing order. Not that you haven't already done that, but it's time to go back and re-visit all the possible causes.

Timing chain is most likely not a problem- you state the timing marks are steady with a light and it starts easily. Neither one of those would indicate a timing chain problem.