expo

My Feedback: (20)

Oh oh. I think I made a mistake and allowed another flier to adjust my hi speed needle on my .91 4 stroke. It ran great but the next flight there was a power loss and when I brought it back in it appeared the needle was in too much. No amount of adjustment could bring the power up to where it was.

Before I dig into it, is it salvagable? What parts could have been ruined? What should I check for? I'd appreciate any advice. Thanks for any replies Expo

mcarleno

What make and model?

Probably a new Piston Ring. Possibly a new sleeve --- but I would try a ring first as they are usually pretty cheap.

da Rock

Sometimes the connecting rod bearings get screwed by a lean run. You can often work the prop by hand to feel for that bearing slop. But it's harder to feel a ringed engine. On ringed engines, if you're really wanting to check the feel, you need to press something like a wooden toothpick against the piston to hold it immobile while you feel the movement from the prop. And you can pull the backplate to see it on the crankpin.

4cycles are often very hard to adjust by ear. Don't you wish you'd had a $25 tach and checked that needle setting before the flight? Of course, that expert would probably have been insulted had you checked his setting.

Richard L.

My Feedback: (24)

Yep, the piston ring might have been fried and the cylinder wall might have been scored. Replace the piston ring as suggested and check the cylinder wall lining.

da Rock

Is it salvagable?

Oh yes....

If it's a ringed engine, and the ring is screwed, it's not hard to replace. And a ring is fairly cheap.

But if you decide to replace the ring, very definitely check out the connecting rod fit. Because they're also not very expensive and can easily be replaced when you're swapping out the old ring.

BTW, 4cycles aren't the easiest engines to assemble and disassemble. If you're not mechanically inclined, I'd think about sending it to one of the reputable engine guys for the work.

Rcpilot

My Feedback: (78)

Not to rub salt in the wound but,

Thats why I don't let people touch my engines. Someone reached over and tweaked the needle valve on my ST2500 once, and it immediately locked up. Spun the conecting rod bearing. It was a brand new engine, and I was putting a couple tanks through it before I flew it. I was running it about 1/2--3/4 turn rich, and this guy walks up and says something about too rich for an ABC engine (it's a ringed engine BTW) and he whacked on my needle valve before I could stop him.

About 2 seconds later..........SCREACH!!! And it locked up tight. [:@]

I wanted to knock the guys teeth out. I was so mad. Haven't ever been that mad at the field. You can't exactly run down to the LHS and buy a new rod for a ST2500.

Get a tach. Tune your engine yourself. It's a bit intimidating at first, especially with a 4-stroke because you can't really hear it when it's too lean--like you can on a 2-stroke. But, learn to get comfortable with it. You'll be better off in the long run.

Ed Smith

Before you rush out and spend money on new parts, check the valve clearances. The engine may have overheated and affected the clearance. The check is at least free.

Ed s

expo

My Feedback: (20)

Some good advice guys-thanx. I should have told you that it's a Thunder Tiger and maybe not a ringed one. I'll have to check the owner's manual. I never checked the valves so while I'm at it, I will.

Another question: This carb has a screw with a spring on it for setting the carb idle. I can't get it low enough to allow my kill switch to close the throttle. I've even taken out the screw and there's still an air gap. It will stumble but won't usually quit . any suggestions?

Rcpilot

My Feedback: (78)

To make it quit:
Hold the plane, and then hammer the throttle real quick and pull it back to idle. One quick motion--full throttle and back to idle--as fast as you can. That'll kill it.

You might have an air leak. The carb should close all the way. Are you sure the carb is hanging open a little bit?

I've got a carb for a TT .91 4-stroke here if you want it. It's only got 1 gallon of guel through it. You can PM or email me if interested. I'll sell it cheap.

expo

My Feedback: (20)

RCpilet,
I'll check it out and let you know if I need the carb. There seems to be something else holding the throttle open. Disassembly tomorrow.

Sport_Pilot

If adjusting the valves doesn't help and, if you have good compression and the bearings are in good shape, then you may simply have some trash in the needle causing it to run lean and not allow it to richen up. If the compression is weak then you have either worn out the rings or glazed the sleeve. If the sleeve is glazed then compression may slowly return. Try adding some extra castor oil to the fuel to seal the rings and fly it for a while. If it doesn't improve then remove the head and check the sleeve.

expo

My Feedback: (20)

Some results: The book sez both valve clearances were to be between .003 and .012. I and they were Exhaust: .009 and INTAKE: .017. Both were brought down to .005--that should help. It did a lean backfire before the takeoff which may have changed things also.

The reason the throttle wouldn't close was that the set screw that rides in the barrel slot preventing the barrel from coming out was stopping it from closing also. Really, there was nothing wrong with the way it was installed, but it restricted the closing. I added a small washer shim under it so the *** didn't go in the slot as far and it closed it up slightly.

I added a few ounces of castor to the mix until I get things back to normal.

Thanks again guys!!

Sport_Pilot

Check the intake manifold screws, they may be loose causing a lean irratic mixture.

MerlinL14

I believe that is the throttle stop screw, not the LSN.