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Jetting the Stock Carburetor for performance motors can involve a lot more then just setting up a stock carb for a stock motor. If you bought the stock carburetor for your stock VW you probably won't need the following information. You might just want to change the main jet size. If you have a slightly modified motor then read on. Stock Main jets where generally on the lean side.
If you plan on racing in a stock class where you must use the standard Solex Carb then this information will help you get started in the correct direction.
If you have access to a exhaust analyzer with the air/fuel meter your job will be a little easier. Set your jetting a little rich (11.5-12.1:1) on acceleration and about 12.5:1at high RPM.
If you don't have the analyzer then you need to have a reference point to start with. (We would always end up checking out the car with this method after tuning on the Dyno.)
To get this Starting Reference Point, Get a passenger to help you with the spotting and find a stretch of deserted road. You need a starting point to start all runs. With the same passenger in place each time and at a pre-determined starting speed you must start accelerating from the exact same spot traveling at the exact same speed to begin each test run. Depending on your motor select a speed, a little slower then your top speed and take notice at the point along the road you reach this speed.
(It can also be helpful to check the time with a stop watch) Make note of the speed and time and proceed with your jetting. After each run do a repeat run and log the time and speed.
We would recommend increasing the main jet size until the speed starts to drop off and then back down two steps smaller.
Main jets can easily be replaced or you can ream out stock jets. Always compare new jets with new jets and reamed jets with reamed jets due to the slight camphor on new jets.
Jet drills are not the same as number drills so don't try to use them. We recommend using the taper reamers. To check the size as you ream, measure the length of the reamer protruding through the jet.
Solex jets are measured in hundredths of a millimeter and come in steps of 2.5 (120, 122.5, 125, 127.5) The larger the number the more fuel the jet will flow. Some jets are measured in flow so don't mix two types of jets. (Weber progressive & Holley Weber for example). Note: VW 120 main jet has a 1.200 mm hole.
Normally you will change jet size by one or two sizes at a time. For example a tuned exhaust would be changed one size at a time, Bigger pistons, high lift cam, and higher compression might warrant stepping up several sizes.
If you have a fuel pressure regulator installed, start out with about 3# pressure.
Along with the acceleration runs you should take spark plug readings on the #1 & #3 cylinder.
When taking a plug reading you must run at least 1/4 mile at the desired throttle position. At the end of the 1/4, push in the clutch and simultaneously kill the ignition. Don't change the throttle position until the motor stops completely.
You are looking for a light Tan or Gray coloring on the insulation. Black is rich, white is too lean. Some fuels give slightly different coloring. Instead of gray you might see light brown, but white is always too lean. Remember that a dark or black plug can give poor performance and poor economy but a white or white blistered plug can burn holes in pistons and size pistons to cylinder walls. So if you error be sure it's on the rich side.
When you get the main jet pretty close start changing the air correction until you have a mid-range slightly rich. Then check the full throttle setting again.
Accelerator pump jets are in the end of the Accelerator pump nozzle, and can make a big difference when the gas pedal is smashed down. This jet regulates how fast the quantity is injected, (Note. How quick not the quantity). You can play around with their size but be sure it is pointing in the right place. You want the squirt between the butterfly and the carb body, not hitting the butterfly.
The accelerator pump's injection quantity is adjustable. On 30
PICT-3 carbs, the adjustment is made by installing the cotter pin through a
different hole in the connecting link. The 34 PICT-3 carb's have a
bell-crank with an adjustable segment. On 34 PICT-4 carb's a spring-loaded
screw is used to adjust the quantity.
Make sure the throttle is full open when the gas pedal is fully depressed. This might seem very basic
but it happens so often, we had to mention it.
Remember as the Venturi size increases the velocity of air through the carb decreases. When the velocity of air falls below a certain level the fuel and air cease to be mixed in the correct
propositions.
Many times good low end torque will help turn faster lap times then all out horse power.
This is just a start and to be good at carb jetting you need to study up on many of the subject we
just slightly skimmed over.
TOO RICH > Black sooty plugs / Sluggish performance / Black exhaust smoke / Motor labors at an
Idle / Smell of fuel.
Caused by > Too large main jet / Too small air correction jet / Float level too high / Clogged or
restrictive air filter / Excessive high fuel pump pressure / Sticking needle and seat.
P.S. Remember the choke should be set
to come on the smallest amount possible for your climate. It helps starting
a cold motor but excess is not good for your engine. Setting
the Choke
TOO LEAN > High running temperature / Detonation / White blistered spark plugs / Flat performance
/ Back firing through the carb.
Caused by > Too small main jet / Too large air correction jet / Air Leaks / To little fuel pressure or
restricted fuel inlet.
Note: The number of washers and the thickness of the washers under the
needle & seat, sets the
float height and fuel level in the float bowl. With the level the
distance from the top of the carburetor body (gasket surface) to the surface
of the fuel should be 19.5 */- 1.0mm or .767" +/- .040"
Washers are available in .50mm (.020"), .80mm (.031"),
1.00mm (.040") and 1.50mm (.059")
Question w/Answer Cleaning Intake Heat Risers of Carbon ->*Click Q&A on Idle Jet Throttle Bushing Repair
New DP Carb-->*ck
---- New SP Carb-->*ck ---
SP to DP Carb
Adapter->*ck
--- Rebuild Kit-->*ck
How
to hook up vacuum Hoses
Fuel Pump->
order
---
Solex Main
Jets->
order
---
Solex
Idle Jets->
order
--- Acc. pump Jets->
order
Jet Reamers #V3257 --
$52.95
->
*ck
Jet Gages .45 -1.5mm - $32.95
->
*ck
Jet Gages 1.5 - 3.00 --- $29.96
->
*ck
Weber Replacement 30mm Rebuild Carb Kit
"Quote"
$21.95
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All Questions should be E-Mailed to Vic's