i have a 66 289 and underload it back fires thought carb. i have new lifters cam and pushrods timechain and plugs.full msd wires digital 6 box and distributor.ui am aslo running 650 doulbe pumper,with funnel web intake.the only thing i have not look at is heads.what does a motor do when you have bent valve.

 

did it start doing it from the begining or did it come into in slowly?

good chance the cam got wiped out during brake in... take the valve covers off and run it, make sure all the rockers are moving the same.

you can take one plug wire off at a time at the cap and drive it ... find the one that has the backfire..

also try the 351W firing order. might have a 351W cam in it.

 

does sound like a wiped cam lope. could be something less serious though. backfiring threw the carb means its running lean.

 

Bent valves means they typically hit the piston and cause major damage....probably not your situation. Typically, backfire up through the carb is an ignition timing issue, but, assuming that you have a cam with a max rpm of 6,000, the 650DP is a bit big and assuming you have vaccume secondaries, its probably way too much fuel. There are a couple of ways to address this, most through trial and error. To start,


A typical 289, 6,000 rpm calculates out at around 500 cfm, meaning the fuel jets are big, you typically want to reduce the jets about 50% of what the cfm differential is, meaning, about 12%- start with the secondaries, try that and then the primaries if needed, but assuming your idle mixture screws are set at abut 1.3-1.5 turns out, the primaries should be fine as is.

Next, if needed, delay the opening of the secondaries. If you are running an automatic, it is preferred to have vaccume secondaries to mechanical (and the opposite is true is you have a stick).

If it is still occuring, then i would really start checking the ignition timing, advance, etc.

 

i did up the motor and run it for a little while then parked it.when i dug it out of storage i put on the new carb and msd.it started doing the backfire under load,so i changed cam lifters push rods and plugs,it runs smooth it only does it when you punch it does it a couple of times then clears up.

 

let us know what you find out. im having the same issue. would like to stretch this engine a few more miles while im getting the new one ready to put in.

 

First off there could be a broad range of things causing your problem and with the minimal amount of info we have, it is a crap shoot on what the problem could be. Before you jump the gun first check items that delt with the carb and msd installation (assuming you drove the vehicle somewhat before you "did it up" and assuming you didn't have the problem before you parked it). Although the carb may in fact be too big and could in itself be the problem. I am guessing the DP carb may have mechanical secondaries and not vacuume.

Have you checked/adjusted the fuel bowl levels? checked the idle/mix screws? confirm no vacuume leaks anywere? Checked fuel pressure? (if running a aftermarket pump or an old pump, high pressure or not enough), vapor lock? how is your fuel line run? confirm that the plugs/wires/cap/timing and MSD wiring are all ok.

Obviously by your description the cam/lifters/pushrods and plug change did not change the issue. By your description you installed these just to hopefully cure the problem? Concentrate on the things you did do before the problems started.
Perform/check all primary adjustments and settings. you maybe suprised at how after double checking everything and small adjustments or settings can make a difference.
good luck
Dave

 

what kind of vacuum do you have at the distr. is it ported or manifold...?