YACC - Flat spot 5500 - 7000 after hard acceleration

MN- · June 27th, 2018, 07:14 PM

Yet Another Carburetor Conundrum...

My ride an 01 250 bone stock except:
Modified airbox (loose battery box for removal)
2 washers under needles ~1.5mm
Added fuel filter
Prior to last year (spring 2016) rebuilt petcock & lined the tank, (tank was punctured at fairing mounts)
Idle at 3.5 turns. Idles smooth
Starts in 1 sec w/ no choke
slides operate smoothly.
Needles have no ridges.
No perceptible stumble revving in neutral.
No perceptible vacuum leak - hoses look good

flat spot in the 5500 to 7000 range after a hard full throttle pull to 10k+
Then backing down to the 6k range (60 mph) it stumbles. ~1/4 throttle
If slow acceleration to 6k seems to stumble less.

Started happening last summer. Prior running was great.

New plugs and 1 new plug boot - for some reason the boot was lifted and the arcing burned through the beryllium copper thread retainer.

Valves adjusted previous spring. 2k miles since. Minor tweek

In the last 2 weeks I have rebuilt the carbs 3x. 2nd time was a 2 hr soak in Berryman. Probed, blew out, spray carb cleaner all passages.

Even adjusted floats to 18.5 mm in case it was a flow issue (no change)
Old float needles... I have some new ones ordered

Bike runs great when cold but after 4 min it will stumble while cruising.
Seems to run great when on the throttle,, Great pull after 7k and up.
Stays strong at 80 mph for 3 minutes +

Have driven with the fuel cap open ( no change )

Any Thoughts

Scouring other posts has not gin=ven me any new directions to try.

jkv45 · June 28th, 2018, 06:26 AM

Quote:

Originally Posted by MN-

Yet Another Carburetor Conundrum...

My ride an 01 250 bone stock except:
Modified airbox (loose battery box for removal)
2 washers under needles ~1.5mm
Added fuel filter
Prior to last year (spring 2016) rebuilt petcock & lined the tank, (tank was punctured at fairing mounts)
Idle at 3.5 turns. Idles smooth
Starts in 1 sec w/ no choke
slides operate smoothly.
Needles have no ridges.
No perceptible stumble revving in neutral.
No perceptible vacuum leak - hoses look good

flat spot in the 5500 to 7000 range after a hard full throttle pull to 10k+
Then backing down to the 6k range (60 mph) it stumbles. ~1/4 throttle
If slow acceleration to 6k seems to stumble less.

Even adjusted floats to 18.5 mm in case it was a flow issue (no change)
Old float needles... I have some new ones ordered

Bike runs great when cold but after 4 min it will stumble while cruising.
Seems to run great when on the throttle,, Great pull after 7k and up.
Stays strong at 80 mph for 3 minutes +

What do the plugs look like?

Why are you so far out on the Idle Mixture screws? Typically 2.5 to 3.0 is adequate.

How old is the gas?

Does the fuel filter create a low spot in the fuel system?

Seems like it's rich in some spots. Starting without choke, and running fine until hot suggests that as well. I'd pull one of the shims and back the idle mixture down. Set the float level back to 17mm.

Did you remove the idle mixture screws, check the o-rings, and remove/clean the Pilot Jets?

Diaphragms check out ok?

Any chance you over-oiled the air filter? Try running temporarily without an air filter - any better?

I'd also sync the carbs if you haven't.

MN- · June 28th, 2018, 04:46 PM

Quote:

Originally Posted by jkv45

What do the plugs look like? Grey

Why are you so far out on the Idle Mixture screws? Typically 2.5 to 3.0 is adequate. at about 3.5 but there was not a sharp edge to any peak

How old is the gas? 2 different tanks regular & super non oxy.

Does the fuel filter create a low spot in the fuel system?
Yes, but bypassing the filter has no effect

Seems like it's rich in some spots. Starting without choke, and running fine until hot suggests that as well. I'd pull one of the shims and back the idle mixture down. Set the float level back to 17mm.

Did you remove the idle mixture screws, check the o-rings, and remove/clean the Pilot Jets? Yup. I'll change them when I get the carb kit (o rings and screws)

Diaphragms check out ok? Looked good to me

Any chance you over-oiled the air filter? Try running temporarily without an air filter - any better? I'll try that tonight. but I have not oiled it in quite a while as this bike does not get ridden much & it looks clean

I'd also sync the carbs if you haven't.

Done 2 years ago but will do it again when I rebuild w/ gasket kit.

Thanks for the input.

MN- · June 28th, 2018, 06:12 PM

My reply earlier today seems to have not made it....
3.5 turns on idle screws. No sharp edge to the tune.
Plugs look grey.
Gas is new. Had standard regular then drained and replaced with non oxy super.
Fuel filter is larger (2" diameter 3" long) bypassed it with no changes.
Cleaned all jets / ports.

I have carb kits on order. I will reset the floats and remove 1 washer, replace idle screws springs & o-ring. (rinse and blow out everything)

Carbs synced prior to last year, but will check after this rebuild.

Diaphragms look good.

I'll try without the filter tonight / tomorrow but it looks clean and not saturated.

greg737 · June 28th, 2018, 08:22 PM

Maybe something's up with the coasting enricher? It's usually a foolproof/bulletproof item on these Keihin carbs but it might explain part of your problem, the part where your engine stumbles when resuming after coasting down from a hard acceleration.

greg737 · June 28th, 2018, 09:03 PM

Also, the RPM area you're talking about, starting from around 5,000, is a designed-in weak spot in the EX250's ignition system.

If you look at the Service Manual you'll see that the EX250's ignition system starts out at idle with 12 degrees of ignition timing advance and the CDI box is programmed to move the ignition advance to "all in" which is 42 degrees of ignition advance by around 5,000 RPM.

The EX250's ignition advance curve is set this way because the bike's CDI is a simple, old-school, 2-dimensional curve type of box. There are no compensation inputs (like you get with a modern motorcycle ECU or a modern car).

42 degrees of ignition advance works fine (or at least okay) with normal throttle through the 5,000 to 7,000 RPM range, but when you're full WOT through that rev range the mixture is running right at max-horsepower air/fuel ratio (probably right about 13.5 to 1 with well-tuned carbs). In that rev range with a rich air/fuel mixture 42 degrees of ignition advance is too much advance.

But the Kawasaki engineers had to make a compromise call when they set the parameters of the CDI box ignition advance curve.

So the 5,000-7,000 RPM hesitation exists and while properly tuned carbs will make things run as well as they can, it can't completely fix it.

MN- · June 28th, 2018, 10:14 PM

When I get the rebuild parts I'll take a closer look at the enricher when I have the carbs apart. Seems reasonable that there may be a problem there.

As far as the CDI, do they partially fail? weak spark mid band? The fact it seems strong at high rpms to me suggests it is working well.

I can understand a 'flat spot' at the timing crossover point on this simple system. But my engine is actually missing / misfiring.

The coils measure in spec.

Thanks for all your ideas to check out.

jkv45 · June 29th, 2018, 06:56 AM

Quote:

Originally Posted by MN-

My reply earlier today seems to have not made it....
3.5 turns on idle screws. No sharp edge to the tune.
Plugs look grey.
Gas is new. Had standard regular then drained and replaced with non oxy super.
Fuel filter is larger (2" diameter 3" long) bypassed it with no changes.
Cleaned all jets / ports.

I have carb kits on order. I will reset the floats and remove 1 washer, replace idle screws springs & o-ring. (rinse and blow out everything)

Carbs synced prior to last year, but will check after this rebuild.

Diaphragms look good.

I'll try without the filter tonight / tomorrow but it looks clean and not saturated.

Not sure how you got that big of filter in there without creating a low spot in the system, but I don't think that's the problem here.

Did the plugs have a tan look to them? Typically they are either white (lean), tan (correct), or black (rich).

Next time out, if the problem persists, think in terms of throttle position instead of RPMs. Narrowing down the throttle setting (1/8, 1/4, etc) where the problem starts can help isolate the cause if it's carb related.

Interested to see how it runs without the filter.

MN- · July 2nd, 2018, 09:53 PM

The carb Kit arrived Saturday. Sunday was rainy so I rebuilt the carb once again.

The Plugs look great. Light tan. see attached pic.
Attached a pic of how I installed the larger fuel filter. I replaced it with a smaller one.

probe and soaked the jets. Rinsed all vacuum lines (carb cleaner) and inspected for cracks (none).

Pulled the washers from the needles. One stack measured 1.56 mm the other 1.71. One stack had a .83 the other a .81. I used only those washers when reassembling the needle / diaphragm. So the needle lift is ~.82 mm on each.

Re-adjusted the floats with new valves to 17 mm.

Replaced the idle screws/springs/washer/o-rings. One side was missing the washer.

Cleaned & oiled the air filter.

Idle screws are at about 2.5 & 3 -- I am really only seeing a slight dip in the idle speed.

Starts easily w no choke.

_Much_ better performance around 6K but when light cruising it still seems like it is occasionally missing (but much better). You can feel the torque curve start to pull at 7K... no quite as big as the 2 washer setup.

No missing when the throttle is > 25%

The light cruising is on the straights with the throttle between 10% & 25%

Since I can cruise at 80 for long periods of time, fuel delivery should not be the issue (with and without the gas cap open)

I'm most likely going to drive it like this for a while.

I'll try different idle screw settings to see if I can tune it out.

Other Thoughts?

MN- · July 2nd, 2018, 09:54 PM

The filter Picture did not make it...

I'll try again

jkv45 · July 3rd, 2018, 07:08 AM

Quote:

Originally Posted by MN-

The filter Picture did not make it...

I'll try again

Is this the current, or previous, filter set-up?

In this photo it looks like the fuel line is slightly pinched where it leaves the petcock and could cause a restriction.

There just isn't much room for an in-line filter. Did you check the one at the carb inlet (https://faq.ninja250.org/wiki/How_do...fuel_filter%3F)? I know some people recommend just running one of those instead.

I would also push the other line up snug to the petcock and re-position the clamp.

MN- · July 10th, 2018, 08:20 PM

It's back... huge stutter... like the ignition is shorting or there is air bubbles in the last stage of fuel delivery.

From the pictures of the plugs it seems it is not too much fuel.

It only happens when lite cruising at speed between 45 & 65 with the throttle at about 20% (high vacuum). When you crack the throttle (at all) the problem goes away.

It also seems to get worse with gusts of wind on the front... and occurs less when in traffic (wind break lighter engine load).

Since the most recent carb rebuild I have filled the tank with super non oxy, temporarily bypassed the filter, and ridden with the tank cap open. (put on over 100 miles)

The filter currently on the bike is much smaller than the one in the picture with no constrictions and less of a drop below the carb inlet.

I'm stumped.... It is pretty much unrideable between 45 & 65...

DannoXYZ · July 10th, 2018, 09:10 PM

remove washers from needles

MN- · July 11th, 2018, 06:53 PM

Thanks to all for the input.

I will give that a shot. (removing the last washers)

My concern is that this is new (ish) behavior.. (It started last summer)

I have had the bike for a total of 4 years after it being a 'barn find'.

I spent a winter fixing broken plastic, checking valve clearance, changing the plugs, replacing brake and clutch switches, checking that things were in spec (plug wire, coil resistance...) Rebuilt the carbs.... The dried fuel and other gunk would not have allowed it to run.

The carb was modded by the previous owner (just the 2 washers under each needle)

The bike was working great... none of the stuttering... smooth acceleration.

Until last summer... Started on a hot day... and got worse through the 50 mile ride. As it cooled in the fall the problem lessened. Now in the heat of summer in is as bad or worse than ever.

I have removed a washer from each of the carbs in the last couple of weeks with one in each carb remaining.

I will try removing the remaining washers in the next week or so.

(It just does not seem that this is the problem)

I'm in Minnesota with temps lately ranging from 80 to 95 and occasionally with oppressive humidity.

June 27th, 2018, 07:14 PM	#1
MN- ninjette.org member Name: Peter Location: North of Minneapolis, MN Join Date: Dec 2015 Motorcycle(s): 2001 Ninja 250, 2006 Vulcan 900 Posts: 10	YACC - Flat spot 5500 - 7000 after hard acceleration Yet Another Carburetor Conundrum... My ride an 01 250 bone stock except: Modified airbox (loose battery box for removal) 2 washers under needles ~1.5mm Added fuel filter Prior to last year (spring 2016) rebuilt petcock & lined the tank, (tank was punctured at fairing mounts) Idle at 3.5 turns. Idles smooth Starts in 1 sec w/ no choke slides operate smoothly. Needles have no ridges. No perceptible stumble revving in neutral. No perceptible vacuum leak - hoses look good flat spot in the 5500 to 7000 range after a hard full throttle pull to 10k+ Then backing down to the 6k range (60 mph) it stumbles. ~1/4 throttle If slow acceleration to 6k seems to stumble less. Started happening last summer. Prior running was great. New plugs and 1 new plug boot - for some reason the boot was lifted and the arcing burned through the beryllium copper thread retainer. Valves adjusted previous spring. 2k miles since. Minor tweek In the last 2 weeks I have rebuilt the carbs 3x. 2nd time was a 2 hr soak in Berryman. Probed, blew out, spray carb cleaner all passages. Even adjusted floats to 18.5 mm in case it was a flow issue (no change) Old float needles... I have some new ones ordered Bike runs great when cold but after 4 min it will stumble while cruising. Seems to run great when on the throttle,, Great pull after 7k and up. Stays strong at 80 mph for 3 minutes + Have driven with the fuel cap open ( no change ) Any Thoughts Scouring other posts has not gin=ven me any new directions to try.

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June 28th, 2018, 06:12 PM	#4
MN- ninjette.org member Name: Peter Location: North of Minneapolis, MN Join Date: Dec 2015 Motorcycle(s): 2001 Ninja 250, 2006 Vulcan 900 Posts: 10	My reply earlier today seems to have not made it.... 3.5 turns on idle screws. No sharp edge to the tune. Plugs look grey. Gas is new. Had standard regular then drained and replaced with non oxy super. Fuel filter is larger (2" diameter 3" long) bypassed it with no changes. Cleaned all jets / ports. I have carb kits on order. I will reset the floats and remove 1 washer, replace idle screws springs & o-ring. (rinse and blow out everything) Carbs synced prior to last year, but will check after this rebuild. Diaphragms look good. I'll try without the filter tonight / tomorrow but it looks clean and not saturated.

June 28th, 2018, 08:22 PM	#5
greg737 - Name: - Location: - Join Date: May 2009 Motorcycle(s): - Posts: A lot.	Maybe something's up with the coasting enricher? It's usually a foolproof/bulletproof item on these Keihin carbs but it might explain part of your problem, the part where your engine stumbles when resuming after coasting down from a hard acceleration.

June 28th, 2018, 09:03 PM	#6
greg737 - Name: - Location: - Join Date: May 2009 Motorcycle(s): - Posts: A lot.	Also, the RPM area you're talking about, starting from around 5,000, is a designed-in weak spot in the EX250's ignition system. If you look at the Service Manual you'll see that the EX250's ignition system starts out at idle with 12 degrees of ignition timing advance and the CDI box is programmed to move the ignition advance to "all in" which is 42 degrees of ignition advance by around 5,000 RPM. The EX250's ignition advance curve is set this way because the bike's CDI is a simple, old-school, 2-dimensional curve type of box. There are no compensation inputs (like you get with a modern motorcycle ECU or a modern car). 42 degrees of ignition advance works fine (or at least okay) with normal throttle through the 5,000 to 7,000 RPM range, but when you're full WOT through that rev range the mixture is running right at max-horsepower air/fuel ratio (probably right about 13.5 to 1 with well-tuned carbs). In that rev range with a rich air/fuel mixture 42 degrees of ignition advance is too much advance. But the Kawasaki engineers had to make a compromise call when they set the parameters of the CDI box ignition advance curve. So the 5,000-7,000 RPM hesitation exists and while properly tuned carbs will make things run as well as they can, it can't completely fix it.

June 28th, 2018, 10:14 PM	#7
MN- ninjette.org member Name: Peter Location: North of Minneapolis, MN Join Date: Dec 2015 Motorcycle(s): 2001 Ninja 250, 2006 Vulcan 900 Posts: 10	When I get the rebuild parts I'll take a closer look at the enricher when I have the carbs apart. Seems reasonable that there may be a problem there. As far as the CDI, do they partially fail? weak spark mid band? The fact it seems strong at high rpms to me suggests it is working well. I can understand a 'flat spot' at the timing crossover point on this simple system. But my engine is actually missing / misfiring. The coils measure in spec. Thanks for all your ideas to check out.

July 10th, 2018, 08:20 PM	#12
MN- ninjette.org member Name: Peter Location: North of Minneapolis, MN Join Date: Dec 2015 Motorcycle(s): 2001 Ninja 250, 2006 Vulcan 900 Posts: 10	It's back... huge stutter... like the ignition is shorting or there is air bubbles in the last stage of fuel delivery. From the pictures of the plugs it seems it is not too much fuel. It only happens when lite cruising at speed between 45 & 65 with the throttle at about 20% (high vacuum). When you crack the throttle (at all) the problem goes away. It also seems to get worse with gusts of wind on the front... and occurs less when in traffic (wind break lighter engine load). Since the most recent carb rebuild I have filled the tank with super non oxy, temporarily bypassed the filter, and ridden with the tank cap open. (put on over 100 miles) The filter currently on the bike is much smaller than the one in the picture with no constrictions and less of a drop below the carb inlet. I'm stumped.... It is pretty much unrideable between 45 & 65...

July 10th, 2018, 09:10 PM	#13
DannoXYZ ninjette.org certified postwhore Name: AKA JacRyann Location: Mesa, AZ Join Date: Dec 2011 Motorcycle(s): CB125T CBR250R-MC19 CBR250RR-MC22 NSR350R-MC21 VF500F CBR600RR SFV650 VFR750F R1M ST1300PA Valkyrie-F6C Posts: A lot. MOTY - 2018, MOTM - Nov '17	remove washers from needles

July 11th, 2018, 06:53 PM	#14
MN- ninjette.org member Name: Peter Location: North of Minneapolis, MN Join Date: Dec 2015 Motorcycle(s): 2001 Ninja 250, 2006 Vulcan 900 Posts: 10	Thanks to all for the input. I will give that a shot. (removing the last washers) My concern is that this is new (ish) behavior.. (It started last summer) I have had the bike for a total of 4 years after it being a 'barn find'. I spent a winter fixing broken plastic, checking valve clearance, changing the plugs, replacing brake and clutch switches, checking that things were in spec (plug wire, coil resistance...) Rebuilt the carbs.... The dried fuel and other gunk would not have allowed it to run. The carb was modded by the previous owner (just the 2 washers under each needle) The bike was working great... none of the stuttering... smooth acceleration. Until last summer... Started on a hot day... and got worse through the 50 mile ride. As it cooled in the fall the problem lessened. Now in the heat of summer in is as bad or worse than ever. I have removed a washer from each of the carbs in the last couple of weeks with one in each carb remaining. I will try removing the remaining washers in the next week or so. (It just does not seem that this is the problem) I'm in Minnesota with temps lately ranging from 80 to 95 and occasionally with oppressive humidity.