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Old 06-14-2009, 10:18 PM   #1
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Default Carb - Impossible tune.

Long story short, I have an OS.21TM that will NOT tune, at all.

Symptoms have included but are not limited to:

-Top End screw seems to control idle mixture as well as top end. Needs to be set excessively lean for engine to gain any RPM above an idle.
-Continuous blue smoke flat out, but overheats anyway.
-Forever increasing engine temp (300+)
-Lots of oil dripping from pipe at idle
-High fuel consumption
-unstable idle, or no idle
-spontaneous leaning out and warbling after full throttle
-high-lean idle after overheating
-stalls when jumping / landing
-truck rolls over and massive amounts of fuel are sprayed from exhaust pipe

I have spent 5 days trying to tune this engine. It previously ran fantastic.

Shouldn't the high end needle have nothing to do with idle, or starting, because now it does, in a major way. At a normal setting, the engine will idle, but immediately bog and stall when throttle is opened, unless leaned excessively...

Any thoughts? I have replaced the fuel lines, exhaust and tank gaskets, torn the carb apart 3 times and checked the head seal. Compression is good, cylinder and piston look great.
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Old 06-14-2009, 10:29 PM   #2
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Is any fuel/oil leaking from around the clutch?
Sounds like you have an air leak, and that is a likely culprit.
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Old 06-15-2009, 06:30 AM   #3
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Yes the high speed needle will affect the low speed too. Just look at where the fuel goes into the carb. It goes through the HSN first. Then the low speed needle.

A couple of possible problems.

1. Bad fuel. If you have a buddy who's car is running fine, get a tank of fuel from him and try it. If not, just go buy some fresh, quality fuel. This really sounds like a fuel issue to me.

2. Reset the LSN to factory settings, richen the top a bunch and start running. Don't touch that LSN until the top end performance seems about right. Then read the tuning bible at the top of this forum to learn how to get the LSN and idle set right.

If those don't fix it, you either have an airleak or a toasted motor.
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Old 06-15-2009, 06:34 AM   #4
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Go back to facotry settings,
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Old 06-15-2009, 07:25 AM   #5
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Sounds like an air leak. Excessively rich but still overheating.
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Old 06-15-2009, 07:40 AM   #6
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just out of curiosity - how many gallons have run through this mill?

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Old 06-15-2009, 09:14 AM   #7
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I've kinda got the same prob on my buggy right now.It will idle all day long...revs up beautifully but then dies as soon as you let off the throttle.I've tried raising the idle and went both ways with the low side
I am getting near the bottom of my fuel bottle so I'm kinda thinking it's the fuel
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Old 06-15-2009, 09:21 AM   #8
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Sounds to me like you're tank pressure isn't constant. Perhaps you've got a clog in your stinger? I've been having tune wandering issues (like my last main my car went rich/lean on me twice... bog bog bog, lean) and I'm betting I have something in my pipe that plugs the stinger from time to time, making for a really rich mixture due to extra tank pressure. Perhaps something similar to this is happening to you? The strangest of events to me would have to be the excessive fuel at tip over. That sounds like somehow your pressure line is sucking fuel out of your tank -- what on earth!?
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Old 06-15-2009, 09:23 AM   #9
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Around 1 gallon through the engine. OS is good enough not to include any factory settings in their manual or on their website, so I'm assuming all needles flush with the bodies are factory. If that is the case, it won't run at all at factory, until the highspeed needle is turned in about 400 degrees. At which point it'll run insanely fast, with lots of smoke, but it'll warble when coming down from high revs and eventually overheat. Which to me sounds like an air leak. When decelerating, you have zero throttle, but it's leaning itself out,so it must be getting air form somewhere. Does that make sence?

Bought new fuel, no difference.

Read the tuning bible. I was excited to read about the first few paragraphs that pretty much sum up my symptoms, but that didn't help.I have also tried tuning backwards, starting with the high needle, but there is no low setting that will stop the warble, and leaning out... Though it continuously spews out fuel. LSN seems to effect how long it warbles or until it stalls after coming off of high rpm.

Motor has been getting hot, but pinch, compression and general appearance of the piston and cylinder sleeve look good. IT does run with huge asmounts of power when lean. So again, it must be an airleak, but where?

What do I need to do to track down an air leak? Someone mentioned the clutch with fuel / oil. I don't think it's leaking, and can't think of anywhere else it could leak other than the seal around the carb, which should be completely sound. Any tips? I have spent almost a week trying to figure this out.
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Old 06-15-2009, 09:31 AM   #10
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Check around the backplate, seal it just to be sure.
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Old 06-15-2009, 09:40 AM   #11
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[QUOTE=Orangetang;5940033].I have also tried tuning backwards, starting with the high needle, but there is no low setting that will stop the warble, and leaning out... Though it continuously spews out fuel. LSN seems to effect how long it warbles or until it stalls after coming off of high rpm.QUOTE]

Tune your high speed first then low speed needle. If your tuning the low speed first your probably to lean which will cause the overheating even when your constantly richining the top...
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Old 06-15-2009, 10:01 AM   #12
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how are your exhuast gaskets..i had the same problem once and had a bad gasket from the manifold to the pipe..
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Old 06-15-2009, 10:12 AM   #13
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Have you ever touched the mid speed needle on that motor?? If you have take all the needles out all of the way and rescrew them in to factory setting that should fix your problems. Happened to my friend
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Old 06-15-2009, 10:23 AM   #14
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Quote:
Originally Posted by Orangetang View Post
Around 1 gallon through the engine. OS is good enough not to include any factory settings in their manual or on their website, so I'm assuming all needles flush with the bodies are factory.
My O.S. motors have always come with default carb settings in the manual. On mine, they were 3 turns from fully closed on the main needle, flush with the body on the mixture screw (mid range), and .5mm out from flush on the low speed needle.

I've found that I generally leave the mixture screw alone unless it's got decent bog from held mid throttle to full throttle and adjust the lsn "to keep it sane" while I dial in the main needle to near peak speed with good smoke and temps not running away with some sustained hard back and fourth passes. Then I set the lsn based on 5 seconds of idle and slamming the throttle wide open, adjusting richer till it bogs, then leaner until it stops bogging and accelerates smoothly. As always, I re-check lsn and mixture needle whenever I change the hsn or idle when actively tuning. Once my tune is right all around, my changes for differences per track or day are done via the main needle only.
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Old 06-15-2009, 12:56 PM   #15
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I used some Permatex Ultra Copper high temp rtv silicone gasket maker to seal the backplate, pullstart case, exhaust manifold gasket, carb body and high speed needle / fuel inlet seals and area. I took pictures of everything I did and was going to post them here, but my laptop insists they are corrupt or damaged, while the D90 displays them without any trouble....

After taking everything apart it looks like the front bearing might be weeping oil. I'll try to order one and keep an eye on it. Hopefully what I did today fixes or helps with this problem.

The exhaust gasket almost always leaks badly. I have the a Trinity pipe, and it seems that there is no gasket available that fits it properly. I've been using the OFNA .18 seals, which are still a little too big, and tear 100% of the time after a little run time. I used the RTV silicone to seal it into the header, when I install it on the engine I'll use some RTV silicone on the engine end.

OS manual doesn't specify, and the manual doesn't exist on their website.

Here is the only picture it let me transfer... Before I did any sealing.
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Carb - Impossible tune.-20090615_4690.jpg  
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