ATVConnection.com ATV Enthusiast Community - Stock Carb Mods

Stock Carb Mods needed to be cleaned up and summarized.
Oldmanracing was killed and thus to edit,
I had to start a new topic.
The stock carb can flow enough air to generate 60 to 65 horsepower. MrHP has generated 65 on a customers 730. If you are building a motor beyond 65HP, a different carb would be required.
I thought I would post my recommendations for the stock carb.
These suggestions are only a guideline that has worked for me.
Different altitudes and temperatures will require jetting adjustments.

Step 1 is for LID ON and Step 7 is for LID OFF.

Step 1 AIR - LID ON (Trail Riding and Water)

The snorkels running under the gas tank cover is a major restriction.
Go to the local hardware store and buy some sump pump hose.
Some sell it bunk in 6ft sections or a box of 24 feet.
If you have to buy 24ft, you will have enough to do 4 quads.
At around $10 this is $2.50 per set. Cheap mod.
Even though tubing is same size as stock, adapter end is .100 larger.
OD of adapter end is around 1.700, with end of tubing to first ring measuring just under 1 5/8.
Pull out your stock snorkels from airbox.
Remove clamps and pull out tubing from adapter.
Install new tubing into the large end of airbox adapter. Little WD will help.
This is a snug fit and no clamps are needed. End of tubing will fit into adapter groove.
From the groove in airbox adapter to end of tubing should measure 12in. About 34 rings on mine.
Run new snorkels along side snorkel to carb, where old tubing ran, and install small end into the airbox.

Benefits:
1- More air because tubing is shorter(minus 9inches), not under cover(hood) and less restriction thru adapter.
2- Tubing has larger ribbing which reduces the shock waves from reversion.
Reversion refers to reversed air flow, or in simpler terms, its when air in the intake runner reverses
direction for a split second. The condition is caused when a burst of pressure escapes into the
intake runner from the cylinder during valve overlap. Reversion creates resonance shock waves
which exit the open end of the tube at various rates depending on engine speed. To help maintain
inertia in the airbox and resonance, thus increase air flow.

Latest Airbox Mod:
I did not like the way air enters the box. I felt that the air caused too much disturbance in mid-range. As the air on each side of the filter, enters the filter, they are crashing together, some air is pushed into the carb and some is pushed to the rear. This caused negative pressure to vary too much. Not good for cv carb.

Solution:
Since I have the airbox adapters mounted already backwards for the sump-pump hose, I took the stock inlet hoses and cut them from adapter end, up 20 rings. I then installed them into the inside of the airbox. Air now travels to the rear of airbox and all the crushing now forwards air only towards the carb. Mid range even sounds different and hits harder. With new spring and this mod we had much more mid range velocity.
Tubes mounted inside the airbox is what the ducati's use in their racing machines.

Step 2 Throttle Slide Spring

You need to cut your spring to an overall length of 3 1/8in.
Older DS's had to be cut 3 to 5 coils to come to this length.
The new springs are 3 1/8 from the factory.
Do not go shorter, static height is 2 7/8, you need alittle load so slide does not bounce/flutter.
If you do not like to cut your spring, than order a 05 spring. They are inexpensive.

Note: More spring information-10/28/05
All factory springs appear to be made out of .032 spring wire.
2000/2001 had a pitch of .140P and were 26/27 coils. Length 3 5/8
New 2005 has a pitch of .120P and are 26/27 coils. Length 3 1/8
When we cut the old springs 3-4 coils, we had a length of 3 to 3 1/16
When you tests the force of the cut old spring to the 05 spring you get
very similar gram forces throughout compression.
Summary:
If you check your spring and it is 3 1/8 with 26/27 coils, this is OK.
If you check your spring and it is 3 1/8 with 24 or less coils,
order the new 05 spring.
If your spring is over 3 1/8 long and has 26/27 coils, cut to 3 1/16 to 3 1/8.

Step 3 Throttle Slide
This is where I have gone and a few of you will get nervous.
On the bottom of the slide are 2 vacuum ports.
I drilled 2 extra vacuum ports. Starting with a small size and testing.
The size I ended up with was .125 for all 4 vacuum ports.
At this size I could get the slide to rise all the way up, even with the lid off.
You need to measure and drill the extra ports in the same relationship as the first 2, just
on the other side.
The surface is not flat so use a sharp pick to start a pilot. Drill first a 1/16 hole.
Turn slide over and look for best location. If hole is not centered between ribbing, you can install 1/16
drill and side cut to best position. Then final drill with .125. Stock ports just need to be re-drilled.

Step 4 General Jetting

I prefer to use the Dynojet Kit.(Moose is the same)
The needle is up to .011 thinner in mid range. All around riding gives better throttle response.
For drag only applications the stock needle is very adequate.

I would like to do a review of the differences between the Stock and DJ Needles.
Note: Both needles are 63mm long.
I am going to compare the stock needle in the 5th groove to the
DJ in the 3rd groove(from top down)

Stock DIA. DJ DIA.
IDLE: .0985 .096

The DJ is richer at idle. But you only have to turn out the pilot screw 2?-3 turns(45 pilot).Some early carbs had a 40 pilot, too lean, install a 47.5 if you have to purchase new.
We have all found to reduce backfiring at a high speed slow down and for best offline performance, the pilot screw had to be turned out 4 to 6 turns and also a larger pilot jet for the stock needle.

For all purposes, when used as above, they produced the same results.

Normal Driving Range: This covers the next 8 mm of travel
.089 .0895
As you can see both needles are delivering a very similar fuel path.

Accelerating Mid Range: This covers the next 10 mm of travel.
.077 .071
As you can see the DJ needle becomes very aggressive at this point.
This is a very critical stage. This is when you are accelerating and everything most
work together. On a cv carb THIS IS YOUR ACCELERATOR PUMP, it works great, but your slide needs to start to open quickly to supply the required air. If you do not open quickly you will become very rich and your motor will stumble. Jetting is a combination of fuel and balanced air per rpm range.

This is where the DJ needle is the winner. You just have to give your motor more air,
and then throttle response and torque take a good jump.

Top Range: This covers the last 12mm of travel
.0435 .048

This is where the stock needle is very aggressive. It tapers .0028 per mm compared to the
DJ at .002 per mm. For drag only, where the rpm?s are kept in the upper range, for that application maybe the stock needle is ok to use.

Overview:
The DS is a 2 spark plug motor that by design is a mid range torque monster. Give it a better air/fuel curve in mid range and it really comes to life.

The original aftermarket cdi only raised the rev limter. If they were to have an aggressive timing curve, the stock needle just could not supply the needed fuel. Now that the new cdi?s have changed there timing curves, if you use the DJ needle you can possibly raise the needle more and get even a bigger torque hit. More fuel also needs more air.

Stock CDI:
Needle clip in 2nd groove from top.
3rd groove if exhaust system is over rev design. Testing required.

CDI with noted aggressive timing. I have a new Big Gun and works fine.
Needle in the 3rd groove from top.
4th groove if through testing works.

Main Jet:
Start with the Dynojet 175 and tests.
Note ? If you can run with the needle higher, you may have to jet smaller.
These needles are very tapered and it is better to start leaner(main jet) and then get larger.
Try to get needle height set first before finalizing main jet.

Step 5 - Breather Tube/Balance Tube

The clear hose that runs from the carb to the beginning of carb snorkel no longer has a purpose when
slide control has been altered. I have disconnected the hose at the airbox end and installed a unifilter.
The barb going into the snorkel has been covered with a soft plastic cap and clamped with original hose clamp. Too much negative pressure is being generated below the slide bladder causing the slide to rise too slow. Conflicting pressures on both sides of the bladder not good. Throttle response appears to be better in the lower gears, better torque and flutter control.

Step 6 - DS Float Maintenance and Pilot Cleaning

1- If you have inconsistent main jet setting, float height adjustment may be the problem. If you find your jetting always rich, that is, after a trail ride the plug looks gas soaked, and black is the color on end of plug and porcelain is dark brown; adjustment to the float is required.
I feel that the factory spec is a little on the rich side. The spec is 10mm(.394).
Trying to bend the tab and hit this dimension is like hitting a moving target.
I have found more consistent results using this procedure:
With float bowl cover removed, turn carb upside down.
Note that the floats are formed with a crowned end surface. How can you take a measurement off a surface that is not flat - You can?t.
But there is a edge just below this crown that is straight.
I take a first measurement to this edge, at the pinned end. Write down reading
I take a second reading to this edge, at the unpinned end. Write down reading.
Subtract second reading from first reading.
My Spec: 0 to 5/64(.080) That is - The second reading should be equal to first reading or lower to as much as 5/64(.08). Mean reading of .040 or 1 mm.
I have used this spec for 3 years and have not had inconsistent readings or plugs fouling out.
If you are doing power testing runs and find that top end is inconsistent. That is, sometimes strong and sometimes flat. Float height is the issue.

2- Pilot Cleaning
If your motor starts hard or not at all. Pilot circuit is dirty or clogged.
Remove pilot screw, be careful not to loose small spring and washer.
With a air compressor, that has a rubber tipped nozzle, blow air into the
orifice that the pilot screw was removed from. Spray alittle carb cleaner
into orifice and blowout again. Gas has a tendency to dry-up and leave deposits
around the pilot air/fuel passage. Try to clean twice a year or when having starting problems.

3-Needle Assembly Into Slide:
The injection molding of the vacuum slide sometimes had a slight mismatch in its production.
With the needle and washers removed from slide, look down with good light and where the washer will rest, does your bottom look round or can you see a slight flat lip that is raised and thus looks like like the bottom is D shaped. Many slides seem to have a D shaped bottom.
If you have a D bottom:
Take the thick metal washer and grind a flat. Usually about half the width to the inner hole is find.
Install the thick washer first to align in the D.
Then install thick nylon washer.
Then needle with clip in 4th or 5th(stock needle) or 2nd or 3rd(DJ needle) from top groove, and thinner washer on top of clip.
Press in plunger with small spring attached.
When assembled and you turn the slide over and look at the needle going into the slide, you will notice that the needle is positioned in the middle and has equal space around the needle. This is important otherwise the needle will be dragging in jet needle and wear out of round. Also the better it is positioned, the better fuel will be drawn up the jet needle and the fuel will atomize better.
Final Check:
Push needle lightly up into slide body. There should be a slight movement. Spring will return needle when released. If running in the 5th(Stock) or 5th/6th(DJ), you may find that there is no movement.
Warning: If no movement, the assembly may come apart while riding.
Fix: Grind 1/32-1/16 off top of needle and re-assemble. Check again for movement.
I have found that the slide length varies. Excess shrinkage is the issue.

Step 7 AIR - LID OFF (Race and / or No Water) - This is only been successfull with my modded emulsion tube and or exhaust system per elevation.

Lets review why the stock carb runs lean at high rpm's with lid off:

The factory built the DS to meet emission and speed regulations.
For many years all builders hated cv carbs because they needed negative pressure created by a closed air box to help slide and fuel control in the carb.

So how do we out smart all the above.
First, improve the exhaust system.
Second, rebalance the carb to match the motor needs - stock carb mods.
Third, control the negative pressure inside the carb, not in the airbox. - this is new.
Air going threw the venturi generates negative pressure, slide rises and fuel is drawn up the needle jet. Before fuel can rise quickly it needs to be atomized, thus the emulsion tube adds air to the fuel. Where does the air come from, threw the air jet on the right front of carb air intake. Side where throttle cable is mounted. Other side jet is for the pilot jet system.
But what happens when you allow outside air to enter the venturi area of a carb that has not been threw a tapered orifice?
It now becomes a air bleed. It now reduces the negative pressure in the venturi and has rpm's rise it starts to lose its ability to draw fuel up fast enough to match the air requirements. Fuel mixture starts to run lean.
The air bleeds control the mixture mostly at mid to high rpm. The air bleeds are best described as a controlled air leak in the main circuit. The air bleeds work by "bleeding off" this vacuum, which draws fuel through the main jets. A larger air bleed will make the mixture leaner at high rpm by bleeding off more vacuum or "signal."
How do we adjust for this condition? Install a smaller air bleed to richen the high rpm mixture by bleeding off less signal.

The fix:
The main air jet in the DS is a number 120.
The main air jet in a 2003 or 2004 Quest or 2005 Traxter is a number 40
Part number 707200187
120 is actually a .037" hole.
40 is a .022" hole

The 40 still supplies all the air needs for the emulsion tube, but maintains the negative pressure to supply the needed fuel for high rpm's with the lid off.
For times that we are riding with the lid-on, jet accordingly, but there is added power at high rpm's also. With lid-on, I run 2nd groove on needle and a 180 DJ main.

With the air snorkel removed from the front of carb, when looking straight into the carb venturi, the air jet is at the 3 o'clock position. Same side as the throttle cable. You will need a small screwdriver to remove the jet. Cost is around $8.00
When reinstalling, do not overtighten, small threads.
This will richen the mainjet by about 2 jet sizes and not allow the top end to lean out.

Note: To run with the lid off, remove the gasket also and if the 3 rubber clamps are loose, remove them also.

Special Note: The balance tube does not have to be disconnected. With no negative pressure from airbox, the reversion effect does not effect slide motion.

Step 8 Set RPMs to 1450

The butterfly in carb will cover the pilot orifice slightly and cause motor to start poorly.
If motor does not start quickly, you will load the motor with excess fuel and if ignited may backfire and turn the motor backwards. NOT GOOD This will hurt your sprag clutch and they are expensive.
To set the rpm's without a tach:
When the motor is warm and you shift into gear-
If goes into gear very smoothly and no sounds or jerking - Rpm's too low
If goes into gear and makes a light clunk sound - Rpm's just right
If goes into gear and makes a hard clank sound and motor/quad moves - Rpm's too high

Step 9 Hightower/OMR Jetting
Here is some info for general jetting compensations.

Jetting rules of thumb for travel and/or seasonal changes

Compression Ratio Correction Formula:

CR at sea level - (Altitude / 1000 x 0.2)

Example for 9.7:1 piston (Stock)

9.7 - (3,000 / 1000 x 0.2) = 9:1

Stock DS runs well with 89 and above 3000 87 octane.

Main Jet Corrections, rules of thumb:

Altitude correction - drop one 5.0 increment (two jets-155 to 150) for every increase of 3000'

Temperature correction - increase one increment of 5.0 (two jets-150 to 155) for every 30 deg. drop

Humidity correction - increase one increment of 5.0 (two jets-150 to 155) for every 30% drop

Needle Adjustment - Example - If running in the 3rd notch from top, sea level,
When running above 5000ft - lower to 2nd notch or try 85 octane

Step 10 Setting Pilot Screw

Set screw out 3 turns. Start motor and let idle or go for short run.
To set pilot screw motor must be hot. Turn idle down till almosts quits running.
Turn screw in till lugs or stops running- Turn off motor - turn screw in and count turns.
Go back to 3 turns and start motor.
Turn out till motor lugs or stops running.Turn off motor - turn screw in and count turns.
Subtract in reading from out reading and that number divide by 2.
Take that number and either add to low reading or subtract form high reading.

Example: In reading 2 turns - Out reading 8 turns 8-2=6/2=3
Add 3 to 2 = 5 turns or subtract 3 from 8 = 5 turns

The key is to have the motor running at the lowest rpm without turning off and motor hot.

Reset idle speed to 1400rpm

Step 11 Overview of Air Fuel Ratio

Bottom- Step 10 will give you a average setting. This is good for fast trails or duning. I like to set the pilot screw out 1/2 to 3/4 turns more from average for drag racing. Being a 2 spark plug head, the DS can take alittle more fuel at bottom and generate that extra torque for launching. At drags you are not idling alot and plugs do not have enough time to load up/ start to fuel foul.
For slow speed/tight trails I will turn in the pilot screw 1/2 turn in from average. A long slow speed trail ride for 3 or 4 hours will start to fuel foul the plugs. By leaning out alittle really helps keep them clean for that type of riding.
I am not concerned in what the a/f ratio is because I tune for the application and what the motor is telling me its needs. I would think that the leanest is around 14.5 and the richest is around 16.

Mid Range- Remember that this is a 2 spark plug head and loves to make mid range torque. A/F ratio should be in the 12.5 to 13 range.

Top- The DS revs out better if the top is starting to lean out alittle. Try to get a A/F ratio in the 13 to 13.5 range

Step 12 Maintenance of Choke
Pull the choke plunger out of the carb.
Clean the plunger with carb cleaner and then spray and wash out the cavity in the carb that the plunger goes into.
Wipe off excess cleaner so as not to discolor any metal.
Spray WD-40 into cavity and onto the plunger. Reassemble.
Do this twice a year.

Replace the factory breather tubes that run on each side of carb and are tied together above the carb. Buy enough and re-route the tubes down to the swingarm hole where the carb overflow tube exits. Tie the two tubes together just in front of hole.