Engine re-built - need help with tuning.

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jc7622
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Engine re-built - need help with tuning.

Post by jc7622 »

I just got my bike back together. It's 2005 KDX200 hybrid. It has a srock carb, V-Force reeds, PC Platinum II pipe, RB head mod for pump gas and the jetting is one down on both the main and pilot (did the jetting a long time ago - can't remember what they are). I am at about 1000' and it is about 70 degrees now.

I just rebuilt it and had the cylinder re-plated and had RB do the head. I took it out to an empty field to try it out. When I just give it a little throttle it runs great and is very responsive. For example when I go from zero-to-1/4 throttle it is really responsive and powerful - amazing. When I go from 1/4-to-1/2 throttle it still runs very good. When I go from 1/2-to-full throttle it has good smooth power and response and really rips.

The problem is when I go from zero-to-full throttle in one twist of the grip it bogs really bad. It bogs so bad that it almost shuts down. It is so abrupt of a bog that it almost threw me over the bars the first time. It just ain't happening if I try to give it any more that 1/4 or 1/2 a twist. It didn't do this before.

When I'm in the right gear or even a gear too low (high revs) it is more accepting of a big roll of the throttle without bogging. If I am in a gear too high (low revs - lugging) and give it a big roll of the throttle it bogs really bad. It wasn't doing this before. I must be missing something.

I can tell it is gonna be a sweet running bike, but this bogging when I give it all the gas is puzzling me. Any ideas where I should start to fix it? Could it be all jetting? Does it sound like I put the KIPS together wrong?

edit: The little spring that holds the little ball against the KIPS gear is missing. Could that be it? See post below.
http://www.kdxrider.net/forums/viewtopic.php?t=6608
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Post by Indawoods »

Lean bog....
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Post by jc7622 »

>|<>QBB<
Indawoods wrote:Lean bog....
Ya know, it felt a little bit off after I built the hybrid. It was just every so slightly less responsive as when it was all KDX with a closed airbox. I wondered if the open KX airbox threw it off. Still it wasn't this bad.

The only variables now are the re-plated cylinder, new Wiseco piston and RB head. The head is the big change. Could it throw off the jetting that much?

Where do you recommend I start? Main jet? Needle?
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Post by Indawoods »

I start by raising the needle one clip (Good field technique).... solved my lean bog. If that doesn't do it... go up one main size.
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Post by GS »

I second Inda's sage advice!!
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Post by jc7622 »

Current jetting is 155 main, 45 pilot, stock needle. I'll try the above and see how it works.
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Post by fuzzy »

May also need to turn in your air screw a touch

Also, FWIW, when you're very close like this keep in mind that the motor will run a fraction leaner (on the top end especially) when the motor is hot compared to cold.
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Post by lemmy »

But couldn't the kips opening too quickly be causing a problem since he doesn't have the spring in there?
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Post by fuzzy »

Yes....I see that 'edit' now...I would certainely fix that first.
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Post by canyncarvr »

The difference that detent ball makes in KIPS activation is negligible. FRP used to reduce that tension as a performance improvement. Quit doing it 'cuz it didn't do anything.

The detent is more to keep the KIPS stabilized than it is to time anything. It will also tend to make the opening a 'singular' event..enough force built up to move past the detent will more likely keep it moving.

At some point, the speed of the twist (from idle to WOT) will matter and will come under the heading of 'throttle control'. There is a reason some carb applications have accelerator pumps! Some have a couple! :wink:

If you could do it 'instantly..a throttle change from idle to WOT is going to result in a bog.

'It hurts when I do that!'

...don't do that.


A lean bog changes with engine temp..as fuzzy alluded to.

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Post by KarlP »

Patience, Man!
This motor has a replated cylinder, so I guess new piston and rings. I should think it needs a bit of breaking in.
Air box changes will effect jetting requirements. So may new piston and rings and an RBD cut head.

I think Inda is on the money. A small change is in order and his suggestions are good.

On my bike I can be idling along in third, about 11 mph, and go to full throttle. It'll pull clean without stumbling. I got it there by making a number of small changes over time. This board was most helpfull in that process.
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Post by fuzzy »

Yeah, you should be able to wick a 2-stroke open and have it run squeaky clean, but that air screw is gonna have to be right...

As CC said, don't try this on a 4-stroke with a non-pump carb...Accel pump carbs are for air compressors(4-strokes)

CC thx for the elaboration on the PV
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Post by scheckaet »

I have the same setting 155 main, 45 pilot (I think) but I have head and carb from RB and run the DEK if that's any help
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Post by canyncarvr »

I'd be curious to find out how many KDX'ers can do a basic throttle response check for air screw adjustment and have perfect response..considering the following 'modification' to the process:

From idle (throttle closed..cable slack), slow walk speed and at least 2nd, maybe 3rd gear, to WOT as-fast-as-you-can-flick-it.

And how many times does that situation come up in trail riding anyway? I don't remember the last time I had my bike at WOT. With a trail sight-distance of sometimes a few feet, sometimes absolute zero..and no traction situation that would handle it...why do it?

I'm saying I don't see the worry to a 'zero to full throttle' response 'problem'.

I'm NOT saying such couldn't be resolved or sorted out. I just don't see the point, I've never tried it 'cuz I don't care.

I don't worry about my bike's wheel balance at speeds over 100mph, either. :hmm: Maybe I should................

OK folks! After MUCH adieu, searching, and ferreting out..I have found the final word on carb theory. Presented HERE by (evidently) a close relation to B.O., this is a 'no-teleprompter' version of the way things are.

Pay close attention! :prayer:

(If this piques your interest (as it should), I leave it to you to find part 1!

:mrgreen:

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Post by jc7622 »

Honestly I don't know how often I do it, but I guess I do it a lot because I did it right off the bat standing on the pegs leaning over the front fender. When it didn't take off like a bat out of hell I nearly went over the bars.

I think it is more common in a racing situation or a panic situation when you are trying to save your ass from crashing. Either way I guess that is part of my riding style. I hope to get out this evening or tomorrow to see if I can fix it.

There is a cross county race this weekend that I want to ride in. Each lap is about 9.5 miles but about 1.5 miles of the course goes around a road racing circuit. I better buy some elbow pads in case I take a tumble on the asphalt.

This is the race:

http://www.okgoldrush.com/

And this is the road racing course where it's held (this is going to be fun):

http://www.hallettracing.net/

And check out this dude on a street bike at Hallett. His speed through the turns will be fater then my speeds on the straights:

http://www.youtube.com/watch?v=Yrn2INXn ... re=related
Last edited by jc7622 on 11:56 am Oct 30 2008, edited 1 time in total.
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Post by canyncarvr »

I'm not saying that looking for a tuning resolution to the situation is bad, a waste of time, or anything of the sort, but that I wouldn't be surprised if it was hard to get to and I've never had the opportunity to worry about it. I can't imagine getting any traction in such a situation either.

If it affects your riding or riding style...go for it!


I DO want to make sure my bike responds off-idle to 1/2 or so (basic air screw response test) 'cuz I am in that situation often. Same sort of thing, in that, if it's not right you can take a trip over the bars.

Some 2-smokes do have accelerator pumps. Mostly on sleds..that being largely a weight/load issue. Dirt bikes aren't generally 5-600lbs, waist deep in snow.

..although, 2-smoke sleds seem to be going the way of 2-smoke bikes.

BTW...I noticed an ad on the Yamaha site that said Stewart is riding Yamaha next (this) year?



The 'carb theory' vid was just funny.

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Post by jc7622 »

Yep. Stewart to Yamaha, Reed to Suzuki, Villipoto to the 450's. As far as the two-stroke sleds go ... doesn't Palin's husband race sleds? It would be nice to have a rider (or a first husband rider) in the White House. And speaking of two-stroke bikes falling out of favor, the FIM in Europe has changed the rules that there will be do difference between 2sts and 4sts in any class. This means that 250 2sts and 250 4sts will race in the same class, etc, etc.

I didn't watch the vid. I'll check it out.
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Post by KarlP »

Third gear seems to be the most usefull in this WOT test.

My procedure (which may not be the "official" procedure, or even the right thing to do) is to mark my grip for 1/4, 1/2 and we know where WOT is. That's where the throttle stops. :grin:
I worked on getting the response to 1/4 throttle clean, mostly pilot and air screw. Then I worked on getting the 1/2 throttle clean, mostly with the air screw and needle position.
When the response to 1/2+ throttle is clean, the response to full throttle is clean, too.

CC is right, I'll seldom go from "idle" to WOT throttle, but "idle" to 1/2 or 3/4 all the time.

When I'm testing, it is not "idling" in third, it is a bit over idle RPM, with throttle closed. I think I used 11 mph according to my enduro computer.

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Post by canyncarvr »

I use the term 'idle' to mainly mean throttle closed. But it doesn't apply to closed AND much of any RPM.

Exaggeration to make the point: There isn't a lot of finesse to tuning for a situation that involves a closed throttle, 3rd gear, 30mph!, and a quick wick to WOT.

I'm not attempting to point out a distinction without a difference. Knowing how your bike responded before, there is no reason you should not expect it to be tuneable to that level now.

Still, depending on the load, the gear, and the engine speed of the bike, I think it is reasonable to expect it to be less than perfect response-wise in a closed throttle to WOT situation.

Next time I go riding, I'll do some 11mph, 3rd gear, closed-to-WOT 'tests' to see what happens.

If my backwards M5B hooks up, I WILL be flat on my back in a heartbeat.

Send me your address so I'll know where to send the medical bills.

:shock:

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Post by jc7622 »

I got a chance to make some changes. Unfortunately I was riding on some empty lots in a neighborhood, so it was just some very low speed tests - trying to be very quiet.

Going from a 155 to a 160 helped a lot. I raised the needle and I'm not sure yet if that was an improvement or not. I need to do some more testing somewhere where I can really do it right, but I am definately on the right track. Thanks for the advice everyone. It still bogs if I am lugging a couple gears too high and crack the throttle wide open. I know, I know what are you doing lugging two gears too high.
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