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
