Finally, a carb tuning HOW-TO

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RobH

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I think one of the hardest things for a nitro newbie is tuning his carb. I'm not a expert so read on at your own risk. I would, however, like to share my experience on the topic so that maybe you wont make the same mistakes I did.

I thought i was doing everything right. I tuned my HSN first and the LSN needle second and the idel set screw last. The temps were always good (230-250). When I took my R/C out to the track, i got a pretty stiff wake up call. My engine seemed to lack the bottom end found in the other engines on the track. I say "seemed" because, as luck would have it, I re-tuned my engine a week later and found a drastic error my tuning methods.

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RobH's Carb Tuning HOW-TO:

The first thing you need to do if get your RC to stay running under rich conditions. Start with the low speed needle (LSN) and high speed needle (HSN) rich enough that the engine bogs down constantly, but lean enough that it doesn't die. You need your RC to run before you can tune it.

Next, as we've all read before, get the engine up to temp. Under rich conditions, the engine won't get that hot. The warmer you can get it, however, the more accurate the tune.

Before we start on the needles, let me point out that it's important that you never let the RC have less than a 1/2 tank of gas at any time during the tuning process. If you run the tank to the bottom, you will get a false since of lean / rich as the RC will naturally lean out as it gets to the bottom of the tank. As you tune the engine, be sure to top off the tank each time it hits the half way mark.

Now you can finally start turning needles. Again, as you've read before, you adjust the HSN first. The HSN directly effects the engine's performance and temperature while it’s at WOT. When the HSN is too rich, you will hear an odd noise at WOT. The engine looses speed and can be best described as bogging down. That noise is an indication that the HSN is too rich. Make a couple of WOT passes and listen for the noise. Continue to lean the engine in small increments after each set of passes. Eventually, the noise will go away and you will notice an improvement in the speed at WOT.

You're not done with the HSN yet. You should still see lots of smoke coming out at WOT. That smoke means health. The smoke is made by the oil in the fuel and if you don't see smoke, you're damaging your engine because it isn't lubricated enough. SMOKE SMOKE SMOKE SMOKE! Did I say SMOKE enough? Continue to lean the HSN and pay close attention to the smoke at WOT. If the smoke is gone, you need to richen it up until you have a visible stream of smoke at WOT.

The HSN is all about WOT. You're looking for smoke at WOT and you're listening for bogging down at WOT. At this point, we don't care about acceleration and smoke and bogging during acceleration. The HSN only pertains to what you see and hear at WOT.

If you tuned the HSN correctly, the RC should still be slow and it should still bog down during acceleration. At WOT, however, you should see a visible trail of smoke and you should not hear any sort of bogging engine noise.

The LSN is next, but we need to talk about "loading up" and "cleaning out" first. The best way to test a LSN setting is to mash the throttle down and "gun it". If the RC bogs down and acceleration poorly, you know the RC is too rich. There is something you must that that will mess you up. If your RC sits at an idle for any amount of time (10 or more seconds), excess fuel will build up in your carb. When you hit the throttle, that excess fuel gets pushed into the engine and makes it look like a rich condition. To avoid this, you need to "clean it out" before you test your acceleration. If you RC has been idling for some time, pick up the RC and put the hammer down. Just mash it and let the wheels spin up to full revs then let go. The blows out all that excess fuel and you're now ready to test your LSN setting.

From a stand still, mash the throttle all the way down. At this point, the RC should bog down and really sound unhealthy during acceleration. Do this a few times before making a small clockwise turn on the LSN. Repeat the process by mashing down on the throttle a few times and listening for the bogging down. Eventually, the bogging will go away. At this point, you're getting close to the "sweet spot". It's now time to take your temps. Run the RC a few passes accelerating quickly and hitting top speed. Afterwards, take the temperature (point your temp gun squarely at the glow plug for the most accurate reading). At a full tank, you should not exceed 230-240 degrees. If you run the tank out, you can expect temps as high as 260-270. In my book, that's fine. You wont run for very long on an empty tank anyway.

The next setting is the idle set screw (ISS). This is a real no-brainer. There is no prize for the guy that can idle at the lowest RPMs. The goal of the ISS is to keep the engine at a low enough RPM that the clutch does not engage, but high enough that it's doesn't die. Find someplace in the middle where you think it sounds good and you're done.

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The first time I tried this, it felt like I had bought a new engine. I now had a ton more speed an acceleration than before. I had seen this speed before, but it was at unsafe temps. Now, I run a sweet 230-240 with ton's of acceleration and top speed.

As it turns out my biggest problem was incorrectly tuning the HSN. I saw the RC bogging down under acceleration and figured the HSN was still too rich. By the time the low-end stopped bogging down, the HSN was way too lean and the engine wasn't producing any smoke at WOT. Furthermore, I had to compensate by leaving the LSN very rich. This kept the RC at a stable temperature but at the cost of bottom-end performance.

Happy Tuning,
RobH
 
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Wow rob, that had a lot of good information there. I read that whole post and i learned a lot about tuning. That answered a lot of questions i had too. I am going to retune my engine tomorrow and try all that!!:banana:
 
Ok, good post. In a nutshell that is the correct method. I pretty much do the same thing.
Now for my question. I have the stock 21BB on my Savage. I have finally gotten it to where it decided to stay running and even restart (most of the time). One thing I have noticed with this engine, that I have never had before, is that if I make a hard WOT run and back off the throttle I can hear the engine speed up like it is leaning way out (like when you pinch the fuel line) but it doesn't die out. No matter what I do, I can't stop this. If I richen up the mixtures this goes away but it then runs really bad. I still have the WOT smoke and a slight hestitation from a stand still full throttle start.
I can't figure this engine out and have several times contemplated tuning it with a ballpeen hammer.
 
Originally posted by LarryA
Ok, good post. In a nutshell that is the correct method. I pretty much do the same thing.
Now for my question. I have the stock 21BB on my Savage. I have finally gotten it to where it decided to stay running and even restart (most of the time). One thing I have noticed with this engine, that I have never had before, is that if I make a hard WOT run and back off the throttle I can hear the engine speed up like it is leaning way out (like when you pinch the fuel line) but it doesn't die out. No matter what I do, I can't stop this. If I richen up the mixtures this goes away but it then runs really bad. I still have the WOT smoke and a slight hestitation from a stand still full throttle start.
I can't figure this engine out and have several times contemplated tuning it with a ballpeen hammer.

I my my eninge won't tune I ussually consider my next upgrade to be a huge rock to beat the poop out of it.
 
Excellent information RobH. :bow: I know this cleared up a lot of my questions and it will do the same for others. :thumbsup:
 
I give up with the junk HPI carberator! I was about to just launch the Savage in the corner and got a thought. Checked the SH engine that I stole the head off of for the Savage and found that the carb from that fit the HPI 21BB. It is now on and all went pretty well up until my failsafe kicked in due to low battery power. The initial testing went better than the HPI carb has run so far. No more MSN! Top end and bottom end, as it should be. I have a better smoke trail at WOT, cooler running temps (about 275 max), and no more strange engine noises when I back off WOT. Maybe I'll get lucky and finally get to run this things a couple times without doing nothing but trying to make it run.
 
That cleared alot of my questions. Thanx for the ver useful info.
 
I'm glad you guys found the info useful. :)

Larry, The OS 20e made all the difference when transplanted on my 8-port. That Savagw .21BB and the Hyper .21 are "budget" engines. I think the carb is one of the places they feel they can skimp. :\ Glad to hear you got it running better.
 
It is appearing so. Time will tell. I can't afford an engine now so what I have must do. I was going to use an OS 20E but without machining the base it will not fit the HPI crankcase. I'm not doing that because it is off an OS 21RG that still has pretty much life left. If I machine it then I have no carb for that engine. The SH carb seems like it is better, anyway it came off a $230 8 port long stroke engine. I can't use that either, no pull start and no way to crank it up. I'll make due or park the Savage until I can replace the engine.
 
Mods, I think you guys should sticky this or make it as a archives...:write:
 
It's not really mods, it's more like just thinking up what you have to do to make something work with what you have. Or in some cases, to fabricate stuff you think might be as good or better than what is outraugously exspensive and overpriced to buy.
 
Originally posted by RobH
If your RC sits at an idle for any amount of time (10 or more seconds), excess fuel will build up in your carb. When you hit the throttle, that excess fuel gets pushed into the engine and makes it look like a rich condition. To avoid this, you need to "clean it out" before you test your acceleration. If you RC has been idling for some time, pick up the RC and put the hammer down. Just mash it and let the wheels spin up to full revs then let go. The blows out all that excess fuel and you're now ready to test your LSN setting.

It can also cause you to roach the mill. No load rpms, ESPECIALLY wot no load rpms are a great way to damage the mill, throw a rod, stretch a rod, as well as any number of other problems that no load high rpms can and will cause. So I strongly disagree with this portion of your tuning how to, and would not recomend anyone do this, sorry Rob
 
I know it sounds horrible, but, when I have my Buggy, Drake or NTC3 sitting up on the starter box I blast the hell out of the throttle a few times before I slap it on the ground. This is of course after it was idling for a couple minutes. For the same reason too. To clear the load-up.

Long term, no-load, hammer grabbing is definately bad. But if you run rich, its almost essential to clear before you run.
 
How do you think people do backflips?? Go medium speed and blast the throttle in the air. I don't see anything wrong with picking up the car and slam the throttle till it gets to max rpm. If you did that for five minutes, yeah, then I do.
 
Originally posted by LarryA
It's not really mods, it's more like just thinking up what you have to do to make something work with what you have. Or in some cases, to fabricate stuff you think might be as good or better than what is outraugously exspensive and overpriced to buy.

What I meant was that the moderator or the admin's should make this a sticky or archives so other can look at this in the future. It's just to save time and make it easier for other....:hi:
 
Rob, I thought your advice was brilliant.

I notice though that you tune your engine from the top down, what's the difference to the bottom up.

The reason, when I got my buggy (2nd hand) it would just wouldn't run, so I had to start from scratch (with little to no experience) but a bit of research pointed me in the direction of:-

1. Set tickover first with idle jet
2. Set bottom end by pulling the trigger and getting the pickup right
3. Set the top end so there's still smoke at full chat
4. Check bottom end again for cleanest acceleration
5. Check top end again

(and get one of those digital thermometers off ebay - the little palm sized ones for $30)

I just found this, Josh Cyrul tells how to tune your engine:-

First off, I raise the idle about ¼ a turn from where it was previously run. This is just to keep the engine from stalling when I start going through my routine. Next, I will fire up the engine “blipping” the throttle a few times until the engine is up to a reasonable temperature. Some where in the 150 degrees range. Now, I will richen the top end up a full turn. Holding the carburetor wide open I will continue to richen it up till the engine nearly stalls. If it does stall that is ok, just lean the engine back in slightly so you can get it to re-start. My reason for doing this?? This helps flush out any dirt or residue (or after run oil) out of the carb and engine. Now that things are cleaned out we can begin to tune the engine to race trim.

First off, I’ll set the bottom end. To do this I will listen to the idle. If the idle slows down and the engine stalls I will lean out the bottom end needle. If the idle speeds up then I will richen up the bottom end needle. Usually, I set my engines to idle at least 1 minute before the idle slows enough to stall the engine. Once the idle stays consistent for 1 minute I will lower the idle as much as possible until it stalls. From that point I will raise it back about 1/8 of a turn. The lower you can set the idle means lower engine temperatures, better fuel mileage and better operation of your clutch since a high rpm idle can slightly engage the clutch causing excessive heat and wear. Overall, your engine should idle nice and consistent for at least 45+ seconds. I’ve had engines idle for a whole tank when set-up properly!!!

Next it’s time for the fun part! The top end needle, where all that straight away speed and power comes from. This is more of an adjustment for out on the race track but here’s a way to get it close on the bench. I begin by opening the carburetor to full throttle (never for more than 2-3 seconds before returning it back to idle during this leaning out phase). Listen to the engine and lean it out until it sounds like it is achieving a nice clean sound. This is a critical adjustment that will greatly decrease the life of your engine if you get in a hurry here so take your time. Once you get it to where you think it is close you have to make sure that the engine won’t lean bog or hesitate (say after a long straight, hard braking and then hard acceleration to the next corner). Open the carb to full throttle for 2-3 seconds, then close the carb for 2-3 seconds, then back to full throttle for 2-3 seconds. Your engine should rev nice and clean through the 2nd full throttle blast. If it hesitates at all then your engine is too lean and must be richened up on the top end needle.

Most importantly, listen to your engines. A lot of people fall into just watching the smoke that comes from the exhaust and tuning the engines that way. This way is ok but if for some reason you use different fuel (with different oil content or mixtures) it becomes much more difficult to make your engines perform consistently from race to race and in different weather conditions from just the sight of smoke. Learn how your engine sounds and responds to your throttle input. Good luck and have fun!!!

Josh Cyrul
 
Great find, and great information. I would not expect Rob to respond. He has long since departed this site.
 
Maybe I'm stupid asking this, but can someone explain what WOT means?
 
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