Hi Ian, I thank you again. All your thoughts plus those of your colleagues are a real life line for me, as there are only 3 H36 Dimonas in Poland (including one grounded due to some certification problems). I will be reporting the results once the new throttle shaft is put in.
Best regards Jarek On Sun, 1 Dec 2019 at 22:38, Ian Williams <[email protected]> wrote: > Hi Jarek, > > > > Good luck with the carb repair. I will be interested to see or hear the > result. > > Looking at your pictures I would say that the wear factor is very high. > Basically very similar to the wear I found here. > > Re the red silicon … you may also discuss with an industrial chemist … or > Rob Thompson, but basically what happens is that uncured red silicon (or > any silicon sealer) tends to react with the carbon in fuel and creates long > polymer strands (like micro rope) These strands will block anything and > create real havoc. > > For example, the problem I had (and you do tend to learn lot with > experience) was .. I had made a fitting into the fuel tank so I could > install a fuel dip stick… and used some silicon to ensure a good seal > (didn’t need it in the end because I later installed a MGL fuel computer > system). Some apparently dropped down into the fuel outlet at the bottom of > the tank. > > Anyway, after a few hrs, during one flight the engine lost power. > > After the initial panic subsided, I took apart the fuel system. And didn’t > find a lot wrong HOWEVER, poking a piece of wire into the banjo fitting > and gauze screen at the bottom of the tank I found a blockage of soft tacky > stuff. It was a “mass” of strands that had built up enough the block the > tank outlet. > > May sound weird … and it was but it happened. > > But overall I do like the little Limbach engines … they are very simple > and by themselves quite reliable if you look after them … like regular > checks of the tappet clearances. > > Another thing I am the maintenance engineer for 2 Grob 109’s. I > thought I would compare the fuel flow from one with the Dimona… both are > quite similar in design. Setting it up, the pump gave a few spurts then > stopped. So the pump had failed. It is the same Hardi pump as installed > on the Dimona’s. The life for those pumps in a Dimona is 1500 hrs …. > I am sure this one was on the Grob for much longer. > > But when you think about it, it is easy enough to check the mechanical > pump … by turning off the electric pump and doing all the ground running. > Prior to turning it on and flying away. However apart from the pump’s > noise, there is really no other way of checking the electric pump…. Which > was the case with the Grob 109. > > The MGL fuel computer flow sensor is installed close to the electric > pump. When it is on, the measured fuel flow almost doubles because it is > affected by the pump’s pulsing so that is a very basic way we can check if > the electric pump is operating. > > > > I hope that all helps > > > > Best regards > > Ian Williams > > New Zealand > > > > *From:* [email protected] *On Behalf Of *Jarek Steliga > *Sent:* Monday, 2 December 2019 7:04 a.m. > *To:* Laurie Hoffman via dog <[email protected]> > *Subject:* Re: [DOG mailing list] Stromberg 150 CD3 air leak around the > throttle axle > > > > > > Hello, please let me respond to each post separately. > > > > Rob, > > > > While rough running I have never experienced any detonations, just a > serious drop in RPM and generally things seem to go out of whack. > > > > > > Ian Mc Phee, > > > > I believe mine are the old aviation leads (plugs were recently replaced). > I did suspect they might have been broken so I borrowed a similar set from > a friend who has FALKE. That other set did not seem to make any > difference so I put mine back on. I am going to run the engine in the dark > as you suggest though. > > > > The TUMA tap is no longer there. But I realize that it is high time for me > to check up the fuel flow since I have still neglected this measurement and > am still not even sure if that electric pump does or doesn't work. > > > > > > > > > > Ian Williams, > > > > A new throttle shaft is on its way to me. Since I am unable to lay my > hands on any spare carbs the rebushing may become the only option. Your > comment gave me some new hope. I am definitely going to share the results > once things get done. Why should the red sillicon be kept 100 miles away > from the fuel system? > > > > > > I hank you all very much. > > > > > > Best regards > > Jarek > > > > > > > > > > > > > > > > On Sun, 1 Dec 2019 at 00:45, Rob Thompson <[email protected]> wrote: > > My experience with worn throttle shafts is that they cause an overall > leaning of the mixture but not sudden changes to rpm with the throttle > position constant. > > What you are hearing can be a good indicator of what is going wrong. What > does it sound like when the RPM drops? > > Rob > > > > Rob Thompson > > 0429 493 828 > > > > > > > > On Sun, Dec 1, 2019 at 10:01 AM Ian Williams <[email protected]> wrote: > > Hi Jarek, > > Just may be able to help you there, as I have had a similar issue > > I have just completed a 500 hr inspection of our Dimona. It is one of the > early ones … (one of 10 bought by the Thai Air Force in the early 80’s) > > The engine was reconditioned in Germany about a year prior to all 10 being > sold by the RTAF. However the aircraft did not fly again with them prior > to being sold. > > The engine was half installed … ie refitted but not completed. > > One carb was new while the other had been used. > > The aircraft has done around 450 Hrs now in New Zealand. I have had a > couple of engine problems … one where the magneto condenser became very > leaky .. happened just after take off and the engine didn’t like it at all. > The other was a fuel problem which as it turned out was self inflicted > (note .. keep red silicon 100 miles away from any fuel system) > > > > So back to the present. Because of some corrosion around the engine and > baffles, during the 500 hr inspection I took the engine out and corrosion > protected everything. Then engine back in. > > The first test flight went well with no apparent problems … However the > second and subsequent flights … not so. > > What was happening was at around 500 to 800 or so feet after T.O. the > engine would loose about 400RPM for about a second, then pick up again. > Later it would do the same randomly even in coarse pitch but a bit longer. > > I thought it was a fuel problem so took out the tank, flushed all the > lines and checked the fuel flow (took off the fitting to the RHS carb and > ran the electric pump for 30 secs) got 28 to 30 L/hr which I thought was > OK. I have an MGL fuel flow sensor which has a small orifice so gave that > a good clean as well. > > After putting everything back, another test flight … still the same > Bugger. > > I did think it could be an plug lead problem but if a plug stops firing > you sure do know about it so had to rule that out. > > So I took both Carbs to a bloke about 15Km from where we are. He is an EX > Air Force engineer specialising in fuel systems. He took a look at the > carbs and said that the throttle shaft of one carb … he wouldn’t allow that > on a car. … the shaft was worn but didnt seem excessively so to my limited > experience. > > Apparently there is available a kit to repair these. However also the > bearing surfaces (just part of the carb body really) can be replaced with > Teflon bearings he said … so basically quite repairable. > > It seems that even a slight leak around the throttle shaft can upset the > operation of the plunger with the needle attached … which is quite > surprising. > > I just happened to have a couple of “new” carbs in stock so installed them > after checking the diaphrams condition. Did a test flight yesterday and > all was good. I did lean the mixture a little as the CHT’s were quite low > (100 to 120 degrees C). Of interest, the engine started a lot easier and > in the past had a tendency to stop on roll out after landing (even with the > idle RPM set to around 900 RPM) … but no issue now. > > So finally … yes, rebush the throttle shaft / bearings. Any leakage > around them seems to have a significant effect on the operation of the CD > control. > > Hope that helps > > > > Best regards > > Ian Williams > > New Zealand > > > > > > > > *From:* [email protected] *On Behalf Of *Jarek Steliga > *Sent:* Saturday, 30 November 2019 10:30 a.m. > *To:* Laurie Hoffman via dog <[email protected]> > *Subject:* [DOG mailing list] Stromberg 150 CD3 air leak around the > throttle axle > > > > > > > > Hello, > > > > I am still struggling with my Limbach rough run at RPMs around 3000. Since > my max static RMPs are ca 2700 the problem only becomes apparent on take > off (I actually had to abort one take off while being already ca 100 m > above the ground - a rather hair raising experience). > > > > After having replaced a perforated metering needle piston diaphragm with a > new one I am back to square one in my detective work of what gives. > > > > The next suspect I am zeroing in on is a badly worn out brass throttle > axle combined with the visibly enlarged hole in the carby body this axle > protrudes through. Can the air passing through this opening affect the > formation of underpressure inside the carby to the extent of producing > effects above described? > > > > Any comment will be greatly appreciated as always > > > > Regards > > Jarek > > > > > > > >
