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New exhaust - will I need to rejet?

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Basil Moss
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New exhaust - will I need to rejet?

Post by Basil Moss on Sun 24 Nov 2013, 1:53 pm

Hi chaps, so I'm doing a big service in a few weeks, valve clearances are due, and while I've got the bike off the road I'm going to exchange the exhaust for the Art Pro 2 from my old bike. What I'm wondering, as I've already fitted a K&N air filter, is will the fuelling need adjustment after these alterations?

  If so, how do I go about it, what are the options and how much will it all cost? I'll look at the condition of the plugs to give me a general idea, but I assume that only really tells you about the state of the idle mixture - what does a dyno run entail, and how does one use the results?

  The new bike has much better compression than the old one, and generally runs much better (bar an annoying rattle), but my last one seemed to have a tad more "poke" especially at higher speeds - is this just because it ran better with the aftermarket exhaust, or could the carbs have been altered to make it work better?
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G3o
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Re: New exhaust - will I need to rejet?

Post by G3o on Sun 24 Nov 2013, 3:15 pm

By changing the exhaust you will allow better flow out of the engine meaning you get less contamination of the fuel with every revolution. Essentially giving you a richer mixture.

By fitting a free flowing air filter you allow more air to be sucked into the cylinder with every revolution. Giving you a leaner mixture.

The two combined should mean you're pretty much set to go. You will be slightly out but nothing worth worrying about. :]

dado_dassler
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Re: New exhaust - will I need to rejet?

Post by dado_dassler on Wed 29 Oct 2014, 11:16 pm

got a question about the same.
i recently bought myself a cb500 with leo vince exhaust.
the previous owner seemed pretty ill and he said he only used it on very short rides from time to time. sometimes he would let it run idle but mostly it stood still in his garage.
now on my first ride today i could hear very short-term spluttering from the exhaust. mostly it would happen when i approach crossroads, at the time when i let go of the throttle.
i bet it's running rich and i don't find it annoying since the sound from it isn't that loud.
i'm going to adjust the carb air-fuel mixture after my next ride.
will the adjustment affect the spluttering from the exhaust or do i have to get hold of another air filter for it to go away?
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stormbringer
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Re: New exhaust - will I need to rejet?

Post by stormbringer on Thu 30 Oct 2014, 6:08 am

@dado_dassler wrote:i'm going to adjust the carb air-fuel mixture after my next ride.
will the adjustment affect the spluttering from the exhaust or do i have to get hold of another air filter for it to go away?
Hmmm...

Out of curiosity: How do you plan to 'adjust the carb air-fuel mixture'?

dado_dassler
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Re: New exhaust - will I need to rejet?

Post by dado_dassler on Thu 30 Oct 2014, 11:17 am

my plan was to adjust the pilot screws on the carbs and the one for idle speed, but i've no idea whether it affect the exhaust splutter or not. i'm doing the adjustment regardless of the splutter, but i'd like to know ahead if that's going to cure it or not.
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jonny10
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Re: New exhaust - will I need to rejet?

Post by jonny10 on Thu 30 Oct 2014, 12:57 pm

@Basil Moss wrote:Hi chaps, so I'm doing a big service in a few weeks, valve clearances are due, and while I've got the bike off the road I'm going to exchange the exhaust for the Art Pro 2 from my old bike. What I'm wondering, as I've already fitted a K&N air filter, is will the fuelling need adjustment after these alterations?

  If so, how do I go about it, what are the options and how much will it all cost? I'll look at the condition of the plugs to give me a general idea, but I assume that only really tells you about the state of the idle mixture - what does a dyno run entail, and how does one use the results?

  The new bike has much better compression than the old one, and generally runs much better (bar an annoying rattle), but my last one seemed to have a tad more "poke" especially at higher speeds - is this just because it ran better with the aftermarket exhaust, or could the carbs have been altered to make it work better?

There seems to be some debate about just how much modding you can do without having to adjust fuel/air mixture.
If the exhaust you're fitting is a freer flowing/less restrictive one than the present one then, possibly, yes - more air in & more out "could" (and this is where the debate arises), theoretically, lean out the fuel, which could result in a hot running engine & poor performance. 
I'm not convinced that the "hot" part applies to the CB which is a cool running engine to start with.  
Anyway, try it and satisfy yourself first. Then check the plugs after a good blast as soon as you've killed the engine, without her tickin over. That'll tell you what the mixture is like when she's running on the main jet. You'll know then if she's running lean and you might be able to tell from performance anyway, eg if she is lumpy/bogging when you open the throttle.
If you do have to adjust the fuel then increase the main jet sizes; check on ebay - they're not that expensive and it's not that hard to do.
The alternative is to bring her to a shop/tuner who will be able to dyno test before & after and rejetting to suit & balance the carbs etc.
The dyno guy will be able to give you graphs showing torque & bhp numbers throughout rev ranges for different fuel/air mixtures, therefore identifying the the optimum mixture for best performance.
Dyno time doesn't come cheap (as I found out myself) and best to price around to at least get some idea.
No doubt others will be along to help too....
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jonny10
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Re: New exhaust - will I need to rejet?

Post by jonny10 on Thu 30 Oct 2014, 2:19 pm

Forgot to say:
You should really clean the plugs first before takin her for a run at good revs; this would give you a clearer picture of what they're like when the bike is running on the main jet.
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badseeds
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Re: New exhaust - will I need to rejet?

Post by badseeds on Thu 30 Oct 2014, 11:43 pm

A plug chop. Find a long, straight piece of road that you know well and are comfortable speeding down. Cane its spuds right up to the red-line in every gear, then kill the engine at 11. Coast to a stop, examine plugs. ;-)
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eternally_troubled
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Re: New exhaust - will I need to rejet?

Post by eternally_troubled on Fri 31 Oct 2014, 1:17 pm

@badseeds wrote: Coast to a stop, examine plugs. ;-)

Easy to do on a single-cylinder dirt bike or on my old air-cooled
Ducati where you can get at the plugs easily, less easy on a CB500.... *possible* but less easy.

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