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**princesirohi** · 02-18-2014, 09:58 PM

Re: Carburetor Jetting and tuning for the correct AFR.

[MENTION=32286]psr[/MENTION] ji: can you explain the complete internal construction and working of a CV carb with the help of pictures. specially for p200ns. nothing urgent, but it is a request.

**gyan_recl350** · 02-19-2014, 02:21 AM

Re: Carburetor Jetting and tuning for the correct AFR.

Originally posted by psr View Post

If the carburetor has a FUEL adjustment screw ie., adjusting the Amount of fuel then turning Clockwise make the AFR LEAN and Vice verse. If the AFR screw is an AIR Adjustment screw then turning it clockwise will make AFR RICH. The AFR adjustment is for fueling from start to approximately 1/3 of max RPM .
Here is a representative image to explain things in correct perspective.

[ATTACH=CONFIG]136906[/ATTACH]

Thank you for your input sir. I humbly say that, this turning thing and air screw and fuel screw thing I knew and my bike's carb is BS29 and it has fuel screw.
Sir all I wanted to know if if this fuel screw (suppose carb with fuel screw) decides the A/F mixture only for the pilot jet circuit is which is normally closed to 1/4 throttle?

OR

When the needle circuit and main jet circuit comes into picture beyond 1/4th of the throttle, is this fuel screw still decides the mixture? But I guess I got the answer and it is NO, I hope I am right?

Now sir tell me, while tuning the carb(suppose with fuel screw) why we look out for that peak RPM at which we stop turning the screw, how does that logically translate to A/F mixture being correct at "this" number of turns?

----consecutive posts auto-merged-----

Originally posted by princesirohi View Post

@psr ji: can you explain the complete internal construction and working of a CV carb with the help of pictures. specially for p200ns. nothing urgent, but it is a request.

You might like to see this:

GadgetJq.com is for sale | HugeDomains

http://www.gadgetjq.com/keihin_carb.htm

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**princesirohi** · 02-19-2014, 04:14 AM

Re: Carburetor Jetting and tuning for the correct AFR.

^^ thanks for the link.

regarding your querry, i think the fuel screw controls main jet and not the pilot jet.

and last time i discussed this with bajaj service manager for pune region, in CV carb, he told me it is neither fuel screw, nor air screw, it is VCS (Volume Control Screw).

**nirvan** · 02-19-2014, 09:34 AM

Re: Carburetor Jetting and tuning for the correct AFR.

Originally posted by psr View Post

It can never be done accurately...better to buy the jet size as required..

My Impulse came with a jet that was too large, leading to poor performance (smoothness, acceleration), poor mileage (15-18 kmpl), a spark plug that was always soaked with black unburnt fluid and black smoke from the exhaust.

After months of going back and forth and trying various other remedies, I finally settled for a Honda Unicorn main jet, since no other 150cc Hero anything jet was available. I changed to the Unicorn main and pilot jet and all seemed well, except the top speed and the w-o-t acceleration.

Here's the story:

Originally posted by nirvan View Post

On an Impulse, I bought a 2011 Impulse in May 2013.

I soon realized to my horror that the Impulse was giving me about 15 kmpl. Several visits to several mechanics later, things were pretty much the same, kmpl went up to 18 tops with all the vain efforts from the various experts. The bike would let out blackish smoke when revved and the spark plug was always black and oily. Idling was poor and uneven. The guys at the showroom that sold me the bike looked at me as if I were cuckoo when I told them I was getting 15 kmpl. They were looking at me as if I had lost my marbles and/or as if I was some manic freak whose sole purpose was to wreck perfectly efficient Honda-tech bikes. I didn't ride the bike much and that didn't help the diagnostic process either. (I've done 2200km in 7 months on this bike).

In desperation, I opened the carb up a few times, cleaned it to the best of my ability and generally sprayed the life out of it. Jet size was something I kept in mind, but replacement jets were never available so I left the jet in there. I replaced the spark plug, air filter, engine oil and cleaned the life out of everything that could have been the culprit.

Eventually, I decided to replace the jets; albeit I used the jets from a Unicorn which were easily available in Ramkote unlike the Hero stuff; nothing available anywhere from the Impulse/Hunk/CBZ-X parts bin. The main jet from the Unicorn had an orifice which was pretty much half the dia of the jet in the Impulse I bought. Long story short; my Impulse is now capable of delivering a reasonable mileage and idles smoothly.

I rode to the showroom I bought the Impulse at the far end of town just to show the people there the gaping size of the orifice in the main jet. They were sympathetic and perhaps relieved that their freaky 15kmpl customer had somehow figured things out for himself and was out of their hair.

I recently did a couple of 250km highway runs on my Impulse and found that the bike is really unhappy to be pushed beyond 90 and refuses to go beyond 95 unless there's an incline. With a new spark plug, air filter and a carb running rich, I was sure it was something to do with the main jet being too small. I looked up the internet and found that the Impulse uses the biggest main jet of the Unicorn 150cc spawned breed while the Unicorn itself uses the smallest (115 to 128 IIRC).

I've been looking around for an Impulse/Hunk/Xtrreme main jet in vain for the last couple of weeks and seem to have exhausted all my options in Hyderabad. My next option is to buy another Unicorn jet and drill it out, since NO Hero spares dealer stocks any 150cc jet. I now have these questions:

1. Can I try jets from the Yamahas and/or Pulsars etc.? Anyone know their sizes and if they will be compatible?

2. Can anyone please source an Impulse main jet for me? I'll be deeply indebted and grateful if you can help me.

3. Is there anyone in Hyderabad who can drill jets with reasonable accuracy?

Thanks!

**phanikar** · 02-19-2014, 12:09 PM

Re: Carburetor Jetting and tuning for the correct AFR.

Originally posted by nirvan View Post

2. Can anyone please source an Impulse main jet for me? I'll be deeply indebted and grateful if you can help me.Thanks!

PM your number please.

**gyan_recl350** · 02-19-2014, 12:13 PM

Re: Carburetor Jetting and tuning for the correct AFR.

Originally posted by princesirohi View Post

^^ thanks for the link.

regarding your querry, i think the fuel screw controls main jet and not the pilot jet.

and last time i discussed this with bajaj service manager for pune region, in CV carb, he told me it is neither fuel screw, nor air screw, it is VCS (Volume Control Screw).

Welcome friend but sorry I differ from you, the fuel screw is for pilot jet circuit but my doubt is if beyond also. Yes they say volume control as ultimately it is metering the fuel but still it would be called fuel screw.

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**gyan_recl350** · 02-19-2014, 12:17 PM

Re: Carburetor Jetting and tuning for the correct AFR.

Originally posted by nirvan View Post

:

1. Can I try jets from the Yamahas and/or Pulsars etc.? Anyone know their sizes and if they will be compatible?

Which carburetor is there on Impulse?

Sent from my X10i using xBhp Connect mobile app

**psr** · 02-19-2014, 12:39 PM

Re: Carburetor Jetting and tuning for the correct AFR.

Originally posted by gyan_recl350 View Post

Thank you for your input sir. I humbly say that, this turning thing and air screw and fuel screw thing I knew and my bike's carb is BS29 and it has fuel screw.
Sir all I wanted to know if if this fuel screw (suppose carb with fuel screw) decides the A/F mixture only for the pilot jet circuit is which is normally closed to 1/4 throttle?

OR

When the needle circuit and main jet circuit comes into picture beyond 1/4th of the throttle, is this fuel screw still decides the mixture? But I guess I got the answer and it is NO, I hope I am right?

Now sir tell me, while tuning the carb(suppose with fuel screw) why we look out for that peak RPM at which we stop turning the screw, how does that logically translate to A/F mixture being correct at "this" number of turns?

----consecutive posts auto-merged-----

You might like to see this:

Care & Feeding Of The Keihin Carb

Technically for understanding the functions of various carburetor components working, A Changeover of different jets,needle position are discussed as though they have Cut Off points...In actual real time use they are inter related....The Idle circuit Continues to work at WOT also. The needle position Does have an influence on proper standing start , and the combination is responsible for the overall FE.
To answer your specific query as to why adjust the Idle AFR at peak RPM.....Normally as I had said before the idle circuit Actively contributes till 1/3 of max RPM, so to get the AFR correct till the specified RPM , it is normal practice to set the RPM at about 1/4 to 1/3 RPM and adjust AFR screw for max RPM ...This guarantees that fueling is good from start to cross over point of Needle taper, and no drop in acceleration at mid to high RPM happens...Some times this will result in less FE and if it happens a 90 degree turn back to reduce AFR is done.

Originally posted by nirvan View Post

My Impulse came with a jet that was too large, leading to poor performance (smoothness, acceleration), poor mileage (15-18 kmpl), a spark plug that was always soaked with black unburnt fluid and black smoke from the exhaust.

After months of going back and forth and trying various other remedies, I finally settled for a Honda Unicorn main jet, since no other 150cc Hero anything jet was available. I changed to the Unicorn main and pilot jet and all seemed well, except the top speed and the w-o-t acceleration.

Here's the story:

I recently did a couple of 250km highway runs on my Impulse and found that the bike is really unhappy to be pushed beyond 90 and refuses to go beyond 95 unless there's an incline. With a new spark plug, air filter and a carb running rich, I was sure it was something to do with the main jet being too small. I looked up the internet and found that the Impulse uses the biggest main jet of the Unicorn 150cc spawned breed while the Unicorn itself uses the smallest (115 to 128 IIRC).

I've been looking around for an Impulse/Hunk/Xtrreme main jet in vain for the last couple of weeks and seem to have exhausted all my options in Hyderabad. My next option is to buy another Unicorn jet and drill it out, since NO Hero spares dealer stocks any 150cc jet. I now have these questions:

1. Can I try jets from the Yamahas and/or Pulsars etc.? Anyone know their sizes and if they will be compatible?

2. Can anyone please source an Impulse main jet for me? I'll be deeply indebted and grateful if you can help me.

3. Is there anyone in Hyderabad who can drill jets with reasonable accuracy?

Thanks!

The Impulse uses 149cc engine same as Unicorn and uses a CV carb...The Carburetor is Keihin and so only Keihin jets will be suitable. Karizma OE jet is 125 and is priced approximately at 250/-
Keihin jets are mentioned by their hole dia and so a 115 jet is supposed to be 1.15mm size hole...
Which jet size do you want ? PM me and I will see if I can help.

**gyan_recl350** · 02-19-2014, 01:04 PM

Re: Carburetor Jetting and tuning for the correct AFR.

[MENTION=32286]psr[/MENTION] Sir thanks a lot for the detailed reply, I get the reason now

Sent from my X10i using xBhp Connect mobile app

**nirvan** · 02-19-2014, 01:16 PM

Re: Carburetor Jetting and tuning for the correct AFR.

Originally posted by phanikar View Post

PM your number please.

PM sent. Thank you.

Originally posted by gyan_recl350 View Post

Which carburetor is there on Impulse?

I'm not sure of the model number but as PSR has outlined below, it is a Keihin CV carb that is found on all the Hero 150cc bikes, the Karizma and the Honda 150cc bikes.

Originally posted by psr View Post

The Impulse uses 149cc engine same as Unicorn and uses a CV carb...The Carburetor is Keihin and so only Keihin jets will be suitable. Karizma OE jet is 125 and is priced approximately at 250/-
Keihin jets are mentioned by their hole dia and so a 115 jet is supposed to be 1.15mm size hole...
Which jet size do you want ? PM me and I will see if I can help.

Thanks for offering to help! To be honest, I'm not sure of the original size. Searching here on xbhp reveals that the size is 132, but now I'm confused because you say the Karizma has a 125. I read elsewhere (can't find that post now) that the Impulse main jet size is 128. Logic tells me the Impulse cannot have a main jet bigger than the Karizmas' so I would guess perhaps 120 should be the size that would match.

References:

Honda Unicorn - Cosmetic and Performance mods - please give your views (Pics on Pg 9) - Page 7 - Team-BHP

Hero Impulse ownership experience - xBhp.com : The Global Indian Biking Community

http://www.xbhp.com/talkies/motorcycle-ownership-experiences/21084-hero-impulse-ownership-experience-167.html#post1022949

Share what you think about the motorcycle you own or have owned in the past.

Hero Impulse ownership experience - xBhp.com : The Global Indian Biking Community

http://www.xbhp.com/talkies/motorcycle-ownership-experiences/21084-hero-impulse-ownership-experience-54.html#post850265

Share what you think about the motorcycle you own or have owned in the past.

**phanikar** · 02-20-2014, 12:13 PM

Re: Carburetor Jetting and tuning for the correct AFR.

Originally posted by nirvan View Post

I'm not sure of the model number but as PSR has outlined below, it is a Keihin CV carb that is found on all the Hero 150cc bikes, the Karizma and the Honda 150cc bikes.

Thanks for offering to help! To be honest, I'm not sure of the original size. Searching here on xbhp reveals that the size is 132, but now I'm confused because you say the Karizma has a 125. I read elsewhere (can't find that post now) that the Impulse main jet size is 128. Logic tells me the Impulse cannot have a main jet bigger than the Karizmas' so I would guess perhaps 120 should be the size that would match.

No buddy, could be 128/132 also. For Eg: Karizma jet is 125 where as CBZ extreme is 128. Have checked with my sources to procure one for impulse. Even the HMC guys are not sure what are the jet sizes for Impulse

Have asked a friend to open the carb and check for the correct size.

**nirvan** · 02-20-2014, 01:19 PM

Re: Carburetor Jetting and tuning for the correct AFR.

Originally posted by phanikar View Post

No buddy, could be 128/132 also. For Eg: Karizma jet is 125 where as CBZ extreme is 128. Have checked with my sources to procure one for impulse. Even the HMC guys are not sure what are the jet sizes for Impulse

Have asked a friend to open the carb and check for the correct size.

Yes, it does appear that way as I'm finding out! Thanks for trying to help.

**rohit.ganga** · 03-03-2014, 12:21 PM

Re: Carburetor Jetting and tuning for the correct AFR.

Hello Friends,
I think I have scrwed up my AFR setting, I am facing some problems, hope someone will be able to guide me.
It all started after service from a local guy, I had real bad experience with the Bajaj Service guys here in Bangalore. I had tried this guy out last time and he had done a good job, this time around needed the vehicle in a hurry and looks like he has messed up the AFR. I tried my hand at the adjustment myself after reading through the forums and looks like I did not follow something correctly. Let me explain the problem.

Engine starts fine, RPM hold steady at 1.2K, then once the engine heats up the RPM goes high and does not come done on throtle release. This is happening during gear shift also. I think that the engine is running lean and this is what is causing this high RPM.

I also noticed that when the RPM is at 3K and the AFR turned clokwise and completely closed, the engine does not stall and keeps running and the RPM hold steady, this lead me to believe that the Engine is running real rich. Can someone please help me with this...

This issue is with a Pulsar 220 DTSi.

Please help!!!!

**gyan_recl350** · 03-04-2014, 05:03 PM

Re: Carburetor Jetting and tuning for the correct AFR.

Originally posted by rohit.ganga View Post

Hello Friends,
I think I have scrwed up my AFR setting, I am facing some problems, hope someone will be able to guide me.
It all started after service from a local guy, I had real bad experience with the Bajaj Service guys here in Bangalore. I had tried this guy out last time and he had done a good job, this time around needed the vehicle in a hurry and looks like he has messed up the AFR. I tried my hand at the adjustment myself after reading through the forums and looks like I did not follow something correctly. Let me explain the problem.

Engine starts fine, RPM hold steady at 1.2K, then once the engine heats up the RPM goes high and does not come done on throtle release. This is happening during gear shift also. I think that the engine is running lean and this is what is causing this high RPM.

I also noticed that when the RPM is at 3K and the AFR turned clokwise and completely closed, the engine does not stall and keeps running and the RPM hold steady, this lead me to believe that the Engine is running real rich. Can someone please help me with this...

This issue is with a Pulsar 220 DTSi.

Please help!!!!

Looks like your idle is set too high as you said once you close the fuel screw completely still the engine does not stall so rich and lean is completely thrown out from the scene here as the screw is closed. What it means the idle RPM is set too high that your needle circuit is coming to play even when fuel screw is completely closed. Pilot, needle and main jet they all come into picture at different throttle positions and since idle is set so high that needle circuit is getting involved even before required and it makes the bike not to stall.

Just adjust the idle RPM when the engine is hot and running and then see the difference. You can take help of tacho to adjust it.

**yes!rajib** · 11-21-2014, 07:49 PM

Re: Carburetor Jetting and tuning for the correct AFR.

Hi. Anybody can tell me where to get mikuni 15 size pilot jet in india??? Or which bike consist of this jet as pilot jet??? Exept fz16. Plz hlp me out.. Also thunderbird has 15 pilot jet is that true??

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Carburetor Jetting and tuning for the correct AFR.

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