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] <mailto:[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] <mailto:[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] <mailto:[email protected]> On Behalf Of Jarek Steliga Sent: Saturday, 30 November 2019 10:30 a.m. To: Laurie Hoffman via dog <[email protected] <mailto:[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
