Hi all,

i know it's not a mustang, but i was hoping someone might have a little advice
buddy with BBC changed cam to a compcams xe268. used new timing gear set, new lifters, used same rockers and pushrods. used correct run in with lots of lube and zddp

since then it has gobs of power but on very quick accelleration from idle (without gear) it backfires through the carb. basically fully flooring the pedal in neutral from idle.

i thought ignition, which is set to 12-14 initial (stock heads) and 34 max at 3000. no problems starting hot. so that seems ok

further he bent a pushrod with the rocker slapping against the valve cover. i was on holiday. so from what i hear he bent it back and opened up the valve cover (grind).

any idea what can cause such a backfire (if ignition related, should we advance or retard?) retard would make more sense to me (theoretically) ...

 

My first thought with this is a bad exaust lifter causing the exaust valve to not fully open. At lower RMPs the exaust is able to fully scavenge but at higher RPMs it isnt and the left over gasses are being forced into the intake when the intake valve opens. The bent pushrod is a big clue as they are supposed to bend and save the rest of the valve train from damage if there is something binding.

If it was me i would take a very close look at everything to do with the bent rod. Lifter, rocker arm, spring, valve, lifter bore, etc...

Hope this helps.

 

1. The balancer could have slipped and the timing is retarded just enough that if you whack it then it pops.

2. There could be a vacuum leak at the manifold to head surface, that will cause a lean backfire through the carb.

3. Rats have heavy valvetrain and issues with coil bind/broken springs. If its bending pushrods then its either being over revved or has a bind in the system.

4. Flat lobe on the cam. Even with proper break in and ZDDP you can flatten a lobe on any flat tappet cam.

5. Plug wires on 5 and 7 switched. GM numbers the cylinders differently and the firing order is 1-8-4-3-6-5-7-2 with number one being the passenger side (for you guys) opposite Ford. If you number it like a Ford, then its the early 289/302 order with number 5 being where you start. Make sense?

 

Thanks for all the ideas ... i'll recheck on the lot whenever I'm back at the car.
he always helped me with my Mustang (I use his pit and advise), so I owe him big ;-)

he mentioned as well now that the bend one was #1 (chevy numbering), caused by valvecover (rocker hittting), further he mentioned that at break in the #6 and #8 (the ones next to each other) had a red/orange header, which resolved itself somehow. not happening anymore, i didn't get to see that either

I was thinking the backfire can be caused as well by shot accellerator pump or similar (lean)

I don't believe in gone lobes and firing order yet, since the engine seems to run perfectly well at other occasions. but maybe a crossfire. I'll check on it anyway .. who knows

 

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