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Carburation issue

Zakrov
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Carburation issue Empty Carburation issue

Post by Zakrov on Sun 30 Dec 2018, 8:04 pm

Hi!

Here's the thing, i run k&n filter on my bike and an universal megatone (cafe race exhaust) on it, the thing is the bike backfires a lot as soon as i close the throttle on hi revs, i know that due to the modifications i've done its normal. I've install a 25 cm db killer plus fiber glass also.

I've been reading some discusions on the forum and the conclution i've made is that the best set up for the carbs are to keep all jets standard, and maybe set the needle 2 shrimps. Also something i've noticed on my bike is that as soon as i put chocke the bike dies, and then when i start the bike cold, the bike run at 1000 rpms and when it's hot t reaches 1500 rpm.

any better idea?
Carburation issue 2018-111
ceejay
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Post by ceejay on Sun 30 Dec 2018, 8:12 pm

I'm not sure about your carb questions (someone with more knowledge will be along to help i'm sure) but the bike idling at 1000rpm when cold and then 1500rpm when warmed up sounds normal to me - at least that is roughly the same as my bog standard cb500!
Jameshambleton
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Post by Jameshambleton on Sun 30 Dec 2018, 8:30 pm

Sounds about spot on to me
eternally_troubled
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Post by eternally_troubled on Tue 01 Jan 2019, 4:32 pm

You are probably right about leaving the jetting alone.

How easily does the bike start without the choke?

On my bike you don't need the choke unless the temperature is less than 10 degrees and even then it isn't really *necessary* for starting until it goes below 5 degrees, approximately.

I don't know what the temperature is in Spain, but if it's above 10 degrees then you probably don't need the choke - a bit of throttle can be used to keep the bike running once it has started.

I find that if you use the choke when it is too warm you can cause the bike to stall, so I suspect you are just seeing the normral behaviour.
Zakrov
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Post by Zakrov on Tue 01 Jan 2019, 4:40 pm

@eternally_troubled wrote:You are probably right about leaving the jetting alone.

How easily does the bike start without the choke?

On my bike you don't need the choke unless the temperature is less than 10 degrees and even then it isn't really *necessary* for starting until it goes below 5 degrees, approximately.

I don't know what the temperature is in Spain, but if it's above 10 degrees then you probably don't need the choke - a bit of throttle can be used to keep the bike running once it has started.

I find that if you use the choke when it is too warm you can cause the bike to stall, so I suspect you are just seeing the normral behaviour.
Thank you for you answer!
I don't even have to give some throttle to start it, fires right up! Due to the filter i've installed and the "free flow" of the exhaust the bike performs not really good on low rpm, do you think that it would be possible to improve it ?
Zakrov
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Post by Zakrov on Tue 01 Jan 2019, 4:44 pm

@ceejay wrote:I'm not sure about your carb questions (someone with more knowledge will be along to help i'm sure) but the bike idling at 1000rpm when cold and then 1500rpm when warmed up sounds normal to me - at least that is roughly the same as my bog standard cb500!

My question is if there's  any solution in order to reduce the backfires of the bike. closing the mixture screw 1/4 ?
eternally_troubled
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Post by eternally_troubled on Tue 01 Jan 2019, 4:56 pm

@Zakrov wrote:
Thank you for you answer!
I don't even have to give some throttle to start it, fires right up! Due to the filter i've installed and the "free flow" of the exhaust the bike performs not really good on low rpm, do you think that it would be possible to improve it ?

It's not easy and it's not always possible to 'fix' these kind of problems, even with the right equipment (dyno etc).

As you mentioned it might be worth adjusting the needle position - this has the advantage of being fairly easy to undo if it doesn't work!

The original exhaust has more back pressure which can help some of the low rpm problems - might be worth putting in a more restrictive baffle and see what happens?

Also, when you say 'low rpm' what do you mean?
Zakrov
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Post by Zakrov on Thu 03 Jan 2019, 9:05 am

@eternally_troubled wrote:

@Zakrov wrote:
Thank you for you answer!
I don't even have to give some throttle to start it, fires right up! Due to the filter i've installed and the "free flow" of the exhaust the bike performs not really good on low rpm, do you think that it would be possible to improve it ?



It's not easy and it's not always possible to 'fix' these kind of problems, even with the right equipment (dyno etc).

As you mentioned it might be worth adjusting the needle position - this has the advantage of being fairly easy to undo if it doesn't work!

The original exhaust has more back pressure which can help some of the low rpm problems - might be worth putting in a more restrictive baffle and see what happens?

Also, when you say 'low rpm' what do you mean?

The range between 2500-4000. i'm planning on changing the exaust in January, there's a Shop in Barcelona called Endy Exhaust, they make custom exhaust from scratch and road legal, i'll go there and see if i can find one that suits my bike.
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Post by eternally_troubled on Sat 05 Jan 2019, 7:24 pm

Well, all CB500 are pretty lumpy below (about) 3k, so I wouldn't worry about that too much. It should be useable above that.

If you go to a place that makes custom exhausts I would have thought they would have (or have access to) a dyno...
cb500rider
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Post by cb500rider on Tue 15 Jan 2019, 10:40 pm

@Zakrov wrote:Hi!

Here's the thing, i run k&n filter on my bike and an universal megatone (cafe race exhaust) on it, the thing is the bike backfires a lot as soon as i close the throttle on hi revs, i know that due to the modifications i've done its normal. I've install a 25 cm db killer plus fiber glass also.

I've been reading some discusions on the forum and the conclution i've made is that the best set up for the carbs are to keep all jets standard, and maybe set the needle 2 shrimps. Also something i've noticed on my bike is that as soon as i put chocke the bike dies, and then when i start the bike cold, the bike run at 1000 rpms and when it's hot t reaches 1500 rpm.

any better idea?
Carburation issue 2018-111
When it's backfiring a lot then it could be one of your spark plugs isn't firing properly and, therefore, excess fuel could possibly being burned off as it enters the extremely hot exhaust system. Usually any bike will have a lower RPM than normal when running cold at first, but after a short while the engine has time to warm up then the RPM settles at the manufacturer's recommendation after taking off the choke.

Also, remember, that you've changed the bike manufacturer's standard setup for which the Carbs were originally setup for. and so you will need  to have the Carbs re-tuned for the new current setup of K&N filter and that Cafe Racer exhaust system you've installed to finally get the correct air & fuel mixture to prevent backfires on deceleration !

Here's a good website on the above mentioned, but if you're unsure then take it to someone that knows what they are doing like a bike mechanic or someone you trust that actually knows what they are doing.

http://motorcyclemd.com/deceleration-backfire-tuning-your-carb/

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