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Old 03-15-2014, 03:39 PM   #1
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Default os 12 xz tuning help

Hey, got an NT-1 with a 12 xz. After a wot run, the engine revs stay kinda high when I let off, then a few seconds later the idle drops down to normal. Engine is running fine otherwise, doesn't seem too lean on either needle. There is no mechanical linkage problem.

One thing I noticed though is that without the reducer, the air gap is about 1mm. The manual seems to indicate it should be 0.5mm without the reducer but I'm not sure if that means it should be .5mm with the reducer?? The gap with the reducer in is very small.

Friend has a mugen with a 12xz, his air gap without the reducer seemed to be about 1mm also. His engine doesn't seem to have the problem and his HSN is about 1 turn leaner on the top. LSN about the same as mine.
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Old 03-15-2014, 07:01 PM   #2
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try leaning out the "low" speed needle , that's the one that works mixture at idle
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Old 03-15-2014, 08:45 PM   #3
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try leaning out the "low" speed needle , that's the one that works mixture at idle
I'm not the best at tuning but can usually get it to run good, but more on the safe side. I can tell you, do not judge your settings with someone else's regardless of motor, no 2 are the same.
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Old 03-15-2014, 09:16 PM   #4
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With the low and mid speed needles, set them back to factory settings.
If you don't know what the factory settings are, it's quit easy: they are both set flush with the carburettor. Once you've done that, retune it again.
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Old 04-08-2014, 11:16 PM   #5
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double post. sorry.
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Old 04-08-2014, 11:16 PM   #6
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Well bringing back to factory setting helps but it won't help him to tune it right.
reving high and then low at idle is due to rich LSN like the gentleman said.
When you look at the carb and notice you barely have any gap? Don't worry cuz it's totally normal. 0.5-1mm gap is normal depending on your setup and engine w/o venturi. So take it off and look at it again. See some gap? Then dun worry. You will have to adjust it later again anyway.
Solutions:
1. Set the gap w/o venturi between 0.5-1mm (just eyeball it. You can't measure it accurately anyway). Then put restrictor back on.
2. Tune the engine. (check the tuning bible here)
3. Revving high and then low at idle? Start leaning out LSN till you hear constant revving at idle.
4. Close the carb gap till RPM lowers. (there are many methods to check if LSN is set right. Pinching fuel tube, counting seconds before engine starts puttering out, listening to sound, checking smoke, etc.. Ur judgement)
5. Run it again and fine tune. Aim for 200-260 F. Dun let it four cycle. If over 260, dun be concerned too much cuz it can be totally fine depending on your engine brand and setup. 200-260 is just a guide line and that range is safer than, say, 300.
There are tons of details you need to know in between. Research and keep asking questions.
Good luck.
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Old 04-08-2014, 11:54 PM   #7
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Quote:
Originally Posted by snuvet75 View Post
Well bringing back to factory setting helps but it won't help him to tune it right.
reving high and then low at idle is due to rich LSN like the gentleman said.
When you look at the carb and notice you barely have any gap? Don't worry cuz it's totally normal. 0.5-1mm gap is normal depending on your setup and engine w/o venturi. So take it off and look at it again. See some gap? Then dun worry. You will have to adjust it later again anyway.
Solutions:
1. Set the gap w/o venturi between 0.5-1mm (just eyeball it. You can't measure it accurately anyway). Then put restrictor back on.
2. Tune the engine. (check the tuning bible here)
3. Revving high and then low at idle? Start leaning out LSN till you hear constant revving at idle.
4. Close the carb gap till RPM lowers. (there are many methods to check if LSN is set right. Pinching fuel tube, counting seconds before engine starts puttering out, listening to sound, checking smoke, etc.. Ur judgement)
5. Run it again and fine tune. Aim for 200-260 F. Dun let it four cycle. If over 260, dun be concerned too much cuz it can be totally fine depending on your engine brand and setup. 200-260 is just a guide line and that range is safer than, say, 300.
There are tons of details you need to know in between. Research and keep asking questions.
Good luck.
It's called a FAUX IDLE...

Too rich on the bottom, too high on the idle.
Rich idle mixture pulls the idle revs down until you go WOT and then leans out making idle mixture have more air.

Best practice:

Richen the top needle 1-2 whole turns to ensure ample fuel pressure

Check for all air leaks, plug condition etc.

Turn on radio, check all endpoints, braking etc and use a rubber band to hold carby shut

With 2 whole turns of HSN you cant rev at all you'll flood.

Slowly lean the bottom mixture as your hear the idle slightly rise, drop the idle screw until you can give it tiny blips without flooding and listen to the idle behaviour

Also consider the status of mixture when engine stalls. If it starts immediately, and then stalls after a few second, then it's likely it flooded on the bottom mix, so a little leaner on the bottom and drop idle screw again

You should hear the engine nicely idling with a wet "plop" without stalling.

Then put it on track and dial in the HSN.

I did this with a friend with an OS on the weekend and he too had a bad plug. It was tricky with teh OS but it purred for the rest of the afternoon


PS: And do what SNUVET SAID!!
There is a chance you are too lean on top, but this is the most common cause of a hanging idle, other than wrong endpoints
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Old 04-09-2014, 07:45 AM   #8
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We got it tuned out. I closed the gap to around .75 roughly leaned out the hsn to make it scream on top end. Then came back to adjust the lsn and got it to have power on the bottom and not idle goofy. Then ran it and checked temps then came back and just tweaked the hsn a little and its good now. Has alot of power now and no goofy idle.
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Old 04-09-2014, 06:17 PM   #9
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thanks all, good info, makes sense.

As chaos said, we leaned the low quite a bit and closed the air gap and it's running much better now. So it def had the "faux idle" issue. Much faster and doesn't even run much hotter.
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