I have only 50 psi compression on gilera runner 4stroke 125

A D-50 with a carb - that is a few years old and probably lots of miles on it.
If you know you has gas and spark to the plugs then either there is not enough compression (engine worn out) or the timing of spark to compression stroke of the engine is wrong.
Make sure the timing belt is ok
Then check compression - the lowest cylinder should not be less than half of the PSI of the highest cylinder and they should be about 125 PSI or more. If it is down as far as 100 PSI it may still run but is very weak. Be aware also that Compression guages can vary significantly in their readings.
While you are checking cylinder #1 - check to see if the distributor lines up to #1 tower on the cap while the compression stroke pushes air out.
Let me know if this helps you at all

I think either way that the head needs to come off.

First you must check to see if engine is mechanical sound by performing a compression test. remove the spark plugs install a compression gauge in the spark plug hole and crank engine about 4 cycles compare readings to all cylinders reading should be around 125-150 psi readings in the 50 psi range possible jumped or broken timing belt. If engine is mechanically sound test for spark at the spark plugs use a spark plug laid against valve cover and have some one crank it over look for the spark to jump the gap. A no spark problem is internal in the distributor.

A new engine would be about 180 psi for each cylinder. As the miles add up and wear occurs, compression will get lower. The important thing is that compression is even and balanced-lowest compression reading should be within 75% of the highest cylinder's compression. Some Toyota engines had a minimum allowable compression of about 135 psi, but if compression is balanced, you could have average compression as low as 125 psi and engine would still run good, some loss of power but still acceptable, and oil consumption would not be that bad.
If cylinder compression is below 120 psi as an average, you should be looking into a rebuild event-rings and valves need attention. If any cylinder is below 100 psi, it may not even fire, certainly will produce no power, and could be a head gasket failure, very worn rings, or valve problems.
Remember the important thing is to find compression readings that are very close to each other. I have an '87 Sentra with 75,000 miles(true!)-lowest comp. is 160, highest is 165 psi-runs great, good power. Also a '79 Datsun pickup (with god only knows how many miles-about 200,000 maybe). Compression is about 125-135 psi, fairly well balanced-noticeable loss of power, very little oil burning, still runs fair to good-dependable, at least.
That should give you an idea of what to look for. Good luck.

If cylinder compression is too low, the car may not even start because of it. Have a shop do a compression test or a leak-down test to check the internal health of your engine. A good engine should have pressures of 150-200 psi in all cylinders and be close to each other. A worn or high mileage engine may have pressure as low as, say 125 psi, but if still balanced (close readings), would still run decent. Compression below that, especially below 100 psi, is serious trouble. ignition spark refuses to ignite the air/fuel mixture and you get a misfire-a non-working cylinder. Low compression can be caused by worn piiston rings or poor valve sealing. Low compression in 1 or 2 cylinders can be a valve problem, burned valves or valves not seating properly.
A leak-down test will check each cylinder and if there is a problem, the test will find the cause of the problem.

GOOD COMPRESSION SHOULD BE 120 - 175 PSI IF COMPRESSION 100 PSI ACCEPTABLE. 90 PSI LOWER CYLINDER WEAK IF COMPRESSION 50 - 20 BLOWN HEAD GASKET.

bad rings you can add a teaspoon of oil the the cylinder in question and redo the compression test if the compression goes up then the rings are bad if the compression stays the same i would be looking at a burned or bent valve or bad head gasket.

around 180 psi per cylinder.

leaking valve or head gasket leaking

that is a ground issue, check the grounds or negative connection of all the lights. what is happening is that the other lights are getting their ground from the other lights that is why when you swithc to that the light, the rear one goes off.

tnx for using fixya,

drcool