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Old 05-27-2014, 07:33 PM
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Looks like the Fx will be in the Usa this week!!

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Old 05-27-2014, 11:51 PM
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I went to the local track this weekend to test the engine a little more. I've got rid of the high idle, but I'm still looking for the sweetspot of the LSN.

I found it a little bit rich on the bottom so I leaned it a bit. I could lean it about 9hours and it would still accelerate without bogging, but during idle it sounds a little unstable, but it keeps running.
Does anyone have tips for setting the LSN of a short needle carb? I've always had long LSN carbs in the past and could tune them very well.

It's not that my FX runs bad or anything, but I would like to get it as optimised as possible.

Runtime was only 7:30 on technical high grip astroturf track with short run ups. But this was with the LSN on the rich side. Didn't time it after the adjustments, but it felt like I could drive it a little longer.
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Old 05-28-2014, 03:03 AM
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Originally Posted by morgoth View Post
I went to the local track this weekend to test the engine a little more. I've got rid of the high idle, but I'm still looking for the sweetspot of the LSN.

I found it a little bit rich on the bottom so I leaned it a bit. I could lean it about 9hours and it would still accelerate without bogging, but during idle it sounds a little unstable, but it keeps running.

The bottom needle won't change your acceleration, lean bog is from your HSN. Your HSN is the main fuel flow to the engine like a graden hose. If you turn the tap on you richen it or if you turn the tap off you lean it. The LSN will make a difference to acceleration but the top is where you need to start. The bottom needle is for the idle but does play a part in the overall tune. You really need to start fresh and reset your idle gap first before working on the needles, if that's not set right you will be chasing a false tune.
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Old 05-28-2014, 04:43 AM
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Originally Posted by KP Graphix View Post
The bottom needle won't change your acceleration, lean bog is from your HSN. Your HSN is the main fuel flow to the engine like a graden hose. If you turn the tap on you richen it or if you turn the tap off you lean it. The LSN will make a difference to acceleration but the top is where you need to start. The bottom needle is for the idle but does play a part in the overall tune. You really need to start fresh and reset your idle gap first before working on the needles, if that's not set right you will be chasing a false tune.
The HSN isn't a problem. The engine is running crisp and clean.
But when I'm adjusting the LSN, I can turn it in/out without noticing a very big difference. If I do the same with the long needle LSN, then the engine would run super rich while accelerating or it would lean bog if it's to lean.

A long LSN is more sensitive, but I know how to tune them because I go by performance and sound.
With the FX engine (short LSN) this adjust doesn't do much once you started accelerating. From what I noticed, the LSN does mainly influence the idle (duh) and the transition from being offpower to onpower.

So what I did is lean it a much as possible and the keeps running during idle, but the idle doesn't sound as stable as the idle from my OS Speeds.

I'm trying to find the sweetspot for the LSN on this engine. The HSN isn't the problem.
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Old 05-28-2014, 05:23 AM
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Originally Posted by morgoth View Post
The HSN isn't a problem. The engine is running crisp and clean.
But when I'm adjusting the LSN, I can turn it in/out without noticing a very big difference. If I do the same with the long needle LSN, then the engine would run super rich while accelerating or it would lean bog if it's to lean.

A long LSN is more sensitive, but I know how to tune them because I go by performance and sound.
With the FX engine (short LSN) this adjust doesn't do much once you started accelerating. From what I noticed, the LSN does mainly influence the idle (duh) and the transition from being offpower to onpower.

So what I did is lean it a much as possible and the keeps running during idle, but the idle doesn't sound as stable as the idle from my OS Speeds.

I'm trying to find the sweetspot for the LSN on this engine. The HSN isn't the problem.
Do you know what the idle gap is set? Just wonder if the idle gap is a bit to big to start with. That would make the LSN a little less sensitive. Also what does your idle sound like? Is it stable, idle up/down. Just trying to get a bit of a picture in my mind to try and help you, cheers
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Old 05-28-2014, 05:31 AM
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Originally Posted by morgoth View Post
The HSN isn't a problem. The engine is running crisp and clean.
But when I'm adjusting the LSN, I can turn it in/out without noticing a very big difference. If I do the same with the long needle LSN, then the engine would run super rich while accelerating or it would lean bog if it's to lean.

A long LSN is more sensitive, but I know how to tune them because I go by performance and sound.
With the FX engine (short LSN) this adjust doesn't do much once you started accelerating. From what I noticed, the LSN does mainly influence the idle (duh) and the transition from being offpower to onpower.

So what I did is lean it a much as possible and the keeps running during idle, but the idle doesn't sound as stable as the idle from my OS Speeds.

I'm trying to find the sweetspot for the LSN on this engine. The HSN isn't the problem.

typical short needle carb tuning, quite different then long needle tuning.this is exactly what killed all the OS guys when OS went to the short needle.....

Anyways yes the LSN basically only control idle mixture , everything else is tuned off the high end....Even a bog out of the corner is handled by the HSN... Basically use the LSN to set the idle mixture and that is all you use it for....Forget about using the LSN for any sort of driving performance other then if your so far off the engine is either starving or drowning.....Idle mixture only ..
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Old 05-28-2014, 05:31 AM
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Yearh, the Hsn can be waaay too lean without you know it, if the lsn is too rich with a too high idle speed.
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Old 05-28-2014, 05:42 AM
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Originally Posted by KP Graphix View Post
Do you know what the idle gap is set? Just wonder if the idle gap is a bit to big to start with. That would make the LSN a little less sensitive. Also what does your idle sound like? Is it stable, idle up/down. Just trying to get a bit of a picture in my mind to try and help you, cheers
I'll open it up tomorrow or friday and have look. I think it's in the ballpark. The engine had a nice 2 stage idle with about 20 seconds of difference between the first and second stage.
Then I leaned it at the end of the afternoon because my runtimes were quite low, it smoked a little less when going on throttle.

Originally Posted by Maximo View Post
typical short needle carb tuning, quite different then long needle tuning.this is exactly what killed all the OS guys when OS went to the short needle.....

Anyways yes the LSN basically only control idle mixture , everything else is tuned off the high end....Even a bog out of the corner is handled by the HSN... Basically use the LSN to set the idle mixture and that is all you use it for....Forget about using the LSN for any sort of driving performance other then if your so far off the engine is either starving or drowning.....Idle mixture only ..
Does it makes a lot of difference in runtime? I was actually trying to lean it to see if my runtimes would improve.
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Old 05-28-2014, 05:50 AM
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Originally Posted by morgoth View Post
I'll open it up tomorrow or friday and have look. I think it's in the ballpark. The engine had a nice 2 stage idle with about 20 seconds of difference between the first and second stage.
Then I leaned it at the end of the afternoon because my runtimes were quite low, it smoked a little less when going on throttle.



Does it makes a lot of difference in runtime? I was actually trying to lean it to see if my runtimes would improve.

yes and no.... the needle itself wont..but if you over lean the LSN it may fool you into over rich the HSN which will in turn hurt mileage........ Best bet with these is do everything off the HSN first, the follow up and trim up the tune at Idle with the LSN......These will tune pretty much the complete opposite of a long needle OS......the LSN has almost zero effect on anything but the idle mixture, and its only used to balance out the tune after the HSN has been set............
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Old 05-28-2014, 05:53 AM
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Actually I made some findings when I broke in a few FX engines: With the stock 7 mm venturi, I only used the lsn when I heatcycled the first 5 tanks of fuel. I could turn the Hsn mayby 1 turn in or out without the mixture changed when the engine ran 10000 to 14000 rpm in the bench.
So I did have to use the Lsn to set the mixture at the lower rpms

Last edited by Lille-bror; 05-28-2014 at 06:11 AM.
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Old 05-28-2014, 06:12 AM
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Originally Posted by Maximo View Post
yes and no.... the needle itself wont..but if you over lean the LSN it may fool you into over rich the HSN which will in turn hurt mileage........ Best bet with these is do everything off the HSN first, the follow up and trim up the tune at Idle with the LSN......These will tune pretty much the complete opposite of a long needle OS......the LSN has almost zero effect on anything but the idle mixture, and its only used to balance out the tune after the HSN has been set............
Thanks. That's how I do it with the FX.
I was just stunned that adjustments on the LSN hadn't a big influence.
I'll check my idle gap and it's too far off I'll start with 0.5mm and don't bother too much with it if the engine runs good.

Also, I'm running 0.5mm shims with 25% nitro. Would I gain runtime if go back to 0.4mm?

Originally Posted by Lille-bror View Post
Actually I made some findings when I broke in a few FX engines: With the stock 7 mm venturi, I only used the lsn when I heatcycled the first 5 tanks of fuel. I could turn the Hsn mayby 1 turn in or out without the mixture changed when the engine ran 10000 to 14000 rpm in the bench.
So I did have to use the Lsn to set the mixture at the lower rpm
Would that be because the crankcase is flooded with fuel? I had the same thing, but when I cleared by reving it 1 or 2 times to 1/3, then it sounded pretty crisp.
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Old 05-28-2014, 06:19 AM
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You would not gain runtime with a lower head shim, you would loose a little.
If your idle is not stable then sounds like you are still too rich on the bottom.
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Old 05-28-2014, 06:39 AM
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Originally Posted by Frank L View Post
You would not gain runtime with a lower head shim, you would loose a little.
If your idle is not stable then sounds like you are still too rich on the bottom.
It was stable, then I leaned it 3/4th of a complete turn to see I would gain runtime. It kept idling, but idling was going up and down a little bit. It rather sounded that it was starving then being too rich. But it didn't behave very different on the track. Just a little less smoke when going on throttle and slighty more crisp.
I'll richen it up the next time I'm on the track.

I'm going to leave the 0.5mm shims in and switch to a 6.5mm restrictor for this weekends race and I'll see how that goes.

Thanks for the advice!
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Old 05-28-2014, 06:48 AM
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Originally Posted by morgoth View Post
Would that be because the crankcase is flooded with fuel? I had the same thing, but when I cleared by reving it 1 or 2 times to 1/3, then it sounded pretty crisp.
Dont think so. When I heatcycle I dont rev the engine during the first 7 tanks of fuel. I set the idle gab and then adjust the mixture so its just a little rich of optimal setting and then readjust the idle gab juuuuust a little to gain the desired rpm. So, I dont touch the Hsn but adjust the mixture with the Lsn when the engine runs the first 5 tanks of fuel at low rpms.

Hope it makes sense
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Old 05-28-2014, 07:03 AM
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seriously aren't we talking about how to tune these for on track performance not breakin settings ?
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