Help! My Carb is Jetting! Why?
11 April 2009
9 Comments
Twin carbs are abit new to me and would like to try and get to understand them better.
ive sort of grasped the concept into the idle,linkage setting etc but strugling to understand the jetting side of things.
Is there a ‘law’ to it?..ie if you run this with this fuel etc you need this jet?
For example..im running a 1915 with kad40’s..009..plan to ditch this for a vac advance dizi….mechanical fuel pump and run on 98 fuel
my bus has a flat stop on gear change and will sometimes pop and sputter on aceleration…but no allways??
You can jet them "by the book" but that’s always a rule of thumb – it will get you close but if you want to get it perfect then you need to take it to a rolling road or put the engine on a dyno and have somebody who knows what they’re doing set up the jets for you.
Get the Bob Tomlinson book for Webers or Dellortos, that has all the info you’ll ever need.
Thanks Fella
Planning a dyno day at raceshak in order to get my bus running sweet.Are the books still readily avalible or would a wanted thread be needed?
Would you agree that carbs need vac advance or is it a case of suck you in and buy one of our dizzys?
Would i be better off changing to an elecki feul pump or are mech ones just as good?
Thanks Fella…sound advice
Wana take alook at me carbs/timing before she goes down the track @ Bib Bang
If the car is driven regularly then the stock pump will be fine. If it often gets laid up for 2 weeks or more then an electric pump will give you the advantage of quicker starts as you dont have to crank the engine for ages to prime the carbs.
Dave.
Still available AFAIK – Amazon, eurocarb, VW Books etc. Well worth the money.
Dave.
thanks again fella
flatspots with kads can usually be fixed by tweaking the acc. pump lever (or advancing the timing a little?). jetting is, generally speaking, matched to the venturi size etc, hence kads are roughly jetted/set-up to deal with most applications right out of the box.
dig through the pages at http://www.lowbugget.com/main_page.html and you’ll find some good advice.
i’ve typically found that (on stock’ish cam’d motors) by tweaking the acc. pump action and running as much timing advance as possible (8-10 deg on a good 009), the flat spots can be fixed. yes, kads tend to be jetted a little over rich ‘out-of-the-box’, but they’re pretty easy to tweak with a little work.
if your splutter persists, check your ignition system carefully..
i can’t comment on some of the advice as i have no in-depth experience in that area…
but, in my own personal experience, i have almost always managed to tweak the carbs/timing to work with a good 009 dizzy without a noticeable ‘bog’ from idle.
But, if you have the time/parts/cash to follow that route, i’m sure you’ll end up with a better running (and more efficient/economical) motor. I’ve tended to concentrate on day-to-day driveability over economy/efficiency in the past, so i’m sure there are more efficient routes out there!
regarding the fuel pump issue – i’ve always used a stock pump with kads (even on my ‘hot 1641cc’ project, which broke into the 15-sec bracket in a heavy cabrio bug). i’d only suggest an electric pump on a hi-po drag car with giant carbs, which might require a long, full-throttle third/fourth gear blast…. a good-quality, stock fuel pump will be more than adequate!
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