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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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#34 ·
Re: FAQ - Carbs (candm)

Quote, originally posted by candm »
can i use the bosch fuel pump and accumulator with carbs? i have a 2.0 liter 16 valve motor going in a 79 rabbit with motronic but might switch to carbs. both the accumulator and fuel pump are brand new. thanks

Absolutely not. You will blow out all the seals and gaskets from the carbs. The FI pump operates around 80 psi. Carbs use between 4 and 7 psi . Anything higher is damaging to the carbs and definitely a fire hazard.
 
#35 ·
Re: FAQ - Carbs (noypiesky)

Quote, originally posted by noypiesky »
Anyone know where I can get parts for dellorto's? Specifically for 45 DHLA? TIA.

If you haven't tried CB Performance they have a lots of Dell parts. Alternetely check out http://www.dellorto.co.uk I ordered my chokes from them and they showed up in like 4 days. I was really impressed.
 
#36 ·
Re: FAQ - Carbs (the12for12)

Sorry about that, I've had some **** going on lately that forced me to give in to my brother's demands to trade him my Rabbit for his 92 GTI. Since I've done this and won't have any money in the near future because of medical bills, I will be unable to undertake any big projects.
Oh well, maybe my brother will come by and ask the same question and you'll
at him.


Modified by sirdUbsalot at 5:03 AM 11-16-2004
 
#40 ·
Re: FAQ - Carbs (the12for12)

Quote, originally posted by the12for12 »
A.IGNITION
However, the MSD has a nice built in feature that is not found while reading the instruction manual. By reversing the magnetic leads, the MSD can give you a nice timing advance curve. You simply set the ignition at you want at idle (stock is around 10-15 degrees BTDC). As you blimp the throttle, you will find that the timing jumps to around 20-35 degrees BTDC. ,

I found this in a search, and I thought I would comment.
I don't know for sure what is going on inside the MSD box to cause this, but I have a guess. If my guess is right, this probably isn't the best idea for controlling advance.
As the trigger wheel aproaches the sensor, a positive voltage is generated. As the trigger nears the point where it is closest to the sensor, the voltage rapidly drops and goes negative. (This is the event you want to capture) As the wheel moves away from the sensor, the negative voltage increases as it tends toward the steady state of about 0V.
If you switch the wires, then you get a voltage response that goes negative, jumps positive, then settles to zero. The MSD unit is probably triggered by a negative going zero crossing. If you have it connected correctly, this corresponds to the point where the trigger wheel is closest. This part of the voltage output curve is very steep and doesn't vary much with RPM -- so the output from the detection circuit is pretty stable.
Switching the wires means that the detection circuit will *not* trigger on the nice crisp transition because it will be a "positive going" transition (starts negative, and goes positive). Instead the circuit will trigger sometime later as the voltage settles back down to near zero. Unfortunately this part of the curve both varies with RPM and isn't very steep. The "varies with RPM" part is what makes it advance the timing. The fact that it isn't a steep curve means that slight variations (including the temperature of the electronics, external "noise", etc.) can have a significant effect on when the circuit will trigger.
This may be a bit far afield for a carb FAQ, but I thought I would add it. Keep in mind that this is again just my guess based on my understanding of how the ignition system works. And while I'm pretty confident that switching the polarity of the wires is *not* a standard operating mode -- if it works, and it's consistent..go for it. Just be aware of potential problems (I would expect inconsistent advance values)
-Steve
 
#41 ·
Re: FAQ - Carbs (Stephen Webb)

First off, if you have a fixed dizzy, you can't advance the timing from the MSD. First off, measuring timing with a timing light is near impossible with an MSD because of the multi-spark to measure the static timing at idle. With a fixed dizzy, I just set my timing above 4k rpm where the multi-spark function of the MSD box doesn't occur. The multispark feature of the box is only below 3000 or something IIRC. I'm not quite sure what is being seen to make the statement that the MSD actually provides a timing curve.
 
#42 ·
Re: FAQ - Carbs (the12for12)

alrite folks would you please be able to give a bit of info on pierburg 2e2,s as mine is playing up.
My car is a golf mk2 1.6 driver and i would like to know if a weber 32/34 dmtl is a direct replacement. Any help would be greatly appreciated. thanx in advance
 
#50 ·
Re: (brainjerauld)

That's the ignitor. That little black thing right under the rain tray on the driver's side.
Wire the dizzy to the ignitor, then wire in the MSD between the ignitor and the coil. You need the ignitor because the MSD cannot trigger off the OEM hall sender in the dizzy. Then you will likely need a MSD 8980 timing computer to give your car some low rpm retard. Set the mechanical timing to 30-32 deg BTDC, and then the MSD 8980 will retard it 20 degrees at idle so that it starts and runs easier.
Good luck.
 
#51 ·
Re: (RaceJetta)

OK,.... I tried the diagram over and over, not missing a thing and still NO SPARK!!!!!
If I get rid of the MSD and just wire the ignition module, coil, and distributor, will that work??? and how do I do it? This seemed like it would be simple
but still no spark!!! There must be a way!
 
#52 ·
Re: (brainjerauld)

Back to the FAQ
Is there a good source to buy a Carter 4070 pump? I can't seem to find one around here. Also I need to buy parts for my DHLA's, maybe someone could post up some links for good sources for Weber and Dellorto carb parts.
Thanks, lots of good info here. http://****************.com/smile/emthup.gif


Modified by Flintstone at 9:04 PM 10-31-2005
 
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