i just rebuilt my 1986 350 and when i started it up it ran great, idle was smooth, and sounded great. after running for for about 3mins the motor started running rough and backfiring. any ideas??
i check the spark plugs wires they are not crossed.

the motor does have a roller cam and roller rockers, the motor was blue printed and balanced and ran on a dyno great before install.

this motor does have accell 32lbs injectors and a high flow after market fuel pump, could it be that??

did you check the timing

most likely the timeing

If it fired & ran fine it's probably not your timing. Pull your valve covers off and check out the travel on each of your rocker arms. A cam lobe could have gone flat. Look for a rocker arm not moving too much. Also perform a compression test. If you have a weak cylinder it will show up there...

I bet it's the cam. A racer friend of mine was telling me about oil specifications that changed recently. The current oil does not have lubricating ability the old oil had. He says that the oil used for run-in is critical. He had an engine flatten a cam in 8 min. I hope it's something more simple in your case. I would like to know what you find out.

Since it is 86 it could be either a roller cam or flat hydraulic. If it is a flat hydraulic it would be a real good place to look.
The only way to be certain is to pull the valve covers and check each lobe lift by putting a dial indicator parallel to each push rod at the top of the rockers. If any of the values falls short of the cam lift spec. then you will need a new cam. Remember that there will probably be 2 lift height measurements, 1 for intake and the other for exhaust...Not all cams have 2 different heights but many TPI grinds do.
You need to turn it by hand to get the highest reading for accuracy.

It could be running too rich, check the fuel mixture.

You may have wiped out your new camshaft. If it is a flat tappet cam, it's imperative you coat the cam's lobes with molydenum disulfide grease and also run the G.M. oil supplement in your oil (full of zinc) to help prevent camshaft lobe failure. Pull the valve covers and measure the amount of lift at each rocker arm. If one (or more) is less than the others, that's what happened. You flattened out a cam lobe that operates an exhaust valve.

If its a flat tappet cam it MUST be broken in properly so you wear a lobe off.
Yes i did it on my first cam install, the hard way. For me it was more a clicking noise than anything else.

If you DID wipe your camshaft (and lifters) out, you might consider going to a mild roller cam. The roller cams aren't all that costly, but the roller lifters are; about $300. And you'll need a set of shorter hardened pushrods as well. But with a roller cam, there is NO need for any kind of break-in procedure.

Knowing that roller cams were the modern way to go, I put an Erson 214/214 .480 lift roller cam in my '82 back in 1995. Combined with my Competition Cams 1.6 roller tip rocker arms, I've got a healthy .513" lift at the valves. With the cam's short duration and high lift, I've got stump pulling low end power.

i just rebuilt my 1986 350 and when i started it up it ran great, idle was smooth, and sounded great. after running for for about 3mins the motor started running rough and backfiring. any ideas??
i check the spark plugs wires they are not crossed.

the motor does have a roller cam and roller rockers, the motor was blue printed and balanced and ran on a dyno great before install.

this motor does have accell 32lbs injectors and a high flow after market fuel pump, could it be that??

How long did the motor run for during the dyno pulls, more than 3 minutes? Did you reuse all of the 23yr old emissions equipment, such as hoses, egr etc after reinstalling the motor? Did you rebuild or replace the heads? Did you replace or reuse the distributor? Once the motor has cooled down and you restart it, does it immediately start backfiring or does it run smooth until it kicks into low idle and then start doing this again? It is possible, as many have suggested, that you toasted the cam, however, more information is needed on exactly what you did or didn't do as to replacement and what is still original vs now rebuilt. I've seen dry rotted hoses, bad egr's, broken or crushed wires/connectors during engine removal/installation, and many other things cause headaches after replacement. Not to forget, did you make sure everything was hooked back up?