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Old 12-13-2004, 10:25 PM   #1
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Default After reving, High idle....Whats wrong?

I have a RB V.12 and am using 20% nitro. After I rev up the motor, the RPMs stay very high for a second or two and then drop down to normal. Sometimes the RPMs drop right down. Is the problem tuning or do I have an air leak somewhere. I'm going to tear down the engine look everything over and seal with sealant, but I wanted other opinions on what it could be. It is running quite rich. Lots of oil and smoke. Any ideas?
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Old 12-13-2004, 10:32 PM   #2
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Default Re: After reving, High idle....Whats wrong?

You are probably running the low end needle rich and to make the engine idle properly, you probably also open the idle needle big... I think that could be the problem.

Lean the low end needle (LSN) in small steps and when the idle is high, lower it (i.e. close the idle gap at the slide carb neutral position). Give the engine some revs and observe the idle. Repeat the above till you get a stable idle.

That's of course assuming your high end needle (HSN) is already tuned correctly.
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Old 12-13-2004, 11:00 PM   #3
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Thanks for the quick reply. I was thinking that could be the reason. I'll try that before I go tearing the engine apart. Maybe I could sneak out of the office tomorow and tune it durring lunch.
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Old 12-15-2004, 02:57 AM   #4
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With the RB V12 I usually find that 4 turns out on the main needle (from fully closed/leaned), and 2 turns out on the low speed needle are good starting points from which to tune.
RB`s website also has some interesting points on it.....notably they make comment that tuning becomes more delicate if you use a fuel with greater than 16% nitro content.......and also that a 0.1mm shim be added for every 5% nitro you go higher than 16%.

Cheers
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Old 12-15-2004, 02:58 AM   #5
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Oops, forgot that website....
www.rbproducts.com
Hope this helps
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Old 12-15-2004, 07:16 AM   #6
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my personal tuning guide-


This assumes you are getting fuel to the carb and have a good glow plug and ignitor!!

Always start with the top end when tuning!! Top end adjusment affect the bottom end settings, so never adjust the low end needle first.

FOR TOP END TESTING (you may need a friend to help with this one):

1). Pinch test: Hold the vehicle off the ground and hold the throttle steady at about mid to 3/4 throttle- then pinch the fuel line (midway between the carb and tank) for ONE second. The engine RPM's should increase slightly and return to normal without stalling.

If it stalls- too lean
if RPM's do not increase- too rich

2). Spit test- OK, spit (water) boils at 212F, so spitting on the head is a rudimenatry way of checking temp.

If the spit evaporates away in 2-3 seconds you are golden!!

If it boils off immediately- I means sizzles in an instant- you are most likely too lean (HOT)- richen it up a little.

If it just sits there and does nothing, you are too rich- lean it out a little.

3). Another way is to monitor temps with a temp gun. A temp gun is the best investment you'll make in Nitro RC'ing. At $75 dollars it may allow you to save your engine someday. It will always come in hand tuning.

4). The last and hardest method is plug reading.

- Plug wet and shiny- playing it safe- probably blubbers at full throttle.
- Plug dry and shiny- getting there- if it runs strong, leave it here!!
- Plug has a dull sanded look and not pitted at all- just right!!
- Plug has a highly sanded look and is pitted and possibly distorted- STOP- replace the plug and richen it up. The element may break off and damage the engine!!

Either way, the engine should emit a good amount of smoke while "pinning" the throttle. Keep temps in check if you are a beginner- I'd say less than 220F for racers new to nitro. Ask someone who has a temp gun to check for you if you don't have one.


LOW END TESTING

1). Pinch Test- after the engine is warm and the top end is set- bring the vehicle to an idle. Pinch the fuel line midway between the tank and carb and see how long it takes for the engine to stall.

2-3 seconds is optimal
less than 2-3 seconds- too lean
more than 2-3 seconds- too rich


2). Temperature tuning the low end:

If you have a temp gun- get the engine warm and check the temp. Let's say it's 200F. let the car idle after a good hard run and see if the temp goes up or down while idling. If the idle temp increases- it's too lean on the low end needle- richen it up a little. If the idle temp decreases- it's too rich- lean it out.

Do this until temp remains roughly the same at idle when you pull it in after a good hard run.

3). Idle speed testing

Bring the engine to an idle after a good hard run-

- if the idle creeps up and keeps increasing- it's too lean
- if the idle decreases and stalls- it's probaly too rich and loading up the plug.


Either way you go about it- the engine should be able to idle for 30 seconds without loading up when you nail it.

GOOD LUCK and happy tuning!!

Corey
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Old 12-16-2004, 06:57 PM   #7
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Thanks for the help guys. I have a temp gun and have been running nitro for a few years now. Sometimes I dont race for 8 or 9 months and I can't remember stuff. Must be getting old. I'll be track testing tomorow and let you know how it goes.
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Old 12-21-2004, 01:03 PM   #8
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Just wanted to let you know how it went last weekend. I get the carb retuned on Friday and it ran flawlessly. AndyT, I used your setup as a baseline and it was really close. I actualy had to richen both the low and high end needles about 1/4 turn each. I even had to adjust the mid range adjustment a little as it was opening up the HSN too soon and making the car jump fowards.

The original problem was with the fuel. I had switched to Trinity/HPI fuel MFG by wildcat. I though everything was working great. Once the air temp got too high I just couldn't keep the motor running cool enough. Then I finally wised up and switched back to Odonnels (the oil residue is really nasty). Thats when I started to have the idle problem, but the motor ran fine. I guess I had just richened it up so much that it was way out of tune.

Ran in a 2 day race over the weekend, and it ran at 240 to 260 without a hitch. Now I just need to learn to driver better.
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