Backfiring through Carb. HELP!!!!!!!!!!!!!!!


New Member
Apr 1, 2005
89mustang with 306 E7 rebuilted heads. split cam 471. lift flat tappet, coustom built 650 QuickFuel carb. had a bad lifter and replaced run no miles with bad lifter hear it in shop on start up. But now i have a backfire though carb when i try to accerate for idle sitting in driveway and wrost on the road. If advance timing go way a little but still there but car backfires on start up then. DO I NEED TO JERK OUT ENGINE AND PUT A NEW ONE IN.
  • Sponsors(?)


Founding Member
Aug 25, 1999
Le Mars , Ia
Try advancing the timing and see if it stops. A manifold/vacuum leak can also lean it out enough to backfire. Double check that there are no gasket leaks and the vacuum lines are going where they are supposed to go and not crossed up.


StangNet's favorite TOOL
SN Certified Technician
Mar 10, 2000
Dublin GA
Backfiring out the intake is either a valve stuck open or a lean mixture or spark plug wire(s) connected to the wrong cylinder(s). Check compression on all cylinders and then look for vacuum hoses loose, cracked, or misconnected.

Sticking valves: If a intake valve is bent, has a bad spring or is misadjusted, the engine will sometimes backfire through the intake. Use a vacuum gauge connected to any convenient spot on the intake manifold. Run the engine at 1000 RPM & look for 18-21 inches of vacuum with a steady needle. A problem intake valve will make the vacuum gauge needle sweep 5-10 inches.

Finding vacuum leaks

Small vacuum leaks may not show much change using a vacuum gauge. The range of "good readings" varies so much from engine to engine that it may be difficult to detect small leaks. The engine in my first Mustang pulled about 16.5" of vacuum at 650-7250 RPM, which I consider rather low. It was a mass market remanufactured rebuild, so no telling what kind of camshaft it had. Average readings seem to run 16"-18" inches at idle and 18"-21" at 1000 RPM. The only sure comparison is a reading taken when your car was performing at its best through all the RPM ranges and what it is doing now. Use one of the spare ports on the vacuum tree that is mounted on the firewall near the windshield wiper motor.

Use a squirt can of motor oil to squirt around the mating surfaces of the manifold & TB. The oil will be sucked into the leaking area and the engine will change speed. Avoid using flammable substitutes for the oil such as propane or throttle body cleaner. Fire is an excellent hair removal agent, and no eyebrows is not cool...

The vacuum line plumbing is old and brittle on many of these cars, so replacing the lines with new hose is a good plan. The common 1/8” and ¼” vacuum hose works well and isn’t expensive.

The PCV grommet and the power brake booster check valve grommet are two places that often get overlooked when checking for vacuum leaks. The rubber grommets get hard and lose their ability to seal properly. The PVC grommet is difficult to see if it is correctly seated and fitting snugly.

Diagram courtesy of Tmoss & Stang&2birds

Vacuum leak due to slipped lower intake manifold gasket...

Ask Nicoleb3x3 about the intake gasket that slipped out of place and caused idle and vacuum leak problems that could not be seen or found by external examination. Spay everything with anything you have, and you won't find the leak...



  • mustangFoxFordVacuumDiagram.jpg
    58.3 KB · Views: 146