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Old 07-29-2003, 12:37 PM   #1
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Default Ways to track down an air leak

I have a relatively new Sirio .12 SG engine that I am having fits getting to idle much less tuned correctly

I am pretty sure I have an air leak or fuel leak somewhere in the Carb. The reason I say the carb is if I put another Carb on it, the engine acts so much better.


Symptons:

When it gets started, it is so rich you can hardly keep it running wihtout blipping the throttle. When it gets up to temperature, and say you go full throttle for a fews seconds (2-3), as you let off, the engine is still revvingvery high. Almost anything you do to the needle settings doesn't change the temp at all. Also, you can screw the low needle in all the way and the mid needle in all the way and the thing still runs and feels very rich at the bottom end. I didn't even think an engine could run that way. Another sympton is once I stop the engine (put hitting the fly wheel), after about 30 seconds or so, I see air bubbles in the fuel line. It seem to not want to keep the fuel pressure.



Suggestions:

I need some suggestions to determine how to find the problem. I had already sent the carb back to trinity and they replaced the middle needle o-rings but that didn't help.
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Old 07-29-2003, 01:02 PM   #2
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A problem with the carb of the sirio is a very common problem. If you can get a nova carb for it it will run great.
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Old 07-29-2003, 01:20 PM   #3
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while i have yet to try this, some of the guys use wd40 and spray it on any area that could cause an air leak (carb needles, orings, front bearing) one at a time. if the motor bogs or dies then there is a leak.
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Old 07-29-2003, 04:24 PM   #4
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Rodney, I had the a similar problem last Sat. at Fernando's, when I replaced the turbo head button for the standard plug button on my Sirio. I didn't have the new head button seated properly, even though the head appeared to be on rght... you might check that.

I also have some stuff in my toolbox, that is just for sealing air leaks...see me at the track next time, you can use it.
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Old 07-29-2003, 05:22 PM   #5
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Al's suggestion is a good one and it is close to what we use for diagnosing intake leaks on full sized race cars. I use Berryman's Chemtool or some other carbeurator cleaner and spray small blasts near the base of the carb and other motor seals while the motor is running. The carb cleaner is combustible and if you have an intake leak, the pitch of the motor should run a little higher for a bit and then come back down.

Once you have found the leak you need to clean the area SUPER clean and then dry it. Once you have done that you can seal it with an o2 sensor safe silicone RTV from the auto parts store. I use Permatex Ultra Blue and after sealing everything you should let it dry for 24 hours for a full cure. BTW, make sure you keep the RTV out of the part of the motor where air/fuel needs to pass.
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Old 07-29-2003, 05:39 PM   #6
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Quote:
Originally posted by Speed De
A problem with the carb of the sirio is a very common problem. If you can get a nova carb for it it will run great.
I discovered a badly leaking carb on my brand new RB Rody 3 port while breaking it in last week. The carb is Nova based, so don't think that all new Nova carbs are perfect either. Each and every carb is different.
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Old 07-29-2003, 05:40 PM   #7
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Just make sure the carby cleaner doesn't contain petro chemicals that will swell and damage the rubber orings.
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Old 07-29-2003, 07:25 PM   #8
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Thanks for the feedback. I will try the suggestions and let you know how things work.

For the time being I am using a friends Nova carb and am having abosultely no problems with the engine now. It is definately in the carb.
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Old 07-29-2003, 07:41 PM   #9
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Default Ways to track down an air leak

Guys;

I find that using some kind of spray on a running engine to trace air leaks is NOT a good idea.
No matter how careful you are, some of the chemical gets drawn into the Carb and effects the engine, so you get a false indication.

Better just to seal everything or change the Carb as others have suggested (I would seal anyways too).
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Old 07-29-2003, 07:48 PM   #10
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You could just use a spray bottle containing methanol or d-alcohol.
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Old 07-29-2003, 09:42 PM   #11
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Denatured alky is combustible too. There should be no problem "attacking" the o-rings. Mechanics have been doing this for decades on your automobile and the little carbs are not really any more sensitive. Heck, the methanol and nitromethane are just as damaging to rubber as anything else.
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