Hey everyone,i hope i can get some help with this. I have a 1991 f150 5.0 4x4.i purchased it from a friend of mine.It was her husbands truck and when he died it sat for about 2yrs. It has 102,000 miles on it. It runs rough ,jerks,and backfires through the drivers side manifold.check the timing and cant pull a mark on number 1,however, it will show mark correct if i put it on number 8.i tried adjusting the timing and firing order and it will not run.It is running rich,the lady i bought it from said there has been nothing done to distributor or timing chain,They purchased it new.it is not pulling any codes,Any ideas? Thanks for any help

 

Make sure you are using cylinder #1 for the timing light and the correct non-HO firing order. Also verify the plug wires are routed to minimize crossfiring:


Be sure you remove the SPOUT plug to disable the computer controlled timing when checking and/or setting the base timing.

 

i did,checked and double check firing order,cant see the timing mark if i put it on number one.i get the timing mark showing if i put it on number 8, was wondering if maybe it jumped a tooth?

 

With the #1 spark plug out turn the engine by hand a with a screw driver feel for top center. Then look at the timing marks to check if the balancer did not slip. A 15/16” socket fits the balancer bolt.

 

Timing chain could have jumped, distributor gear roll pin could be sheared or the outer hub on the balancer has slipped. Best advice is verify cylinder #1 is at TDC on the compression stroke, then verify the timing pointer is correct on the balancer.

One more thing to check...be sure you are using the actual timing marks on the balancer and not the big slot...

 

well, i was using the big slot,found the timing mark and timing is fine. back to zero with this truck. thanks alot,at least i can rule out a timing issue,

 

Well, at least you ruled out the ignition timing
Sounds like you are lean to me (the backfiring)
Check all the vacuum lines going to the vacuum tee and the plenum
Check the upper intake / plenum gasket for being cracked
Sometimes you can just look down from right above the upper intake manifold and see the cracked gasket coming out
Good luck