Flat spot in rpm band

[Freefire]

Hey guys,

I'm in need of help diagnosing an issue with my 00 klr250 and I know y'all can help. Recently picked up a bike from one of America's heros that served in Iraq and Afghanistan. It sat up for a year and wasn't running when I got it.

After a quick carb cleaning by my old man, we had it running. It idles fine, but acceleration is flat, even slows down, between 3.5-5k rpm. At 5k and above, the bike takes off.

I don't know what it can be. I thought I heard that the carb works more on throttle position rather than rpm, but I could be wrong.

Any suggestions? All advice is appreciated.

[Freefire]

Some clarification, I can downshift to increase rpms past the flat spot and hit the acceleration band in any gear, so it isn't gear/transmission specific as best I know.

[SilverBullet]

So irregardless of what gear you are in you have the same flat spot at that rpm?

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[Freefire]

I can creep past it in first but it is a wall on every other gear.

[chaissos]

Sounds (to me) like there are pathways in the carbs that didn't quite get clean.

[SilverBullet]

Originally Posted by chaissos

Sounds (to me) like there are pathways in the carbs that didn't quite get clean.

But as he mentioned above throttle position doesn't always equal rpm's. That is why I asked the gear question. 3-5K rpm will be a different throttle position in each gear. If flat spot is really rpm consistant and irregardless of throttle position then I would be more suspect of the electrical system, i.e. coil, CDI, stator, etc.

Cheap and simple first. Put a new spark plug in it and report back. Also verify correct plug not just an exact replacement of what's in it now.

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[SilverBullet]

Also while you're installing the plug check all carb hoses. I assume this is a CV carb so needs good engine vacuum to operate. Bad or kinked hose and/or engine vacuum leak will effect operation.

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[chaissos]

True...but throttle position doesn't necessarily dictate which jets are in use, that's normally changed by the engine RPM automagically. There are vacuum parts that control some of those, on some carbs. I'm not all that familiar with this specific bike. A thorough carb cleaning still may sort it out, using cleaner and compressed air. No wires on the jets, or the size may get messed up.

I haven't messed with this stuff in quite a while (think 1982). I did play with my bikes, but the newest of them was pretty old (my Yamahas, both 2 and 4 strokes).

[SilverBullet]

Nothing automagic on fuel delivery unless you have fuel injection.

over simplification:
0 to 1/4 throttle is pilot jet
1/4 to 3/4 throttle is needle jet
3/4 to WOT is main jet

needle jet moves up and down with throttle twist but needs assist from vacuum on CV carbs to move.

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[Freefire]

I appreciate the guidance and suggestions! I'm inexperienced with carb work but will make it my mission this week to figure it out.

[Moekazi]

Check the diaphram for splits and also that he put the needle shims back together in the right order. If you hadn't found any vacuum leaks, raise the needle a little.

[focus frenzy]

carb needs cleaning and a bump up on the pilot and adjusment to bleed air will fix that.

mt GS had a bad bog spot in that RPM range and it turned out the PO had the wrong pilot jets in it and other settings were off.

[Kawiguy]

Better safe then sorry,
go through the carburetors again,

Get them properly synced
make sure there are Absolutely no vac leaks before the sync,

Spray starter fluid on boots and all around if you have an increase in idle then you have a leak.

Check rubber diagrams inside the carb make sure there are no tears etc.

[Ben Eberle]

Originally Posted by SilverBullet

Nothing automagic on fuel delivery unless you have fuel injection.

over simplification:
0 to 1/4 throttle is pilot jet
1/4 to 3/4 throttle is needle jet
3/4 to WOT is main jet

needle jet moves up and down with throttle twist but needs assist from vacuum on CV carbs to move.

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That's true of the old fashioned slide/needle carbs but CV carbs need both full throttle and high engine rpm to be on the main jet. The vacuum that raises the slide does not come from the intake manifold behind the throttle but from the venturi vacuum that comes from air flow velocity under the slide.
At lower RPMs the carb may be on the needle jet even though you are using full throttle. Essentially, higher rpm equals higher air flow equals higher venturi vacuum which lifts the slide until the air velocity in the venturi is low enough to not lift the slide farther, hence the name "constant velocity".


When cleaning the carbs, don't overlook the air jets and passages.

[Freefire]

Ok I pulled the carb, with some trouble from the previous owner for screwing up 3/4 of the screws holding the bowl, and soaked it in some cleaner.

Drilled the mixture plug, removed the pilot jet, and removed the main jet and holder, but didn't see the needle jet collar.

Is it possible for the bike to run without it? Maybe the po tried messing with carb and forgot to replace or did I lose it?

[Moekazi]

It will run on idle, but will bog out as it comes on the needle because it's getting too much fuel.

Are you sure it's just not stuck up there? On some carbs they're press fit and shouldn't be removed. It's about $30, so i hope you find it.