Hi,

I have purchased two new G51 Super Tigers from Tower Hobbies to run in a Topflite Cessna 310. I mounted up both engines and have set them up identically down to the linkages.
I started both engines today, and out of the box, one runs exactly as it should, and the other runs like a pig. It surges, doesn't burn very much fuel (leaves a big mess) and idles terribly. Mid range gets a little more like it, but low end is not very good.

Now both engines regardless have had the same amount of fuel put through them (about half an hour of running), and while the good one has fine compression, the poor one has noticeably lower compression. I lifted off the head to check the seating on the cylinder, retorqued the head down and checked the rest of the engine as well. I did notice fairly uneven wear on the cylinder wall from the still bedding in ring. I noticed what appears to be a small hole in the cylinder head to perhaps locate the ring in it's land. I also noted the poor alignment of the maching jig for the head has caused the holes drilled for the head bolts to erode through to the sidewall into the relieve machined for the cylinder liner. I don't believe this causes a compression loss however as it is not a machined sealing surface.

also on further inspection (and strangely enough) the compression is better in the reverse direction than the forward direction.....any explanation for this??

Now my question is, should I persevere with this engine or send it back for warranty work? With a twin I'm concerned that the performance may never match the other engine, and that if the performance disparity is apparent now, it must have a poor machining tolerance somewhere in the manufacturing process.

With a gallon of fuel through the engines, one is fine and the other is poor.

Both were run in identically on the same day in the same operating conditions, fuel was identical and all set up procedures were as well. All from the book using the directed 3 turns initially for the first tank. Fuel filters are fitted and I cleaned the needles as well when it was running poorly to remove that option.
I'm guessing poor quality checking from the factory on this engine now, and am off to buy another rather than fiddle about with spare parts. At least I can part it out for future problems on the two operating engines.

Coulda put the extra towards a pair of OS engines I guess.....

If the engine still hasn't "come in", it can certainly be looked at by us to see what's wrong. How does it run when it's leaned to just rich of peak RPM, and the idle mixture adjusted to give a good idle and transition? That's one thing you haven't said. What fuel, what propeller, what RPM is it running?

The fuel is 10 percent nitro, 18 percent coolmaster mixed fresh. Prop is an 11x8. Plugs OS no.8.

I further ran the engine but it never did pick up it's compression. I'm not sure why but I have had very little to do with ringed two strokes before. Plenty of time on ringed four strokes and dozens of ABC's.

It was run in as carefully as any of my previous engines strarting splutteringly rich at first and leaning out over time.

As I am too far, and the engine is too inexpensive. i won't really consider sending the engine in. So I have pulled it down for a look before putting it in the cupboard for parts. the only thing I have noticed is uneven polishing from a clearly out of round ring or bore, the non-polished areas having a bronze effect. Everything else seems in order, the ring sits in it's land, and is not cracked or missing a piece. The piston top has a small hole where the pin to stake the piston ring has clearly been pressed through.
Rod is fine, everything looks fine (nicely engineered these days the internals too). So all I can guess is a poorly sealing ring.

I am experiencing the exact same problem you are describing with a set of G51's that I am putting on a Seagull Dual Ace. One is running half decent, and the other will barely run due to low compression.

Debris Field, did you run it anymore before putting it away for parts?

No I've laid it up carefully for future scavenging!

I now have however got a replacement piston and liner on the way which i hope will cure the issue. The replacement motor is just fine and after matching the other engine for run time first has performed sweetly since.

Cheers

Mick

I was hoping that it was a low speed idle setting, i still do, but my g-51 is doing the same as both of yours, does this mean a bum product line?
I ran it with the exhaust disconnected, and got better results oddly enough, I think I was getting far too much fuel in the engine, with the exhaust connected Raw fuel was spewing out, at the recommended High speed idle setting for break in, I have managed with much effort to run two tanks through it, but I am starting to think its a crap engine....oddly enough, I have a ST 75 that is bulletproof and unstopable, as well as a ST G2300 that is the same, no problem, maybe a flaw in the ST 51 series?

I swapped carbs from the good engine to the bad (both being same age I figured it was a good step to try) but no, the same problem persisted.

I sent mine into Hobby Services on Wednesday. Hopefully they can remedy the problem, or at least tell me what is wrong with the engine.

Thanks for the solution to the problem Bill, they're both running as sweet as with an equal amount of time on each.

Maiden flight tommorrow....fingers crossed...

Okay what was the problem?

I think I missed something.

Hobby Services replaced my engine under warranty. There were no comments as to what was wrong with it. I have not run the new one yet.

Since I was waiting to run both engines on the twin, I went ahead and purchased another new G-51 while the original was in the mail. After a few tanks of fuel the new G-51 performed much better than the first two with a gallon through each.

Bill had the ring, piston and liner replaced for me.
Once this was done the engine ran faultlessly using the same break in, it's now a nice strong engine with a great transition.