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Author Topic: Resetting Stock Idle Speed  (Read 7774 times)
ducpainter
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« Reply #15 on: September 05, 2019, 10:51:36 AM »

If the slides don't lift via vacuum they'll run crazy rich. Verify slide lift before you go further.
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« Reply #16 on: September 05, 2019, 02:30:39 PM »

Remove the airbox lid & filter and watch the slides as you rev it.
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« Reply #17 on: September 09, 2019, 12:12:19 PM »

Update: I've taken the carbs all apart and reassembled once more.  I know you're not supposed to change more than one thing at a time for proper diagnostics, but--well I'm tired of taking the carbs on and off at this point hah. 

I changed everything. New jets all around from my jet kit, I had previously left them in a little bag in the hopes that the previous owner had it tuned to run.

When I pulled the pilot jet (a size 40) I noted holes in the sides of the jet which I had thought were stock.  upon closer inspection, it looks like these are DIY... I presume this was the culprit for the improper fuel flow in the pilot circuit. 

Here are the jet sizes now (Previous  / current):

Main Jet:  155 / 140
Pilot Jet: 40 (with holes)  / 40 (no holes)
Starting Jet: 70 / 70
Needle:  3rd position down / 4th position down
Idle mixture screws: 3.5 turns out

Also, the airbox "swiss cheese" holes are masking taped over to better emulate stock

Tweaked the idle speed a bit and Eureaka! The bike now idles!  But it's still not running quite right... I get a bit of backfiring through the airbox, I haven't run it that much yet, but I noticed it at about half throttle.  The 155 main jet might have been better after all.  I'm going to run it for awhile first and put fresh gas in before I make any other changes though, maybe I can get a better read out of the plugs after a few miles.

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« Reply #18 on: September 09, 2019, 01:00:24 PM »

Well done waytogo.
A good baseline now. People get those pilot jets confused as there are two types as you found.
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« Reply #19 on: June 16, 2020, 05:11:42 AM »

Ancient history at this point, but after a winter of break time, I'm back at it.  Swapped the main jets back from a 140 to the 155 and wow, what a world of difference.  Throttle response is vastly improved and, while it's not running perfect just yet, it's damn close.  It idles fairly well after a quick warmup with the choke, though it definitely sounds slow. 

I think I may need to adjust the needle position.  First I'll probably just increase the idle speed a hair and take it for a cruise and see how it feels--

Thanks everyone who helped out so far!  I wouldn't have had a clue where to pick back up if this thread of our conversations didn't exist for me to reference!
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« Reply #20 on: June 16, 2020, 08:04:55 AM »

 waytogo Was the jet kit a Dynojet kit?
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« Reply #21 on: June 16, 2020, 08:55:46 AM »

No, I just used some jets I had lying around the garage; most of the jets I have come from cheap carb rebuild kits ordered online.  Used stock size jets from a rebuild kit and a 155 from another set of carbs I had
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« Reply #22 on: June 16, 2020, 09:23:50 AM »

I asked due to DJ jets different calibration to oem, a DJ155 is about oem 145. Also try and use genuine Mikuni jets instead of no name jets. So you are using 155 Mikuni jets with a drilled airbox or stock? 140 normally suits stock closed lid, 155 drilled or half open and 160-165 for fully open lid. Are the needles oem or from a DJ or Factory Pro kit? If your fuel has significant ethanol content then that may explain the need to jet up as ethanol leans things out.

People have run into tuning issues with cheap import or pirate jet kits (and carbs for that matter). With those kits (and pirate carbs), established tuning data may not translate across. I don't touch the stuff.
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« Reply #23 on: June 18, 2020, 05:31:02 AM »

Thanks, I'll keep that in mind.  I'm working on a budget, so I'm gonna make the no name jets work. 

Probably 155 Mikuni's but I suppose they could be from a DJ kit.  They are used and appear to be pretty good quality brass.  Airbox is drilled with a K&N filter, but, as mentioned earlier, I've taped off the holes to better emulate stock settings. 

I was reading up a bit on the particulars of mikuni carbs via this handy webpage: http://www.iwt.com.au/mikunicarb.htm#:~:text=The%20pilot%20jet%20is%20the,to%20around%201%2F4%20throttle.

It sounds like at this point a little tweaking of the pilot circuit should get me where I wanna be.  I believe I need a richer mixture there.  Could someone tell me: Would I turn the pilot adjustment screws in or out to achieve a richer mix?

Thanks!
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« Reply #24 on: June 18, 2020, 01:18:39 PM »

Out waytogo
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« Reply #25 on: June 19, 2020, 04:49:54 AM »

Quote
Probably 155 Mikuni's but I suppose they could be from a DJ kit.
 I'm working on a budget, so I'm gonna make the no name jets work.

So you/we are guesssing here, same w pilots. Hard to advice when not knowing the parts....

1. Give us pics, all jets & needles and we`ll sort out what is Mikuni or DJ.
2. Or, buy, despite budget, a DJkit for open airbox.
« Last Edit: June 21, 2020, 04:27:55 AM by greenmonster » Logged

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« Reply #26 on: July 25, 2020, 09:32:53 AM »

Alright folks, another update!  So I raised the needle and I'm now running on the 5th position from the top.  These needles are not stock. Cheepos from some kit. Currently running:

Main Jet:  155
Pilot Jet: 40 (no holes on the sides)
Starting Jet: 70
Needle:  5th position down
Idle mixture screws: 4 turns out

Airbox DIY holes are taped off

Bike idles strong now and sounds good.  But, I'm getting too much gas.  When I choke it (which it does not need to start) it smokes to bejesus.  after awhile of idling and a bit of throttle, there is a small small amount of smoke.  Its whitish greyish and I presume it to be excess gas burning-- not oil.  The throttle response is swift and snappy, but it continues to rev high 2 or 3 seconds longer than it should.  Pulling the clutch lever, strangely enough, lowers the high rev.  Any Ideas which circuit is running too rich?  I'm thinking idle screws or needle. 

4th position needle and 3.5 turns out had my engine cutting out when I snapped the throttle and only starting with choke.  I think the carbs are likely a little out of sync, so I'm trying to find a time to run out and borrow a decent monometer setup.  When it smokes, I can also see a bit of smoke wofting from the lower cylinder exhaust where it meets the engine head---I think this will need a new crush gasket; this is the side also smokes much more prominently in the instance of smoke.

Thanks all
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« Reply #27 on: August 10, 2020, 07:36:01 AM »

Making progress on this, tried several configurations of needle position over the weekend.  Can someone tell me what the stock needle is for these BDST 38 Mikunis for the M900? 


I have a couple different needle types from various sets of these carbs and rebuild kits and I don't know which is which...

I have one that has a long skinny taper and another that is wider with a short stout taper at the end.  (can't quite get my image uploaded...). 

More info for those who want to dig in:
So far, I've had the best performance with:

-the longer skinny needle at 4th position from the top
-155 main
-70 secondary
-3 turns out

with this configuration, the bike runs great in 1st gear, pretty good in 2nd/3rd, but bogs when I snap the throttle twice on the second snap. 

My hypothesis was that this could be a symptom of rich condition (I have a hard time believing the floats could be emptying so fast and starving the engine for fuel).  Maybe the second snap was flooding?  I never did stall though...

Switched needles to see if I could get any better performance, but the bike was real jerky at quarter throttle and worse at half with the fat needle. (much too lean, I think)

Went back to the long skinny taper, and switched the main to a 140... fired up right away, but when I snapped the throttle it died.  Hasn't stared back up yet from there. strange since I don't think the main has much to do with the starter circuit...I'll check my diaphragms and try again with this config.

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« Reply #28 on: August 11, 2020, 07:07:36 AM »

Still guessing w needles & jets.
And guessing rich/lean.
Try plug reading: When you reach a problem throttle/RPM,
kill engine, check plugs, black=rich, white=lean.
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« Reply #29 on: August 11, 2020, 07:43:39 AM »

Thanks,

no tach, so RPMs are by ear only.  Plugs say lean sometimes, rich other times.  I'm trying to find the right balance with what I've got. 

I can see it running rich with the smoke out the exhaust when I raise the needle way up.  I can also see excess fuel residue in the airbox in these instances.  I haven't found a middle ground yet.  Slight adjustments from rich condition throw me into territory of white plugs and starving engine... 

I'm thinking I may have a washer on the wrong side of the needle's e-clip.  I've also got a washer below the needle assembly in the slide (or is this a shim?). looking at the stage 2 dynojet kit instructions I'm gonna try to replicate with my 155 main... wish I could figure out which of these needles is stock.
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