Backfire at high rpm

[bandit]

Got my car tuned up to where I want it and it runs great now except when I was out today and stomped on it and it hit higher rpms(5500+) it backfired twice. Not the loud shotgun backfire but loud enough. I did just switch to a mechanical fuel pump so thinking it may be running out of fuel at high rpm but not sure. I'm just running glasspacks off of the headers for exhaust but I never had the backfire before. Been doing a lot of tinkering with timing and carb settings so it could be anything. Any ideas what causes backfire only at high rpms? Runs great otherwise.

 

[kiwigtx]

I had that once a few years ago...turned out to be one bad spark plug. Changed one plug and problem went away - **the fuel was building up, then occasionally firing, causing the backfire. Not noticeable at idle or low speeds.

 

[coloradodave]

Make a couple of acceleration runs on a deserted section of road. At the end of the last run, slow down and shut the motor off without any idle time. Pull a couple of the easiest plugs to get a look at the color. It's much more difficult today because of the poor fuel quality, but you can still tell if a plug is lean by the interior porcelain being white. OR, shift at 5,300 rpm...lol

 

[snakeoil24]

Are you running lean? It can cause a back fire or "Pop"

 

[62 Dart Convertible]

Most common cause would be lean mixture. Had it on a smallblock years ago - richened primary and secondary jetting and hello, more power and no high speed back firing.

Could also be caused by weak/incorrect valvesprings, or even ignition crossfire (bad cap/leads). Is you ignition 100%? I'd start there first.

 

[bandit]

Timing by feel. My timing marks are off. But I have it all dialed in to where it drives really well except for this high rpm issue.

lol, yeah I need to shift sooner. I start peeling out and get carried away. Just an issue that has popped up where it wasn't doing it before so want to correct it.

I was thinking about upping the jets too but just wasn't sure if I am dealing with timing or fuel issue when it is isolated to higher rpms. Guess it can't hurt to try bigger ones.

 

[snakeoil24]

Change your4 secondary jets first. Go up 10 percent in size. But eliminate what 62 dart said first. check ignition for cross fire in cap, then spray spark plug wires with a water mist at night and look for arcing. Also check compression each cylinder shopuld be within 10 percent of each other. Once you eliminate the easy also try misting the coil and see if it shorts.