Carb tuning help - mikuni flat slide

[max power]

Got a 91 Honduh XR250L with a punched out bore, white bros exhaust system, K&N and a mikuni 32mm flat slide carb.  

When I got it, the carb was hillbilly connected to the intake with a piece of tire innertube and a XR400 air tube.

I sent the carb to XRSONLY in California to add an air box adapter causing me to have to buy an XR250 air box tube and they found the jets to be way big.  

I believe the jet now is 145   

Problem is, when at cruising speed, say half throttle, it stumbles   

Called XRSONLY who said I need to raise the needle 1 spot. Let in more air.  Did that.  Still stumbles at half throttle.

Do I go the other way?  Is it a air/fuel screw adjustment issue?

What's my next move?

 

[Bubba]

The main needle doesn't meter air.  It meters fuel from the bowl, typically at throttle openings from 1/8-1/4 and up.  Most of the low speed operation is metered by the idle jet.  The main needle meters more fuel as the slide and needle is raised because it's tapered and you're pulling it up out of the main jet orifice, increasing fuel flow into the carb throat.  So, if you RAISE the needle--which means you LOWER the clip one or more positions on the needle--then you're richening the mixture, which sounds like what is needed and what the experts recommended.  If you lower the needle by RAISING the clip, then you've leaned the mixture further.  One other thing that I'm not sure about is that when changing the main jet to a large or smaller size, it may be necessary to match the needle to the new jet size if the taper isn't a proper fit.

Edited December 18, 2017 by Bubba

[Killer_kaw]

I have no experience with slab sides but lots of tuning on the zrx’s. When tuning those a cruising speed stumble is usually solved by adjusting the mixture screws or a larger pilot jet. Throttle response stumble when opening it up usually requires richening the main jet by  raising the needle(lowering the clip on the needle). Or a larger jet. Could be apples to oranges comparison on carbs. Sounds lean though

[max power]

3 hours ago, Bubba said:

The main needle doesn't meter air.  It meters fuel from the bowl, typically at throttle openings from 1/8-1/4 and up.  Most of the low speed operation is metered by the idle jet.  The main needle meters more fuel as the slide and needle is raised because it's tapered and you're pulling it up out of the main jet orifice, increasing fuel flow into the carb throat.  So, if you RAISE the needle--which means you LOWER the clip one or more positions on the needle--then you're richening the mixture, which sounds like what is needed and what the experts recommended.  If you lower the needle by RAISING the clip, then you've leaned the mixture further.  One other thing that I'm not sure about is that when changing the main jet to a large or smaller size, it may be necessary to match the needle to the new jet size if the taper isn't a proper fit.

I guess I had the description incorrect.  It has been a few weeks since I talked to xrsonly about the issue, but he did tell me to raise the needle by dropping the clip one spot.  It does  seem to run better but still stumbles.  

1 hour ago, Killer_kaw said:

I have no experience with slab sides but lots of tuning on the zrx’s. When tuning those a cruising speed stumble is usually solved by adjusting the mixture screws or a larger pilot jet. Throttle response stumble when opening it up usually requires richening the main jet by  raising the needle(lowering the clip on the needle). Or a larger jet. Could be apples to oranges comparison on carbs. Sounds lean though

I'll see if in can find the mixture screw and where it should be set. 

 

Thanks guys 

[Steve Butters]

LBTS 

[2talltim]

WOT all the time..... Fixed...MODs should close this thread.

Edited December 18, 2017 by 2talltim

[max power]

Found this.  Apparently the Mikuni flat slide was designed for a 2 stroke application but is a common mod on 4 strokes. 

Interesting read   

 

http://www.odysseypilot.com/index.php?topic=3731.0

[max power]

According to the above guide, mid range fueling is controlled by changing the circlip on the needle.  

[Killer_kaw]

Definitely a different animal. That guide should help a lot. 

[dorset]

is the stumbling right at half throttle, or a bit below or above?   if you mark the twist grip barrel at 1/4, 1/2, 3/4, and full rotation, you can figure out pretty closely which circuit is the one giving you trouble. the metering overlaps, and so a lean stumble at exactly half throttle could be the clip position, the needle taper, or lingering effects of the needle/needle jet diameter:

this old chart gives you an idea of which circuits dominate at which throttle positions:

"straight diameter" is referring to the size of the annulus between the straight portion of the needle and the needle jet. you can vary each independently to change that.

there are two screws-- the idle adjuster screw is knurled, up high on the body, and raises and lowers the slide to change the idle speed. and the idle air screw is brass with a slot, over by the bellmouth. it's just a needle-- turn it in to richen the mixture, and out to lean it.

 

Edited December 21, 2017 by dorset

[max power]

It seems to be at about 1/2, but I can't tell ya for sure because it's dark every time I test.  It happens at cruising speed, which on a 250 I'm guessing to be around 1/2 throttle.  I will mark it and test this weekend.  

Very well could be the needle taper as it was the the one I started with.  

Thanks.  

[ReconRat]

Is this a stock exhaust? Modded exhausts can sometimes choke at intermediate flow rates. And if you want to confirm a lean or rich condition at the trouble rpm, ride it at that rpm, pull the throttle in and kill the engine. Pull the plug and read the condition. But yeah, needle, needle position and/or main jet. You said they changed the main jet for you? Yeah, might try one size bigger. Not saying they lack experience, but.. that's changing a main jet without confirming it with a road test. Let the idle screws balance for the idle and off idle. They don't deliver enough to make a big change in the intermediate. And even one blocked up little bleed air/fuel passage anywhere in the carb can cause fits. Last resort, pull carb and blow air both ways through every passage you can find, at least twice. btw, I love Mikuni, never did like CVs.

 

Edited December 22, 2017 by ReconRat

[ReconRat]

Or looking at it another way. Stumbling at half throttle, is either the needle not clearing the main jet soon enough, or that main jet is too small. If the top end seems solid, without straining for fuel, I'd work with the needle position first. If it does strain for fuel at max rpm, confirm fuel flow rate first before changing anything.

edit: confirm WOT by doing the plug reading to see a lean condition at that WOT. Might be hard to notice a performance drop at top end rpm.

Edited December 22, 2017 by ReconRat

[dorset]

remember that half throttle won't be the main jet-- it's the needle jet that the needle goes in and out of. the needle jet drops into the casting from above, and you can see the top of it when you look through the bellmouth, with the needle stuck into it. the main jet is immersed in fuel in the float bowl, and screws into the bottom of the needle jet to hold it in.

before you buy anything, drop the clip all the way to the bottom groove on the needle. that will be as rich as that needle/needle jet can go, and if all is well, it will be *too* rich, which means that the correct mixture is within range of what you have. if it's still too lean in the bottom groove, you'll have to start testing richer needle/needle jet combinations. that's not hard to do, but may be unnecessary.

to tape your throttle, just wrap a ring of masking tape around the throttle barrel and stick another piece on the throttle housing with a mark in the middle. close the throttle and mark a line for zero on the tape on the throttle barrel. then open it all the way and mark another line. divide the space between them with three evenly-spaced marks, and you will have marks for zero, 1/4, 1/2, 3/4, and WFO.

the needle/needle jet controls mixture in the half-throttle rang, while the main jet has essentially no effect below 3/4 throttle.

[Gixxus Christ!]

All very good advice. I'll throw mine in the ring.

 

 

Tune your carb from the top down. You need to have the right main jet dialed in before you need to worry about needle position.  Yes, you can adjust needle height to get it to run right mid range but If your main jet isn't right, you'll be sacrificing top end. Dial in main jet with WOT runs and plug chops, or get a wide band 02 sensor and afr meter and check it that way. Once main jet is correct you can confidently adjust needles and pilots and all the other fun stuff. 

[max power]

 OK, let me try to answer all the questions that I didn't know well enough to answer in the first place.

It does have an aftermarket exhaust. White brothers E-series. It has the bigger header off of the 250 R. The main jet was reduced from something Ridiculous like a 190 to a 145 which, according to XRsonly (who have been modding these bikes for 30 years ) is what it should have with my particular set of mods which are very common but I sent it to them without the needle included.  The needle clip was in the middle so I dropped it one. Didn't change the stumbling issue. It runs great at wide open throttle and at low RPMs just stumbles at cruising speed. 

 

 And thanks guys. I appreciate all the information and will be processing it when I have a chance to get my hands back on the bike   

[max power]

And of course I will be reporting back my findings

[ReconRat]

Yeah, bear in mind I haven't worked on any carb for quite a while. Just trying to recall what the heck happens. Sure have seen errors and changes to main jets though, to fix problems. I've never had to change a needle jet or needle, other than position, even with a large overbore. Maybe I missed something on that one. Dunno, if it runs good you're done.

edit: also just noticed a Harley forum saying to tune carb from the bottom up to main jet. Different carbs and different priorities, I guess...

Edited December 22, 2017 by ReconRat

[ReconRat]

Mikuni flat slide tuning guide: http://www.mikuni.com/fs-tuning_guide.html

Bingo!

Poor mid-range performance: " Flat throttle response in the mid-rpm range is seldom caused by either an over-rich or overly lean condition. Flat mid-rpm performance is more likely due to the effects of the cam or exhaust design. If the needle size is incorrect, it will normally reveal itself as poor mileage (too rich), slow warm-up (too lean) or light detonation when accelerating moderately from around 2500 to 2900 rpm (again, too lean)."

Yeah, I know... it's a Harley Mikuni flat slide.

Also...

http://www.mikuni.com/pdf/hsr_tuningmanual_021003.pdf

http://www.mikunioz.com/tuning-tips-and-manual-downloads/

http://www.iwt.com.au/mikunicarb.htm

edit also: And yes, it says never change main or pilot jets trying to fix a mid range problem. You can verify their performance, but they won't fix the mid-range. If you link to the correct carb on the mikunioz site, you'll find a tuning manual guide specific to your carb. Simple instructions.

Edited December 23, 2017 by ReconRat