While breaking in, the engine suddenly revs up? Help?

[dj_sparky]

Ok, so while I'm breaking in this brand spanking new STS .30 with the heat cycle method, I hit the throttle less than halfway, let go, then hit it again, and let go, and then suddenly it won't rev down. As if the idle gap didn't go back -- but I'm pretty sure it DOES go back, yet it keeps revving at high until I hit the brakes.

This happened earlier in the break in cycle as well, but stopped after I loosened the LSN a bit (Screwed it out.). Any idea what it could be? A leak where the idle screw is? But it stops revving after I hit the brakes.. So, it does sound like the idle gap simply isn't closing, but last time I checked, it did close yet it revved high. Is it possible that it's slightly open, and since I'm running it with the wheels in the air, it gets stuck in 2nd gear and revvs high? I dunno. It should go back to idling, but it just revs high.

And since it goes back to normal idle once I hit the brake, I don't think it's a leak anywhere (Would be the carburator screws I'm guessing -- since it happened and stopped after playing with the LSN at a certain location, then again when I opened the idle gap a bit. Haven't tried again, since I had to screw the engine screws tighter. Unrelated issue.) -- or am I wrong?

To break it down, why doesn't it go back to idle when I let go off the gas, and the idle gap goes back to almost closed? I didn't get to check whether or not it went back to normal (Almost closed. Normal rev.) this time, but last time the idle gap went back to normal but kept revving high (And I'm sure that the idle rev is OK/normal.)

What could this be caused by, what should I look out for?

Just went out and tried revving it a couple of times, and it is normal now (After I set the idle gap screw back to normal/previous location.). Is it possible that a certain combination of screw locations could be letting in some air in the carburator, resulting in this? But if that was the case, I don't think braking would do any good since it would still be letting in air. I have been having lots of issues with the electronics in this though, but I think that was caused by the battery wires. I used to loose connection constantly with the car. Using another battery now of course.

 

[Rolex]

If it stops revving when you hit the brakes, you need to adjust the idle with the brake fully applied and the trim knob all the way down.

 

[MotoGod]

+1 to what Rolex said. ALWAYS adjust your idle with the brakes on. Pull your carb boot off and turn on your car electronics (engine off). Pull the throttle and watch your carb slide. Try to mimic the high rev condition with the engine off. See if the slide is binding, or just not going back to idle position.

I always run throttle return springs just to be safe.

 

[dj_sparky]

I would also think that was the cause, but this isn't a consistent problem. It randomly happens when I adjust LSN or something on the carburator. (It has only happened twice.) Then when I turn LSN or idle gap screw back, it goes back to normal. It doesn't seem to be an electronics problem., but I could be wrong. I will double check the electronics in the next break in session.

 

[Metalhead Matt]

Might be a tear on the o-ring on the lsn as well. Moving it may cause it to spread, allowing way to much air into the mix.

 

[dj_sparky]

Thought so too, but then again, when I brake, it goes back to normal idle -- if it was an air leak, it wouldn't go back to normal, because braking wouldn't control the air leak and it would just keep revving. Idle gap is at normal, but hitting the brake lowers the rev. That's sort of weird, unless a tiny tiny gap in the idle gap is open and is the cause of the high rev, and braking lowers it. But still, why the high rev cause of a tiny gap?

Because braking lowers the rev to normal, it has to be an idle gap issue. Right? But when I checked when it first did this (Only happened twice.), the idle gap was at a normal 1mm or less, but still braking lowered it to normal, and it didn't even move the idle gap I believe.

 

[Scrogg]

Throw a rubber band or two on the carb slide. You are on the right track. Those carbs can be a little sticky when new. Good luck with it, I hope you got a good one. Those damn engines either run great and break the crank, rod or piston around 2 gallons or run like poop and break at 2 gallons.....The last STS .30 I had my hands on refused to break in, finally it just cracked the piston after we opened the third gallon.