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Thread: Tuning on a Micarb VM24 Carburetor of Bullet350 during High altitude-Steep Slope ride

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    Rookie caravans's Avatar
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    Default Tuning on a Micarb VM24 Carburetor of Bullet350 during High altitude-Steep Slope ride

    I was riding on a steep high mountain ghat somewhere in south India & found my motorcycle's engine straining itself on the steep slopes, unable to pull myself & the baggage, even though it earlier performed & still performs meticulously good on plains & during down-slopes. So I decided to do some detailed study of my Motorcycle's Carb, the Micarb VM24 Slide type Carburetor & decided to find out the steps in the art, since even though I know of Carb tuning under normal operating conditions, this high altitude tuning appears to be a kept secret like how to reverse-swing, apart from the few general ideas like increasing air etc.

    The below are the doubts on which I would require expert advice & description which could make this a clear understanding of things to do on this carb during mountains riding;
    1. To increase the pulling power of a motorcycle on a steep high altitude, what should be the approach of carb tuning on a VM24 carb?

    2. If the Pilot Jet air-screw is made LEAN as per requirement, does it BETTER / INCREASE the low-end Torque?

    3. Is this effect of Pilot Jet air-screw setting change profound only up to 1/8th Throttle range or also above it?

    4. Can increasing the pilot jet size help in increasing low-end torque? If yes, is this a normal measure before a high mountain trip or should be done only when the present Pilot Jet size has been tested to its limits?

    5. Since on a steep incline ascending, throttle would be generally ABOVE 1/8th (i.e, above Pilot Jet range ), then for riding purpose solely, is Pilot Jet air-screw setting of any help in decreasing engine loading & consequently increasing pull ?

    6. If above answer is NO, then presumably, will the Needle Jet (1/4 ~ 3/4 Throttle) setting also need to be made LEANER by lowering the needle inside the carb?

    7. But then, Needle Jet setting cant be done on an emergency basis. It requires one to open cab top, remove slides & accessories before changing the needle clip position to higher (& thus needle lower), which may also require removal of the fuel tank.
    So, if a ride involves both low plains riding & steep high mountain riding on the same day, what should be the approach to Needle Jet setting ?

    8. Since on steep slopes, throttle wont be generally above 3/4th, the Main Jet doesn't play any significant part in pulling. Right ?

    9. Larger main-jet only increases power in the high end & has no or insignificant infuence on low end or normal rpm range ? True ?

    10. On the VM24, when viewed from the RIGHT of the bike, apart from the Idle-Stop Screw (BIG/at the center) & the pilot air screw (extreme right/just before the engine), what is the 3rd screw (on the extreme left/ located on the carb body)? What is its function ?

    The above questions are entirely related to Steep slope riding under engine load.

    Experts views in this regards would be highly appreciative & a great help for people like myself to become more sure about solo trips to higher altitudes.

    Regards,
    Caravans.
    Happy Thumping.

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    Query approved and moved.
    (Been There Done That) x 3.25

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    Exclamation Guidance required

    Can any Gurus shed some light on this topic. I haven't seen a single reply on this thread. Guidance from the Seniors will be much appreciated.

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    Ha..Ha..Well, when I wrote that, I sure would have felt the dilemma that you probably feel right now; anyways, albeit a bit too-late to clear up a mess I should take responsibility to have started , I will do it now. Isn't there the proverb 'better late than never'?

    Now, a VM24 carb is a VM24 carb, nothing special about it ! As you might already know, there are 3 Jets in this carb that gives responses at different slide positions which is controlled by the throttle cable & lever. The Pilot-Jet works at idling & start-up throttle positions, the Needle-Jet works at above start-up to 75% throttle position (i.e, most of normal running) & the main-jet works at above 75% to full-open throttle position (i.e, at high speeds).

    So, basically when-ever normal riding is involved, it's the NEEDLE JET that matters. Now increasing power means burning more fuel. And to let more petrol to pass needle jet at a particular rpm means a larger opening. In VM24, its done by lifting the Needle to a higher position by lowering the needle holding pin.

    A catch to remember is that in this carb, the Main-Jet is placed below & hence earlier in the fuel circuit than the needle jet. Hence a smaller (below 90 in most 350cc CI) size Main jet will restrict the petrol enough to affect the needle jet. A stock main jet with needle-jet position adjustments is just ENOUGH to let a 350cc CI Bullet to ride from Leh to Kanyakumari; lots of people have done that including myself.

    In a higher altitude where Oxygen is less (& which I found to be totally insignificant in any mountains of Southern India, so mostly in the Himalayas), the bike at normal tuning will run rich for obvious reasons causing plug fowling by carbon deposition. So obtaining more power vs running the engine optimally in such a condition is definitely a compromise between the two parameters.

    I found that from Keylong to K'La (max altitude possible) it was enough for me to keep reasonable good running & no plug fouling plus enough power by lowering my Needle for 1 notch plus a higher rpm setting. And at lower altitudes with steep slopes I either made the needle to normal slot or one slot up to increase fuel flow. The increased carbon on such-a-case will burn off since the engine is under load while rising. Some people also do advance the timing before taking manali-leh road although I didn't do that myself. However I have seen some bikes panting for air even with their needles lowered to the leanest. In such a case (although I dont see its significance as I could ride myself to the highest motorable point in the world without much carb tuning change apart from the obvious) a smaller main-jet could help to lower fuel-flow. Another desperate measure that I have seen bikers take when nothing works is opening the air-filter in very high altitudes (I have heard a certain Stelvio Pass in the Swiss Alps to be a notorious place in this regards).

    However when irregular Idling (not running) is concerned as in any good altitude mountain, there are two ways of counteracting it. One, which I do on most low mountains is to slightly (ONLY JUST) increase the rpm. This helps in running up as well, whereas while descending, one must always keep an ear for the PING sound declaring too lean mixture. The second solution is obviously to make the Pilot Jet slightly leaner (only enough) to let-in more air during Idling. This solution is more probable in case of high altitude idling. I guess pilot jets of larger size would pass more air & have bette effect on drastic cases of low oxygen, however I rarely (rather never) felt requirement for a pilot jet change ever. Turning it to leaner plus increasing idling rpm was always enough to let the engine idle at the highest altitudes.

    So, now I guess the contradicting aspects of high altitude tuning are more clear; 'more' cause unless you try it yourself, you would never really know completely.

    And where's the next ride?

    Take care. Ride safely.

    Happy thumping,
    Caravans.

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    Default Re: Tuning on a Micarb VM24 Carburetor of Bullet350 during High altitude-Steep Slope

    Could you please advise the carburetor jet sizes for better fuel economy for Chennai for a 2008, CI 350 Std Bullet.

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    Default Re: Tuning on a Micarb VM24 Carburetor of Bullet350 during High altitude-Steep Slope

    Std. VM24 Jets are P:25, M:90. No changing there for std performance in most conditions.

    To obtain more-extreme mileage (not recommended for the CI's inherent bad lubrication system), you may lower the needle by 1 step.

    Also experience has shown that "timing" has more influence on mileage than carb jetting. Within the standard jet sizes, a bike with faulty timing can have a worser mileage of about 10kmpl compared to one with good ignition timing. So for better mileage, set your points (if your's have CB) correctly first. Too-much retarded usually gives poor mileage. Too-much advanced will lead to heating & pinging issues & overall piston damage.

    Take care. Ride safe.
    Anirban.

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    Default Re: Tuning on a Micarb VM24 Carburetor of Bullet350 during High altitude-Steep Slope

    thank you. I have the same jets and settings. I get around 28 kmpl. With the same settings earlier I got 35 kmpl which was alright. Can you please suggest a practical way to set timing and tuning. My bike does not have RPM. I think connecting a electronic RPM meter will help in easy setting. Please suggest.

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    Default Re: Tuning on a Micarb VM24 Carburetor of Bullet350 during High altitude-Steep Slope

    28kmpl is not bad mileage for a 350cc ci bike which I assume has covered 80K to 90K kms.
    In newer condition, they always give better mileage. Hence there can be many reasons for this. The general are worn out piston rings, worn gaskets, worn points assembly causing retarded timing, worn ignition-advance mechanism etc. Less probable if you service correctly are dirty air filter, clogged oil filter, clogged carb jets, leaking pulse air valve tube (if your bike has one) etc. So you need to first identify the cause.
    Also, frankly, how to set timing is beyond the scope of this type of communication. You however can obtain a workshop manual from any source to go through steps for static timing & then, dynamic timing (which is must & most important).
    You will need a non breakable straight metal rod to insert through the plug hole & a good multimeter / bulb & a mm scale & pencil/pen to do static timing. Jist is to set the points to open exactly at the moment piston reaches 0.8mm before TDC. You need to check this a number of times to be sure that basic setting has been achieved.
    Next, you need to do ping timing. For this you need to advance the timing in slow degrees until you can hear the engine pinging at 3rd-4th gear with above half throttle while riding. Next you retard the timing slowly & reach the point when it no longer pings even with throttle opening above 50% in 3rd-4th gear. Then retard it very slightly more & tighten the screws. That's timing for a cb points bike.
    Tuning, since you say you have stock jets & assuming needle is in 3rd, you only need to set pilot screw which only affects idling & very low speed riding. It is air-bleed type, so IN means rich, OUT means lean. Standard position is 1.5turns from fully closed which may vary by a quarter to half turn from bike to bike. Never go beyond 2.5 turns out. If you feel rpm still increases after 2.5turns, you need to go to the next pilot jet size or you will damage the spring tightness.
    But again, you need to go through the manual for clear understanding of steps before doing any setting change.

    Also, you don't need an RPM meter for setting timing or tuning the carb of a Std CI 350. Believe me, it only comes with experience & you will need to commit a few errors & be corrected (or be guided by a experienced person first hand) to learn how to maintain these two.

    Take care & ride safe.
    Anirban.

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    Default Re: Tuning on a Micarb VM24 Carburetor of Bullet350 during High altitude-Steep Slope

    thank you. My bike has done sixteen thousand kms. By adjusting the idling speed and timing it has now come to 31 kmpl. I am hopeful with proper adjustment it can be brought to 35 kmpl. I have removed the pulse air valve as I have been advised by mechanic. My earlier bullet used to give 39 kmpl on regular rides. Not accelerating too much, gentle riding around 50 kmph.

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    Hello fellas....I am planning to buy for royal enfield 500 standard. ....can any one tell me the avg problems...nd how to fix it.....thanks

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