residue on the inner carb surfaces

[Birdseyeview]

I'm trying to accomplish a pneumatic sync on my 912 ULS Bing carb and it is running so rough when I hook up my carb mate it doesn't let me make any progress toward syncing because the vibration overcomes the float needle valve and fuel vents out the carb (left carb only). This was part of another previous forum discussion and I thought I had it resolved when I lowered the float levels in both carbs, which worked fine until I hooked up my carb mate. I'm attempting the p-sync at 3000 rpm. When I removed the carbs to do another internal cleaning I noticed a thin film of some dark sticky residue on the internal carb surfaces downstream from the butterfly valve. It came off with a quick squirt of carb cleaner but I'm wondering where it came from? It seems like there's only two possibilities: 1) Something coming in from the air cleaner or 2) something coming in from the fuel tanks or lines. The engine only has 2.6 hours total run time. During my gas tank install into my wings last year I did coat the inside of my tanks with the Bill Hirch gas tank sealer that some on the forum suggested would protect my older fiberglass tanks from ethanol. It appears to be surviving fine when I look into and probe the internal surfaces of the gas tanks. I did put a few gallons of ethanol gas into the tanks a couple of months ago when I did my first fuel flow test but I drained it all out after the test and have only used 91 octane and no ethanol gas ever since. The fuel lines I'm using are all the thicker black rubber fuel lines that were supplied by Kitfox. I have a clear polyurethane header vent tube. Has anyone seen this type of residue buildup inside their carbs? If so, is this normal?

[jrevens]

I can comment about the carb sync Larry. It sounds as though you might not have done a good mechanical sync adjustment before doing the pneumatic sync. Is that the case?

[Birdseyeview]

John, I've been over the mech sync both in my mind and rereading the manuals and rewatching the videos several times and I believe I'm doing that part right. The experts on the Rotax forum suspect that I may have an underlying problem in the carbs that is contributing to the rougher than "normal" idle and preventing the P-sync. I'm at my whits end on that front so I've decided to take it into the nearest Rotax overhaul facility and have them look it over. I'm only 85 miles away from LEAF. Hopefully they'll find something I missed or that I screwed up in my recent carb overhaul attempt. I haven't looked this up to confirm it but someone recently told me that the word "Carburetor" is derived from a French word that translates into "DO NOT DISASSEMBLE". I'm believing that it may be true.

[Av8r3400]

Do a search for “brown sticky stuff” or “BSS”.

this is release agent from your fuel tanks most likely.

[Birdseyeview]

Mangy Fox, Thanks for that input - reading the threads on the BSS was very enlightening and a couple of them described my issues to a tee. While installing my wing tanks a couple years ago I coated my 2001 vintage tanks with the Bill Hirch sealant after reading some threads about it vs the cream issues. This was before I installed the tanks in the wings. I never rinsed the tanks with acetone at the time and it appears that the Bill Hirch sealant didn't prevent the fiberglass release agent from dissolving out and turning into the BSS. I plan to remove my wings now and rinse with acetone then let them air out for a few days. Some of the builders in the threads swore off using auto fuel after their experience with the BSS. However, once the tanks are rinsed and harden up shouldn't they then be free of the BSS and auto fuel safe? My home base airstrip sells both 100LL and non-ethanol auto fuel so it is very convenient to use the auto fuel here and I like the extended oil change interval it allows. What's your opinion about using auto fuel after the acetone rinse?

[avidflyer]

I would contact the tech people at Bill Hirch sealant and see what they have to say. Will the acetone dissolve the sealant? I had a question about KBS coatings that I used to seal a tank on my Kitfox and tech people there were very happy to help. JImChuk

[Birdseyeview]

Jim,

Thats a great suggestion - Thanks