Recommendations for off set key.

[J.S.@SMS]

I have recently started rebuilding a gx200 clone engine and I would like to know which flywheel offset key to use. They come in 2, 4, 6, 8, 10 and 12 degrees offset.
Some of the things that I have done to the engine are:
Governor delete (all governor components gone and hole drilled to 1/4 and tapped for a 5/16-24 bolt)
New rings
New gaskets
New valve seals
New carb (Ruixing)
Cleaned up/slightly shaved the head
Honed the cylinder.

I'm planning on leaving valve train and crank assembly stock. I would like to get a key which would move the peak power into the 4500-5000 range. Would this work or is it just a waist of $7.50 +s&h.

 

[KartFab]

peak torque or peak hp? If it is peak hp, then you are fine the way you are. The only time you want to deviate from stock timing is if you get a performance camshaft, and it will specify on the "cam sheet" or "cam card" that comes with the camshaft, what you need to do for timing, valve lash, and compression.

As a general rule of thumb, timing can be advanced from stock. Stock being about 20-23 DBTDC on clone engines. For very mild cams and builds, you can advance to 26-28 DBTDC. If you are going with a middle of the road cam that has increased duration compared to a mild, but isnt all out. Generally a "best overall cam", you are looking at about 32 DBTDC. Finally, if you want to go above that, you typically will have an all out, high lift and duration cam, and run on methanol, then you may be seeing 34-36 DBTDC for timing.

If it were me, and i wanted to shift the torque curve to a higher rpm range with the stock cam, i wouldnt advance to more than 26-28 DBTDC. If you advance timing, you are making the peak hp occur at a higher rpm range, at the expense of lower rpm peak hp. With these engines, you are basically going to be FLOATING THE VALVES at 5500 rpm anyway. With stock timing, you will rev up there really fast and be bouncing those valves pretty hard. No point in advancing in my opinion.

Also, if you want to make peak HP at a certain rpm, you will really need to look at camshaft profiles, timing, and header design in order to acheive that. It isn't super simple. But like I said, camshaft is the thing that really determines timing, that and if you decide to switch to methanol you add 2-6 degrees on top of what you would do with gas.

None of these are hard rules. Just observations from a hobbyist/engine tinkering fool.

 

[J.S.@SMS]

Thanks for your help. Planning on building this engine later on so i will hold off on ordering the offset key until then. And I am aware that the valves will float in the mid 5500s. I am counting on that to keep the engine from blowing up. However will be using the limiter screw on the throttle to keep the engine from floating the valves.

 

[mckutzy]

You want to float the valves, as that will be the new limiter... If you change the stock valve springs to heavier ones... then youll need to change the con-rod, as that will blow up first....
Stock rod, keep the stock springs..

Limiting the throttle will potentially allow the TC/Cent clutch to not get to full rpms and cause premature damage, overheating ect....

 

[bob58o]

First things to do are get a better air filter and exhaust.

Read this from Sneaks (Gary Costanza)...
https://www.kartpartsdepot.com/product_p/001.htm

This is about an old Predator 212, but should be similar to a gx200 clone.

"In order to meet EPA standards you will find these clones are jetted very lean. In fact, they are too lean for what we want. Usually, the clone’s main jet is #70. I have found that #85 - #90 works better at most elevations and should get you in the ballpark and on your way to making safe and reliable power.
The other jet of concern is the pilot jet. And that is located just under the idle screw adjuster. Normal pilot jets are in the range of .014”. We want .020” to richen up the
bottom end, and help with acceleration off idle, and through the low part of midrange just before the main jet takes over.
... Then the air box and stock filter to be replaced by a K & N racing style air filter and adaptor. Next, let’s complete this motor’s breathing improvements and get rid of that very restrictive exhaust system and spark arrestor for an open pipe into Dr. Dyno’s exhaust analyzer sensors. Add some of your favorite oil and we are ready for our appointment with the good Dr.
Our Predator fired right up without hesitation and ran smooth for its 30 minute break in. The new jetting was much to her liking with smooth acceleration and crisp throttle response. Ungoverned, the Predator was very anxious to turn more rpm’s than the manufacture would like to hear about. After warming up to a head temp of 275˚ I laid the hammer down and let Dr. Dyno take over. The manufacturer stated 6.5 horsepower at 3600rpms. With the improvements we did our test results were closer to 9 horsepower at 3600rpm with maximum torque pulling 14.10 ft. lbs. at 2300rpms. The maximum horsepower of 9.83 was achieved at 4700rpms. Power started dropping off at 4900rpms and we hit a wall at 5600. I believe some valve float was limiting our revs after that."

 

[J.S.@SMS]

You want to float the valves, as that will be the new limiter... If you change the stock valve springs to heavier ones... then youll need to change the con-rod, as that will blow up first....
Stock rod, keep the stock springs..

Limiting the throttle will potentially allow the TC/Cent clutch to not get to full rpms and cause premature damage, overheating ect....

I know that the valve float can act as a limiter. That's what i will be using to keep the engine from throwing a rod. And like i said i will be building the engine in the future (I'll come back to that later) at which point I will do a billet reciprocating assembly (rod crank and flywheel) and upgraded springs. And I will not turn the limiter screw on the throttle plate all the way in, just enough to keep the engine around 5000 rpm.

And bob58o, I am planning on building this engine later and will definitely put an air filter and header pipe and a mikuni 26mm carb hooked to a long (6") intake runner bent into a smooth 90 degree angle for more low end torque. However, before I do that i want to break the engine in properly and put about 25-50 hrs. on it. This will test the quality of the engine after the rebuild.