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Old June 24th, 2019, 03:40 PM   #1
1MNeary
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08 250R miss under load at high RPM

I have an 08 ninja 250 as the title states, and it recently developed a miss at about 8-9000 rpm. It runs perfect up to 8000rpm and when I take off it will hit redline no issue in first gear, you can barely detect a miss in second, and from third on you get a miss at 8000rpm and it doesn’t want to rev any higher. If you cruise in sixth it starts stumbling intermittently at about 7000 and it gets more frequent as rpm increases until about 8500 where it bogs and won’t go any faster.

Mods: slip on muffler off a big bike, snorkel removed, jet kit(unknown jet sizes), emission delete(done correctly).

Gearing is currently stock

I bought the bike with all mods except emission delete with 8500 miles on it less than a month ago. It ran pretty good for 100 miles but felt a little rough(6000rpm stumble, choppy idle). I did an oil and filter change, put fresh cr8e plugs in it(didn’t adjust gap), ran a fuel system cleaner through it, and did a clean and re oil on the factory filter. The bike ran PERFECT for 700 miles, lots of spirited riding, several 100 mile highway trips at 80mph, then the other day it starts doing the problem I described.

I pulled the tank off and performed the mod where you enlarge 3 holes on the gas cap( I live in FL my bike was beeping constantly), drained the tank, verified no rust or foreign debris, cleaned the filter(it was clean), and checked all hoses on the tank and carb. I also verified the plug wires were snug while I had it apart. I put 1/2 gallon of fresh 93 octane in the bike and took it for a ride, the issue persists.

When riding the bike slowly into high rpm the stumble is felt less but still exists, and still will not go any faster in 5th and 6th. When the choke is applied full the bike backfires constantly and runs worse, at 1/2 choke it does the same thing but runs pretty much the same until you let off the gas, then it backfires. At 1/4 choke it runs no different and also backfires after letting off at wide open throttle.

I have searched all day and found lots of people with this issue but not allot of clear answers to what the fix is. Anybody had this issue and resolved it?
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Old June 25th, 2019, 01:49 AM   #2
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Hi Matthew!

You inherited bike with many mods not done by you, so they are unknown. Let's look at some potential problem areas:

1. petcock. Perform a flow-test. Unplug petwork fuel-inlet and vacuum hose where they go into carbs. Aim fuel-hose into measuring cup. Apply some vacuum to smaller hose and time fuel-flow for 30-seconds. How much fuel did you collect?

2. petcock filters. Remove petcock from tank and inspect two filters attached to inlet tubes.

3. inline-filter in fuel-inlet hose before carbs. Make sure filter isn't inserted backwards. It will crumple in on itself if reversed and block flow.

4. Jet-sizes. You may have non-factory jet-sizes. From factory, jetting is way, way too rich, so there's no reason to go larger, but lots of people automatically install massive jets anyway. Racers install free-flow full-exhaust and actually switch to smaller jets for maximum-power. Pull main and pilot jets from your carbs and verify their sizes.

5. slide needle. May also be non-factory. Factory Pro has shipped pre-gen needles with their new-gen jet-kits. Not even close match. So remove slides from top of carbs and post photo of needles.

6. carb vent-tube clear. At back centre of carbs, there is plastic T with port aiming up and backwards. Verify vent hose that goes from carb-vent down to rear-shock is clear and you can blow through it easily.

7. really clean carbs while you're there. Use some guitar wire to floss through all holes in main-jets, emulsion tubes, pilot jet's holes. Blow through with high-pressure air to push all crumbled crud through. The fuel-circuits inside the carb-bodies should be flossed from each end as well. This is hidden secret stuff that most people miss. https://www.ninjette.org/forums/showthread.php?t=147048
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Old June 25th, 2019, 06:56 AM   #3
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Quote:
Originally Posted by 1MNeary View Post
When the choke is applied full the bike backfires constantly and runs worse, at 1/2 choke it does the same thing but runs pretty much the same until you let off the gas, then it backfires. At 1/4 choke it runs no different and also backfires after letting off at wide open throttle.
Try removing the air filter temporarily and see how it runs.

You may need to go through the carbs and correct some "mods" and over-enthusiastic jetting changes.
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Old June 25th, 2019, 07:15 AM   #4
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Originally Posted by jkv45 View Post
Try removing the air filter temporarily and see how it runs.

You may need to go through the carbs and correct some "mods" and over-enthusiastic jetting changes.
very well put, sir

I'd go on to advise checking and correcting the spark plug gaps (OP admitted to never checking them in his fist post) and checking plug caps and wires and electrical connections at the coils while you are there. Possible you've got some misfiring at certain RPM's?
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Old June 25th, 2019, 07:05 PM   #5
1MNeary
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FIXED

For anyone who has this issue, here’s what happened on my bike.

First problem, previous owner cut the snorkel back to much which causes the intake hose to sit incorrectly in the airbox, after inspecting the box the hose was kinked which limited the air to the carbs.

Second problem, like an newb I didn’t gap the plugs correctly and the gap on the plugs was about .022 (suppose to be .030-.040). I assume the combination or running rich and to small of a gap(weak spark) caused the plugs to foul out. High rpms would result in more fuel but not more air due to the kinked hose, and apparently the spark was still strong enough to burn the fuel off up until 8000rpm when the amount of fuel coming in could no longer be efficiently burned off by the weakened spark. Apparently 800-1000 miles is now long it took to foul the plug explaining why it ran well for a little while.

Potential third problem, the cylinder 1 coil was very loose and was probably vibrating at high rpm. Don’t know if this could cause a miss or not but i tightened it up before test riding so I added it in as an issue corrected.

I installed the new plugs at a .037 gap because I know it may be running rich, I will report back if the issue returns but right now it is running amazing, I was probably down 3-5hp the whole time I have owned it because it is way faster now than it has been since I bought it.

Side note. The first set of ngk cr8e plugs I installed came gapped at .022 and the set I just installed came gapped to .030, seems this is a lesson of never trust the plug gap out of the box the gap plug to plug isn’t even that consistent.
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Old June 25th, 2019, 07:16 PM   #6
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MOTY - 2018, MOTM - Nov '17
Wow! Good work! How did plugs look? Here's mine after about 2-hrs on track at WOT 80% of time (left, stock bike). Right is my race bike with full-exhaust and smaller main-jets.

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Old June 26th, 2019, 05:30 AM   #7
1MNeary
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Wow, that’s crazy for just a track day. I will take pics of the old plugs this afternoon.
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Old June 26th, 2019, 06:01 AM   #8
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Originally Posted by DannoXYZ View Post
Wow! Good work! How did plugs look? Here's mine after about 2-hrs on track at WOT 80% of time (left, stock bike). Right is my race bike with full-exhaust and smaller main-jets.

And that looks a lot better than stock! What size Main Jet are you running?

People just don't believe it could be possible that it's jetted RICH from the factory, and always want to increase the Main Jet size.
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Old June 26th, 2019, 06:47 AM   #9
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Yep, I went down a size on the mains when I tuned mine with a full exhaust.
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Old June 27th, 2019, 03:39 PM   #10
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These were the plugs after 800ish miles.
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Old June 28th, 2019, 05:51 AM   #11
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Did you try riding without the air filter?

Those plugs are rich overall.

The carbs need to come off and be gone though most likely. Chances are the jetting is richer than optimum or adjustments are off.
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Old June 28th, 2019, 10:52 AM   #12
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Originally Posted by jkv45 View Post
And that looks a lot better than stock! What size Main Jet are you running?

People just don't believe it could be possible that it's jetted RICH from the factory, and always want to increase the Main Jet size.
I went down to 96 mains from factory 98. But that's not complete story because factory jetting is both lean AND rich.



With full exhaust, we pick up about 2.5hp. But is still leaving about 2.5hp on table due to super rich high-end. Lean mid-range is out of range for pilot-screw adjustment. Shimming stock needles just slides entire curve to left, making rich high-end occur sooner. Actual correction requires this.



Not obvious is Dynojet's needle is slightly thinner in midrange compared to stock and it adds fuel. Combined with fat thicker tip which removes high-end fuel. This combined with one size smaller jet reduces high-end fuel enough to pick up an additional 2.5hp for +5hp total over stock. Now we've got Ninja 300 power without the extra weight.
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Old June 28th, 2019, 12:10 PM   #13
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interesting read. still fighting my 6000-7000 miss. starting to run out of options
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Old June 28th, 2019, 01:56 PM   #14
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Originally Posted by 1MNeary View Post
These were the plugs after 800ish miles.
Unless you were at full-throttle 8000+RPMs for most of those 800ish miles, something's seriously wrong. Definitely pull those carbs and verify they're set up with factory jetting.

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Old June 28th, 2019, 03:48 PM   #15
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If you're not at 10,000 RPMs, you're just wasting time!

Kidding!

Sorta...
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Old June 29th, 2019, 09:23 AM   #16
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interesting read. still fighting my 6000-7000 miss. starting to run out of options
At what throttle opening is the problem happening?

Narrow that down and it will be easier to correct if it's carb related.
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Old June 29th, 2019, 09:37 AM   #17
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At what throttle opening is the problem happening?

Narrow that down and it will be easier to correct if it's carb related.
https://www.ninjette.org/forums/show...0&postcount=15
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Old June 30th, 2019, 06:30 AM   #18
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I'll post there.

Post a link to the entire thread, I can't find it.
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Old June 30th, 2019, 06:34 AM   #19
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I'll post there.

Post a link to the entire thread, I can't find it.
https://www.ninjette.org/forums/showthread.php?t=329169
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