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tim9227

Bubbles!!!!!!!

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Has anybody had any trouble with bubbles in fuel lines and how did you cure it?

Just swapped my Walbro wb37 to a Tillotson HL360a on my Cors-air Black devil hoping to cure the problem.

Fuel ratio is good all the way through the rev range but if I allow revs to drop below 3000 rpm then vibration causes fuel to froth in carb , goes lean and cuts out.

One squeeze on primer bulb and it fires up again.

Appreciate any thoughts on possible cures.

Thanks

Tim

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Does fuel line go vertical up to carb ? Any loop or u-bend can cause a vapor lock of air.

What inline fuel filter are you using ? I had 2 duff ones with tiny pin holes in middle, where 2 plastic halves were joined. No fuel leaked out, but tiny bubbles of air got in when carb was sucking fuel up. This type of problem would be made worse with any type of obstruction to the breather vent.

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Also worth checking your primer bulb for cracks/holes- ive discovered that these begin to degrade over time, and generally need replacing every 2 years or so- regardless of use.

I had a problem with bubbles on a machine before- solved it by reducing line lengths, removing the in-line filter and replacing the fuel line tank weight with a bronze sintered weight/filter. This should work- but only if you pre-filter all fuel going into the tank, and regularly clean the carb.

GD

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Thanks for suggestions ,

Been doing a few mods and current fuel system consists of:-

In tank fuel filter - primer bulb - external pulse pump - fuel header tank situated above carb with bottom outlet feeding new Tillotson HL360a carb and a top outlet doubling as vent and return to tank.

All seems a bit OTT but it eliminated the bubbles in the fuel line to the carb but the original fault of carb going lean and cutting out when left to tick over for about 30 seconds

still persist.

Am I missing something? ( apart from this great flying weather)

Thanks

Tim

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Small bubbles per say are not a problem and very common. They are due to fuel vapourising in the line due to ambient temperature. All my engines have used an arched fuel line approach to the carb without problem. However if the arch is too large a vapour lock can occur. Most problems ultimately prove to be an air leak usually from a perished primer bulb. I removed all of mine and changed to a single breather pipe which I blow into to pressurise the tank and then prime the carb (Miniplane setup).

I would advise going back to the WB37 and ditching the primerbulb if the vibration was less with that carb.

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Stripped fuel system back to basics and refitted Walbro carb.

Tick over is a lot smoother and feels a bit more powerfull but still having problem of motor cutting out on tickover which is a pain when descending as it will only restart with a prime.

Any ideas anyone?

Thanks

Tim

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Check the gasket between the carb and the engine block- if there is a small hole feeding back into the carb- make sure that this isn't blocked- or the gasket dosen't block it. I found it necessary to make this hole slightly larger on the factory gaskets to get the perfect idle. When it is blocked, the metering membrane onthe carb dosen't function properly. Sometimes the gasket can be fitted upsidedown, thus blocking the hole too. If there is not a hole here, perhaps there is a pipe from the crankcase to the carb- again check for blockages, or holes in the pipe... or bad fittings at the ends.

This will have more of an impact at lower revs- but generally will make the engine run rougher if the metering system is not 100%

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On 26/07/2012 at 22:04, tim9227 said:

Stripped fuel system back to basics and refitted Walbro carb.

Tick over is a lot smoother and feels a bit more powerfull but still having problem of motor cutting out on tickover which is a pain when descending as it will only restart with a prime.

Any ideas anyone?

Thanks

Tim

Hi Tim, did you ever get this sorted? Im having the same problem now with air bubbles. Ive replaced everything and so no joy. Cheers

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