I have my first st engine a G90. It was given to me by a friend who said he could not get it to stay running. So far, I have found that the needle valve retaining screws keep coming loose and allowing air into the carb and the muffler has a loose piece at the back where the exhaust comes out. This engine has about a gallon of fuel through it. As long as the needle valve screws are tight it seems to run ok. A little locktight should fix this problem. My question is- should the muffler end piece be loose and move around? There does not seem to be any way of tightening it up that I can see. You can even hear it rattling when the plane flies over.

Thanks
Jim

Sounds to me like the baffle is loose. You can leave it as is. It seems like the newer ST´s have this problem with the muffler. Better yet, go buy an aftermarket muffler and you will be happier. With my 2 ST´s I have had nothing but trouble with the mufflers.

As far as the retaining screw one of my ST90s doesnt have one bc it came off in flight. I have seen any noticable difference in performance. You could JB weld it in place without any problems.

With the ST90s I have seen they seem to be pretty sensitive about tank location. Of all the ones that I have seen that wouldnt stay running it had to do with the tank being too high.

Also my 2 st90s seem to be a lot happier on good ol´ 5%. I tried 15% and didnt notice a big jump in RPM but did have more trouble getting them to run as well.

By needle valve retaining screws, do you mean the two small brass screws on the side of the carb where the high speed needle valve is mounted? If so, then definitely tighten those down, but before you do that you might want to remove the whole carb, look at the spraybar from the underside, and make sure that the small slit in the center of the spraybar is centered before you really tighten things down. If those two screws are loose then it's possible for the spraybar to rotate and throw your low and high speed needles out of whack. Good luck with the engine. I have one and it's really starting to grow on me. They take a little tinkering with at first, but once you get the hang of it they're really great engines.

Dave

If thats the case I misunderstood. Those screws are very important and I would out some loctite on them ASAP. I was thinking it was the throttle stop screw that comes in from the top that has the spring on it.

The screws I was referring to were the two that retain the high speed needle valve assembly in the carb. I will take Dave's advice and remove the carb and check the spraybar before I locktite the screws in.

So I should not worry about the muffler end piece? Is this normal for these mufflers? Toss it and get a pitts? What do you guys think?

Jim

You can run the muffler as is no problem. Its just annoying to hear that clanking all the time and everyone who sees you fly will come up and tell you something is loose on your plane!

If you take the muffler off from the stack you can actually fold up the baffle and take it out so that it wont rattle anymore. Just be careful not to scratch up the inside of the muffler at the neck or you will have a tough time getting it on and off the stack.

The pitts muffler is an option or tuned pipe or ultrathrust or jett muffler. I recently upgraded mine to a jett. I had a macs pitts on mine for a while and it ran well. Some folks have to crimp one of the exhaust stacks closed in order to get good backpressure.

Of all the options the pitts muffler is probably the cheapest and its easier to install. With my jett muffler upgrade I had to do some major mods to the fuse to accomodate the long muffler.

Pulled the carb and needle valve out last night and sure enough, the slit for the spraybar was a little sideways. Centered the slit in the spraybar so that it was pointing down. Locktited the retaining screws and put the carb back in. Locktited the carb nut as well. Will see how it does tommorrow.


Jim

Use M2.5 flat washer and spring washer under each screw head. Flat washer will keep spray bar cone in place. Spring washer
will help to keep screws tight forever.I have 7 Super tigres from G90 to G3250 and it works on each of them for years.

On the stock muffler stack , make sure you throw gasket away and RTV the surfaces for trouble free service.

You might want to go to the ST website and read about rotating the spray bar for best transition and all... Straight down is not always the best position.. I would not use locktite here in case the spray bar does need adjustment...

None of my ST carbs ever needed Loctite on those screws to keep them in place. It makes me wonder if they are the right screws for the threads in the brass carb piece. Of course, using Blue Loctite won't hurt a thing.