Idle surge on initial start?

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amstel78
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Idle surge on initial start?

Post by amstel78 »

Good morning!

Got a couple of carb-related questions that I was hoping someone would be able to answer.

First, I've noticed that on a cold engine start with the choke off, the idle RPM will surge to a higher point then remain there until the bike warms up. Once warm, idle drops to what I think is a normal idle range. It does not fluctuate up or down. If I start the bike with the choke on, the idle will obviously be high (but not as high with the choke off) then once warm, will almost drop to a point of dying unless I turn the choke off. Once the bike has been brought up to full operating temp, idle is smooth and steady and does not surge or fluctuate. Is this normal, or do I have a lean condition somewhere when the engine is cold?

Second; I'm not quite sure what the jet sizes are but I think they're probably stock. I'll know later this week or next once I have time to take the carb apart. However, I've been trying to tune for best low-end throttle response. Once the bike has been allowed to warm up after a cold start, I still notice a bit of bogging or hesitation when cracking the throttle open in 1st through 3rd gears. Not really a hesitation, but it feels rough. Once throttle gets to the 1/2-1/3rd open range, engine power at higher RPM feels fine. I wonder if this behavior has something to do with the condition present in my first question above? Once I've ridden it for say 5 minutes, the near-closed throttle response begins to improve to the point that I can bring the front wheel up if I blip the throttle in 1st and 2nd gears .

I started with the air screw out 1.5 turns, and believe I have it around 2-2.1 turns out now.

FWIW, I usually warm up the bike by first allowing it to idle on its on for 1-2 minutes in its present condition. I do not manipulate the throttle unless it begins to die (choke on). Once warmed a bit, I still allow it to warm up a little more by riding it slowly in 1st or 2nd gear near the house until I get to the trail. Once I get to the trail, I'd give it moderate throttle until I feel that hesitation or bogging I described above begin to dissipate. Once that's happened, then I can to go WOT.

Sorry if I'm being long-winded and if this is normal for 2 strokes. Still used to 4 strokers...
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rbates9
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Idle surge on initial start?

Post by rbates9 »

It is always a good idea to check your jetting if you haven't but from what you describe sounds about normal.
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amstel78
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Idle surge on initial start?

Post by amstel78 »

rbates9 wrote:It is always a good idea to check your jetting if you haven't but from what you describe sounds about normal.
Yes, I plan to check the jets once I get back from vacation. I'm leaving next Saturday and I have a very rough work week ahead of me so no time to do it until then. I purchased stock jets from RMATV since I didn't know what the PO has in it now although from how far I have the air screw out, and needle clip in the 3rd slot, jet size should be OK.

As a side note, when I start the bike cold with the choke off and idle begins to climb, I'll usually just turn the choke on to prevent the engine from over-revving itself before it has a chance to warm up. Is that OK to do?
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Slick_Nick
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Idle surge on initial start?

Post by Slick_Nick »

If it starts right up no choke, chances are its jetted too rich.
'00 KDX 220R
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