1990 Chevy Cheyenne running badly

You need to have the codes puled to see what is wrong making the check engine light on. I would suspect a vacuum leak. With the light on your engine is going into what is called open loop. Wehn it does that it richens the fuel mixture up. Tha is why it runs better- but also gets relly poor fuel mileage. Have apro look at it he can get teh codes and tell you how much it will be to fix it.

If you are getting a code 15, then check your sensor wire for corrosion. Some of hteearly ones leaked and sent antifreeze up the wire corroding them and givne a bad reading and screwing up the computer. One othr thing I forgot is if this is a TBI unit they blow out the gasket beneath the tbi and cause a large vacuum leak which will cause a miss. This is an easy replacement as all you need do is remove an dreplace th gasket. Try to get eht thick gasket for the bottom. Also if it is also a TBI then the hose leading to the map sensor collapses after it ages. It gets soft and sucks together. The map sensor is either near the coil or on the firewall. That also can give you trouble and are easy to fix/repair.

Yes there is a set timing if I rmeember correctly there is a brown wire to disconnect on the firewall and then set the timeing8degrees btdc I think. Then reconnect it. But the gasket under the tbi will give you a hcek of a leak, with the engine running, spray choke cleaner around the botom of the tbi unit. If it smooths out then that is your roblem. It only misses when the light is off vecause it istrying to add more fuel to stop vacuum leaks. When it is on it's in open loop and automatically richens the mixture.