cylender 7 low compression

[debobf]

i have a 89 F250 with a 351w "5.8". all cylenders test good but number 7 witch reads 60 dry and 90 wet, i also have alot of blowby, putting gunk into my air box, the engine also has a upper rattle right near #7,
now i bought this truck after it has been sitting for over 4 yrs i was wandering if it might just need a good clean out or should i go through the whole thing..... she has 150,000 miles on her