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Power jet tuning advice?

Discussion in '2 smokers' started by Fan_Of_Iwata, May 26, 2020.

  1. Fan_Of_Iwata

    Fan_Of_Iwata n00b

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    Hello all,

    https://advrider.com/f/threads/beginners-guide-to-two-stroke-jetting.1254185/ What a lovely thread about 2 stroke tuning that i read for about three times earlier today!

    However i wish to get some advice regarding how to tune the Mikuni power jet kit installed on my TM38 flatslide carb and Mikuni recommends that original main jet should be reduced by 25% and the reduction will become the size of the power jet.

    So in my case my bike is currently using a #380 main jet so i trimmed it back by 25% which now #95 will become the size of the power jet however this is a conversion from VM36 roundslide to TM flatslide with power jet kit so i started off in a richer side by installing a #320 main and a #100 power jet then go from there..

    If the engine ever shows a rich/lean symptom at WOT which most likely will occur..should i be making the adjustment on the main jet first and leave the power jet alone until i have find a suitable main jet for it or should i be making changes on both simultaneously using the aforementioned Mikuni's formula? and for the future changing condition, should i only make changes on the main jet, power jet or both?

    Jack.
    #1
  2. snarlyjohn

    snarlyjohn Human money repellant

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    The answer will depend on what you are trying to achieve by using a power jet. As you probably already know a P/J alone will provide no more peak power - in fact it will reduce the peak very slightly because of the slight reduction in mass airflow it causes. Despite this there can be reasons to use them - one is that they can be solenoid valve controlled to lean out the very top end of the powerband, thereby increasing EGT and extending the over-rev slightly. Another use for them is as a way of reshaping the fuel curve. You can adjust the height of the nozzle to set the throttle opening at which the P/J becomes active, and you use the mainjet and powerjet sizing to set the amount of extra enrichment when the P/J starts flowing. Some people like them because it allows you to adjust the fueling at WOT without affecting the A/F at throttle openings less than WOT at all. Personally I avoid them if possible, but if you have to use one to cover up a lean situation at full throttle it'll be a matter of setting the mainjet to provide enough fuel for 1/2 to 3/4 throttle (the MJ has quite an influence when running on the needle/needle jet) and use the P/J to fill in the top end. Trial and error in other words, for both jet size and height. And keep in mind the powerjet is a very poorly named device - there's no power in them, they're just a tuning tool that's very often completely unnecessary.
    #2
  3. Fan_Of_Iwata

    Fan_Of_Iwata n00b

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    Thank you for your detailed explanation maet :) It's the answer that i have been looking for that nobody could provide. I truly appreciate it :-)

    No i'm not trying to achieve any gain in horsepower with it but simply looking for the procedure to tune it to suit my Yamaha IT250 since the power jet kit has been installed already when i purchased it brand new a while back.
    #3
  4. Fan_Of_Iwata

    Fan_Of_Iwata n00b

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    How would i be able to find a suitable main jet for mid-range throttle since that part is mostly controlled by needle jet/needle clip position?
    #4
  5. snarlyjohn

    snarlyjohn Human money repellant

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    This is the tricky bit - if the engine can be tuned correctly with the needle, needle jet and main jet then there will be nothing to be gained by using the power jet. The correct powerjet size would be zero. I guess the correct procedure would be to tune the engine using zero P/J fuel. Once it's jetted as well as it can be you could try shifting a small proportion of the top end fuel to the powerjet; say a 90:10 split.
    If the performance improves you could try a little more; say 80:20, and so on until the performance no longer improves or begins to get worse. If however the performance is worse with a small proportion of P/J fuel the correct response is to revert to the conventional jetting, and ideally remove the P/J altogether. I've had very good results with TM38s and haven't had to crutch them with a powerjet, and that's a good thing because the P/J has a definite effect on the TMs excellent air flow capacity. I wouldn't use a powerjet just because it was already fitted any more than I'd pull all my teeth out just because I was given some free dentures. In case you haven't noticed I really don't like powerjets but will admit they sometimes have a purpose. They can be a good way of providing sufficient top end fuel with methanol, where the tip of the needle would otherwise be the restriction at WOT. But they shouldn't be used as a band-aid for lazy tuning.

    Mostly is the key word - the main jet has a small but very noticeable effect right down to about 1/4 - 1/3 throttle.

    Short answer - verify that there is a real need for a P/J first before spending any time on it.
    #5
  6. Fan_Of_Iwata

    Fan_Of_Iwata n00b

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    I noticed it already at your first reply that you're not a fan of the power jet and you're not being bias with it either :-) I will have a go at it in Mikuni's "magic" formula to see how it turns out and if it's decreasing the performance rather than gaining any (ha! be lucky if it does) i will simply revert to the OEM VM roundslide.

    I have never ever experienced a P/J before as i'm always using VM roundslide or TM flastslide in my all machines and all are tuned correctly and precisely without it...looks like Mikuni has got me for this one with their gimmick hahaha
    #6
  7. snarlyjohn

    snarlyjohn Human money repellant

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    Don't swap the TM for the VM if the powerjet turns out to be not necessary - a TM38 flows as much air as a VM40. The only downside is they are a pain in the arse when it comes to changing needles or clip settings. Just remove the PJ and plug the holes.
    #7
  8. Fan_Of_Iwata

    Fan_Of_Iwata n00b

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    Okay i will do that if it turns out to be unnecessary.

    I looked on Mikuni's catalog and there are four sizes of needles which are available for use in TM36, 40 and those TM pro series carbs. I have discussed with other blokes that upgraded the VM to TM in their ITs and they're so pretty happy with the stock 6FJ41 needle that came in the TM but i'm not so sure about the starting point in needle jet for the flatslide since the VM was using N-8 which was on the very lean side of the needle jet range by Yamaha. So i have bought a series of richer "P" and "Q" needle jet because i heard that these TM tends to have a much leaner mid range apparently.
    #8
  9. bark sampler

    bark sampler Been here awhile

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    Interesting discussion. I like Power Jets on Lectrons as they don’t have main jets, but never tried one on a Mikuni. My experience is the same in never seeing the need on a Mikuni. I do remember in the late 80s & 90s they were common mods for elevation changes and tuning, but went away as they don’t even work great for that. I could see value having P/J tuning for running flat out on lake beds to prevent a seizure.

    On 2-strokes my experience is the main only effects the last 1/4 of the throttle where 4-stroke designed carbs can have more range of impact. That’s a generalization, but I think there is usually more restriction in the needle section in 2-strokes than a 4-strokes.

    When I’m faced with one of these blank page problems where there isn’t good data on the carb/applications, I like to start from the bottom up. Start by tuning the air screw and pilot, then the slide cutaway, then the needle diameter, then the needle position, then the needle jet, [edit: then the power jet based on length of tube WRT throttle position] then the main. Using marks on the throttle as noted in that first post link (very nice!).

    Some like to start top down. It can be more efficient, but if your main is off by 5-10% lean, you may be starting with stuck pistons, especially if it’s a new build/piston. Working towards full throttle is safer and easier in my mind to sort through but may have some revisions at 1/2-3/4 throttle.

    Y’all are probably way past this stuff and the questions were more P/J related so I’m preaching to the choir here, but sometimes I need to remind myself too!
    #9
  10. Fan_Of_Iwata

    Fan_Of_Iwata n00b

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    You're fine mate! Any additional information is good to soak it up into the head and save it for later on.

    Yamaha was the first manufacturer to introduce the P/J roundslides into their TZ's and later on in the '80-'81 IT175s but soon removed from the '82 model onwards.
    #10
  11. bark sampler

    bark sampler Been here awhile

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    Good memory! I forgot about the TZ’s using them. I’ve seen them on the 175’s. That’s my main experience, really second hand. I’ve heard of them plugging or sucking air. The hot (end of life) ticket for that piston!
    #11
  12. Fan_Of_Iwata

    Fan_Of_Iwata n00b

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    My dad had a IT175H back when it was new in 1981...a great bike and it's the 175H that what gave me the idea to try it on the 250K (whether it works or not at least it's a good experiment)

    There was a trick back then for riders in high altitude to plug the P/J and use the main jet only since with the P/J it was running extremely rich in high altitude which air is thinner but people were often forgot to unplug it when riding at or near sea level and resulted like what you said above.
    #12
  13. greasyfatman

    greasyfatman Long timer

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    Good info appreciate your post.

    I was told the PJ in the modern YZ bikes prevents over rev actually makes a rich condition. Disabling the PJ gets it to rev further but unsustainably lean.

    What’s your take? Was I wrongfully informed.
    #13
  14. greasyfatman

    greasyfatman Long timer

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    Could you elaborate on slide cut aways? How do you know to go ip or down? Or what are the affects?
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  15. snarlyjohn

    snarlyjohn Human money repellant

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    I think it's important to get the main at least in the ballpark before working on things like the slide cutaway. The mainjet size has a small but definite influence even at fairly small throttle openings. The way I like to do it is this:

    1, Get the engine to start, idle and run at small throttle openings - just in the ballpark, don't spend time trying to get perfect at this stage.
    2, Do the mainjet at WOT, using some care to look for lean conditions that aren't corrected by more MJ (ie. needle or NJ restriction at WOT)
    3, Do the midrange - 1/3 to 3/4 throttle, needle and needle jet
    4, Revisit the bottom end and fine tune - pilot jet, screw, possibly slide cutaway to fine tune the transition from pilot to needle. More cutaway is leaner, less is richer. I'm too stingy to buy a selection of slides so I usually just buy one with a low cutaway and file it to suit. Often the slide doesn't need to be altered.

    Once you've done this it'll be very close, though after a couple of hours of riding to get a better feel it you might decide to make very small adjustments here or there.
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  16. snarlyjohn

    snarlyjohn Human money repellant

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    That doesn't sound right, though I don't have a YZ here to check. According to the manual the PJ closes at 8500rpm, which makes more sense.
    #16
  17. greasyfatman

    greasyfatman Long timer

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    What sign would I have that I need both more air( from 1 to 2 cut away) and more fuel. Tuning an 80cc two stroke vm16. This was the next thing I was going to try to see what happens.
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  18. bark sampler

    bark sampler Been here awhile

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    I find slide cutaway is useful when you can’t tune in at 1/8 to 1/4 throttle but it does come in to play at up to 1/2. It can be very useful with VM’s to clean up partial throttle where changing the air screw/pilot is below where problem is and needle effects a wider range you don’t want to mess up.

    On an 80, you may not spend enough time at that throttle position, so it may be better off with some other adjustments. But if it’s fat at 1/4 throttle, that “1” slide is pretty rich.

    I learned a lot about slide cut outs riding open 2-strokes with VMs. Can-Am 400’s needed a 3.5-4 slide and the biggest cut out Mikuni made for a VM 40 was a 3 so we filed/ground out the difference. Getting that 1/8 to 1/2 throttle just right makes them so sweet. If you tried to fix it with the pilot, it would be lean off closed throttle and not fix it. Doing it all with the needle/needle jet helped but flattened out 1/2 to 3/4 throttle leaving a hole that made it feel like a 250.

    Here’s the classic diagram.
    upload_2020-5-28_20-11-30.gif
    #18
  19. greasyfatman

    greasyfatman Long timer

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    DC91D62E-B17D-4198-83FF-786C3263BB77.jpeg
    with the auto clutch and how we ride them, I feel they don’t have enough off the bottom. They are well ported and high compression. I am making an intake for a vm20. But would like to get the 16 right on two of the.

    if you went up in slide opening did you also go up in pilot jet?
    #19
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  20. bark sampler

    bark sampler Been here awhile

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    I usually change the pilot if the air screw is less than 1/2 or more than 2 turns. But the slide is useful if changing the pilot or air screw doesn’t seem to work as it comes into play at a tick more throttle.

    A lot of times people try to tune with pilot and needle position but that doesn’t cover the 1/8 -1/2 throttle well. That’s where slide cut away or more commonly needle base diameter + needle jet opening diameter play into it. On those 400’s people would put in super small pilots thinking that was the problem, so on a used one I went UP in the pilot (back to stock) and bigger/leaner on the slide. Issue fixed.

    Those BW look like fun! If you have an extra slide, it’s easy enough to carefully mark above the current radius and grind or file it to the next size. Just take material off the center and none on the edges and taper it in. I think a 1 means the radius results in it 1 mm open compared to flat. 2=2mm, etc. Start with 1/2 a mm and see if it runs better.

    Man, we fell off Power Jets, think I took us to this rabbit hole...
    #20
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