Mikuni TM Flatslide Carburetor Help!

[PredatorOfThePredator]

Hey guys, I’ve been having issues from the beginning on my predator 301cc build. My engine is built to the max as much as possible (rod, cam, valves, etc.) and I’m having issues with it running properly. The engine won’t start & idle unless I give it gas and have my idle screw about 90% in. And then my idle is too high and engaging my clutch. Once it’s running I’ll back it off to a comfortable idle and when I start riding, it’ll die if I let off the gas even for a second. I’ve adjusted my jets down from 260 to 180, changed spark plug, and still running rich if I check my spark plug. The last thing I’ve done is put a 160 jet in and couldn’t pass 1/4 throttle or it would bog, and plug is still black. In terms of power the best the engine has ran is with a 260 jet, with idle issues and black soot on spark plug. My carburetor is a Mikuni TM Flatslide 28MM. Thanks for any input

https://offerup.co/7iCSMre5N4



---------- Post added at 07:32 PM ---------- Previous post was at 07:31 PM ----------

 

[KartFab]

You need a 160 main and 17.5 pilot. You also need to understand that the main jet does basically nothing until 3/4 to full throttle and that you need to tune the idle circuit, then midrange with needle clip position THEN main jet.

---------- Post added at 07:09 PM ---------- Previous post was at 07:03 PM ----------

The nr racing website lists it as a 260 but it’s just a typo. I have ran a VM22, TM24, and TM32 the 160 is right about where it should be, you just need to tune the other stuff. http://www.nrracing.com/product-p/28mm-jet-kit.htm


If none of that works, then you might have someone who messed it up before you. There are plenty of hack and slash port jobs that kill port velocity and can lead to an engine with poor fuel signal.

 

[PredatorOfThePredator]

Life saver man! I’m going to swap my idle jet and main jet back down. What clip position would you say? Carb came at second from the top, I moved it to the highest position (needle sitting as low as possible)

---------- Post added at 08:13 PM ---------- Previous post was at 08:12 PM ----------

I am the one and only owner, everything bought from manufacturer.

 

[Brianator]

I'd start with the needle in the middle position if it has 5 slots or as it came if only 4 slots. Dropping the needle by raising the clip leans it out while raising the needle enrichens it. Start with the mixture screw at 1.5 turns out from lightly seated, being an "air screw" (because it's on the intake side of the carb) by screwing it in you're enriching the mixture and screwing it out leans the mixture.

The goal is to size the pilot jet so the mixture screw's ideal setting is between 3/4 & 2-1/4 turns, if it's not you need to resize the pilot jet. Less than 3/4 needs a larger jet while more than 2/1-4 needs a smaller pilot jet (the opposite jetting applies to carbs with a fuel screw - motor side of carb - but the procedure stays the same). Best way I've found to set it is to increase the idle, play with the mixture screw until you have achieved the highest and smoothest idle then bring the idle back down to ideal. If off idle acceleration isn't crisp, play with the mixture screw a little and see if it cleans up. The pilot circuit is most effective from off idle to about 1/4 throttle then the needle comes into play, all circuits must be tuned individually using their effective throttle positions and sometimes then fine tuned to work together. When adjusting the mixture screw always allow up to 30 seconds for the new setting to take effect and always make smaller adjustments, if looking at it like a clock then 1 to 2 hour adjustments are good.

I learned most of this the hard way with the TM34ss on my Quadracer. I found that the carb is time consuming to set up but once it is you rarely have to play with it outside of tuning it for tempurate differences during the day (cool in the morning, hot in the afternoon - mixture screw) and for the different seasons (summer/winter - full rejet).

Here a Mikuni jetting chart that should help you visualize how the circuits work and interact with each other, as you can see the pilot circuit is always doing something so if it's too rich, you could have a bear of a time with the rest! Sorry for the novel, hope it helps some. Brian

 

[PredatorOfThePredator]

Update

I installed the 17.5 idle jet, what a difference. Idles much better. Dropped my main jet down to a 160, still blowing black smoke. Dropped down to a 130, a little better response but still chokes up if I try to take off too fast, need to ease into it. Dropped to a 110 and ran like complete crap. Advice?

---------- Post added at 10:30 PM ---------- Previous post was at 10:25 PM ----------

To add to what I mentioned above, with the current set up of 17.5 pilot and 110 main jets, needle as lean as possible, when I adjust my idle in any position other than all the way out, it cuts off.

 

[Brianator]

Id put the 160 back in (better to be rich than lean) and play with the settings of the needle now that you've made progress with the pilot. Move the needle up one notch at a time and note differences, no harm in going all the way to rich then back down if there was a setting you liked better. Like KartFab said, start with the pilot, then the midrange (needle) then the main. Worry about fine tuning the main jet once you're able to run up to WOT. Believe it or not you can run without the main jet if your emulsion tube is threaded in the carb body (should be on that) and keep it under 2/3 throttle position.

 

[PredatorOfThePredator]

What exactly is throttle valve cut-away

 

[Brianator]

It's the profile at the bottom of and of the lower entry side of the slide and you do not want to mess with it! It's been designed to work as it is very well and only needs to be changed for another or modified for full out professional racing. What's your mixture screw set at?

 

[PredatorOfThePredator]

I may have messed with the throttle valve cutaway in the past. What setting is it supposed to be at? I also messed with the mixture screw in the past, what’s a good baseline to start at?

 

[Brianator]

I'd like to direct you to the lengthy post I wrote above as I went into alot of detail and am not typing it out again! Lol

 

[PredatorOfThePredator]

Here’s a picture of the 2 screws I believe you are referring to.

https://www.dropbox.com/sh/qf2b2qcb6wwkv48/AADMikN9mpQ_1mH71jMTjVV6a?dl=0

Lolll don’t mind the name

---------- Post added at 11:05 PM ---------- Previous post was at 11:03 PM ----------

Ah! Now I’m understanding what you meant by mixture screw

 

[Brianator]

The one on the side of the carb with the Arrow is the mixture screw. What you're pointing to in the carb mouth I believe is the PowerJet with is designed to help balance things out while running WOT for extended periods but i cant confirm that is what it is, i think i have a manual for my carb I'll check it out quick. Didn't see a name btw, maybe It's for the best! Lol

---------- Post added at 11:21 PM ---------- Previous post was at 11:08 PM ----------

I cant confirm what that fitting is at the moment, I'll see if I can dig something up tomorrow but for the meanwhile....

Have a read, its "Spankys jetting guide", something I learned ALOT from. This is for 2 strokes but applies to all carbs of that style, more so those with a "air screw"

http://users.conwaycorp.net/virtus/page2/speed/jetting.htm

Reading the tuning manual for the Mikuni HS40 will take you further down the rabbit hole and you'll start to understand that it's even possible to have a needle that isnt ideal for your application....

 

[PredatorOfThePredator]

Just adjusted my mixture screw to 1-1/2 turns out. It was seated all the way rich! Also brought the clip to position 3 of 5, right in the middle, and main jet up to 160. Gonna start her up in 30 minutes and let you know how it goes (currently at work right now, just running outside to adjust as you mention)

---------- Post added at 11:31 PM ---------- Previous post was at 11:23 PM ----------

Also, fantastic article! Couldn’t have been explained better

 

[Brianator]

I hope you're pleasantly surprised! Lol. I always try to find a "baseline" where it will run and idle so I can always go back to that if I've stayed too far and try again. If you have to err it's always best to err on the rich side, work your way towards lean, then take it back a notch towards rich and it should be good (that applies to all circuts). I agree, Spanky put together a killer crash course!

---------- Post added 03-14-2020 at 12:15 AM ---------- Previous post was 03-13-2020 at 11:34 PM ----------

That fitting in the mouth of the carb is an "air jet", I'd imagine it's best left tightened down and left alone. I found it in a great parts explosion/list for your carb :

https://www.mikunioz.com/flat-side-carbs-non-pump/tm28-parts/?v=3e8d115eb4b3

 

[Brianator]

Way to leave me hanging man....lol

 

[DirtyDogs446]

I'd start with the needle in the middle position if it has 5 slots or as it came if only 4 slots. Dropping the needle by raising the clip leans it out while raising the needle enrichens it. Start with the mixture screw at 1.5 turns out from lightly seated, being an "air screw" (because it's on the intake side of the carb) by screwing it in you're enriching the mixture and screwing it out leans the mixture.

The goal is to size the pilot jet so the mixture screw's ideal setting is between 3/4 & 2-1/4 turns, if it's not you need to resize the pilot jet. Less than 3/4 needs a larger jet while more than 2/1-4 needs a smaller pilot jet (the opposite jetting applies to carbs with a fuel screw - motor side of carb - but the procedure stays the same). Best way I've found to set it is to increase the idle, play with the mixture screw until you have achieved the highest and smoothest idle then bring the idle back down to ideal. If off idle acceleration isn't crisp, play with the mixture screw a little and see if it cleans up. The pilot circuit is most effective from off idle to about 1/4 throttle then the needle comes into play, all circuits must be tuned individually using their effective throttle positions and sometimes then fine tuned to work together. When adjusting the mixture screw always allow up to 30 seconds for the new setting to take effect and always make smaller adjustments, if looking at it like a clock then 1 to 2 hour adjustments are good.

I learned most of this the hard way with the TM34ss on my Quadracer. I found that the carb is time consuming to set up but once it is you rarely have to play with it outside of tuning it for tempurate differences during the day (cool in the morning, hot in the afternoon - mixture screw) and for the different seasons (summer/winter - full rejet).

Here a Mikuni jetting chart that should help you visualize how the circuits work and interact with each other, as you can see the pilot circuit is always doing something so if it's too rich, you could have a bear of a time with the rest! Sorry for the novel, hope it helps some. Brian

I'm literally having almost the same exact problem. I have a predator 212 I put mod2cam billett flywheel billett rod 18lb valve springs tourque coverter with juggernaut driver high flow top end exhuast and bout the 24mm flatslide carb kit by mikuni. I put all of that on at once and it literally won't idle unless I'm at around 4000 rpm and when I start going and really get on it as I pick up speed it keeps stuttering. I have tried adjusting things to no avail. There is a 50 jet I believe in there and I cant find any jets that are lower then that I feel like this carb is just too big and dumps to much fuel. One thing you said that caught my attention is you talked about moving a clip to lean it out how do I do that I'm literally new to this all so any info or pictures would help a ton. The spark plug is very black.