1990 GMC C1500 Firing order

See pic rotor should point about 11:30 when installed with #1 on tdc.

Then you probably have your wiring screwed up going to cylinders. This is the way chevy's have fired for 30 yrs. Are you sure you had #1 on tdc? Probably not.

Bring engine up to #1, remove spark plug and put finger in hole and either bump starter until you feel compression there should be a mark on harmonic balancer where it will line up with timing mark on timing chain cover. I do not have any pictures of this. IF you are unable to do that I highly suggest having a friend or someone who is familiar with cars to come over and help or do it for you. It's pretty simple to do. If you look closely at pic I sent you will see timing mark on front of engine diagram. However it will be small so try clicking on pic to enlarge it. The mark on balancer will go all the way across balancer. Once you get to start then use a timing light to time it. I'm sure you can't use the stock specks due to spark knock so it will be hunt and peck so to speak to get the right timing done.