88 zuk with a carburetor. ----- used to get 25 miles per gallon or more. Now around 17. Starts great, runs good, may be down on power a little. Is this a carb problem? Petro works had rebuilt carbs for allmost $ 500 . Should I go for this?

Check the tune-up stuff, and the compression, I think there is
also an O2 sensor that is part of a feedback system on the carb
that might be having an issue.

If it's running good and starting good, I would not just change
out the carb, as it seems to be working the way it should, and
8 MPG loss is a bit but not to the extent of a bad carb, definitely
check out the O2 and tune-up before spending that much on a carb

Wild

I will bet that you catalytic convertor is plugged. 

I had the cat back pressure checked and put on a new o2 senor also. New plugs, cap , fuel filter, air filter. Everything but a compression check. Thought it starts too good for bad compression. Turn the key and it just pops to life.

I just lost 100 mpg per tank because I had to put on  a more restrictive ( quiet) muffler to comply with a new noise ordinance in our township. Used to get 250 miles on a tank  and since I put on the new muffler I only get 125 miles per tank.  I still have the header (no cat) OEM muffler and 2" all the way to the back.

I had the cat back pressure checked and put on a new o2 senor also. New plugs, cap , fuel filter, air filter. Everything but a compression check. Thought it starts too good for bad compression. Turn the key and it just pops to life.

Did you put on new plug wires too and a distributor rotor?  If not, do it.
$500 for a rebuilt carb?!?!?  Get a $30 rebuild kit from the auto parts store and do it yourself.  Although, if it starts and runs good, the carb probably isn't the problem.
Have you adjusted the valves lately?
Have you checked the timing?

Yep new plug wires and rotor. Had the valves adjusted 10,000 miles back. As to rebuild kit for this thing have heard that this thing is very complicated, anyone done a rebuild on one of these???

Rebuilt the carb, is that what you are asking?  It isn't bad to do.  This is the only car carb I have dealt with, but it doesn't seem all that complicated.  Just remember how it comes apart, so you know how it goes back together.  If you get you a copy of the samurai FSM (factory service manual) it has diagrams in there that show how the carb goes together (still doesn't hurt to lay out the parts in an organized way,  when you take it apart though).  It also shows you how to adjust everything, once you get it put back together.
I'd check compression too.  Maybe there is one or two cylinders that are really low, or maybe they are all kinda low.....it could still start and run ok with either of those issues.
Could also be a burnt valve (burnt valves will cause low, or no, compression).
Did you change or do anything to the engine or anything else, right before you noticed the MPG loss?

I suppose I should check the compression it is the only thing I haven't done. --------------- Thanks for the input guys.

Keep a close watch on the valves, they tend to tighten instead of
loosen, which can cause burned valves and depending on the mileage
of the engine, could require a total tear down and rebuild. Been there
and done that on my 1.6 from neglecting the valve adjustment

Wild