LSN and Idle speed

[Flying Squirrel]

As a relative new guy I'm looking for advice/guidance on setting the LSN and in particular it's relationship to the air gap in the carb. Ie, if I lean out the LSN a touch the RPM increases and I therefore lower the throttle for a steady tickover, I'm not sure I'm doing it right.

 

For clarity, the engine is an inverted force .46 2 stroke and occasionally cuts when closing the throttle. The idle can also be a little inconsistent, slightly fast for a few seconds before dropping.

 

Members of my club have tried to help but the consensus comes to an on board glow being the answer, I'm loathed to do that as I think it should run fine without, maybe I'm wrong.

 

Centre of tank is just below the spray bar, no leaks in pipework etc running optifuel 12%

[Denis Watkins]

A pressure leak does not have to show a wet patch

As the pressure increases with a the increase in throttle, a quick sshhhh of leaking pressure will interrupt running.

This problem sounds like plumbing, even with new pipes and new tank, check plumbing again.

Also, the 3 Force I helped fit all leaked pressure at the over complex Banjo fitting.

Cured using correct sized spanners compressing the washers tightly.

[Engine Doctor]

Also is the engine run in ? A good idle won't happen if engine is still on the tight side.

Let the engine idle for about 20 to 30 seconds then open throttle smartly. If engine coughs and splutters and is a bit smokey then the idle screw is too rich. If engine hunts then cuts its a weak mixture. Adjust by tiny amounts and repeat until you get a decent Pick up. 

Also a cold plug will cause poor idle an Enya no 3 or OS no 8 are both general use medium heat plugs and reliable quality.

Edited September 1, 2022 by Engine Doctor

[Nigel R]

As above, when the engine is run in, adjust the LSN to get the best pickup. Use throttle trim to adjust idle speed after that.

 

[Flying Squirrel]

Thanks guys, I'm pretty sure (as far as I can be) that plumbing is sound. Plug is currently a model technics 3d plug (4 stroke length) but have also tried OS 8, and Enya 3s'

 

Yes, the engine is run in. Generally the engine transitions from a idle to WOT quite smartly but occasionally quits returning to idle.

 

As I alter the LSN and the rpm increases/decreases and I have to adjust the throttle idle position accordingly - I'm wondering if I'm 'masking' one with the other and getting lost, hope that makes some modicum of sense!

[extra slim]

Sounds like LSN is a little lean to me. Engine “hanging on”, when throttling Down is a sign of this. Onboard glows tend to be for rich settings and inverted engines, so not needed here IMO. I would richen the LSN quarter turn and see if it is any different. When I have issues I do 1/4 turns until I get a notable change and then make finer adjustments thereafter. Good luck

[Andy Gates]

extra slim - his engine is inverted so an on board glow will be of some help in this instance.

[extra slim]

Not if it’s lean it wont

[Jon H]

Got a video of the beast in action? Assuming you can keep the camera out of the propwash it is much easier to diagnose with a video. 

[Flying Squirrel]

Not yet, I'll try and take one next oppurtunity. The cutting out often only happens in the air though, even after behaiving for several minutes on the ground - maybe that's a clue to the more educated?

[David Davis]

Is the engine cowled? If so it might be over-heating.

[Denis Watkins]

This Banjo assembly leaks pressure unless tightened to compress the soft washers.

[Gary Binnie]

Sounds to me like the carburettor barrel is not returning to the same position at idle, there is usually a throttle barrel stop screw though I'm not familiar with this engine. Would check that before adjusting mixture.The low speed is adjusted after setting the high speed needle.

 

If there is a stop screw adjust the throttle cable/pushrod to close it with a little bit of tension applied (over travel).

 

That engine looks good quality but some have awful carburettors that can't ever be set to run properly. My ASP 61 low speed adjustment is painful, my ASP 120 was completely transformed by fitting the carb from an OS 90FSR. 

 

[Nigel R]

Denis might be on to something there, has the remote needle thing been checked / tightened?

 

It's amazing what a touch of vibration can do to an assembly like that.

[Flying Squirrel]

13 hours ago, David Davis said:

Is the engine cowled? If so it might be over-heating.

 

 

It is,  and thinking about it, it happens more with the cowl on, either that or I just notice it more as I have to remove the cowl to investigate. I've not noticed it feel overly hot but what temp range should I be looking for if I get the laser temp gun out?

 

 

 

 

12 hours ago, Denis Watkins said:

This Banjo assembly leaks pressure unless tightened to compress the soft washers.

 

 

I did suspect this as mine actually clamps on to a DIY bracket (the original snapped but) but as far as I can tell the joint/joints are tight and sound, I will check again though.

 

 

11 hours ago, Gary Binnie said:

Sounds to me like the carburettor barrel is not returning to the same position at idle, there is usually a throttle barrel stop screw though I'm not familiar with this engine. Would check that before adjusting mixture.The low speed is adjusted after setting the high speed needle.

 

If there is a stop screw adjust the throttle cable/pushrod to close it with a little bit of tension applied (over travel).

 

That engine looks good quality but some have awful carburettors that can't ever be set to run properly. My ASP 61 low speed adjustment is painful, my ASP 120 was completely transformed by fitting the carb from an OS 90FSR. 

 

 

With the engine not running it appears to return OK, there is a throttle stop but this is wound in to allow throttle cut to work.

 

I've had force engines in cars before and they've been OK for the price but this is my first aero one, I fear you may be correct re the carb..

[David Davis]

Cowlings are not my area of expertise but if the engine runs well with the cowling off but over-heats and cuts out when it's on, you've found your problem.  I believe that you need four times the area in the cowl for hot air to escape compared with the inlet hole. E.g, if the inlet hole is 5 cms by 2 cms you would need a total of 40 square cms in the cowl to allow the hot air to escape. You may also need to block off some of the air intake and even fit baffles to direct the air over the cylinder and exhaust.

 

As I've already stated, I'm not an expert. Jonathan Harper of Laser Engines should be able to help and a picture of your installation would be helpful too.

[Gary Binnie]

Good point about the throttle kill, you could try using the stop screw and cut the engine by pinching the fuel line (if it's accessible). If you've still got the same problem then wind the screw out again. If the problem improves it could be poor servo centring/return to idle position or sticky snake/Bowden cable.

 

The 'pinch test' hasn't been mentioned yet, squeeze the fuel line at idle and if the engine dies without the engine rpm rising the mixture is either ok or lean. If the rpm rise briefly before the engine dies the mixture is slightly rich.

 

To adjust my ASP 61 carb I had to turn the mixture screw tiny amounts at a time, it took an hour of stop/start ground running.

 

Might be worth taking the carb apart to check that it's clean with no swarf left over from manufacturing or burrs on the spraybar hole. If it is a single spraybar hole it should be facing 180° away from the air intake.  If it has any O rings check that they have not been caught and damaged.

 

 

[Maurice Dyer]

19 hours ago, Flying Squirrel said:

Thanks guys, I'm pretty sure (as far as I can be) that plumbing is sound. Plug is currently a model technics 3d plug (4 stroke length) but have also tried OS 8, and Enya 3s'

 

Yes, the engine is run in. Generally the engine transitions from a idle to WOT quite smartly but occasionally quits returning to idle.

 

As I alter the LSN and the rpm increases/decreases and I have to adjust the throttle idle position accordingly - I'm wondering if I'm 'masking' one with the other and getting lost, hope that makes some modicum of sense!

I always fit an O.S. F four stroke plug in inverted two strokes. Expensive (what isn't) but works very well on inverts.

[Jon H]

17 minutes ago, Maurice Dyer said:

I always fit an O.S. F four stroke plug in inverted two strokes. Expensive (what isn't) but works very well on inverts.

 

The MT 3d plug he is using is a long reach plug and is good for inverted engines. You arent wrong, the OS F can help, but in this particular instance his plug is ok.