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Old 02-21-2017, 02:17 PM   #1
Yamamama
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Default Jetting question

My At1 has a nasty hesitation right at or about 4000 rpm, carb is clean. I bumped up the main jet to 160 to allow for the new hooker header. Motor screams and run's perfect at all other RPM's Do I need to follow suit and bump up the pilot jet? Currently running the stock #30.
Is there some rule of thumb for increasing both jets equally ?? I would like to try this on my DT 250 as well.
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Old 02-21-2017, 08:02 PM   #2
lha
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If you increase airflow,then the jetting should change across the board,so I'd go to a 35 or 40. My At3 is running a 40 and runs and idles and does well throughout the throttle zones.
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Old 02-21-2017, 09:47 PM   #3
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Carbs really don't know what the RPM is. It's the throttle position that makes a difference in jetting...

If it has the same problem at all throttle openings and always at 4000 rpm it's either electrical (not likely to happen just at 4000 RPM) or the new pipe resonance might not be playing nice with your port timing, etc.

Resonance can wreak havoc with jetting over narrow rpm ranges and is most likely to happen at or near the torque peak.

If you jet rich enough to get rid of the "flat spot" at the torque peak it will be blubbering rich above and below that RPM.

So hopefully your issue is related to how far you have the throttle twisted and not the RPM.
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Old 02-22-2017, 02:28 AM   #4
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Before ruling out jetting maybe try a jetting pass in high gear while holding that (4k) throttle position before shutdown. Might help by making a mark on your throttle grip that corresponds with other marks on your throttle housing representing closed, 1/4, 1/2, 3/4 and full throttle. like this
P1050705 by Ian Jorgensen, on Flickr
That way when you run the bike and find that spot where the bike hesitates then you will know what range of the throttle opening the problem is occurring and you can troubleshoot according to what jetting circuit is most dominant at this time.


Install a new plug before making the jetting pass and get the bike warmed up then take it out for a spin. When doing a jetting pass I try and time my run so I am in high gear at the throttle position desired when nearing my driveway so there isn't far to walk/push the bike when you shut down and coast. Select high gear and reach the desired throttle position and hold it there (note the spot marked on your throttle) then carefully kill the engine and pull in the clutch while maintaining that throttle position. CAREFULLY! I know how difficult this can be with an AT1! Let the bike cool and remove the plug and observe.

4k RPMs, not sure what your exact throttle position would be but would guess that's near half throttle so that may be mainly the needle and possibly throttle cutaway.
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Old 02-22-2017, 09:57 AM   #5
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Default Jetting

Thanks for the input!! Think I'll give the "mark the grip" a try. I suspect the carb settings may need some attention. Another person brought up the timing advance/thingamabob could be the culprit. I can't see anyway to test that theory without replacing the part. Looks like I'll be sidelined from testing again until mother nature says so .
I did find a good source on the Mikuni jets and want to toy with settings on my AT's. Just looking for the best route to diagnose proper combinations of jet's.
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Old 02-22-2017, 07:20 PM   #6
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The advance only advances at speeds over stop. As soon as it starts,it's fully advanced,unlike a car's advance mechanism. It simply makes the motor easier to start.
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Old 02-23-2017, 03:34 PM   #7
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Quote:
Originally Posted by lha View Post
The advance only advances at speeds over stop. As soon as it starts,it's fully advanced,unlike a car's advance mechanism. It simply makes the motor easier to start.
10-4! Hope I never have to replace that part. The few that I have seen pop up have all been very pricey!
Going with your advise on bumping up the pilot and see how that works out.
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