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Old 05-01-2004, 05:57 AM   #1
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Join Date: May 2004
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Arrow Tuning OS Max 12CVX?

Does anyone know how to tune a OS Max 12CVX with 10E carb? The instruction manual says the standard positions are: (i) low speed needle is 0.5 turn in from flush with carburetor body, (ii) high speed needle is 2 turns out.

When set with above, the engine wouldn't start as the low-end is wayyy too rich. I had to turn the low speed needle in 1.5 turns before it starts and idles. Also, when breaking in, the manual recommends to turn the high speed needle in 30deg with every tank of fuel for 5 tanks. So after 5 tanks, it would be only 1.5 turns out. Isn't that too lean?

I have another problem: when I start the engine it idles good where there is no wheel spin when I lift the car off the ground. However, after a brief run, the idle revs up to the point where the wheels spin quite fast and the car moves forward slowly by itself(abit like an automatic car without brakes applied). Any one have ideas?

I appreciate any reply. Thanks.
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Old 05-01-2004, 09:25 AM   #2
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this is how I run-in my CVX about 3 years ago,...

heat cycling:

with the setup you mentioned before (turn the LSN 90degs in when difficult to start):

7 mins running (leave glow-starter for 30 secs to help the engine gain some temp), then stop and cool down (car on a stand, wheel is free, set the idle so the wheel are moving only slightly,... no movement is better), after stopping the engine, make sure the piston is not at top dead center (TDC) by turning the flywheel,... the "tight" position is TDC,... move further till you pass it, then stop the piston (approximately in between the lowest position and TDC) I did heat cycling for 2 tanks

Constant idle:

Lean HSN 1/8 turns, fill tank, start engine and leave it idling for 3 tanks wheel still off-the ground, keep filling), try to set the idle as constant as possible without leaning the engine,... after the third tank almost finished,.. stop the engine (by covering the exhaust outlet) and let cool (don't forget to align the piston out of TDC)

Slow run:

Lean HSN by 1/8 turns every each tank for this run.
try to get the car moving (wheel on the ground),.. short burst of throttle (partial, max 25%), if difficult to move or dies,... lean the LSN by 45 degs, I did 2 tanks on this,.. let cool and align piston

Mid run:

same as above, but you can apply 50% throttlr, remember, short bursts only,... don't let the engine stay on the same rpm range for a long time. did thid for another 2-3 tanks (non-stop), let cool and align piston

Now, after it all cools down,.. remove the plug,... and turn the flywheel,... if you still feel the engine tight at TDC, repeat mid-run procedure,... after installing plug of course

if not very tight (some tightness is good,.. a lot is not good),... you can start to tune it for top performance, start from HSN, then LSN,... don't hold it at high rpm for a long time with the wheels off the ground (I know it's cool,... but overheating may occur,.. and it's bad...) don't get it as maximum as possible yet,... try to run it up and down the straight,... if throttle response feel sluggish,... lean LSN bit by bit,.... if it still revs after you release the throttle,.. LSN might be too lean already,....

as a guide, the HSN setting is normally around the 1.5 turns out +/- LSN is up to your liking but don't be too lean,....

a healthy engine should spurt out thick white-blue smoke on rev,... if only white mist,... probably too lean,... if fuel spitting out (or your eyes becomes watery),... that's rich,...

I hope this helped
HPI Racing Bali
Team HB Bali
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