Running rough after a spell

If you’re using auto fuel I’d say it’s old gas. If you’re using 100LL it’s something else.

I'd do a couple things and retry. 

Auto fuel in this scenario is more a culprit than 100LL. I would drop the bowls and unscrew the idle jet right next to the main jet and clean it out. I do this every annual or any time I pop the bowl off to let's say to weigh the floats. I have found about 50% partially or completely clogged especially after long periods of sitting. Since you're going to pop the bowl off to do this might as well weigh the floats. You're already there removing the bowl might as well kill two birds with one stone. Then after this is done I would put my carb sync gauges on and to a carb sync check and balance.

Even if it's running better I'd still clean the idle jets and put some gauges on it.

I'm leaning on the idle jets with the info we have.

Hi John,

Of tangential interest only;

Like you I live in NSW, Australia (relevant to fuel quality/aging behaviour).

Since retiring I have become involved in resurrecting small engines - pumps/generators/mowers /chainsaws/ etc.

I would guess that around 80% wont start due to old fuel. Sometimes so old, that it has turned to a very smelly treacle coloured/textured substance.

The fix is simple - dismantle the carburettor & thoroughly clean, making sure all jets are open. 

I clean with carburettor cleaner (the cheapest I can find), if mechanical assistance is needed, copper wire to "rod" a jet and compressed air.

Refit carb.

The engine usually starts first/second go & runs just fine.

It doesn't take long for the fuel sitting in a small engine carb to  get to the point where is inhibits fuel flow/starting, may be sitting for 12 months. Repeated idle periods, probably results in a coating of "varnish" on the jets/needle which reduces fuel flow making for difficult starting and rough running.

I advise my small engine friends is to drain ALL the fuel  INCLUDING FLOAT BOWL when not using the engine for an extended period.

Small engines use 91 RON fuel - much more resistant to aging, compared with 98 RON. Rotax recommend a minimum of 95 RON but in my area 95 RON is becoming hard to find, so I use 98 (as most of my flying friends do). 98 RON ages rapidly when exposed to atmosphere (vented fuel tank) so before flying always "freshen" the fuel in your tanks with recently purchased fuel ie top up before flight, not after..

My advice to anyone, knowing they won't be flying for an extended period, is to drain all the fuel from the aircraft, including float bowls.

Roger Lee wrote:

I'd do a couple things and retry. 

Auto fuel in this scenario is more a culprit than 100LL. I would drop the bowls and unscrew the idle jet right next to the main jet and clean it out. I do this every annual or any time I pop the bowl off to let's say to weigh the floats. I have found about 50% partially or completely clogged especially after long periods of sitting. Since you're going to pop the bowl off to do this might as well weigh the floats. You're already there removing the bowl might as well kill two birds with one stone. Then after this is done I would put my carb sync gauges on and to a carb sync check and balance.

Even if it's running better I'd still clean the idle jets and put some gauges on it.

I'm leaning on the idle jets with the info we have.

Hi Roger,

Thanks for the reply.  When is the idle jet not in the mix, as it was doing it at 4000rpm.  I didn't think the idle jet would be a factor at that rpm range?

Sean Griffin wrote:

Hi John,

Of tangential interest only;

Like you I live in NSW, Australia (relevant to fuel quality/aging behaviour).

Since retiring I have become involved in resurrecting small engines - pumps/generators/mowers /chainsaws/ etc.

I would guess that around 80% wont start due to old fuel. Sometimes so old, that it has turned to a very smelly treacle coloured/textured substance.

The fix is simple - dismantle the carburettor & thoroughly clean, making sure all jets are open. 

I clean with carburettor cleaner (the cheapest I can find), if mechanical assistance is needed, copper wire to "rod" a jet and compressed air.

Refit carb.

The engine usually starts first/second go & runs just fine.

It doesn't take long for the fuel sitting in a small engine carb to  get to the point where is inhibits fuel flow/starting, may be sitting for 12 months. Repeated idle periods, probably results in a coating of "varnish" on the jets/needle which reduces fuel flow making for difficult starting and rough running.

I advise my small engine friends is to drain ALL the fuel  INCLUDING FLOAT BOWL when not using the engine for an extended period.

Small engines use 91 RON fuel - much more resistant to aging, compared with 98 RON. Rotax recommend a minimum of 95 RON but in my area 95 RON is becoming hard to find, so I use 98 (as most of my flying friends do). 98 RON ages rapidly when exposed to atmosphere (vented fuel tank) so before flying always "freshen" the fuel in your tanks with recently purchased fuel ie top up before flight, not after..

My advice to anyone, knowing they won't be flying for an extended period, is to drain all the fuel from the aircraft, including float bowls.

Hi Sean

Thanks man.  I only use 95, and AVGAS if I am out on a large nav and have no choice.

I am leaning toward fuel/carb and should get some time to play with her over the break.  Will pull the crabs and have a look.