is intake manifold can affect timing?

These do have a habit of sucking in the inlet manifold, spray a little starter fluid around the manifold, if the revs pick up there is a leak, check this first.

Improper valve adjustment is another possible cause. So would a bad PCV valve and unequal engine compression pressure.

my engine is 5af shim type and no need to adjust valve clearance. I tried to replaced gasket on intake manifold hope this will end up my trouble. thanks for the quick reply.

Shim type does not mean valve adjustment is not necessary.

Shims are adjustable, they do wear. Clearence should be .010 on intake and .008 on exhaust (cold engine). You simply put a new shim in.

But the intake gasket can leak and if large enough will cause significant loss of power.

idling is now good and the engine is fine but i cant set the timing
to 10deg. I plugged 2 vacuum from distributor advance diaphram. 15deg is the lowest degree i can set. I already changed and checked the timing belt and set all timing marks, set the pulley at 0" TDC at the compression stroke and rotor at the spark#1. my current timing is 15degree. How can i reach 10 degree? is my distributor faulty or my engine need to be top overhauled? My engine is toyota 89 model 4af carb with egr system.