Intermittent Backfire

[bordin34]

My 73 400 is backfiring rhythmically at idle and wants to die when put it gear. This happened before then went away now it is back. It sometimes backfires throughout the rpm range and when I go WOT it bucks. One time it backfired and it sounded like a bunch of bees were under my hood.

[Big Lebowski]

  Timing? Vacumn leak, bad exh. valve, streched timing chain, maybe a carb prob.?

[bordin34]

I did some tests today and the car backfires much more under load than no load. I put it in neutral and revved it to about 2000rpm and it may not have even backfired, then I put it in Drive held the brake and brought it to about 2000rpm and it sounded like a machine gun.

[Big Lebowski]

Spark plugs arching? The right firing order? Try a different ECU just for fun?

[tkkruzer]

had this happen to me just recently, while at idle pull the spark plug wires off one at a time and locate which cylinder is miss firing. then do a compression check to see if that cyl. has low compression. check the plug for fowling. you may have a burnt valve, or a bad wire. either way you are going to have to isolate which cyl. it happen to affect.  hope this helps,,,Tom

[bordin34]

All of the plugs were richness fowled. I cleaned them the best I could with a wire brush and brake cleaner them re-installed them. It still ran rough but not as rough. Then I went to adjust the float level, which looks very low, and the gasket between the hex nut and bowl tore and squirted fuel out. So I couldn't adjust the float level.

[bordin34]

I ran a can of seafoam through the engine in 3 doses and it helps make it idle smoother, but it still backfires just as much. I also pulled every plug wire and it still backfired, so either it had nothing to do with a specific cylinder or I have more than one that is backfiring. Would some bent valves cause this?

[FLG]

Have you checked the carburetor gasket for any leaks? Could also be a bad carb, whens the last time its been rebuilt? If you need a guy, i know someone in LI...been doing work for my family for 20+ years;  the man is a master at his craft.

Hows your timing btw?

[bordin34]

Timing was good before could have randomly gotten messed up. I have a timing light and might check this weekend. The carb is a relatively new 670cfm Street Avenger I am guessing less than 5 years. 

[FLG]

How about your fuel pump? or a clogged fuel filter??

[bordin34]

Brand New Fuel filter and I have a new fuel pump that I could install but I dont THINK that is the cause, could be. I guess I might have another project this weekend.

[bordin34]

I also noticed while pouring the seafoam in the pcv valve line thing how much my headers leak at the collectors, its crazy almost as much smoke came out the collector as the tips.

[FLG]

Also, check your distributor cap and rotor. Even if the cap "looks" good, if its old just replace it. Sometimes the caps will get hairline cracks, moisture sits in them, and then the spark jumps around.

[bordin34]

Brand New, less than 25 miles, cap,rotor,wires, and less than 100 on the plugs but they fouled I cleaned them and reinstalled.

[tkkruzer]

well that was what I found in my car,, a bent valve,, just slightly bent enough to give me about 85 psi when I did a compression test. do a compression test, and pay close attention to what the first stroke reads. that is to say crank the engine over about three times, with all plugs out and the tester in one cylinder,,,ex. first crank might be about 85 2nd will come up to 110, 3rd will reach 125, the last 2 readings may be residual but that first one will be closer to the correct reading,,,,do all cylinders and see what the differences are...let me know if any of this helps,,,, Tom

[bordin34]

Vacuum Readings it's a video. When I revved it quickly the needle went left, when I revved it slowly the needle went right. Around 40 is idle.

[Animal]

May be a bad power valve.

[andy74]

Vacuum Readings it's a video. When I revved it quickly the needle went left, when I revved it slowly the needle went right. Around 40 is idle.

vaccum should go down when you give it gas,thats normal-good luck

[tkkruzer]

as you sit at an idle does the needle vibrate much? if so you could have a vacuum leak. on my car , the vacuum gauge would read around 18 to 20 in" of vacuum, but the needle would vibrate back and forth . this could be a bent valve . I hate to say it but if all else fails eventually you will have to pull the heads and have them checked, so before that happens ,get as many 2nd opinions as you can and try them all , alot of mechanic work is ust trouble shooting until you find the problem let us know what goes on,,,Tom