I am having a small problem with a q-40 going lean durring a run. Lets assume that the needle is not moving and there are no holes in the tank or lines, and I am using a tetra 6oz bladder tank....what could it be? It kinda reminds me of a sport plane, not going super lean, but i have to start out too rich in order to finish the run at the proper rpm. Any comments would be appreciated and I thank you in advance.

Jeff Burg

What is your head and sleeve spacing?

Change the glow plug?

What prop / rpm are you seeing?

Blow out your needle assembly, something may be gunked up in there.

Jeff, is this the LS version?? The only reason I ask is that when I got mine just recently, I asked a lot of questions and it appears it's a different animal on the ground compared to the SS version.. I'll let someone who knows more about it provide an answer...

I had the same problem with my polecat about 2 months ago- problem was a very slight leak in the bung ( cap)of my tank- how did i find it- well, I insert my tank backwards( bung towards the rear) and upon removing the cockpit moulding then noticed the fibreglass moulding of the fuselage at the TE of the wing moist with fuel.............did a pressure test and yes that was the problem.
All is ok now. Suspect though that this was made easier by the rearward tank, trevor henderson, NZ

The engine is an old nelson ss, turning a 7.8 @ 22,000 on the ground. It is old and I wanted to just use it for practice so I am know that it is not turning world record numbers. I will double check the tank and blow out the needle valve and make sure the hole is lined up with the venturi. I am not certain what the engine is set up at but will check it out and let you know tomorrow. I purchased the engine used and have been running it the way that it came, I am new to q-40 so bare with me. What is the stroke of a SS q-40 motor. Thanks for the comments!

Thanks,
Jeff

Hi Jeff,

The stroke is 0.706"

The starting point for setting the head is: Deck Height 0.193", Head Clearance 0.016".

Ed S

Starting point ????????????????

thats where I have run my SS nelson Q40s for the past 5 years

22,000 on that prop, is quite a bit low. Either you have a leak somewhere between the fuel tank and the engine, or the motor is just tired. Have you tried other APC props with the same numbers stamped on it? They do vary a little....

Maybe Henry knew what he was doing after all!

Ed S

Sounds like the head might be worn out.

Try squirting a little after run oil around the plug and then turn the prop over,,, if you see bubbles, replace the head and/or the top end.


BV

Dave,
Yeah I am aware that this motor is not a stump puller, but I do live betweem two great lakes and it is very warm (over 90 today) and incredibly humid, its alike like Muncie but more humid. Anyway, the motor is set a .194 and .014. I re-did the tank lines and put a new plug in it and raised the head. I wasn't able to fly it, but it needled a little easier so something I did helped, but i won't know till I fly her. Thank you for your ideas.

Bill,
I did the oil thing and there were no bubbles, but thank you for the help I will use that trick again I am sure.

Jeff

I would also suggest running a lighter prop.

Pick up some 7.4x7.5 and 7.4x7.6 props and get it up to 23,000 rpm. The motor just isn't as happy under that.

Or, if your not racing it, "Deflash" it untill it turns 23k by sanding the TE down about 1/30".

Got some on order thanks for the input!!!!!

Jeff

Jeff, check the pressure nipple on the muffler,, make sure it ain't clogged or that the muffler bolt is closing up the hole when the muffler is bolted up to the case.

Other things to consider,,, is there a lot of fuel escaping between the venturi and the case? Is the spray bar properly positioned and/or is there a lot of fuel escaping out the front bearing while the engine is running?

BV

Bill's right; sometimes you can get a lump of Loctite in the rear bolt hole that obstructs the pressure nipple.

Of course, that wouldn't explain why it seems to get leaner as time goes on ... but it would be a preventive maintenance thing to check anyway.

Also, remember that if the tank position is too low, the fact that it's a bladder tank won't prevent the engine from starving in a hard turn. Perhaps the engine is heating up due to successive hard turns with the tank too low ... unlikely but possible ... if you've eliminated everything else, consider that.

Duane Gall
RCPRO

Flew it this morning and all is well. It was one of those things, I changed alot of stuff. Cleaned out the needle valve and made sure that it was lined up properly with the venturi. Shimed the head and changed the plug. Changed all the lines and changed the way the lines were fed through the fuse. So thanks for all the pointers and sharing your experience with me.