!978 Chevy pickup 454, The timing gear was plastic and sheared 3/4 of the gears.

I replaced chain and both gears with double roller chain and gears. Engine wouldn't start, backfired through carb about every third revolution. In the process of trouble shooting found no spark at times. Changed wires and plugs. Rebuilt the distributor (HEI). Now have a yellow spark that you can hear snap when it hits.

I have switched the wires 180 degrees.

I thought the timing chain was the problem but am now wondering if something else might have caused it to malfunction.

This is my son's truck and he told me the Edlebrock (750, I think) was used when he installed it. The truck used to backfire a little when cold. The day it quit running it just died at a stop light and that was it.

Now when we try to start it there is no indidcation that it is even trying to start. It is just a steady crank and when he pushes the throttle is when it starts to backfire.

If anyone has any idea where I should go from here Please help.

By the way, we are working under a shade tree in a parking lot where he works!

The yellow spark would indicate that the spark is weak.Perform some tests on the distributor,starting with the coil.You can perform a search on the boards and in the knowledge base for more info on troubleshooting distributors.

It sounds like the engine jumped time, it is possible that one of the valves hit a piston.Perform a compression check on all cylinders to eliminate this possibility.

maybe distributor wires coming from starter area are grounded on the headers??? happened to me when i did a cam swap in my camino.... just a thought cuz mine did the exact same thing... Fell your power wires going to your dist. , mine would get hot real quick. pulled the wires off the headers and hey hey it ran great.

You've got a timing problem, if the gears were lined up correctly, the dist. would be the problem, get #1 on compression stroke TDC, rotor should be @#1 wire in cap! Firing order 18436572 HTH

Not neing familiar with the 454, I do not know if the vlaves would hit the pistons or not , if you lose the timing chain/ sprocket. If they can hit you may want to check compression as mentioned by drlindz. Otherwise it sounds as if there is a possibility that the new timing set is off a tooth or so. The distributor should not be off, unless it was removed.
Also you should get a blue spark, as yellow would be weak as mentioned, again, by drlindz. This could be simply because the battery is getting weak from cranking the engine over and over. My thoughts are: if it ran ok up to the point when the timing set failed, it should run after it was replaced. Unless it did bend a valve or valves, recheck the cam timing.
Good luck,
Skidmark

I'm assuming the dist was removed for the rebuild, and the wires were moved 180, start from scratch!

Don't know how much help this will be but....the timing marks line up "dot to dot' at the number six position. If you can check this it will be a start...the dot on the big cam gear is on top at number one fireing position. I usually bump the starter through with my finger on number one hole...spark plug removed. When I feel compression trying to blow my finger away.... I look to see where the dist rotor is pointed...I point it too number one wire. Also the timing mark on the engine cover will be near maybe not right on mark but usually on the tab as a begining spot. As far as the valves go just snug them down too start not tight..and as soon as it runs you can adjust them running. There is a complex way to set them not running but its a lot of work to turn it through for each spot for the adjustment.
It is highly likely that if the engine failed while running a valve or two can be damaged..thus the compression check is a way too find this. This is an old Chevy problem from the fiber wheel days. Backfireing while cold is not unusual.