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Old March 15th, 2018, 11:35 AM   #1
austinninjabill
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Flat spot in power curve between 6-7,000rpm

Thanks to everyone who has provided advice so far as I get this '92 250r running after sitting for 15-16 years.

Still no brain bucket (arriving today: full face Evel Kinievel red, white and blue), but am getting a little more confident zipping around on it. Can now get up to about 65-70 before backing off.

Still takes a long time to warm up without bogging and the choke still has no friction at the lever. And I am having to adjust idle just a bit after every few rides.

All that will get sorted out, however, the bike has a funny behavior that I was wondering whether anyone else has seen/corrected.

It will pull smoothly and strongly to about 6,000 rpm, then the power curve flattens 'til around 7,000, where it suddenly takes off like a banshee right up to redline.

Makes the bike a little unruly/disconcerting to ride, having a "lull" then a "bang" right in the middle of the rpm range. When it hits that 7,000 sweet spot, if you're not paying attention and holding on tight, the bike could leave you sitting on the pavement, which is going to hurt a bit at 50mph.

Guessing this is a carb/air-fuel delivery issue?

Again I am running the following setup:

K&N Pod (what a throaty noise that thing makes)
Slow jet: 40
Main Jet: 110
Shims: 1
Air screws: 2.5 turns out (left and right)

Based on above setup, any thoughts as to what might be causing this flat part of the power curve? Guessing the slow jet isn't the issue once above above idle, same with air screws. So do I need it richer (112 and 2 shims) or leaner (108 and no shims) or something in between?
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Old March 15th, 2018, 12:30 PM   #2
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Do some testing and see how the behavior changes:

1. change needle-shims to 2 washers, how's it behave?
2. change needle-shims to 0 washers, how's it behave?

The thing is to make only one small change at a time and correlate that with a change in behavior. Once you collect enough data, it'll be easier to make further adjustments
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Old March 15th, 2018, 02:01 PM   #3
austinninjabill
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Thanks.

Easier to add/subtract shims than working in the float bowl.

Looks like you have a lot experience given the bikes you own.
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Old March 15th, 2018, 03:54 PM   #4
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yeah just add another washer if that doesn't help you may possibly need to try the 112 size jet but i'm sure another washer will help. At Least you know how much of a dream it is to just rip your carbs out now with that pod filter!
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Old March 15th, 2018, 04:17 PM   #5
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I think there's more to it than just some adjustments. Possibly the carb diaphragms.

Your settings are in the ballpark, and plenty rich.

Do you have a pipe or slip-on?

We ran with a K&N filter and a stock, but leaky exhaust, with 108 Mains, 2 shims, and the stock 38 Pilots about 2.75 out depending on temp. We run 87 ethanol-free, which provides a slightly richer mixture than E10.

Ran like a champ the entire way past redline.
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Old March 16th, 2018, 02:51 AM   #6
austinninjabill
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not sure what you mean by pipe or slip-on.

Exhaust?

If so, it is stock and doesn't appear to leak at the head or elsewhere.

Last futzed with by austinninjabill; March 16th, 2018 at 07:47 AM.
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Old March 16th, 2018, 02:56 AM   #7
austinninjabill
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BTW, given how many bikes some of you guys own, it looks like I had better start expanding my collection of one.

I've got nearly 20 cars, so not sure my wife is going to be big on me starting to collect motorcycles as well . . .

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Old March 16th, 2018, 06:31 AM   #8
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Quote:
Originally Posted by austinninjabill View Post
not sure what you mean by pipe or slip-on.

Exhaust?

If so,they are stock.
I think your jetting is too rich with a stock exhaust.

A 110 Main jet is pretty big with a stock exhaust. 2.5 turns out is probably too rich with a 40 Pilot jet.

It still sounds like the carb is not acting properly though, like the slides are sticking or not lifting at the proper time. Manually check that they rise and fall smoothly (engine off, lift them with your finger and let them fall).

20 cars? My wife would kill me. Bikes are so much easier to store.
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Old March 16th, 2018, 08:13 AM   #9
austinninjabill
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So, on the slow jet you think I need to either go down to the stock 38 or up the turns out on the air screws (more turns out = more air = leaner, right?)

Or maybe I go with an upgraded exhaust/expansion pipe (they still call them that, right?) at the other end of the air-fuel equation?

Got the bike for free, have about $400 in it (battery, carb rebuild kits and new jets, K&N pod and crank breather, petcock, caliper and master cylinder rebuild kits, new brake lines, gallon of muratic acid to flush tank, oil and filter, metal for battery box, couple of spare keys, helmet, etc.), so maybe I can afford a (used) downpipe upgrade.

Also, someone in this or maybe one of my other posts mentioned upgrading to an '08 or later shock and said it was cheap, but partzilla wants $400+ for either a pre-gen or post-gen (is that what the '08+ models are called?)

Yep bunches of cars and trucks (in order of year): '32 Ford, 2x '40 Fords, 2x '60s vintage VW sand rails (what they call a dune buggy here in Texas), '73 vette, '73 911, '81 x1/9, '82 RX7, 2x '86 944s, '86 928, '88 924, '96 Saab 900, '00 1500, '00 Insight (manual), '03 Insight (auto), '04 Land Rover D2, '04 RX8, '04 T&C minivan, '04 Saab 9-3, '07 T&C (converting into a Deora III), '07 Saab 9-3, '08 M35. Nothing newer than 2008.

Damn, that's 24 plus one Ninja 250r now. (and now that I have the bike bug, would like a vintage spoked wheel KZ900).

And still looking for a Spitfire, TR7/8, 914, 320i, 633/635 and maybe an Opel Manta. With those my realistic teenage wish list will be pretty much complete, and then I can die happy.

I think I need an intervention.
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Old March 16th, 2018, 08:40 AM   #10
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Quote:
Originally Posted by austinninjabill View Post
So, on the slow jet you think I need to either go down to the stock 38 or up the turns out on the air screws (more turns out = more air = leaner, right?)

Or maybe I go with an upgraded exhaust/expansion pipe (they still call them that, right?) at the other end of the air-fuel equation?

Also, someone in this or maybe one of my other posts mentioned upgrading to an '08 or later shock and said it was cheap, but partzilla wants $400+ for either a pre-gen or post-gen (is that what the '08+ models are called?)

I think I need an intervention.
The idle mixture screws on the Ninja are fuel bleeds, so turning OUT provides a RICHER mixture. It may be possible that something like 1.5 turns out with a 40 Pilot is the exact same as something like 2.5 out with a 38 Pilot - not sure if there are any other factors going on.

Complete exhaust systems for the PreGens may be getting hard to find. The sound improves, and they are lighter, but there is a minimal performance increase with most.

The New Gen shocks are cheap used on ebay.

Nah - you are fine.
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Old March 16th, 2018, 09:16 AM   #11
austinninjabill
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Oh, so not air needles/screws. Good to know, I got that bass ackwards.

I guess I will start with adding a shim to the needle and going back to the stock 38 slow jet as a first step and see what happens.

Bummer on the exhaust, will look for a used post gen shock.

Thanks for the encouragement to chase my dreams: my wife will appreciate it (not).
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Old March 16th, 2018, 07:41 PM   #12
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Quote:
Originally Posted by austinninjabill View Post
Oh, so not air needles/screws. Good to know, I got that bass ackwards.

I guess I will start with adding a shim to the needle and going back to the stock 38 slow jet as a first step and see what happens.

Bummer on the exhaust, will look for a used post gen shock.

Thanks for the encouragement to chase my dreams: my wife will appreciate it (not).
ninja 300 rear shocks apparently fit and has 5 way adjustable preload
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Old March 19th, 2018, 07:22 AM   #13
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While I hate the air box design, I got a sitting for 5 years bike up and running. All it took was a carb cleaning. I did use screw clamps to replace those springs.
The whole air box thing is a real PITA if you want to remove your carbs. It does seem that once they are clean you don't need to remove them often. Mine pulls all the way to rev-limiter with no flat spots.

Please keep us informed about you progress. Like I said I hate the air box.
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Old March 19th, 2018, 09:38 AM   #14
austinninjabill
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Will do.

Probably won't get to adjusting the carbs for a week or two.

Also thinking about making a little bracket with, say, some strapping to hold the carbs stable and in-place.

Right now, without an airbox to provide any stability, the carbs are "floating": held on the front by the stiff rubber plenums (correct term?) and nothing else on the back.

I am wondering if the action of rolling on the throttle causes the throttle cables to torque the carbs enough that maybe I am getting an air leak at the plenum.

Has anyone with a air filter pod and floating carb bodies seen this problem? has anyone made a bracket to secure the throttle bodies to something other than the rubber plenums?

Last futzed with by austinninjabill; March 19th, 2018 at 11:45 AM.
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Old March 19th, 2018, 10:44 AM   #15
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I saw one that had a base that provided a hard mount. I think they ask $300 dollars, which is a little overboard. Google it and fabricate a copy. That is what I will do if I go away from the air box.
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Old March 19th, 2018, 11:45 AM   #16
austinninjabill
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Couldn't find it. What key words would you suggest?
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Old March 19th, 2018, 11:49 AM   #17
austinninjabill
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BTW, I bought these super-beefy clamps to secure the carbs to the rubber intake plenums precisely because I was worried about them working their way off the plenum without any support on the pod end:
Attached Images
File Type: jpg super beefy hose clamps.jpg (2.7 KB, 49 views)
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Old March 19th, 2018, 12:57 PM   #18
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Good idea!

I just use some lag-screws to bolt battery-box back to airbox. Takes all of 10-15 seconds to separate if needed.
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Old March 20th, 2018, 07:33 AM   #19
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Quote:
Originally Posted by Cafe Racer View Post
While I hate the air box design, I got a sitting for 5 years bike up and running. All it took was a carb cleaning. I did use screw clamps to replace those springs.
The whole air box thing is a real PITA if you want to remove your carbs. It does seem that once they are clean you don't need to remove them often. Mine pulls all the way to rev-limiter with no flat spots.

Please keep us informed about you progress. Like I said I hate the air box.
Quote:
Originally Posted by austinninjabill View Post
Couldn't find it. What key words would you suggest?
Google this part number AK-K2S12-S3 You will pretty pictures of the unit.

I am thinking aluminum sheet stock. Add a little fiberglass cloth to the top side if it needs more support. I have a scrap yard near by that sells all kinds steel, plastic, aluminum, plate, sheeting, tube square pipe you name it by the pound. Way to cheap. I saw some big stainless pipe with flanges on each end. Looking to me like gas tank for a apocalyptical/maddMax motorcycle. Now if I can only find it in aluminum.
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Old March 22nd, 2018, 04:54 PM   #20
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Air box gude!!

Not to start a fight but air boxes can be your friend. Factories spend mucho$ designing them. My buddy owns a small shop and 60% of his business is carb related. He laughs every time a pod bike comes in. Yes they can be made to run and even have more peak power. The downside is never having it run as nice as in stock trim. Just old farts input-between us we have a century of cycling.
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