Ok guys this is my first post and looking for some ideas to help me solve this problem I have an 85 F150 with a junkyard 300. It has the duraspark ignition, no computer control. I put the engine in about a year ago after the old one died. I did not have this problem with the old motor The engine sarts and runs fine. When I am in 3 or 4 and step on the gas. It stumbles almost like fuel starvation. It only dose it when warmed up. It will do it on regular or premium gas. I need to set the timming with more advanced to minimize the problem. I just did a compresion check 170,170,130,145,140,170. Not great but within the 25% rule. Plugs looked fine except #3 had a little bit of tan buildup on it. Things that have been replaced over the year for various reasons. carb, distributor,cap, rotor, wires, plugs, fuel filter, air cleaner. None of these changed anything. I also decarboned the engine with GM top end cleaner. My only thought at this point is I am getting some pre ignition from a peice of crud heating up in the combustion chamber Any thoughts
Thanks Dave

 

What's your timing set at now?

 

You might want to check and make sure your vacum advance is not stuck, also on my 77 F150 with dual tanks I have a stock in line fuel filter and also a filter that screws into the carb so make sure you don't have an extra filter some place that you may have over looked.