Help with flat side tuning

[Portaz13]

Hi guys,  got the old girl running by changing plugs,  valve clearance and following mikuni flat slide tuning guide I found on the net. It runs but really won't have stable idle below 2k, it often hunts and hangs around 3 and drops down.  It's also very rich and blows the occasional flame.  While riding it got better the further I went but still I had to ease throttle to get decent power otherwise it was just flat and popping etc. Read the plugs and they were black.  Settings on carbs are as follows,  needle on clip 4, mixture screw 1 & 3/4 from stop,  all jets Are as per last owners dyno run.  I have receipt where shop changed jets 3 times.  Float level is 18mm to highest point. The accelerator pump Is now just touching at idle and the stop screw us around 1/4 wound out.  When I got a clean run its quite raspy and I haven't ridden an oiler in 20 years and can't really remember how grumpy they were.  It supposedly has cams. Head work and 1100 headers as well as the Dyna ignition stuff.  Any ideas what to try next?  

Scott 

[nlovien]

i'd 1st check there is no sign of fuel wetting in the venturi ( and anything coming out the overflow )- i.e. leaking float valves

and i'd remove the pumper rod ( easy job) so yir focusing on the jets

check the choke valves are closing and seating

check the cables are letting the slides fully close (i.e. there is some cable slack and the throttle cam is going back against the stop

i.e. all the things that might be causes excess fuelling-  oh and put it back to the setting that it was dyno ed at whilst doing this so you have a good base setting

Edited January 14, 2018 by nlovien

[Gixer1460]

Agreed on the pumper - doesn't the manual say something about disconnection whilst base tuning? Just off idle is too early anyway.

[Portaz13]

The manual says set it for 1/4 throttle but I did that and it was flat as hell,  only thing I didn't dry was adjusting the needle as its on 4th clip,  would that make it richer?  Manual says set it on 3rd clip but honestly I'm a little confused as to which way is lean and what's rich 

[Portaz13]

And when you say choke valves seating are you talking about the slides? 

[KATANAMANGLER]

1/4 at least for the pumps. Otherwise you're sqirting fuel into the intake while it's still metering on largely the pilot. 

As for the needles: if you raise the needle ( drop the clip) you make the needle circuit richer by raising the taper in the emulsion tube. If you drop the needle ( raise the clip) you make the needle circuit leaner by droping the taper further into the tube.

This only impacts on the needle circuit.  The main jet meters the fuel into that circuit too but has more influence towards the end of the needle circuit and  WOT. The pilot circuit also influences the beginning of the needle circuit as it transitions between the 2.

Mark your throttle up with datum marks so you can identify 1/8, 14, 1/2 throttle positions. This will help identify which circuit is out. Turn the pumps off for this.

[Portaz13]

What would be the best pilot screw setting to start?  From the Dyno it was 2 turns out but previous owner clearly had issues with plugs fowling on the street hence why he sold it. Everything I read points to less than one turn but would love some idea where to start too.  Thanks so much for replies guys 

[KATANAMANGLER]

From memory the manual says  something like 1/4 to 3/4 . Quite small openings compared to CV carbs which are normally 2 ish. I have one set of 36s set at 3/4 of a turn and another set of 36s set at 1.5. Horses for courses.

What is the engine? What are you using on the intakes? What is your current main jet size?

The slides can wear on flatslides and that can make them leak air on the intake on the idle circuit. It makes for a lumpy idle circuit and some surging at cruising speeds. Obviously it's less of an issue when you are opening the slides up.

The earlier flatslides had plastic slides which wore readily. The later ones had aluminium slides that where less susceptible to wear.

What throttle positions is it misbehaving at?

[Portaz13]

They are metal slides and honestly look near new,  it won't idle properly and hunts,  as in sitting at 1500 then goes to 3000 by itself and hangs there for a bit.  Riding I had to open throttle slowly and it built speed pretty quick but was quite raspy sounding which just could be engine.  It's a 750 L model apparently the Tuner had not seen cams like it had before and it has head work and 1100 headers, don't know any further specs.  If I open throttle quickly it just bogged and woundnt increase rpm.  

[Duckndive]

46 minutes ago, Portaz13 said:

They are metal slides and honestly look near new,  it won't idle properly and hunts,  as in sitting at 1500 then goes to 3000 by itself and hangs there for a bit. 

Points to an Air Leak which needs sorting first

[banoffee]

Are the carbs balanced?

[KATANAMANGLER]

Yup, if its hanging up to that extent I would check the o rings on your intake rubbers. Spray them with some carb cleaner and see if they are letting by.

[Duckndive]

The easiest way to sync the slides is with the carbs off the bike ...and set them with a drill bit under the slide .....

[Captain Chaos]

make sure the valve clearances are set correctly before balancing the carbs

[banoffee]

I had poor idle and part throttle running problems last year. Whipping off the pod filters revealed that one slide was miles out after I’d adjusted the needles with the carbs in situ. Carbs came off, balanced as above, easy, sorted. Needle changes worked perfectly too, result.

[Sheep]

Would I be right in thinking ( I am comparing these carbs with mine) that number 3 carb is set from factory and not adjustable for balancing??...In other words the rest have to be set around this one.

I found changing the air jets helped with low throttle fuelling although my slides rattle which points to worn parts hence a slightly lumpy idle.

Another tip I found useful is obtain an infra red thermometer,this helped me identify which downpipe was getting hotter/colder than others which can also point toward improper fuelling in individual carbs.

 

[nlovien]

the choke valves are the ones linked to the choke knob - horizontal front of main body - i've seen these sticking partially open on mikuni's

tend to find 1/2 to 3/4 turn on idle screws is a good start - initially aim for "will it run" on these alone at the smallest no. turns - note BE CAREFULL with these = prone to jamming the tips into the venturi then shearing them off leaving the tip plugging the idle circuit port

its either no. 2 or 3 ( dam't did this a few weeks back and can#t recall - is this early signs of dementia) that is the non adjustable slide = so you set your  throttle stop to this one - i.e. adjust the stop until this slide doesn't drop anymore - then take up the stop until it just moves the slide - then set the other x 3 to match

with this set and the idles say 3/4 turn - see if the bike will idle after warming and choke off - without fitting the throttle cables - this is my 1st test and I don't go any further until I find out why not  - try and avoid too much winding in the throttle stop adjustment to adjust the idle - a wee tweak only - once you've got a reasonable idle adjust the idle screws 1/4 at a time and observe for the idle speed changing - goes higher keep opening until it starts to drop - goes lower do opposite - simple on paper - can be a pain as the engine heats up but if your getting this response thne you know the basics are good, your confident there's no leaks etc..

another thing I do when setting these up is I have a throttle drum i've sanded down to a smaller diameter = more wrist movement to open the carb = less sensitive for the 1/4 - 1/2 - 3/4 - full marks you use to give the key fueling part that's prominent 

once your confident you have a proper working carb with no leaks on an engine that has reasonably balanced compression and the valve gaps are good = a great help is to fit a lambda sensor and gauge -  helps with the guess work

 

[Portaz13]

Wow so many great replies,  I will take them off and go through it all again,  I did kinda think by eye two of the slides had slightly more gap but could be wrong,  I left them alone as wasn't sure how to adjust. Valve clearance and compression I'm pretty confident in. Haven't looked for air leaks and maybe I should go that first before I pull them up for again. And yes carbs were synced using a manometer to adjust  as best I could,  wasn't aware that one was non adjustable. Great tips guys will report back with findings 

[Foz]

I initially used to set all my slides on the bench using a drill bit, the throttle hung and it was slow to return to tickover. When I initially balanced them on the bike using vac guages they were way out, once I'd balanced them on the bike it would rev cleanly and would return to tickover 

[Portaz13]

What was the method you used on the bike to get it clean?  

[Portaz13]

Some progress,  pulled carbies off and installed unknown condition mikuni cv carbs,  it ran but sounded pretty similar and would need throttle cable adapted some how so I went back to flat slides,. Set mixture screws at 3/4 , adjusted the slides by eye actually and it ran and idled around 1800, still hangs a bit but much better.  Tested for air leaks none present. Used manometer to sync and actually it was pretty close and adjusting made it worse. I then noticed one carbie pushing fuel our of overflow so off they came again. Cleaned needles and seat and ran again.  Idle was OK not perfect but ok. Haven't hooked up accelerator pump or throttle cables yet.  I have a gut feeling I need to flush my tank as filter is showing some crap in it.  I'm also wondering if the popping and flames etc could be a plug fowling.  Even though they are new they have been through a lot of fuel adjustments. Planning on adjusting pump and cables and getting new fuel and going for a run and see if it cleans up a bit. 

[nlovien]

with the carbs off - take a look at what spring return your using - you can select more / less by preloading the return spring, see fi you can sort the hanging throttle by being more positive with returning the throttle by hand i.e. using the return cable - might be your on a softer return spring setting and need a bit more to help the slides seat back

[Portaz13]

Cables are not yet connected however where they were holding throttle shut on either the carbie or the throttle did lower the revs. Can the return spring be adjusted? 

[370steve]

Have you checked float heights?

[Portaz13]

Yes all are at 18mm