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Re: [turbobike] Main jet question

Dave Morrill (david.p.morrill@fairchildsemi.com)
Tue, 30 Nov 1999 08:47:52 -0500

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Hello,
    The pilot jet screw on my carbs has a D shaped head, encased in a
tube, so you cant get pliers on it, and regular screwdriver wouldn't
work.  I took some brake line and ground it's OD down so it would fit in
the tube, and tapped a side of it with a hammer to make a D shape.
Works great, but you can only adjust with the carbs off.  I screwed them
in and it leaned out the low end, so they must be fuel.

    As far as needles, I was assuming you had adjustable needles  (the
ones that slide into the main jet).  Most aftermarket needles have 4 or
5 clip positions and a spacer, so you can really fine tune them.  Stock
ones dont.

good luck,
Dave.

this helped me;




RAW ANT wrote:

> When you ay needle down do you mean jetdown or needle? Is that needle
> down as in a larger needle?
>
>
>    ----------------------------------------------------------------
> RAW M.... F..... ANT

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Hello,
    The pilot jet screw on my carbs has a D shaped head, encased in a tube, so you cant get pliers on it, and regular screwdriver wouldn't work.  I took some brake line and ground it's OD down so it would fit in the tube, and tapped a side of it with a hammer to make a D shape.  Works great, but you can only adjust with the carbs off.  I screwed them in and it leaned out the low end, so they must be fuel.

    As far as needles, I was assuming you had adjustable needles  (the ones that slide into the main jet).  Most aftermarket needles have 4 or 5 clip positions and a spacer, so you can really fine tune them.  Stock ones dont.

good luck,
Dave.

this helped me;
 
 
 

RAW ANT wrote:

When you ay needle down do you mean jetdown or needle? Is that needle
down as in a larger needle?


RAW M.... F..... ANT

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Keihin FCR Carb Tuning Procedures

Turbobike Web Site www.Turborick.com

Keihin FCR tuning page

By Patrick Burns


First!

Before you do anything, tear the carbs completely apart (don't forget to remove the fuel screws), blast everything with carb cleaner, and blow out all passages and jets with compressed air. If you neglect this critical step, don't be surprised when, after fourteen jet changes, you find that a shred of fuel line had lodged itself in some passage or another causing the 'jetting' problem you were trying to solve.

Synchronizing:

Synchronize the carbs on a workbench. Using the shank of a small drill bit (such as a 2mm), adjust the idle speed screw until you can barely fit the base of the drill bit under one of the slides as you would a feeler gauge. You may have to remove the idle speed screw from the carbs and temporarily remove its locking spring to be able to thread the screw in far enough to slide a drill bit in. Do this from the engine side of the slide as opposed to the intake side. Remove the top of the carbs as you would to access the needles. On the throttle shaft in each carb there is an arm which controls slide movement. It is secured by a screw and nut. The nut is used for adjustments, the screw is to lock the adjusting nut in position. Loosen the lock screw of the slide you are trying to adjust, turn the adjusting nut to set the slide so the drill bit just scrapes under, and tighten the lock screw. Repeat for each slide. Be prepared to spend about five times as long as you think it should take, going back and forth between slides to get everything just right. Replace idle speed screw locking spring. Because the carbs must be disassembled to access the adjusting mechanisms, it is recommended to use the above procedure. You could use manometers to synchronize the carbs as you would a set of CVs, but you'd have to start and warm the bike, check vacuum levels, shut the bike off, disassemble the carbs, adjust, reassemble, restart the bike, and recheck vacuum levels. You may very well kill the battery (or your bump start helpers) before you finished synching the carbs this way. It has been my, and others', experience that the lock screws tend to work loose with time no matter how surely you tighten them. To remedy this, after the slides are synchronized, clean the adjusters with brake or contact cleaner, blow them dry, and put a drop of silicone over their tops. You can also use Loctite 290, but it tends to make future adjustments a bit difficult.

Fuel screw:

With the carbs synched, the engine warmed up to operating temperature, and the idle speed set, adjust the fuel screws so that when you rev the bike in neutral and release the throttle, the revs quickly drop to exactly idle speed again. If, when you release the throttle, the revs hang up a few hundred rpm above idle speed, then drop to idle, you are too lean. Turn the fuel screws out. In extremely lean cases the idle will hunt between the proper speed and something above it. If, when you release the throttle, the revs drop below idle speed then pick up, the idle mixture is too rich. Turn the fuel screws in. In extremely rich cases the engine will die after revving the bike and releasing the throttle.

Slow fuel jet:

After setting the fuel screws, if you end up having to turn them in closer than 1 turn out from bottomed, select smaller slow jets. If you end up with the fuel screws turned out further than 2 turns from bottomed, select larger slow jets. Go back and repeat the fuel screw adjustment procedure. You have the correct slow jets when your engine passes the fuel screw adjustment procedure (rev and release) with a fuel screw setting between 1.25 turns out and 1.75 turns out.

Slow air jet:

With the correct slow fuel jets installed and the fuel screws adjusted as above, when you are operating the engine at higher revs and closed to 1/8th throttle (say, while transitioning from trailing to neutral throttle), if the mixture is too rich, use a larger slow air jet. If the mixture is too lean, use a smaller slow air jet.

Slow air screw:

Some FCRs are equipped with a slow speed air screw in place of the slow air jet. This is an advantage in that with it, you will not have to keep a supply of slow air jets on hand to effect changes to this circuit. It also allows much faster changes in calibration as you need only to turn a screw as opposed to changing a jet. When running air filter adapters, you may perform changes without having to remove said adapter, which you _will_ have to do with jets since they will not fit through the air feed in the adapter. The following number of turns out are shown with the slow air jet size they are approximately equal to. Note that the change in jet number is not linear with the number of turns out. The screw adjusters are much more sensitive at lesser openings.

Number of turns out

on air screw

Approximately equal

to this slow air jet

1/4
45
3/8
55
1/2
65
5/8
75
3/4
85
7/8
95
1
100
1 1/8
110
1 1/4
120
1 1/2
125
1 3/4
130
2
135
2 1/4
140
2 1/2
145
2 3/4
150
3
155
Main fuel jet:

In an intermediate gear on level ground, accelerate from low rpm. Smoothly roll the throttle open. If the engine will not accept full throttle by 5,000 rpm, it is probably too lean. Select larger jets. If the engine will accept full throttle before 4,000 rpm, it is probably too rich. Select smaller jets. The proper main fuel jets will allow good power at full throttle when accelerating between approximately 5,000 rpm and 9,000 rpm. (These figures are based on an 11,000 rpm redline. Higher or lower redlines dictate raising or lowering the relative tuning points) This will get you in the ballpark on the main fuel jet. >From there, you should select main air jets. Then go back and find a main fuel jet which allows best mph or best power on a dynamometer. It may be necessary to go back and forth between main air and main fuel jet changes a few times to zero in on a combination which allows good performance.

Main air jet:

With the proper main fuel jets selected, if the mixture at full throttle and full rpm is too rich, select larger main air jets. If the mixture is too lean, select smaller main air jets. If, while selecting air jets, you find yourself more than twenty numbers away from the recommended starting point, you should question your main fuel jet selection. Generally (there are always exceptions), once the slow and main air jets are properly selected for the intended application, they need not be changed again. Varying atmospheric conditions can usually be dealt with by fuel jet, fuel screw, and needle changes.

Needle code:

1st letter indicates taper. Letters toward the beginning of the alphabet are leaner (more gradual taper), letters toward the end of the alphabet are richer (more drastic taper).

2nd letter indicates the L1 measurement. This is the distance from the top adjustment clip to a portion of the needle where the diameter is 2.515 millimeters. Letters toward the beginning of the alphabet are richer (shorter measurement), letters toward the end of the alphabet are leaner (longer measurement).

3rd letter indicates starting dimension or root diameter of needle. Letters toward the beginning of the alphabet are richer (thinner diameter), letters toward the end of the alphabet are leaner (thicker diameter).

Needle codes for 28-33mm (90xxx) and

35-41mm (OCxxx) FCRs are listed below.

Code  Taper  L1mm for

28-33mm 

L1mm for

35-41mm 

Diameter mm 
  56.25  72.20  2.605 
  56.70  72.65  2.615 
  57.15  73.10  2.625 
0d 45"  57.60  73.55  2.635 
1d 00"  58.05  74.00  2.645 
1d 15"  58.50  74.45  2.655 
1d 30"  58.95  74.90  2.665 
1d 45"  59.40  75.35  2.675 
2d 00"  59.85  75.80  2.685 
2d 15"  60.30  76.25  2.695 
2d 30"  60.76  76.70  2.705 
2d 45"  61.20  77.15  2.715 
3d 00"  61.65  77.60  2.725 
  62.10  78.05  2.735 
  62.55  78.50  2.745 
  63.00  78.95  2.755 
  63.45  79.40  2.765 
  63.90  79.85  2.775 
  64.35  80.30  2.785 
  64.80  80.75  2.795 
  65.25  81.20  2.805 
  65.70  81.65  2.815 
  66.15  82.10  2.825 
  66.60  82.55  2.835 
Example 1: A 90FTV needle would have a taper of 1degree 15", an L1 dimension of 63.90mm, a root diameter of 2.795mm, and be intended for use in the small FCRs.

Example 2: An OCEMR needle would have a taper of 1 degree 00", an L1 dimension of 77.15mm, a root diameter of 2.755mm, and be intended for use in the large FCRs.

Needle selection:

1st letter: This portion of the needle controls mixture between approximately 1/3 and full throttle. If the engine fails to respond to main jet changes, you may be too lean on this, causing the tip of the needle to be very large. This would tend to artificially limit fuel flow at full throttle, taking control of WOT fuel delivery away from the main jet.

2nd letter: This generally doesn't need to be played with much. If, when adjusting the clip, you find that you are at the 1st groove and still need to go leaner, you may need to select a needle with a longer L1 dimension. If you find yourself at the 7th groove and needing to go richer, you may need to select a needle with a shorter L1 diminution. This would be the last thing I would change as there is usually another explanation for poor performance.

3rd letter: This portion of the needle has the greatest effect on mixture between closed and 1/4 throttle. If the engine is too rich while gradually accelerating through 1/4 throttle, select a needle with a larger root diameter. If the engine is too lean when gradually accelerating through 1/4 throttle, select a needle with a smaller root diameter. Changing this portion of the needle will move the point of transfer from straight section to tapered section higher or lower on the needle with a richer or leaner straight section respectively. Keep this in mind when diagnosing problems near 1/4 throttle after such a change.

Clip position: This controls mixture between approximately 1/8 to 7/8 throttle, with a greater effect on smaller throttle openings. Higher clip position for leaner, lower for richer. You should work to find a needle which allows you to run the clip on the 3rd, 4th (middle), or 5th position.

Float level: Higher float levels richen jetting, lower leans it, but I wouldn't mess with it. I have caused myself more headaches playing with float level on FCRs than any other adjustment of the carburetor. To set float level, with the carbs off the bike and the float bowls removed, hold the carbs in such a manner that the float pivots are above the floats. Tilt the carbs until the float you are adjusting just kisses its float valve, but doesn't compress the float valve's internal spring. Measure 9mm from the float bowl sealing surface to the highest (lowest with the carbs mounted to the bike) point of the float at a 90 degree angle to the sealing surface. Bend the float tab to correct. Once this is set on each carb, reassemble the carbs and mount them to the bike. Connect the fuel line and apply the fuel pressure the carbs will see during operation be it gravity feed from the tank or pressure feed from a pump. Use a sighting instrument connected to the float bowl drain to view the fuel level within the float bowl on each carb. Record levels. Any major anomalies warrant investigation into float buoyancy and needle valve/seat condition. Small differences (one or two millimeters) should be corrected by readjusting the float level on the affected carb.

Float valve size: These should be sized according to horsepower which will be fed by each carb, and whether the fuel bowls will be supplied via gravity feed or a fuel pump (3psi suggested). Do not use 2.8mm or 3.2mm valves with pressure feed, as the increased area of the float valve will allow fuel pressure to overpower the float's buoyancy, flooding the fuel bowl.

Float valve size

in millimeters

HP per carb

gravity feed

HP per carb

3psi feed

2.0
20-28
29-37
2.4
24-34
37-50
2.8
33-41
Not recommended 
3.2
39-50
Not recommended 


 
 
 

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