Brand new 100 with the reg up front. Anything but idle is great. High speed set, but was having a lot of trouble dialing in the idle (yes I have read the procedure, and as far as I know following it correctly). The engine surges at idle and within 45 sec to 1 min. dies, setting at 2000rpm, but since it's surging so much it doesn't stay there. One thing to note is that there seems to be a lot of air bubbles coming from the reg to carb line. None coming into the reg (hose is new and tight). Air leak in the reg? if you watch the bubbles the surging seems to match the bubbles coming thru. It only takes a click or two up throttle to stop the surge, also the bubbles stop, conincidence? FYI it's mounted on a Hyde soft mount, but I've never had problems with the old 110s on Hyde mounts.
What do you think Troy?

Yes sounds like it might be an air leak....doesn't have to be a leak in the regulator. The regulator gets its pressure on one side from the engine...so it could be an air leak. Regulator gets one side of its pressure from the engine side of things. If there is an air leak in at the rear of the engine it might give air bubbles through the regulator.

Another option is to make the regulator a bit richer. Out CCW is rich. Dial it about 1/2 turn and try it again.

Its normal to have some bubbles at the 2000rpm idle. This is likely from foaming in the tank. what happens is the engine is being fed fuel that is under pressure. When it gets past the regulator the line from the regulator to the carb is no longer under positive pressure its under suction from the carb...only at the 2000rpm idle. SO when the pressure changes from tank positive pressure to the the suction from the carb a Small tiny bubble from foam in the tank will turn into a big bubble you can see. These bubbles go away after the engine goes above 2000rpm because the carb is not longer sucking the fuel...above 2000rpm the fuel is being force fed under positive pressure into the carb. This is why the low end air bleed screw only works at 2000rpm.

But a bunch of bubbles can affect things. You need to decide if its foamy fuel that is causing the bubbles or is it a leak in the engine.

So What I would do is open the regulator about 1/2 turn richer and try it again. See if you can stop the surging. The air bleed screw is only a fine tune for that 2000rpm mixture.

Hyde mount will have nothing to do with it. Its fine on a Hyde Mount.

Go for the Regulator open 1/2 turn and then see what happens. Try to put some foam rubber to mount your tank and not let it touch the bulkheads in the model. This will help the foaming. Also you can check the head, and or carb, air box bolts....make sure they are tight actually a little past snug not overly tight, The casting can get warped if you crank down on the bolts. Always tighten the bolts in a crossing pattern a little at a time like installing a tire on the family car.

This will insure that the part gets tightened evenly and doesn't get twisted.

Troy Newman
Team YS

Thanks for the update Troy, I'll go over all of those things. I tend to shy away from messing with the regulator unless it's the last option.

Same thing happened with my 110, It got better or perfect after 1 1/2 gallons of fuel,nothing worked before that.

Just a comment

Well the engines do take a little time to get broken in but after a couple gallons you should be good to go. What happens when the engine is new and breaking in its making lots of heat...so the mixture needs to be rich to help this problem. As the engine break in a little the heat generated is less and you have to lean the mixture to keep the heat at the right levels.


Remember these engines work very similar to diesel engines. They work on temperature and pressure. They is not a spark plug that makes the fire. So mixture settings are very Dependant on heat generated. If the engine is running too cold or running too hot it will not work on the right mixture settings. Since during a new engine seating and break in period the engine is going from a hot running friction grinding beast to something that is smoothed out and clearanced properly the heat generated is different.


For the most part regulator changes are rare....but on a new engine they will almost always need to be done. The regulator adjustment is not something that will change much but a 1/2 turn to a turn each way is not uncommon between all the engines. This has to due with spring tensions....and machining and all the other things like fuel choices.

You are in Tuscon, AZ so your elevation is higher than most places. Usually this would mean a slightly leaner setting. Local conditions, temps, fuel, load, all these things play a role in the mixture settings. There is no way in Japan at Sea level and 70degs F a setting for 20% Cosmo fuel is going to be the exact fine tuned same setting as Denver Colorado at 6000ft and 30% heli fuel and a 90deg day. It just ain't so.

This being the case its really rare to have to touch the regulator changing elevations. I have however when I lived in Denver had to change it but its really rare

Since the engine is new its likely close and your setting and symptoms show its close....but a little fine tuning is not a bad thing.


Troy Newman
Team YS