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Thread: Recycled Cycle

  1. #491
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    The passages thru the petcock are similar to the fuel filter...less than ⅛" a 7/64" drill bit fits in there with a little wiggle left over.

    I'm gonna need to find a replacement for this I think. I'll try running it first, maybe it'll be ok with the 180 main. I can find a larger fuel filter easy enough...probably right at my local AutoZone.

    Sent from my XT1650 using Tapatalk
    hehe... he's starting to squirm.

  2. #492
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    Quote Originally Posted by kirrbby View Post
    My plug is stock CT70, C7HSA.

    Running the 180 it will rev all the way out, no bogging at all as tested. But from about 2/3 throttle is seems like it's struggling a bit. The feeling I got was that a stiff breeze would keep it from revving out to top speed.

    With the 170 needle near the rich end, it pulls hard all the way...a headwind couldn't stop it. Lean stumble at low throttle and a couple times I felt a stumble maybe around 3/4 throttle. Overall it felt it was running lean.


    The 180 felt good, like the motor was getting plenty of fuel. But I'm trying to keep the needle toward the rich side, because any leaner than middle position is where the low throttle lean spot is a problem. I'm hoping to find that the 175 is just right.

    I want to tune this vm22 in as best as I can, log the info, then I'll put the VM20 back on and do the same. When I put the 20 back on, it may just be the obvious choice... we'll see.

    No signs of a overly hot engine yet. I think I'm gonna be good there...again, we'll see. I'm expecting my registration in the mail shortly...registered with a authentic historical plate....1972. Then I'll be on the road.

    I will look into a cooler plug tho...C8...??
    This follows what I've found...and tried to explain...many times over the years, with this carb feeding less than ~125cc. 170 is probably going about ~.25:1 leaner at WOT, plenty of safety margin if 180 delivers overrich A:Fs, as you're describing. The problem is that off-idle is also leaner and now it's just enough to result in transient lean stumbling for 1-2 seconds...which seems like an eternity while riding.

    Best guess, #180 is giving you WOT numbers around 12.4-12.6, 170 more like 12.6-12.8. There's probably the same amount of difference at part-throttle but the fuel delivery curve is different; IOW the rich/lean numbers crossover between the two different mains...depending upon throttle position & rpm. The stumbles you get are from transient lean spots. At low rpm, there's not much airflow velocity through the venturi and that low vacuum results in lazy fuel delivery. It takes a second, or two, for fuel to begin flowing steady-state; meanwhile, you feed it a little throttle and the A:F ratio momentarily spikes lean...like ~16:1 and combustion ceases, a.k.a. "fatally lean". This is the problem with the VM22 feeding less than ~125cc and that displacement is marginal.

    Three tests you may find worthwhile:

    Upsize the pilot jet, set the airbleed screw ~1.0-1.5 turns out from seated and see if the stumbling is cured. With the factory-installed #30 pilot, this carb will deliver EFI-like throttle response...and be pig-rich, completely impractical. Even just one size bigger than what you have in there now will be too big. This is just a short-term, diagnostic, tool. If an outsized shot of extra fuel, at low speed/light throttle, eliminates the bogging, then you know what it needed. The hard part is figuring out how to get it without flooding the motor at idle. FWIW, I get the best results using a #20 pilot and the airbleed screw 1/2-3/4 turn out from seated. The idle mixture is right on the ragged edge of too rich.

    When you get a low-speed/part-throttle stumble, hold the throttle steady. If the motor recovers, you've verified a transient lean stumble, due to insufficient airflow velocity, a.k.a. oversized carb.

    Repeat the procedure that delivered the lean stumble but, instead of holding the throttle steady, give it some overshoot...like goosing the accelerator pump with an automotive 4bbl carb. That'll move the jet needle to a smaller part of its taper, allowing a bit more fuel into the air stream. If this has a positive effect, then you're even further along the right track.

    The proper solution would be a combination of smaller slide cutaway and jet needles with different tapers than Mikuni ever offered, calibrated to deliver more fuel below 1/3 throttle. Those parts don't exist. So, once you find the best main jet/jet needle height combo, the only tuning trick left is modifying the jet needle and "trick" is a good word for the process. Fiendishly tricky is more to the point. If you want to attempt this, you're going to need a handful of new jet needles and some assistance. The line between transient lean spots and steady-state rich misfire, at part-throttle is as thin as they come. The chances of going too far with the needle modification are significant and once that happens, that needle is . Time to give it another shot, starting with a new needle.

    Best edumacated guess, as of now, you're withing one jet size of optimal. And, the final result will include some needle reworking and developing a little different throttling technique. Optimal, this isn't. It is, however, tolerable...~99% right, with easy starting, full power and normal fuel consumption. The occasional, slight, hiccup will likely be part of the game. And, you'll never be able to just snap the throttle open from low rpm and get EFI-like, instantaneous response. However, above ~30mph, it should be just fine, as in good enough that you'll forget it's slightly oversize and just enjoy the miles.

    As for peak engine & oil temps, you've not yet begun to see those. 10, 20, 30+ miles at 55mph in ambient air temps above 85F are the real test. Sustained speed/power output is what heats an engine. With short duration, thermal lag keeps temps down.
    Some luck lies in not getting what you thought you wanted but getting what you have, which once you have got it you may be smart enough to see is what you would have wanted had you known.

    Garrison Keillor

  3. #493
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    A lot of good info in that last post...thank you again.

    It's a cool and windy day here today.

    I put the 175 in for the first try since I think it'll be my winner. It performed just as I thought it might. Then went up one click on the needle to dead center position. It seems a little rich there, just barely tho, I could live with it.
    Seems I felt the same way about the 180 tho, with the same needle position. So I'll go ahead and try that again too.

    CJ, I forgot to tell you I'm running premium pump gas in everything around here except my old Chevy.
    This bike seems to guzzle the stuff. I'll be interested to see what kind of mileage I get from it.

    I forgot to post the pic of the petcock.

    hehe... he's starting to squirm.

  4. #494
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    At this moment, I think that #170 is going to be the charm...possibly #165 (if it exists) with a richer needle height setting. I'd want the highest jet needle setting possible, to bring in extra fuel as early as possible. It's like 65F right now (with rain headed your way), once temps rebound into the 80s, it's going to run richer due to the reduced air density. And, fuel atomization will be better...also driving the proceedings richer still.

    (If my memory hasn't faded too badly, that carb worked best with #150 main, on your black bike.)

    Gotta see what the A:F numbers are, one day soon...and replace all the guessing with hard data.

    BTW, I doubt that the petcock is a problem. Betcha that the tank can be drained in a few minutes, while the engine would probably need closer to 3/4 of an hour to consume that same quantity of fuel, running wide-open.
    Some luck lies in not getting what you thought you wanted but getting what you have, which once you have got it you may be smart enough to see is what you would have wanted had you known.

    Garrison Keillor

  5. #495
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    My thinking was to start riding it with as rich as I can stand tuned carb while the motor breaks in a bit. But I just looked at my starting mileage that I wrote under the seat...6150 miles on the speedo when this motor was installed. I have put 250 miles on it already, lolol I can hardly believe it.

    Maybe I'll install the 170 instead, at the max rich needle, and with the petcock back in line. I'm thinking those small passages are too small...ONLY when you try running with a main jet that's WAY too big.

    My Lord, I have 250 miles worth of jetting...(carb tuning research :-) into this thing. I can't believe that.
    hehe... he's starting to squirm.

  6. #496
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    No problems from the Petcock.
    No fuel filter for me at the local AutoZone.

    170 main is awesome. Moved up one click from rich. Low throttle stumble is barely there, easily tolerable. Makes me think the 165 is worth a go. Maybe after I find a filter.
    I need to tweak the float level, cause it hits the overflow, just barely, when it's leaned over on the kickstand. Seems to be getting past the bowl gasket a little too.

    Right now, I'm happy. Happy happy happy.

    My bike has taken a bit of a beating through it all. Wrenches on painted bolts, gasoline spilled over everything, wet road spray... Maybe Ill be able to just making the little tweaks now. And get it cleaned and touched back up...Happy.
    hehe... he's starting to squirm.

  7. #497
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    It's been quite an extended battle and, ironically enough, you're now running the same main jet I have in the VM22 feeding my red bikes motor. Similar displacement, similar airflow volume. The big difference is rpm, this motor probably spins another 2-3K and, since a carburetor is inherently self-adjusting (meters fuel directly from airflow volume), even the power difference is neatly accounted for.

    You should be an even happier camper when you consider that winning the war of attrition is not your only reward for all of this effort. I'll guarantee that you now have a much keener sense of what the carb is doing, which will undoubtedly come in handy in the future. And, the motor has had an easy 250 miles. You're halfway through the initial 500. The next 500 beyond that will go very quickly and just in time for summer. Then, it's basically ready for the road.

    Been pretty chilly here, low 60s, today...with periods of rain. With mine, throttle response degrades at this temp, and lower. When temps rebound, you may find your off-idle lean spot has shrunk. 20 degree warmer air will definitely make it run a little richer at pat-throttle.

    For a fuel filter, try a power equipment dealer that sells chainsaws, commercial mowers, etc.
    Some luck lies in not getting what you thought you wanted but getting what you have, which once you have got it you may be smart enough to see is what you would have wanted had you known.

    Garrison Keillor

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