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Engine surging / rough running

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1 week 4 days ago #22084 by chris.
Engine surging / rough running was created by chris.
A few weeks ago my plane (Rans S-19) had its maiden flight. Just after it became airborne the engine began to surge. A Rotax engine specialist visited and detected the the overflow line from the airbox was venting at a position that was too low - about 2 inches below the firewall - and was experiencing a ventuir effect from the propellor slipstream. That problem was easily fixed but the engine then began to run very rough. This time the problem was rubber contamination in the carby bowls, especially the right side though the needle jets seemed clear. I'm not sure where the source is. The gascolator is on the firewall and it's clean. I blew all my rubber hoses out with compressed air before installation so I'm wondering if vibration of the rubber fuel lines on the fuel line barbed fittings would be enough to cause this?

My main question: I had somebody with me the other day who works on car engines. He asked why fuel filters weren't installed just before the carbys given that sort of contamination in the bowls can be a real show stopper. The only answer I could think of was "nobody seems to do this" and I would prefer to eliminate the problem at source but if it's going to be a recurring problem I'm wondering if it might be a good suggestion? I could wrap them in firesleeve. Any thoughts?

Thanks,
Chris

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1 week 4 days ago #22086 by Roger Lee
Replied by Roger Lee on topic Engine surging / rough running
Hi Chris,
First you shouldn't listen to auto mechanic that has zero experience on aircraft or Rotax engines. Okay the easy part is done. :)

When installing new hose it can make a difference on particles coming loose on the type of hose installed, barbed fittings vs smooth, type of cutter used on the hoses and yes a filter (But only up to a point).
You do not need a filter before the carbs. If you have a fuel flow transducer you should have a filter before that, then many have a gascolator which has a fine mesh screen and what most do not know is your fuel pump has a screen. So your offending hose will be the one off the bottom of the fuel pump and anything past that if they are rubber and not Teflon. If you have Rotax carb hose that comes off at the carb crossover tube those red hoses are Teflon and not the cause. Any rubber hose past the fuel pump going to the carbs can be the culprit. It is usually limited and once flushed out tends (not 100%) to be gone.
Your vent tubes need to be equal length and out of any air stream like you mentioned out the bottom of the cowl. If you know the carb bowls are clean now hopefully your done, but I would fly a 2-4 hours and double check them.
This type of issue can be common from doing the install or a 5 year rubber hose change if someone does not pay attention to detail. The article below will help you understand.

Good luck with your plane and have fun flying!

Here read this and this may help with current and future understandings:

www.rotax-owner.com/en/rotax-blog/item/2...ation-considerations

Roger Lee
LSRM-A & Rotax Instructor & Rotax IRC
Tucson, AZ Ryan Airfield (KRYN)
520-574-1080 Home (TRY HOME FIRST)
520-349-7056 Cell

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1 week 3 days ago #22091 by chris.
Replied by chris. on topic Engine surging / rough running
Thanks Roger,

I didn't know the fuel pump had a screen in it. My hoses are Gates Barricade. I've got 5/16th rubber fuel line from the fuel pump to a 4-way brass manifold on top of the engine. It goes out to the 2 carbys and the last orifice is a return line with 0.35 mm jet that goes back to the gascolator. Thanks for the link to that document. Plenty of good advice there too.

Cheers,
Chris

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5 days 4 hours ago #22243 by pouellette
Replied by pouellette on topic Engine surging / rough running
I had what I thought were rubber pieces in my float bowl. Turned out that one of the floats was flaking off pieces. Almost caused engine stoppage during a critical go-around.
Pete

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3 days 11 hours ago #22250 by chris.
Replied by chris. on topic Engine surging / rough running
One of the floats has a mark on it like it has been abraded against a rough surface. I'll keep an eye on the possibility the contamination might be from the floats.
Thanks,
Chris

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