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October 24, 2014, 06:49:21 PM

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Author Topic: CV carb and an open exhaust  (Read 3598 times)

LJRead

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CV carb and an open exhaust
« on: December 11, 2007, 08:07:49 PM »

After reading what Pete Snidal had to say about the CV carburetor, it seems a sort of hands off system.  If you were to open the exhaust, for example, by using a shorter muffler, wouldn't the carb self regulate since the output pressure of the exhaust would cause a higher input pressure into the carb, treating it as a change in altitude?  What do Electra owners with a CV carb do to make the fuel input richer.  Is there a new jetting system available?

bwilson17

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Re: CV carb and an open exhaust
« Reply #1 on: December 11, 2007, 10:01:34 PM »
Install  a bigger jet - I have a 2006 Electra and have put on free flow exhaust and kept the OEM muffler - put in a 115 main jet and that is all to be done according to Classic Motorworks.

LJRead

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Re: CV carb and an open exhaust
« Reply #2 on: December 13, 2007, 06:23:43 AM »

I'm afraid I'm still confused about this.  Snidal says all the AVL lean burn engines have the MIK CV caruretor , and he goes on to mention standard O-8 jet and richer P-8, but no mention of 115, 125 or anything like that.  Am I missing something (besides a brain).

bwilson17

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Re: CV carb and an open exhaust
« Reply #3 on: December 14, 2007, 02:04:27 AM »
I have a CV carb and have installed the 115 main jet myself - You drop the bowl to get to the jet.

LJRead

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Re: CV carb and an open exhaust
« Reply #4 on: December 14, 2007, 08:41:37 AM »

Thanks, I guess they will take ordinary jets

Kevin Mahoney

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Re: CV carb and an open exhaust
« Reply #5 on: December 14, 2007, 02:17:29 PM »
They take everyday Mikuni jets. With an open exhaust (most of the restriction is in the pipe, not the silencer), you do need to richen up the carb. Jetting is one way......except that the CV carb has trouble getting enough air before too long. You can only do so much with it. Our advice is to re-jet and see how it goes. If you can't get it set up right, get a somewhat larger carb such as the 30mm flat slide or the AMAL

cyrusb

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Re: CV carb and an open exhaust
« Reply #6 on: December 17, 2007, 08:30:05 PM »
Don't beat yourself up . I find the snidal  manual makes things more confusing. Most of  the numbered breakdowns don't even jive, and there is only half the names of the provided numbers. I bought that book because the consensus was it was better than the manufacturers manual, Is it? does anybody have the re manual?

LJRead

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Re: CV carb and an open exhaust
« Reply #7 on: December 23, 2007, 01:11:01 AM »

I find the Snidal manual a bit sketchy too.  But I haven't yet purchased the R E one.  I like a good index and find that a bit bothersome also.  There is a workshop manual for my bike (2003 Thunderbird) which I am having sent out.  I think Snidal really tries to help neophyte mechanics, in  the process keeping everything in simpler terms, ,however sometimes we neophytes need more information in greater detail instead of less. 

As I recall, the old Motor manuals for cars used to be pretty good - very step-by-step, with lots of photos.  I don't know whether the R E manual is like that, but it should be.

portisheadric

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Re: CV carb and an open exhaust
« Reply #8 on: December 25, 2007, 12:38:32 AM »
I fitted an unrestricted header and a shorter see-thru muffler to my Lean Burn AVL 2006 Electra X, (you can't get less restrictive than this).
I also fitted a K&N air filter in place of the standard one to assist with flowing more air into the 29mm CV carb,and also removed about a third of the airbox baffle plate.
After a lot of messing about I managed to compensate for these changes by fitting a 120 main jet, removing 0.8mm off of the bottom of the slide and by raising the needle to it's highest position .
On UK unleaded (95 RON) petrol these setting worked fine; providing a much higher top speed along with better mid range acceleration.
However these changes were also done in conjuction with cleaning up the cylinder head ports.   
Hopefully this may provide a good baseline and save you some time should you decide to alter things.

I have recently changed over to a 32mm carb, kept the same exhaust set-up and am now running on a 130 Mikuni main jet, (200 Amal Equivalent).

From a personal point of view the top speed gain made from fitting the larger carb over the modified and re-jetted CV (paired with a free flow filter) does not seem worth the extra outlay if you compare the cost of one new main jet against that of a complete new carb  :( 

LJRead

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Re: CV carb and an open exhaust
« Reply #9 on: December 26, 2007, 06:53:36 AM »

This is very useful information.  I'm a little surprised at the removal of such a small amount from the bottom of the slide.  My interest is to be able to open things up a little and use a different muffler (shorter and with less overhang out back), We at times have a lot of very fine dust here, not so bad as it once was before they  tar sealed the roads, so going to a K & N air filter seems a good move.  I have read that the cylinder head chambers (maybe not the actual ports though) are already very large, but I suppose what you mean is what you said, just smoothing things out a bit for better air flow.

It is also interesting what Kevin said about much of the restriction being in the header pipe and not the muffler.   

Thanks for the tips - as I said, useful.