In trying to spray oil base paint, I get very low fluid flow (if any) from the cup to the gun. I've cleaned the gun and insured the hoses weren't clogged. Changing the adjustment valves hasn't helped. My compressor is set at 30 psi but I have only a 1/4" hose, increasing psi to 40-45 doesn't help. Any ideas?

Let me start by saying these types of paint sprayers are not usually recommended for painting large things like room walls because you're going to spend a lot of time refilling the paint cup. The cup only holds 20 ounces of paint and that won't go very far on your walls. If all your paint isn't mixed and properly thinned at the same time your walls could end up with a checker board look. You would usually be way further ahead to rent an airless paint sprayer.
If you have to use it; first run your paint through a paint strainer, avaliable at any paint or box store (The Home Deopt, Lowes, etc). Even new paint has impurities that will clog paint guns. It's highly recommended you mix in a paint conditioner like Flotrol but not absolutely necessary. Next, you're going to have to experiment with the consistency of the paint to get a good even flow out of the gun. The thinner you have to make the paint to get a good flow, the lighter and more coats you're going to have to apply. It's also recommended to run a paint soaked roller over the wall right after you spray it to even out the paint and remove any overlap marks.

when you pull the trigger can u see the needle moving backward? if so, then it's either the paint is too thick compare to the nozzle size or the passage for paint is blocked. try to take the nozzle, needle and the cup off and give it a good clean!

I have the same gun, and had the same problem. Turns out the solution is the plastic airline between the gun and the paint cup (or in my case, the small hole in the upper fitting that this hose connects to). In this sprayer, because it isn't gravity feed, there is pressurized air fed from the gun into the paint cup. That pushes the paint from the cup into the gun. In other higher pressure systems that use a paint cup, the venturi pull of the air is sufficient to get the paint up the feed tube from the bottom of the cup and into the gun, but this gun specs a max air pressure of 10 PSI. In my case, I got a small amount of paint into the drip tube above the cup, and when it dried I had a clog there. To verify, pull the plastic hose from the top of your gun and also remove the paint cup. If you can't blow air through that tube, you won't be able to get any paint to the gun.

First start with the basics, whats the paint VISCOCITY ON A FORD 4 CUP ??
Or Does it run nicely off the stick you stirred it with??

If its thin enough to go up the tube and lines, whats the fluid tip and air cap combo your using?? maybe start with a EX fluid tip (1.8mm) and 30b air cap (cos that sprays everything!!) and reduce from there.

Whats the pot pressure?? Is it a KBII pot? cos they suffered with the air inlet blocking, so IT SHOWS GOOD PRESSURE, but doesnt have it in the pot!

Does it spray anything?? thinners?

Depends on what you want to do with a spray gun realy.
Gravity is good for easy flowing paints which can be used on things which dont require the gun angle to change much.

SOLVENT BASED PAINTS:
Thin your paint to a bit thicker than milk, fully open the fluid needle travel and without air pull the trigger and see if the paint kind of falls gracefully out of the front, if it just dribbles and rund all over the gun, its either TOO THICK or the fluid tip IS TOO SMALL.

If that works OK try introducing some air gradually and spraying test patterns, even atomisation, (regular paint particles) and a pattern of about 1.5 inches by 6 - 9 inches with every thing set to full on should look like a cigar, if its all one ended your air cap hasa blocked hole, no paint in the middle too much air.

Good luck

You need a re-build kit for your gun.

The needle that pulls back when you squeeze the trigger is no longer making a seal when you release the trigger.
The problem is worse when you spray thin oil-based paints.

Depending on the gun, Sherwin Williams has kits for the Graco gun. That's where we bought ours for years up until I retired from painting.
Home Depot and Lowes may have parts.

again you have to change the fluid nozzle it help me with my spray gun

Be sure to filter the paint you are trying to use so that you aren't dealing with large lumps of paint. You will need to experiment with hose pressure to get proper spray quality, and set the mixture control for a fairly fine mist as the paint is likely to be more fluid. Fan control is set so that you don't get an unpainted space in the middle of the paint swath. Make sure to wash the gun thoroughly after painting. If you are doing a lot of painting, expect some nozzle wear over time as the water base paint is somewhat gritty. Don't forget to trigger 'off' before the end of each stroke and trigger 'on' after starting the return stroke to keep running at a minimum. Good luck!

I spoke to a friend who was a spraypainter in his last job, apparently due to the thickness of the paint, I needed a larger nozzle on the gun. I couldnt get a nozzle to suit this gun so I purchased another gun with a cuple of different size nozzles. All seems to be working fine now.

RECOMMENDED SPARE PARTS FOR EACH GUN * Air nozzle, needle and tips * Packing and gaskets * Hoses and fittings * Gauges * Proper lubricants * Filter screens * Diaphragms for pressure regulators