Hi guys. I have a GMS 76 with a serious case of the "slows", plus an alarming knocking sound from it at idle (similar to the condition previously posted by XJet). It is typically operated with 10% nitro Omega (castor/syn blend) & an APC 12-8 prop. So far it has only had ~ 4 gal of fuel through it.

It starts very easily & idles dead reliably, but previously, the HS adjustment was pretty numb -- little response when near the "sweet spot" -- in fact it really didn't have a sweet spot. When it started to peak, I could turn the NV a full turn with no significant change in RPM then it would suddenly go lean & starve when I finally overdid the leaning out. While the HSN was a ***** to set, transitions were good & the LSN was very easy to set. When it seemed to be running OK, if the throttle was backed off from wide open & the revs dropped, the engine would often not pick up again upon opening the throttle. If I then leaned it out a bit it would pick-up OK, but would go lean a bit later.

In flight, with it tuned somewhere safely rich (?) in the "flat spot" it would often sag in flight shortly after take off & act like it had gone lean, but would then pick up mysteriously at times & go as well as it could (not that the @#$%^& thing is a powerhouse).

After reading this thread I checked the position of the spray-bar hole & it was off-center but not really blocked by the carb body. One thing that was immediately apparent was the very small diameter of the hole. I wondered if it was too small for adequate fuel delivery. --- Sooo, using the nipple as a drill guide, I drilled it out to the ID of the stock fuel nipple.

So far I have only done some bench tests, but tuning seems to have been improved noticeably. It now has a much narrower sweet spot in place of the old wide "dead spot" & it doesn't seem to slow down anymore. It is currently taching @ ~ 11,000 RPM on 10% fuel with an APC 12-8. I don't consider this to be particular exciting, but at least it seems to be holding there.

The knocking at idle seems to be louder if I use all-synthetic lube fuel (Cool Power).

Questions:

Is the knocking sound a common feature of this engine? My previous GMS 76 did not make this sound, although it was a performace pig like this one.

Is it a sign of impending doom?

I have looked at the bearings & they appear to be good -- no play in axial or radial directions. I haven't fully dissassembled the engine, (I prefer not to disturb its innards) but I wonder if the con-rod bushings are wearing.

Does the addition of extra castor oil make the sound go away? I am thinking of adding castor to the Omega to bring the total lube percentage up to as much as 25%.