huge backfire through carb

[baxsom]

so car has started running like crap lately.

it is a 72 454 that now has a holley carb and headers.
ran great for a few months. now it is backfiring like crazy.
it also surges at idle in gear. in park it idles smooth but if i try to rev it quickly i get this monster very impressive backfire through the carb.

so far i have put in new points, cap, rotor, wires, plugs.
dwell is 30 degrees timing is 10 degrees in park with the distributer vacuum advance unhooked. i cant find a vacuum leak anywhere.

what should i check next. because i am about to douse in gas and wait for the next big backfire.

 

[navy2kcoupe]

Possibly a broken intake valve spring. I had a '69 SS-396 Chevelle that did the same thing and it turned out to be a bad spring. I've heard that it's not that uncommon for big blocks to break valve springs. Pull the valve covers and check them out. Relatively easy and inexpensive to do.
Andy

 

[vett boy]

I also had what was a simular issue and mine was timing once and also I had a broken valve spring .

If you've been usin' any of Junkies mountain home made jet fuel that could be another issue.

 

[baxsom]

valve spring, would that make an audible noise? at this point thats all it can be. i just put the carb off of my 68 mustang on it and it started and idled fine but as soon as i gave it some gas BOOM, flame.

i;ll pull the covers tomorrow. just getting too late tonight to mess with it.

 

[baxsom]

ok i cant leave well enough alone so i went ahead and pulled the covers tonight.

there doesnt appear to be anything broken but the #6 intake spring is 5mm shorter than every other intake spring.

every exhaust spring is within 1mm of each other as well as the rest of the intake springs,

would that 5mm on that intake valve be enough to cause an issue with backfiring.

 

[TimAT]

Look at the pushrods too- one bent even a little could cause the valve to not seat/open correctly. And look for any shiny metal around the springs on the heads.

Double check your firing order. Put the old dizzy cap back on, it's also not uncommon to get a bad cap. Same with the condenser.

If I get a plug wire too close to the headers mine runs like crap too. Burns the plug boot and misses and surges. Put a couple drops of dish soap on a spray bottle full of water and spray it from the cap to the plugs. See if you get any change. Check the wire from the coil to the points- any breaks? Make sure the breaker plate is secure in the dizzy.

If your Holley carb is older than "91(?) the power valve is done too. One good snort from a BB and it's toast unless you have a carb with the PV relief built in.

 

[baxsom]

pushrods look ok. i cant see anything that seems out of place other than that shorter spring.

plug wires are not touching the headers and the ones that are close are wrapped with heat wrap. i pulled them all off and didnt see any burn marks.

firing order is correct. cap and condernsor change no luck.

i really dont want to change out a spring. i hate that spring compressor tool. at least its just one.

 

[tenaus]

popping

I had the same problem in my 82 crossfire, suddenly started popping out the drivers side carb when stepping on the gas. Idled fine. Tried evrything but couldn't find it. Took it to a engine rebuild shop and found out the number 8 lobe on the cam was bad. Had a new one put in and it runs great.

 

[baxsom]

that is probably the issue.

i was tired last night when i measured my spring height sitting there that i forgot that the stupid rocker arm will push down on the spring if the valve is opening.

i am a dork. i pulled the plugs out and they were very very black and carbon fouled. they have maybe 3 hours of runtime on them. that tells me too rich. but since its backfiring through the carb, that tells me too lean.

but it is popping out of the passenger side pipe too. damn i dont want to deal with a cam swap.

 

[topless82vette]

Back-fire injection can do all this without even trying

 

[6880 Mike]

...its backfiring through the carb...

Keep it simple. A back fire through the carburetor has to be timing related, whatever the actual cause, since you have an intake valve open when it ought to be closed.

Good luck with the hunt.

 

[baxsom]

well i think i found it, simple but a pain.

i pulled the plug wires off that i just put on a few weeks ago and i guess that they were burned afterall. they had gotten so hot that they were curling up on the boot end. they looked fine when installed but it wasnt until they were laying on the floor that you could see the heat. makes sense. it was running fine until i put the headers on.
new wires and plugs and it fired right up, idled smooth and no backfiring.

i slid that DEI heat reflective tubing over the wire boots hoping it will save them from doing it again.

 

[pttsls5]

If you are using the large tube hooker/ hedman headers use the NGK YR-5 spark plug with the original style braided plug wires. You will never cook another wire again.

 

[baxsom]

If you are using the large tube hooker/ hedman headers use the NGK YR-5 spark plug with the original style braided plug wires. You will never cook another wire again.

is that 454 only for the ngk part # or is that a style. the car has 427 heads on it which require the r44xls style plug.

stock braided wire? i am guessing that is going to come from a corvette parts vendor.

 

[pttsls5]

is that 454 only for the ngk part # or is that a style. the car has 427 heads on it which require the r44xls style plug.

stock braided wire? i am guessing that is going to come from a corvette parts vendor.

Not exactly sure about the 427 heads part but you can get original style dated braided plug wires at Paragon for $130.00. The thing about the original plug wires is they have the exact angle at the plug so there is a better gap between the headers and the spark plug wire end. Take your R44 XLS and compare it to the NGK plug if the threaded part is similar you should be O.K. it's the ceramic resistor part that should be shorter and that will give you more clearance from the header.

 

[baxsom]

i know that this has been a while but i finally got it fixed.
so after installed a new HEI distributer and wires with new plugs and heat shielding nothing. still popped.
i thought that was the problem because it didnt backfire the first few times i ran it after install but it came back. so i went to the next step

new speed demon 750 carb.
out of the box it cranked right up, idled smoothly. on the first test drive after making sure the car was fully warmed up i punched the throttle and all it did was break the rear end loose.
no popping, no backfiring, no nothing but acelleration.

i dont know what was wrong with the holley that was on it but problem solved and i got the HEI now to boot. no sense in taking it off.