YS120 - air in fuel line.

[Malc1]

I have just developed a problem where my YS120 fails to idle properly and I suspect is also running rough causing a tail wag on the T Rex700.

I can see air bubbles in the short clear piece of tubing from between the regulator and the carb and as they pass through you can hear the engine misfiring.

Can't see any fuel leaking anywhere but air has to be getting in from somewhere.
Seem to have been messing around with carb adjustments today which as expected doesn't make any difference.

I'm using both of the QuickUk tank fitting upgrades and an OS bubbleless clunk.

The engine has had about 5-6 gallons through it.

What's the best way to track this problem done?

[Jamin_00]

I'd just replace all fuel tubing at first.

Couldnt this also be the diaphragm inside the reg??

[tonystevens]

Its the crappy tank bung mate, the quk one is no better, just a metal version of the align one., desnt give you an airtight seal even if it doesn't appear to leak fuel. You need to get a Dubro tank stopper and double up the fuel tubing at all the connection points.

[Malc1]

Thanks for the input guy's.

So really out with the tank first and mess around with the fuel lines to start with.

All the fuel tubing has been doubled up already when the engine was installed but the different bung option hasn't been looked at.

I wondered also about the regulator and check valve earlier. Didn't want to really have to pull the engine out unless I have to.

Must say it is a pretty constant stream of air bubbles coming through that tube when the motor is idling.

[tonystevens]

ive seen this with all the 120's ive had or had experience with and ill be surprised if the tank bung isn't your problem. Personally id start with that and the fuel lines. Theres two types of dubro stopper, one has a self tapping screw that tightens into a plastic plate, (I wouldn't use that one as it'll strip the threads before you can tighten it enough to get a good seal) and one that uses a nut and bolt instead, that's the one you want. Also keep all your tubing as short as possible and even double up on the short piece of tubing between the reg and carb where your seeing the bubbles, youll be surprised how slack a fit that is on the nipples!

[waveydavey]

Malc, is it just in the idle that you have this problem. Whats the engine doing, I have similar, bubbles in the line from the reg to the carb. Ive tried everything but can't get rid of them and mine runs oddly in the hover, not so much at idle.

[Malc1]

I noticed that the idle was all over the place when I went down the field earlier and noticed the bubbles in the fuel line which tie up with how the engine sounds.
Changed the plug and had a play with the idle needle as well before spotting this.

The engine seems OK when hovering and flying but it may or may not be 100% right.

I have a tail wag that I can't get rid of and am now suspecting that the motor is not running as smooth as it has done.

Need to try and get the engine running properly before looking at anything though and will check the V Bar log later for any sign of vibration from rough running.

[KevDavies]

I fully concurr with Tony. Ive also seen a YS120 doing the same on a 700. Some reason the tank bungs go porous and draw in air. I think the dubro ones are at Hobby king, and I think ive linked Tony to em in the past, just cant think at,

[Malc1]

ive seen this with all the 120's ive had or had experience with and ill be surprised if the tank bung isn't your problem. Personally id start with that and the fuel lines. Theres two types of dubro stopper, one has a self tapping screw that tightens into a plastic plate, (I wouldn't use that one as it'll strip the threads before you can tighten it enough to get a good seal) and one that uses a nut and bolt instead, that's the one you want. Also keep all your tubing as short as possible and even double up on the short piece of tubing between the reg and carb where your seeing the bubbles, youll be surprised how slack a fit that is on the nipples!

Tony, are you replacing both of the bungs?
The central outlet and the outside bung?

[tonystevens]

mine was a DFC so no central bung, if your using one of these in the centre hole Quick UK Retail - shop-products then that's good they seal up a treat just change the outer hole for the dubro type

[Malc1]

I've got one of these already fitted to blank off the outer hole and as mentioned the Quick UK upgrade on the tank outlet in the middle.

[waveydavey]

When I've checked mine, tank in sink of water one way on tube and the other blanked I had no bubbles at all out of the tank and the pressure would stay in there, at least longer than Louise was prepared to have the sink full of water and nitro anyway

[tonystevens]

Yes Kev linked me to a nice metal dubro type bung but the brass tubing was too large a diameter to use with standard fuel tubing I ended up buying a small Dubro plane fuel tank which had the bung included and used that. Dave, that's how I test my tanks. with all the tubing and fuel filters fitted, but I pressurize the tank way beyond anything it would see from the crankcase pressure and leave it over night, if its still pressurized the next day your good to go! I think this is key to successfully running a YS120, the 91's didn't seem to suffer the same problems

[Malc1]

Would this do or is it too big?

DB400 | Dubro Petrol Conversion Bung

[tonystevens]

That's just a replacement bung, you need the full bung stopper and tubing, ill try find a link for you