I have a 6.5 B&S engine on a RYOBI lawn mower That will not come up to mowing RPM. the mower does not have a throttle cable and the spring on the throttle appear to be intach. The engine starts real easy but will only idle. The engine will speed up and idle down when I operate the throttle with my finger. Whats the fix?

there is a wire connected to the carby at the end of the throttle cable
that wire makes contact with the throttle plate when the engine stop is required
if that wire has fallen of or burnt through or touching the plate it will stop the engine and there will be no spark

The mower decides how much fuel to use depending upon rpm. When it hits a tough patch rpm goes down and the throttle opens up. In this case the throttle opens too far and the engine floods producing the white smoke. The plug is temporarily fouled out and the engine quits. After the gas evaporates the plug will operate normally again.

there is a little tab behind the air filter but facing forward. it is a funny shape. hook the spring end over this tab and you should be good to go

Should see a bracket with holes in it .and a small throttle bracket that has a hook shape the spring locks on to it.

Could be the spring has been stretched when you ran into the bush. With the engine running but out of gear move this spring by hand and see if the engine speeds up If it does I sometimes cut off the end of the spring and pull a piece out and make a new end with needle nose pliers or buy a new spring. I like to repair mine.

I know this is an old post but I figure maybe my solution will be found by someone else doing a Google search and it can help them. Hopefully, you've gotten yours fixed in the 10+ months since you asked this.
Anyway, if you remove the top plastic shield from the mower (two screws hold it in place, plus a small cover near the air filter that needs to be removed) you will see a small spring that attaches to a tab at the front of the mower. If you bend that tab forward it will put more tension on the spring and increase the engine speed. You'll know if you are looking at the correct spring if you lightly press on the spring and the engine speed increases.
It is a good idea to monitor the adjustment with a tachometer. My owner's manual says the engine has a 3,100 max rpm. You don't want to set it above the specified rpm or you'll probably burn the motor up in a short time. I set mine at 3,060 rpm. It's hard to get it perfect but I figured slightly low is better than too high. Mine had been running at 2,800 rpm.
I bought a small engine tach on eBay for around $9. The wire wraps around the spark plug wire 4-5 times and then you can read the engine speed. It also keeps track of how many hours the engine has been running. I figure that will come in handy for oil change intervals so I permanently mounted it to the mower. It almost looks like it came that way from the factory. :-)
Here's the tach I bought: Hour Meter Tachometer 2 & 4 Stroke Small Engine

check the spring tension on the carb linkage

sounds like throttle cable come off

i have a spring thats off

A lot of time when the small motors on these push mowers surge it is a governing spring that is weak or broke. It does not hold enough tension and causes the valve to fluctuate. I have fixed this many time for a relatively cheap amount (under $5).