I have a 1994 750 vulcan and rebuilt the front head with new valve and guides,lash adjusters and still it wont hold compression after i ride for a few minutes

its on the camshaft

Incorrect valve tappet clearances, this will stop the auto deco valve in the engine working.

You may still have gaskets trying to compress on the engine and if you had the valves and seats ground, they may be still seating in at that mileage. As the valves and seats wear, the valve stem raises up reducing the lash. All Ironhead Sportsters have solid tappets and should be adjusted to zero backlash. It's always best to set the tappets on these engine a bit loose rather than a bit tight. Always adjust the valves (tappets) with the engine absolutely cold. Keep in mind that the valve lash is set at zero with the engine cold. As the engine heats up, the cylinders and heads "grow" upwards introducing lash into the valve train. When it cools and contracts, it again should go back to zero lash but not hold the valves open. Make sure you get the locknut tightened properly.

Good Luck
Steve

The mechanic should have given you some idea what was wrong.
The low compression is probably the reason it wont start.
This is usually an exhaust valve not sealing or burnt., or a head gasket blown

Check the valve clearances, if they are adjusted correctly and there is still no compression, the heads will need to be removed and the heads reconditionned and valves refaced, new head gaskets will be fitted when the heads are replaced.




These ideas and also assuming the bike didnt blow smoke or use oil, when it last ran, or make any unusual noises, knocking etc. Which may require further rebuilding of the pistons,rings and bearings.
There can also be other worn or damaged items that cause this , but unknown until the engine is being stripped.

not really. The engine uses hydraulic lash adjusters, so there is nothing for him to adjust. If the engine has been run at all, the valves should be seated. If not, maybe the guides were over-knurled. Can't really tell why they are clicking. Can he provide specifics of what was done to the engine? Does it have new lash adjusters? Were the guides knurled or what was done with the guides? What was done to the valves? Are the springs new? Are the camshafts new? etc. etc.

1. For the 4.3L engines which are equipped with screw-in type rocker arm studs with positive stop shoulders, tighten the rocker arm adjusting nuts against the stop shoulders to 20 ft. lbs. (27 Nm) on 1994-96 models and 18 ft. lbs. (25 Nm) on 1997-99 models. No further adjustment is necessary, or possible.
   
2. For most 4.3L engines which are not equipped with screw-in type rocker arm studs and positive stop shoulders, properly adjust the valve lash. For details on valve lash adjustment, please refer to the procedure in Routine Maintenance . (see below)

4.3L Engine

The 4.3L engines may be equipped with either of 2 rocker arm retaining systems. If your engine utilizes screw-in type rocker arm studs with positive stop shoulders, no valve lash adjustment is necessary or possible. If however, you engine utilizes the pressed-in rocker arm studs, use the following procedure to tighten the rocker arm nuts and properly center the pushrod on the hydraulic lifter:

1. To prepare the engine for valve adjustment, rotate the crankshaft until the mark on the damper pulley aligns with the 0? mark on the timing plate and the No. 1 cylinder is on the compression stroke. You will know when the No. 1 piston is on it's compression stroke because both the intake and exhaust valves will remain closed as the crankshaft damper mark approaches the timing scale.

Another method to tell when the piston is coming up on the compression stroke is by removing the spark plug and placing your thumb over the hole, you will feel the air being forced out of the spark plug hole. Stop turning the crankshaft when the TDC timing mark on the crankshaft pulley is directly aligned with the timing mark pointer or the zero mark on the scale.
The valve arrangement is as follows:



E-I-I-E-I-E (right bank-front-to-rear) E-I-E-I-I-E (left bank-front-to-rear)

1. With the engine on the compression stroke, adjust the exhaust valves of cylinders No. 1, 5 & 6 and the intake valves of cylinders No. 1, 2 & 3 by performing the following procedures:
   1. Back out the adjusting nut until lash can be felt at the pushrod.
   2. While rotating the pushrod, turn the adjusting nut inward until all of the lash is removed.
   3. When the play has disappeared, turn the adjusting nut inward 1 3 / 4 additional turns.
   
   
2. Rotate the crankshaft one complete revolution and align the mark on the damper pulley with the 0? mark on the timing plate; the engine is now positioned on the No. 4 firing position. This time the No. 4 cylinder valves remain closed as the timing mark approaches the scale. Adjust the exhaust valves of cylinders No. 2, 3 & 4 and the intake valves of cylinders No. 4, 5 & 6, by performing the following procedures:
   1. Back out the adjusting nut until lash can be felt at the pushrod.
   2. While rotating the pushrod, turn the adjusting nut inward until all of the lash is removed.
   3. When the play has disappeared, turn the adjusting nut inward 1 3 / 4 additional turn.
   
   

Recheck the valve lash sometimes they are set too tight. Do a compression test if the valve are set ok. If it indicated lower than 80 psi it may be a burn out valve.

if i was you i would take it out,or do a lot of shimming.

did u soak the lifters in oil 1st before installing them? if they are pumped up u bring that cylinder u are adjusting up on compression stroke, then adjust rocker to zero lash and then 1/2 more turn of the adjusting nut.