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FAQ - Carbs

327K views 314 replies 120 participants last post by  joshuaj 
#1 ·
Quote, originally posted by GTaye »
Ever wonder?

(Stolen from Sport-Touring.net):
Basics of Motorcycle Carburetor Operation
The basic secret of carb function is that inside each carb are thousands of tiny gnomes; each with a small bucket. As you open the
throttle, more of these gnomes are allowed out of their house and into the float bowl, where they fill the buckets and climb up the carb's passages to the intake, where they empty their buckets into the air stream.

But, if you don't ride the bike for a while, bad things can happen.
Tiny bats take up residence in the chambers of the carb, and before
long the passages are plugged up with guano. This creates a gnome
traffic jam, and so not enough bucketfuls of fuel can get to the
engine. If it gets bad enough, the gnomes simply give up and go take a nap. The engine won't run at all at this point. Sometimes you'll hhave a single dedicated gnome still on the job, which is why the bike
will occasionally fire as the gnome tosses his lone bucket load down
the intake.

There has been some research into using tiny dwarves in modern carbs. The advantage is that unlike gnomes, dwarves are miners and can often re-open a clogged passage. Unfortunately, dwarves have a natural fear of earthquakes, as any miner should. In recent tests, the engine vibrations caused the dwarves to evacuate the Harley Davidson test vehicle and make a beeline for the nearest BMW dealership. Sadly, BMW's are fuel injected and so the poor dwarves met an unfortunate end in the rollers of a Bosch fuel pump.

Other carb problems can also occur. If the level of fuel in the float bowl rises too high, it will wipe out the poorer gnome housing in the lower parts of the carb. The more affluent gnomes build their homes in the diaphragm chamber, and so are unaffected. This is why the bike is said to be "running rich".

If the fuel bowl level drops, then the gnomes have to walk farther to
get a bucketful of fuel. This means less fuel gets to the engine.
Because the gnomes get quite a workout from this additional distance,
this condition is known as "running lean".

The use of the device known only as the 'choke' has finally been
banned by PETG (People for the Ethical Treatment of Gnomes) and
replaced by a new carb circuit that simply allows more gnomes to carry
fuel at once when the engine needs to start or warm up. In the
interests of decorum, I prefer not to explain how the 'choke'
operated. You would rather not know anyway.

So, that's how a carburetor works. You may wish to join us here next
week for basics of electricity, or "How your bike creates cold fusion
inside the stator, and why the government doesn't want you to know
about it."


Modified by GTaye at 4:59 PM 9-8-2004

http://****************.com/smile/emthup.gif http://****************.com/smile/emthup.gif
Later,
Thomas


Modified by the12for12 at 10:37 AM 6-21-2007
 
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#78 ·
Re: FAQ - Carbs (mark memory)

Quote, originally posted by mark memory »
What other carbs can be used? Some mikunis or delltoros? And what size? Should they be around the 40-45mm range and side drafts?

16v's require sidedrafts, 8v's require downdrafts. typical carbs used on VW's are either Weber's or Dellorto's. a lot of people reccommend the Webers because you can still get parts for them, and I would agree with that statement. I haven't seen any carbs much bigger than the range of 40-45mm.
I used the Dellorto's, and was really impressed with them.
 
#79 ·
Re: FAQ - Carbs (MoGtheMooGle)

Quote, originally posted by MoGtheMooGle »

16v's require sidedrafts, 8v's require downdrafts. typical carbs used on VW's are either Weber's or Dellorto's. a lot of people reccommend the Webers because you can still get parts for them, and I would agree with that statement. I haven't seen any carbs much bigger than the range of 40-45mm.
I used the Dellorto's, and was really impressed with them.

sent im http://****************.com/smile/emthup.gif
 
#80 ·
Re: FAQ - Carbs (MoGtheMooGle)

Quote, originally posted by MoGtheMooGle »

16v's require sidedrafts, 8v's require downdrafts.

Just a correction, 8V's can use sidedrafts just like 16V's. Downdrafts are usually used in factory carb type installations. for performance sidedrafts are the way to go when carbs are concerned on a watercooled VW motor
 
#81 ·
Re: FAQ - Carbs (2L Bunny)

Quote, originally posted by 2L Bunny »

Just a correction, 8V's can use sidedrafts just like 16V's. Downdrafts are usually used in factory carb type installations. for performance sidedrafts are the way to go when carbs are concerned on a watercooled VW motor

for an 8v to use a sidedraft setup, wouldn't you be talking about a x-flow motor where the intake is on the front and the exhaust is on the back? otherwise, if you were mounting carbs on a rear-mounted intake setup, you would have to fab up a manifold so that the carbs would be laying flat.
if the sidedraft carbs aren't sitting level, the fuel would flow down the venturis incorrectly.
 
#83 ·
Re: FAQ - Carbs (Mk2enthusiast)

Quote, originally posted by Mk2enthusiast »
Many manifolds exist for counterflow 8v heads and have been used for years. Also, sidedrafts can function properly at a maximum angle of 7 degrees above horizontal.

Yup, I've got a set of Dellorto sidedrafts on my Race rabbit with a counterflow 8V. If you're ever up in Corvallis you're welcome to take a look.
 
#85 ·
Re: FAQ - Carbs (Dellorto)

when you guys say that you keep the intank pump as well are you:
1- using only the oem intank pump?
2- Using the carter pump and the intank (wired up and pumping)
3- Using the carter pump only and just leavign the intank pump in the tank and using it as a pickup only for the carter?
Also on aside note about msd, i have a 6al box, can i either
1- use my factory digifant dist. with the icm as pictured in this thread
2- use and older vacuum advance distributer and wire it straight up to the msd box?
thanks Andy
 
#86 ·
Re: FAQ - Carbs (Red Robertson)

Quote, originally posted by Red Robertson »
when you guys say that you keep the intank pump as well are you:
1- using only the oem intank pump?
2- Using the carter pump and the intank (wired up and pumping)
3- Using the carter pump only and just leavign the intank pump in the tank and using it as a pickup only for the carter?
Also on aside note about msd, i have a 6al box, can i either
1- use my factory digifant dist. with the icm as pictured in this thread
2- use and older vacuum advance distributer and wire it straight up to the msd box?
thanks Andy

2/3 are the same but people have done all those fuel configurations with success (in conjunction with a regulator).
If using the stock distributor, the icm only amplifies the signal from the hall effect, allowing a timing comupter (e.g. MSD PN 8980) to deliver an advance curve.
 
#87 ·
Re: FAQ - Carbs (Mk2enthusiast)

Quote, originally posted by Mk2enthusiast »

If using the stock distributor, the icm only amplifies the signal from the hall effect, allowing a timing comupter (e.g. MSD PN 8980) to deliver an advance curve.

do you think the 8980 would be the best bet? i liked the idea of the adjustable timing controller pn 8680 (knob on the dash) but does it deliver a curve as well or just a straight adjustment?
would the programable timing controller (pn 8981) be of any benifit over the fixed curve?
but would a vacuum advance dist. provide an equivlant curve?
 
#90 ·
WOW...good info.....well im about to jump feet 1st into the carb world.....i have put a 8v together with a set of solex carbs what would i need to know if i pulled my stock 83 rabbit motor and put this built 8v in??? do i need diff. fuel pump....ect.??? im me please if you have the time and thanks agian guys for all the info
Luke
 
#91 ·
Re: FAQ - Carbs (Red Robertson)

Quote, originally posted by Red Robertson »
do you think the 8980 would be the best bet? i liked the idea of the adjustable timing controller pn 8680 (knob on the dash) but does it deliver a curve as well or just a straight adjustment?
would the programable timing controller (pn 8981) be of any benifit over the fixed curve?
but would a vacuum advance dist. provide an equivlant curve?

All the MSD boxes are fairly equivilent, adjustablity within 20 degrees of your mechanically set full advance. The programmable (8981) merely allows adjustment in the slope of advance and initial timing within, again, 20 degrees of your mechanical set.

Digging around on here, the net, or in a Bentley you should find the curve of an OEM vaccum advance or mechanical advance distributor. I know people have used one from a Saab as well.
If you're serious about dialing in your own ignition curve, the best route would be a distributorless ignition system like Electromotive for example.
Based on this and your budget you should be able to settle on something http://****************.com/smile/emthup.gif
 
#94 ·
Re: FAQ - Carbs (OnTheRunDCI)

Quote, originally posted by OnTheRunDCI »
Ok, question.
It says the carter feul pump needs no fuel pressure regulator but then it suggests one? Which is it, no fpr or with fpr?

I would suggest one, people say its not needed because the max psi rating of the carter pump should not hurt most carbs, but if yould always like constant Fuel pressure, id get a regulator
 
#96 ·
Re: FAQ - Carbs (OnTheRunDCI)

i recently put a holley 1-4 reg set to 3.5 on mine, i was getting too much fuel in the bowls with just the carter...
 
#97 ·
Re: FAQ - Carbs (lehr leistung)

I am not running a regulator with my set-up, but will be adding one to see if it works.
btw: 1.8 12.5 comp, 270cam, holley 350 with #57 jets, carter pump.
I will update and let you know if anything gets better or worse. http://****************.com/smile/emthup.gif
 
#100 ·
Re: FAQ - Carbs (PNW_Dubbin_Ain't_Easy)

the wire goes to 12v key on. (off the coil) if you wnat to see a pic. look in my sig. the one with the webber for sale. that should have a pic of my old one installed.i know it sounds getto, but i used some big nuts as spacers. if you wnat some better pics, let my know.
 
#102 ·
Re: FAQ - Carbs (SilverSLine)

Quote, originally posted by SilverSLine »
Thanks for the info. I have a 91 and am trying to eliminate the Motronic harness and ECM. Do any years have ICM's that plug directly into the distributor? I want to keep it as simple as possible.

any answers?
 
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