aj and phoenix

My Feedback: (2)

I have two older Super Tigre .40 engines that were made in Italy by the markings in the casting. They have the smaller "O.S" style mufflers. They were still in the boxes when I got them. They only have about 15 minutes and 1 hour run time respectively. They are both experiencing the same problem. They idle smoothly, transition is crisp and responsive. But above idle the RPM on both constantly surge up and down. I have tried running both in planes and on the bench with the same results. I have tried different sized props 10x6-10x7, no change. Tweaked needle valves until I turn blue and nothing seems to help. Any suggestions?

frikinstik

i have a ST40, 75 and a 2300. the first and last are italian versions and the carberation on them are tricky putting it lightly. on the 2300 i used 3 gallons of fuel trying to get every thing set right. it idled fine, highended off the scale, but in the mid range it really sucked and chugged. i have to step back and say that my 75 and 2300 are inverted. it took quite awhile to get the NEEDLES set right. at first i tuned it like most other engines, set the high end, then set the low end, go fly and be happy. on this one eveytime i made an adjustment on one needle i had to go back and re set the other. this was key. the low end is very important. keep cranking it in a 1/16 at a time until you get smooth low to high idle transition, then check the high end again. and again and again. the good news is that once you get it in the sweet spot you will know it because it will sound perfect, low and high and it will be time to fly and listen, land and tweek some more. once you get it there in the perfect setting you wont have to touch it again. my 40 was the same but on a smaller scale (obviously), but took much less time. my NEW 75 version had absolutly no problems from break in to now (over 50 flights), the new carberation is better. anyway it sounds like in your mid range is where you are having the problems. i had occilating mid range on the 2300 and noticed that the muffler had 2 exaust ports. plugged one of them and end of the occilating on that one. on another engine (os) i had too much back pressure from one of those dubro, exaust deflectors, and removed it and it worked fine.
i think that your problem might be the exaust portion of the stroke. if not then its in the carberation and turning them until you are blue in the face will pay off. you will love them once they are set.

Bax

My Feedback: (11)

The SuperTigre GS-40 is a ringed engine. It should be run quite rich for the first 15-20 minutes or so before it's leaned. We'd run it in a rich four-cycle mode. Once it's had that much time, we'll start to lean the engine a few clicks, let it run there for about 30 seconds, and then richen back up. We do this about once a minute or so. As we do this, we gradually go leaner and don't richen all the way back. This way, the engine gets the rich-lean-rich-lean progression and is gradually worked towards peak RPM.

If the engine won't hold a needle setting, richen it up and run it that way for a while. The Italian-made SuperTigre engines could take as much as an hour's worth of running before they were ready for flight. Fuel with mostly castor oil for lubrication is preferred in those engines.

For getting the needles adjusted, see the post at the top of this forum.

aj and phoenix

My Feedback: (2)

Does that explain the surging?

Bax

My Feedback: (11)

Surging usually means it's not set correctly. If you can't get rid of surging, then take the engine out of the model and put it on a test stand where everything is set up correctly and in the open. If you don't have surging, then you know it's probably something to do with the fuel system in the model. Perhaps you're getting vibration-induced fuel foaming. Do you use a remote filler? Is it leaking? And so forth. You'll need to do some detective work.

aj and phoenix

My Feedback: (2)

I have tried running them in a plane and on the bench. Same results. I have used the test stand to break-in/check-out some 60 plus engines over the years. Neither the bench or the pane are using remote fuelers just a piece of brass tubing between the carb and the tank. Fuel being used is Byron 15% nitro -16% oil. With the one in the plane doing the surging I pulled the tank and lines out, dunked the empty tank and tubing in water, and pressurized the tank. Found no leaks. Still confused, I can usually get an engine dialed in but these seem uncooperative.

Bax

My Feedback: (11)

16% oil is a bit low. Try 18% or more. We usually run a synthetic/castor blend and 10% nitro. The Italians always recommended 100% castor oil as the lubricant inthe fuel, but we've found that the blend usually works OK.

Hans Dahl Christensen

Hi aj and bill Baxter.

I agree - The older St - engines needs castor ( or ricinus - if you like ) .

A good blend which i have used in these engines is : 5 % castor ( or ricinus ) + 15 % ML 70 oil + 80 & methanol.
On bigger ST : .60 and .71 you could go to 3 % castor + 15 % ML 70 + 82 % methanol.

If you use notromethane in some percentage ( 5 % - 10 % ) it must be the fulle percentage so to say that the lubricant MUST be of the right size !!

Kind regards,

hld - OY - 9901.

aj and phoenix

My Feedback: (2)

I found the problem! I removed the carb again and took the needles out. I looked through the opening and saw nothing of particular interest. Just for grins, I ran a small piece of wire through the spray bar while holding it over a white paper towel. A small sliver of brass came out onto the towel. Evidently, when the opening in the spray bar was cut this sliver was left hanging on in the opening. I ran it on the test stand and it ran perfectly. With this success in hand, I tried the same procedure on the second one and it too had a tiny sliver in it too. It's like finding two new motors!

Bax

My Feedback: (11)

Great to hear you solved your difficulty. We've usually found that a piece of trash causes over-leaning and not the surging that you experienced. The surging is what led us away from the trash in the spraybar. As you found, that DOES make a difference!