1990 gmc timing?

i have set the timing on my1990 gmc 305 truck to no avail

it idles good but the timing keeps changing on its own and makes the engine rattle and blue smoke comes out the exhaust seems to come out the front of engine move the timing and it mostly goeas away until under load could it be the timing chain and gears and how do you tell if thats it

the hotter it gets the more it makes the racket1990 gmc timing?
There is a very simple check for a loose timing chain due to a broken tensioner, worn gears, or a stretched chain. Pull the distributor cap and observe the rotor position. Take a breaker bar and a appropriate socket and put it on the crankshaft damper pulley. If you have a degree wheel put it on the damper pulley else you can just chalk mark the timing mark position at the appropriate time.



Now - slowly turn the crankshaft pulley in a clockwise direction. Watch the distributor and observe that the rotor is moving. Stop turning. Now - mark the damper pulley position with the chalk or observe the degree wheel. Very carefully turn the crankshaft in the other direction and VERY carefully observe the rotor in the distributor. The instant it begins to move STOP turning and mark the crankshaft position again. Measure the number of degrees of rotation of the crankshaft. If there is a lot of slop in the chain then you will have moved the crankshaft ten or fifteen degrees (or more) before taking the slop out of the chain after the reversal before the camshaft began to turn. Get the picture?? If all is well and there is no slack in the timing chain then you will see about three to five degrees of %26quot;reverse motion%26quot; before the distributor begins to turn.



http://www.misterfixit.com/chanslop.htm1990 gmc timing?
Could be the chain, take it to a shop and have it checked out by a professional.1990 gmc timing?
Sounds like its worn out. I usually change my chain after 100,000 miles on my chevy 6 cyl trucks. They are pretty loose. Timing only changes on its on when something else is changing it. Its not hard to pull the front cover off and look at the chain.1990 gmc timing?
Yeah sounds like the timing chain. they start to go bad around 100,000. Just letting you know, if you are going to do the work yourself You will need a harmonic balance remover to remove the balancer. Then romove the bolts on the timing cover. There are moon shaped keys inside a slot on the crankshaft, dont lose them. there are 3 bolts holding the cam gear on. It should come off after you do that. Good luck.1990 gmc timing?
the blue smoke is because you are burning oil. as for the timing issue, i would have to look at that one. sounds like you have some miles on that engine.