hey guys,

so i swapped my 305 for a 350 in my 85 caballero and everything seemingly went smoothly. everythings all hooked up sans the kickdown cable. i have a slight problem though. every minute or so itll back fire a few times.

now i timed the engine to 12 degrees BTC and adjusted the idle and air/fuel mixture to tolerable levels. i use 89 octane fuel and it seems to really backfire when i accelerate. when the cars in motion it jerks, but it also happens in park. its nothing super serious, flames dont come jetting out of the tailpipes or anything but its a very obvious albeit low toned backfire.

any ideas on how to cure this ailness? school just started and having a car is a nice benefactor. any help in advance is greatly appreciated.

For anyone to help you, more information is needed. Your truck came from the factory with a feed-back carb controlled my the ECM (computer). Did you are someone else remove all the emissions equipment. What carb are you running? What distributor is installed. Does it have a vacuum advance or like the original advanced by the ECM?

If it is the original the timing is set a 0 degrees with the 4-wire connector located by the back of the right side valve cover disconnected, and about 650 RPMs. After the timing is set, reconnect the 4-wire connector and disconnect the battery ground cable for a few minutes to clear the ECM codes that will be set by having the 4-wire connector disconnected.

Doug

i removed all the emission controls, put headers on it, no cats a 1406 model edelbrock carb with 600 cfm and electric choke. the distributer is an aftermarket HEI with vacuum advance

without the advance the timing is 12 BTC

Well I'm at a loss offhand for a cure for the backfiring. One thing I can think of is the routing of the spark plug wires. They need to be routed according to the manual or you can get cross firing thru the wires.

I'm not going to attempt to describe the routing, you need to check this out yourself, hopefully you have a manual of some sort.

Doug

they are all in the correct order, but are you saying they cant cross each other?

Okay. I will describe it. On the left bank, wire #3 crosses over #1 about intake manifold level. On the right bank, wire #8 crosses over #6 and #4 at about valve cover level.

Doug

#5 and #7 should cross somewhere down toward the plug end. they are the ones next to each other in the firing order (1 8 4 3 6 5 7 2). 7 is charged with fuel and 45 degrees away from firing when 5 fires. This will be a back fire though the carb as the intake valve is just closing. When 7 fires it can induce a spark in 5 during the exhaust stroke and if you are running a little rich you get a back fire in the exhaust. If the wires are parallel for a little ways they can induce a voltage in the other wire. This is the same principle that the ignition coil works on. Primary windings induce voltage in the secondary windings.

I think I can follow what I just wrote but I'm kind of shaky on it so jump in there and straighten me out if I got it wrong.

Well Neil, we seem to agree that the spark plug wire routing could cause his backfiring problem. You gave a good explanation of why.

Doug

im just a tad confused here, this question should clarify it all up though

are you saying the wires NEED to cross close to each other, or are you just telling me they might be right next to each other and they need to be seperated?

it seems to happen only under heavy acceleration so i can understand the concept just confused on if i should separate those certain wires or purposely look to separate them

Cross the ones we are writing about, When they run alongside each other the wires we are referring to can cause the cross firing.

Doug

I'm not sold on this symptom. Is there a popping sound that sounds like it is comming out of the carb or out the other end at the exhaust? Could this simply be a missfire that you are trying to describe?

its like a popping sound but you can hear it outside coming from the tailpipe and when it happens the car jerks like it looses power

I am still inclined to think this is a missfire. Try this, pull one plug wire off and start the engine. Now it will have an even missfire on the cylinder that you removed the wire from. Listen to your exhaust. Is this the same sound you are hearing only steady rather than random?

If both valves are closed when the plug fires then you have a solid combustion. If one valve is onen when you have combustion the combustion escapes the chamber to the exhaust or carb. Causing a backfire.
He stated he replaced a 305 with a 350 but did not say it was rebuilt. I lean toward a valve sticking open for one reason or another.

Yes canted is correct you can narrow it down to which cylinder if is(if one) by pulling the plug wires one at a time. Finding the one causeing the problem is when the problem stops.
The problem now a days is with HEI distributers you are not supposed to remove the #1 wire without damaging the distributer. Can anyone explain that to me?

the engine is a crate motor, brand new and not rebuilt

Flat tappet or roller camshaft? A popping noise under acceleration is the sign of a cam lobe that's gone bad. With todays oil with little or no zinc this is a common problem with flat tappet cams.

flat tappet cam, is there any verification to this beyond taking out the cam and looking at it?

You would need to remove the intake manifold and remove a lifter.

You can remove the valve covers and turn the engine over slowly. Watch each valve, they should all open about the same amount. If you have one that does not open at all or opens only about half of what the others do, it's bad news.

being a new crate motor i would think the cam would not be worn down but more likely a rocker adjustig nut backed off or a lifter stuck either can cause problem with valves opening correctly as canted said ck how they open all should move the same amount