CorradoMatt

Thanks for the replies. It seems to run ok so hopefully no damage. It's got the metal back plate so as Jon said, it has probably leaked out of the bearing.

Slightly embarrassing question to ask but I accidently flew my new Sig Astro Hog powered by a Saito 82b with the crankcase breather plugged! The breather is connected to a length of fuel tube that exits out of the bottom of the cowl. To stop it dripping oil over the car I plug it with a fuel tube stopper. Anyway, after it's first flight today I realised that the breather was still plugged as I was taxiing back to the pits🙄. The engine ran ok but was a little down on power and other flyers said that it was "popping" slightly. I was wondering if this would have caused any damage and what effect this would have had on the engine internals. I briefly ran the engine afterwards (with the breather unplugged!) and it seemed ok but didn't fly it again. I was wondering if the excess pressure would have escaped through the crank bearing seal and possibly damaged it. Cheers, Matt

Just an update on this. I stripped the engine down as i noticed that the liner was slightly out of alignment with the exhaust ports. The sides of the conrod were slightly worn down but i don't think this is a problem. When i stripped the carb i found that the 2 holes in the fuel nipple were not aligned with the holes in the housing (out by 90 degrees). I used a thin washer to pack it out so that the holes were perfectly aligned. Test run this morning and the throttle cut is now working perfectly. It won't be used but it's nice to have everything working properly. The engine drains the tank in about 1 minute - is this about right? I wanted to check rpm but my tacho battery is dead🙄.

Ok, thanks. So presumably i either need to make a new cut lever which is a better fit or remove it completely and blank the holes by sliding on a piece of tube. Hopefully this will improve the engine performance as well. I'm also using a 7x4 nylon Kavan prop.

Thanks for the replies JD8 and Engine Doctor. This would make sense but what confuses me is that when the spring pulls the lever to the cut position, the holes are not aligned. The holes are inline with the venturi.

Evening all, I recently bought an old Mills .75 (photo attached) that I'm planning on building a Veron Cardinal for. From what I can find on the web (https://modelenginenews.org/cardfile/m75.html), the engine appears to be a Mills S75 with a throttle cut fitted. I have test run the engine but the lever above the fuel tank appears to have no effect on the engine speed what so ever! The lever is joined to a pretty loose fitting collar that rotates above the tank and has 2 holes at 180 degrees to each other. This rotates on a tube which also has 2 holes at 180 degrees. The lever is missing the small tension spring which should hold it in the cut position (according to the website mentioned above). Does anyone have any more information on this type of Mills? I'd be interested to understand how it is supposed to work. Cheers, Matt

Thanks for the replies. As a starting point I made the outlets the same diameter as the original silencer. I have a couple of those pepperpot add on inserts which might be worth a try.

Took the Mystic to the field today to test the mk2 silencer design with the additional baffle and stainless steel wadding. The noise level has now dropped from around 86 to 84db so I am pretty close now to the 82db club limit. The engine has certainly lost the typical 2 stroke petrol engine exhaust pop and bark. I'm now going to change from the wooden Menz 17x8 prop to an APC prop. I'm not sure whether to try a 17x8 APC, 17x10 APC or 18x8 APC. Props at this size are starting to get a bit pricey so I don't really want to buy a few different ones to try! I think i'm going to try an APC 18x8 unless anyone recommends anything else. I've also been looking for alternative silencer designs and saw some threads on the LMA forum about a Sid King silencer design that is based on a gun silencer. I've registered for the LMA forum and will ask for more information. Does anyone here have experience of this design? Matt

Fitted now - I was pleased to see that i have clearance for the ht plug! I'll take reading with the new setup before I change anything else. I am currently running a wooden Menz 17x8 prop. I understand that using an APC prop may help reduce the noise and i could also experiment with the size. I am not sure whether to increase the diameter or pitch (or both) to reduce the noise. Does anyone have any advice on this?

Photos of the new silencer below. I've added a baffle around the exhaust port that will also stop the stainless steel scourer entering the engine. The baffle also adds support to stop the silencer being crushed when the bolts are tightened. Just need to fit and seal it on to the engine and carry out some testing. If this doesn't work i'm not quite sure what to do next other than experimenting with props or possibly fitting a header and cannister.

Thought I should post an update on this. I took the Mystic to the field today and ran the engine with the new silencer. We took sound level readings using the club meter and achieved an average of around 86db which was 2 to 3db less than the standard silencer (Club limit is the usual 82db at 7m). I was allowed to fly it to see how it sounded in flight and although the bark had certainly reduced, it still sounded pretty loud. I'm now going to make a mark 2 version with an added baffle around the exhaust port and fill the inside with a stainless steel pan scourer. I can also adjust the dimensions for better clearance on the ht lead and change how the parts are joined for a stronger joint. I'll post some pictures once i start making it.

Thanks for the replies. I've run the engine with it fitted but haven't taken any db readings yet as i need to do it at the field where there are no obstructions etc. I suspect that the heat of the exhaust may crack the joints as the parts are only butt jointed and then aluminium brazed over the top (not much thickness to the braze material). I think next time I will extend the end plates past the edge of the box section and then run a fillet of braze along the joint. I didn't fit anti-crush tubes to this one but I will with the new version. I have bolted it using Norlock washers and high temp silicone. The alternative is to have larger access holes opposite the exhaust port that i can pass the bolt heads through and only bolt up on the exhaust mating face. I'll then have to fit some kind of blanking plate to cover the access holes. I've got a few ideas about fitting extra baffle plates either side of the exhaust port or fitting another perforated tube that the exhaust feeds into first.

Evening all, I recently finished an old Galaxy Models Mystic that I have powered with an RCGF Stinger 26cc rear exhaust engine. The model has had 2 flights and performs well but unsurprisingly the engine is rather loud with the standard silencer. I have made a new silencer from aluminium using Durafix Easyweld to join the parts together. The new silencer is twice the volume of the standard silencer and uses "stinger tubes" to act as baffles. I have run the engine with the new silencer but have not taken dB readings yet. There are a few things that need modifying (plug cap is just touching the surface of the silencer for one) so i will need to make an improved mk2 version. I have found loads of information with regard to effective canister silencer design but there appears to be very little for the pitts style of box silencers. I was wondering if anyone has any good advice with regard to effective silencer design or could recommend something that has worked well for them? I was wondering if fitting a couple of perforated baffle plates between the exhaust inlet and stinger tubes would make a big difference? I've attached a few photos of the model with the standard silencer and home made item. Thanks, Matt

Hi Rich, Thanks for the advice. I read your message at the flying field so I widened the U/C as suggested and made sure that i wasn't going too fast. This certainly seemed to help as the next take off was much better! Unfortunately it was also my last battery for the day so i'll have to wait until tomorrow to try it again (plus a few minor tailplane repairs to carry out this evening!). The Gyroo has certainly received a lot of attention at the club and a couple of members are now thinking about building their own. Thanks again for all your help and advice! Matt

Hi Rich, I’ve now had 5 successful flights but I am still struggling with take offs! Once airborne I am fairly happy flying circuits etc but it is hit or miss if I am able to take off or do the classic roll to the left and end up in a heap! I’ve taken your advice and stand downwind of the model whilst getting the blades up to speed but I do find that the model starts to pull up on its own and then roll over. I’m wondering if I have too much rear tilt at the neutral position? It seem fairly well trimmed in flight. Matt