Would a bad valve casue back-fire? I got my carb rebuild this weekend and the gentleman there said that the back-fire was being cause by a bad valve. Would that be true? What i've recently done to the engine is: Rebuild heads w/ new pushrods/lifters New Stage 1 INtake Camshaft Distribu.. Help me out guys thanks Marvin :TU:
That's a lean misfire. Most likely cause, vacuum leak. Check all vacuum hoses and carb to intake manifold gasket.
When you say lean mixture, are talking about the mixture screws on carb, I've been adjusting them and still having same results. I noticed my a little tapping on right head, what could be problem. found no vacum leaks idles fine , doesnt backfire until you get on it
No, lean fuel mixtures result from air bypassing the carburetor through a vacuum leak. That air leans out the mixture, causing hesitation and misfire. It can also occur if the power system of the carburetor is not working correctly(rebuild time)
OK, I would double check the firing order, make sure the wires are on the right plugs. Then, I would pull the valve cover where you notice the tapping. You could have a bent pushrod or broken rocker arm. If the valve is not opening and closing correctly, the engine will misfire.
I've done that also. I noticed that one of the valves moves a little faster than the others, and none of the rockers were cracked or was any rods bent.
Do a little more investigating. If one of the valves move a "little faster", it's probably because there is clearance there. If it isn't a lifter, pushrod, or rocker, it could be a cam lobe going flat.
What size discharge nozel are you using on this Holley? Too small a size or a misaligned accelerator pump on a Holley will you a lean condition when you get on it. Start simple. Check all alignments on your carb to include float level, accelerator pump, secondaries and so on. If you can get a hold of a Q-jet for testing purposes put that on there and see if the backfire goes away. Q-jets operate differently than Holley's and don't have all of the variables listed above. Aubrey
Backfiring can also be caused by incorrect timing of spark, possibly the relationship of the cam/crank has changed. If the fuel pump is starting to go bad I think it could possibly backfire.
I agree, timing could cuase this problem. I had the HEI power wire on the dist. disconected one time after a motor swap and that caused a backfire too.
thats just it, I had the quadrajet on there when it started. It first started when I adjusted the mixture screws on front of the carb. trying to lean it up little. Its been backfiring every since
The two guys with the problem, what intake do you have. One of you mentioned a Stage 1 intake. I will assume it is a TA 350 Stage 1. If so, you can NOT just put a square type carburetor on these. They have both bolt patterns, but the secondaries are machined for a Q-jet. The square type gasket for Holleys and Edelbrocks does not seal properly on the sides of the secondaries. You MUST use the metal plate that looks like and is the shape of a square gasket in order to support the gasket. For best results put a modified (cut the center out) Q-Jet gasket down first, then the metal plate, follow with the square gasket and carburetor. 90 % of this discrepancy is caused by that issue. It won't always show as a vacuum leak with a guage at idle, but when you give it gas, such as when accelerating at a traffic light, the gasket flutters, causes a vacuum leak and it backfires up through the carburetor and stalls. Dave
I think what he means is that the timing chain between the cam and crank could have skipped a tooth. The chain would need to be loose and worn out before it would jump though. To check for a loose chain: 1. take off the dist cap 2. turn the motor over with a socket on the harmonic ballancer bolt 3. If you watch the rotor while you turn the motor over you will see it spin around 4. Turn the motor the other way and see how much slack is in the chain by the delay in the rotor turning the other way If you can turn the motor over very much without the rotor moving then the timing will never be very acurate. I still think you should try changing the timing a bit. Did you set it using a timing light? if so what is the initial timing with the vac. advance disconected?
I've never had idle mixture screws cause a backfire no matter how they were adjusted. Do you have a vacuum gauge? if not beg borrow steal whatever... you need one to set up your mixture screws. 2.5 screws out is a good starting point from my experience. I set mine to get max vacuum (off a vacuum port on the carb) when at idle in gear. Block the wheels WELL while you do this. Oh, and set you timing first becuase it has a greater effect on the way it runs than the idle mix screws. Also, once the carb leaves the idle circuit then the idle mixture screws won't make a difference in how it runs. They do however effect the transition between idle and part throttle. Another thing to remember, if you have bad plugs, wires, cap, rotor ect then the motor will run rich even though the carb is fine. This probly isn't your problem but check anyway.
I still think you should try changing the timing a bit. Did you set it using a timing light? if so what is the initial timing with the vac. advance disconected?[/QUOTE] Are you talking about changing timing inside dist. with adjusting points or removing bolt and turning the dist. Thats all I have adjusted is the points. My intial timing was 0 when I first checked it. I was told to change it to about 8 degrees advanced, made no difference. how would you know if the vac. advance had failed or would it even matter. It's idling fine, does'nt seem like much is wrong but its got me confused o No:
