Hey, it's thanksgiving and I am tweeking my VQ P-38.
I really cannot add to the above comments. It does sound like an air leak. I would also consider checking the back plate gasket and the older ST's had a front plate also. Turning the bearing plate allow counter rotation.
I would really go to 15% fuel or check your fuel. Hey, you don't have a check valve in the muffler pressure line do you? Don't. I have also found the ST's may or may not like after market mufflers due to loss of back pressure.
If you decide to change the carb, and I really do not feel this is necesssay, but I did it for more power. Here is a post I did for my club newsletter about getting more power.
Good Luck,
Twinman
"ASP Carbs on ST Engines"
Sep-13-02, 08:13 PM (CST)




My Motto -If it ain't broke, it needs fix'n!
I was in one of our local hobby shops the other day and on the sale table was an ASP 1.08 which I was casually handling. It was then that I noticed that the mounting barrel looked surprisingly familiar. The store owner had in stock an OS 7D carb which is pretty much a standard replacement for the Super Tiger carb. I was able to compare the two and noticed that the ASP carb was a little bigger in the bore area than my ST favorites. Thoughts starting spinning in my head. Would it work? The store owner was nice enough to lend me the carburetor from his own personal ASP 1.08. I took it home and sure enough it fit the bore of a Super Tiger 90 carb opening. (OK so he saw a sale coming!). I should at this point say that both carburetors are similar in design. (Read this bigger than the carb Super Tiger uses in the 60, 75, and 90.) It was interesting to note that the A.S.P. carb casting is very similar to the O.S. The high-speed needle valve and fuel nipple are either purchased form OS or an exact copy. The idle adjustment is definitely from the earlier Super Tiger carbs that used a knurled knob adjustment. Later ASP carbs have gone back to the internal idle screw.

ON to the test

7:00 pm Thursday night. Yes it was dark! (Car headlights will work with photo tachs!) A Very tired Super Tiger 90, with a 13x6 master Air Screw prop, narrow spray bar (note the previous posts for this discussion),tuned pipe, and 15% Rich's Brew synthetic fuel.
Witnesses in attendance, my son Kyle, and Rapter, a fellow club member, who thinks we are nuts. (What does he know, he fly's combat!)

Test Results
Standard Super Tiger Carb - 13,000 RPM
ASP Carb on same engine fuel- 13,500 to 13,600 RPM. Good idle, transition and awesome top end!

Flight Results
Test bed-one very used and heavily repaired Super Stick 60. (Who said two or three tubes of epoxy is not heavy.) It previously took 90% or more throttle to hover. With the larger ASP carb it now hovers at 60% throttle and accelerates straight up out of the hover and fly's straight up almost as fast as during level flight. Let's just say take-offs are quick!!

Conclusions
1. You get an awesome performance increase for a small investment. An ASP 1.08 carb retails for $35-$40. An OS 7D carb costs $65. Same size and parts...hard choice?!..Not!!!
2. You can put off rebuilding an engine due to the new performance. Why bother there's no need!..Well almost!
3. You must increase either the diameter of prop or increase pitch. The RPM increase in dives is scary. You don't want to put a rod through the side of the engine.
4. You will still go straight up with a 14x8 prop, just not as fast. Tried a 14x6 later and liked better.
5. Shrewd Hobby Shop owner suckers me into spending more money!!!! Yes, I gave him back his carb and bought new one!
6. For some reason my fuel use is climbing!

The flight tests will continue on the Super Stick for determining reliability. If it continues as it is now, there will be three Super Tiger 90's in the house (two on a twin engine Ultra Sport 1000 back from the grave.. another story, another time) with ASP 1.08 carburetors. Note, after I first wrote this story for my club newsletter, I did install two of these on a twin with two ST 90's and have never looked back.

Note, later I did install these on the twin ultra sport and loved the increased power. Don't worry, I have carbon fiber in the wing!!! I ran this combination on that Ugly Stick for dozens of gallons and never looked back. Engine was mounted up right.

I installed these carb's on a set of ST 2500's that happen to be in..yes you guessed it..a P-38. Gained 400 rpm with 18x8 props.

More Later!
Off to "fix" something else!!!!