learjet45xp

Hey Bob,

i ran my 60LX for the first time today. Had to use a high torque starter on it! Amazing compression! Anyways, It ran great at both low and high end, but was off in the mid-range. Transiton from idle to full was good ,but when you slowly increased the throttle it would bog down in the middle. I am running the bubbleless jett system and it seems all good. Any suggestions? oh btw...I am running a 10x6 APC and A3 Plug.

bob27s

My Feedback: (19)

Two things ...

First - Give it some time..... its new. Do not adjust anything on the low end mixture until you have at least 20 minutes on it.

If you just receive the engine, perform the run-in with a 9x7 on it ..... not the 10x6. Do no skip the run-in proceedure. Instructions provided with the engine. Put the 9x7 on it, dial up the needle so it is running 18K rpm (will still be a bit rich) and run it there.

Second - the engine will tend to load up a little bit in midrange. It is suppose to do this actually. It is a mixture curve is designed for the higher timing and the tuned exhaust. When the engine stages up and down off of the pipe, it gets in a situation where it needs either more fuel or less fuel than you would normally need. Biggest issue is when throttling down - the tuned muffler tries to keep the engine singing at its own rpm, but you have throttled back and cut off fuel. That creates a point where it gets lean rpm but no fuel to support it. That is actaully where most folks blow a plug. So the carb is set up to support that point. So conversely, that mixture endes up slightly rich when you throttle up - lots of fuel, engine struggles briefly to get up on the pipe. That is what you are seeing.

Get some time in the engine with the lighter prop. Then switch up to the 10x6.

If it is still a problem, you need a hotter plug. Rossi 2 or 3, K&B HP plugs are not bad, McCoy 56/59 are pretty good. The OS A3 is an ok plug, but some folks have struggled with it (not sure why).

Let me know how you make out.


also....... let me know when you received your engine (what date)

Bob

learjet45xp

Actually, the engine is used. The gentleman I purchased it from said he broke it in per-instructions. I noticed that the low speed adjustment was backed out almost 2 1/2 turns from being flush with the nut. Reading through previous posts, it sounds like being flush with the nut is close to correct. Perhaps it was running very rich at the mid-range, thats what it seemed like at least. I will go back to the 9x7 prop and run it there awhile and see how she does.

bob27s

My Feedback: (19)

Get the high end set first.....

Pull the engine down to about 1/4 throttle barrel open.
Adjust the low needle until it runs smoothly and then transitions back and forth from there.

Don't try to adjust it transitioning from full idle. You won't see what you are looking for. The bottom end idle tends to take care of itself.

MJD

My Feedback: (1)

Hi Bob and Lear - I'm not hijacking, but I think I have a question here that is relevant: I've seen debate on starting the engine on idle, cracked throttle, or full throttle. I would say without doubt most sport fliers start at idle or perhaps "high idle", but not everybody. I re-read the instructions for my SJ-50, and it doesn't mention this issue. If the low end is right out of whack so that there is little hope of starting it and getting it throttled up, is it okay to simply it start it cold at full bore?

For my tighter engines, like the Jett and West .50v1, I start them at fast idle then immediately advance the throttle to about one third, pull the plug heat, warm it for 10-20 seconds or so while the launcher picks it up, then nail it for launch. Seems to work, they are still squeaky at TDC.

MJD

bob27s

My Feedback: (19)

Does not make a difference where you start it.

But it is important to give the engine a chance to warm up a bit before making an adjustments to the mixture or anything.

I start the QM40 and Q500 engines wide open (not much choice).
CL and FF engines start wide open.
I start my SJ-46 at a dead slow idle by just flipping it back on compression using the spinner.

Setting up the Jett carb improperly the mid throttle mixture can and will screw up the top end if you turn it in too lean. Then the engine will not run very well at any throttle setting. That does not happen with any other engine I know of. That is why Dub sets the low/mid needle at the shop.

MJD

My Feedback: (1)

I guess that is why it is important to think of it as a mid/low range needle versus low speed. I don't touch the needles until thoroughly warmed up. Actually, they both respond to a low speed backwards flip quite well, although the West is still tight enough that when cold a starter is preferable - when cold it often fires but doesn't make it though compression. Part of that is the clipped pylon props that have little momentum.

Thanks Bob

MJD