18mm mikuni carb. not much luck with tuning

[breno]

Hey fellas,
I ve been playing around with my Mikuni carb to try and get a good result.

ATM im running the .7 main jet and a .25 pilot jet with the needle jet set to the very top I think the air screw is out like 2 turns cant remember exactly..
I would have thought if anything this would be lean condition all roundish.

*Taking off needs full throttle in the bottom end to go and it takes off like it normally would with the speed carb.
*Mid range seems okish
*Soon as it gets to 40km/ph its 4 stroking and blowing abit of blue smoke.

I swapped the motors around on my bike so I could use my other exhaust which I know works fine but I'm getting the same result..frustrating.

These jets arent exactly that cheap but I'm not sure how much lower but Im not sure how much lower to go with the jet sizes..
Cheers for your help

 

[bairdco]

i think the 18mm might be too big. i've got a 16mm on a full on ported, reversed cylinder, custom Harley/Aermacchi head with tiny squishband and custom pipe and the 16mm is still slightly on the rich side.

besides that, is your air filter on? when i hooked mine up for the first time, i had no air filter and it acted pretty much like yours. smoothed right out when i got a filter.

 

[breno]

Hmm strange I've read ppl having success with the 18mm but like you say it's acting like it's too big.. I have a large foam uni filter with the spring inside.
Any other ideas?

 

[bairdco]

only thing i can suggest is keep messing with the jetting. dig up the old threads and find out what they're using.

also, porting, expansion chambers, etc can change the fuel/air needs of the engine. there's really no exact chart for what jets you need, it's mostly just trial and error. i know it sucks 'cause jets ain't cheap, but there's really no other options.

 

[Goat Herder]

Just get some micro drill bits. Solder your jet shut drill it to a smaller size until you get it right.

 

[breno]

All true I spose..
Wats happening def. sounds like a rich condition though right?
If it is I guess my choices at this point would b to source the drill bits then solder up the jets or buy the jets at like $8 a pop I think it was.

Is there any tricks I can try to prove the rich result I dunno like partially restricting the main volume of air flow or something?

The motor has been match ported and intake widened fair bit + some other stuff so it should run really well once I get this dam carby sorted.

 

[DanielMaia]

I also use loctite glue in the main jet, wait 30min, drill the hole i want with micro drill set, and its perfect. My bike has 3000km, down jetted to 65# i think, and runs sweet


How is your idle?

 

[breno]

Interesting DanielMaia doesnt sound like a bad idea. Do you have the 18mm mikuni aswell?
Turns out the micro drills are like $6sumthin in oz and I told the fella I was going to use them for soldering a jet and redrilling it. His advice was to spend the extra $2 and buy the jets as these tiny drills bits are prone for breaking at the best of times. I think its like $150 for the set inc. 19 pieces down to 0.25..

Idle at the moment is good but off the mark is a dog unless i give it full throttle.
Mid range is reasonably strongish but could be better
Approaching the upper end and shes blowin smoke and running like a dog.

Im going to try changing my oil mix its currently 25:1 fully synthetic. Might up it to 30nabit:1 with some fresh fuel.
Also going to solder the CDI connections.

Ive tried different pipes. If the above doesnt work I might go back to the standard carb and make sure the motor is happy though I would have thought it should be fine but it needs to be ruled out before buying anything else..another bugger lol

 

[breno]

Ive read that some people are going as low as a .60 main jet to make it work properly.
Still gotta try that trick with turning the fuel off at wot to see if it clears out for a tad when the fuel gets lower.

 

[breno]

if it needs the slide to be open 3/4 to fully open to avoid boggin down would that be put down to the pilot jet.. I read somewhere it can have something to do with the clip position on the needle. Its at the top atm.

 

[Goat Herder]

I got a custom S6S Morini here while I have not measured the carb barrel I am sure it is 19mm. This is on a 49ish cc size motor . Here at 5500 feet above sea level I had to reject. I took some solder [radiator grade what I had lying around at the moment]and a propane torch soldered it shut from the bottom side. Why the bottom side? Because there was a neat tapered funnel effect at the top side. It was shape like a funnel there.

I then took my micro bits out from Harber freight. They costed like 7 dollars. Here is the fun part. I went through the drill set found the bit that fit the existing jet well then looked further found the one bit under it that had a smigging of play to it in my fingers with a bit O wobble check. I pulled this off with out a hitch and no need for a micrometer. Trust me you are not going to read the writing on a drill bit liken to a hair. My first guess was right dun deal she runs perfect!


Here is one more good funny I had to wrap neatly and perfectly a piece of paper towel around the shank of the drill bit. Why no dice gripping the chuck. Good luck It can be done. I did not break my bit ether I held my tong right me guesses.

Whole thing took 15 minutes tops!

 

[bairdco]

Breno, to answer some of your jetting questions, here's a good link as to what does what:

FAQ rejetting 101

 

[breno]

Thanks for the feedback guys.

Ive ordered a .60 main jet and can/will drill it out if need be.
After reading through the rejetting info. I decided to start from scratch.
I also added fresh fuel and am now running 32:1 fully synthetic castrol oil and 98 octane fuel. I have run 98 before it will run fine on it but usually would run 95.

#it now has a .20 pilot jet with the air screw out 2.5 turns from fully in. Off the mark now is good using only 1/4 throttle or less as it starts to level out the loaded motor it starts to splutter again. To get rid of it i just apply more throttle. in saying that if i go to 1/2 throttle it will still continue to splutter a tad it needs atleast 3/4 throottle to pull it out of the splutter.

#the jet needle is set on the top notch andi havent played with it yet. 1/4 to 3/4 thottle is still not smooth and tends to blow abit of blue smoke during the splutter.

#the main jet is still .7 At this point in time. 3/4 to full throttle under load has no splutter but as soon as the load starts to die off and its reaching 55km/hr it starts to splutter in and out but doesnt gain anymore speed. I know it should have another 15km/hr in it. After a full throttle run from bottom end of the rev range to top end and holding the motor at high revs i killed the engine while holding it flat stick and simultaneously drop the throttle. Pulled the plig out after it cooled to find it is greyish black.

Thanks for reading this ar if you guys have any inout or thoughts about what those above conditions are it would be appreciated if you could add to the thread.

 

[GearNut]

Still too rich. It could be the taper of the needle as well, I.E. it tapers down too quickly.
The needle is in control of the fuel delivery from 1/4 to 3/4 throttle which is why I suggest that. You can fiddle with the clip position of the needle all day long but if the taper of the needle isn't well matched to the needs of the engine you can chase your tail and not realize why.
Also if your particular carb has replaceable air bleed jets, you might want to use one size larger on the main fuel circuit. That will lean out the main fuel circuit a tad as well as make it come on a bit later in the RPM range.

 

[breno]

thanks Gearnut I'll look into the air bleed jets aswell didnt know about that.
Where abouts is the air bleed jets?

Firstly I'll try out the smaller main jet at this point coz thats all I have on the way at the moment. Hopefully that helps it.

But like you said the needle does from 1/4 to 3/4 throttle and thats where the problem is Being at the top aswell theres not alot more I can do in that regard...

I have attached a pic of the back of the carb with no filter, plug colour after the full throttle run and a pic of the jet needle.

I know this is a bit of a long shot but does the taper on the needle look as though it tapers down too quickly?

Thanks again for the help with getting this right.

 

[GearNut]

After looking at your pictures, I can see that there are no replaceable air bleed jets on that carb.
Spark plug does look like the state of tune is too rich, but not awefully rich.
As for the taper of the needle, the differences between them vary by fractions of a millimeter. I'm sorry to say that my eyeballs are not calibrated to that degree.
I cannot look at yours and tell if it is good for your application or not. Then again I have never met someone who could do that anyways.

 

[breno]

I may have to just try this .60 main jet and see how that goes.
I think that will give some direction of which way to go with the needle and maybe even the main jet.
I get the feeling that it may run lean in the top end if its still running rich in the middle it should be a give away that the needle needs to be replaced.
Hopefully they call me tomorrow to tell me the jet is in.

I'm using the 3 prong china plug that came with the kit its a Z4JC thats all i can get off the plug.. Could the plug be a factor aswell should I be running something else?

 

[GearNut]

The plug should be ok, but you can always try an NGK plug. If you have a slant head, check some threads on this site to see which one fits a slant head properly. I have a center head and I run an NGK BPR6HS but if I remember correctly, that spark plug will not fit a slant head properly.

 

[breno]

Thanks gearnut for your reply. I think I'll stick with this plug in that case they havent given me any grief in the past with the minor bolt on mods.

Ive been searching the 4 corners of the internet to try and get some info. on these carbs in lamens terms think I found it..it is mikuni related but it should apply to some other carbs too.
http://klx250s.suncup.net/Misc Docs/Tuning the Mikuni Carb.pdf

 

[GearNut]

Just quickly scanning that tuning guide, it seems to be very well written.
It will go a long ways in aiding you in your tuning efforts.