Installing a Jet Kit
by Justin Neville-Rolfe

Contents

• INTRODUCTION
• COMRESSION
• FUEL SYSTEM
• CARB PISTONS AND DIAPHRAGMS
• IGNITION TIMING
• CARB BALANCE

Introduction:

A dynojet can enhance your Vmax considerably. There is a lean flat spot around 4500rpm designed into the US Import Vmax so that it passes the emmission controls in most US states. This makes it unsmooth in the midrange. The original needle and settings come with a needle that cannot be changed and small main jets. A dynojet kit is normally used to remove this lean action. It has little effect when no other changes are made, but the addition of some competition exhausts and open K&N air filters (or airbox mods -TH) increases the air flow without the increased petrol flow making the bike run even leaner. In these cases, a dynojet is essential to increase the fuel flow to make the bike run OK again. Dynojets do affect fuel consumption, but not much. It will probably knock 2 or 3 miles per gallon off your current consumption.

Back to Top

Things to check before installing a kit if the bike has been running poorly

1. COMPRESSION.

You bike must have the right compression, and it must be the same on every cylinder. The correct compression ratio on a Vmax is 10.5:1. ( Meaured compression should be between 128 psi min and 171 max with no more than 10% variance betwee all cylinders). If you get more than this, it is likely to be coking (carbon build up) on the piston. It is more likely that the compression will be low. It can be caused by several factors
LEAKING THROUGH SPARK PLUG/COMPRESSION TESTER
Check that you have a good fit. Clean the thread and the surrounding area, and if need be, get a new washer.
LEAKING THROUGH RINGS
This is a problem, and you will need to get new rings and pistons. Time for a rebore, which will give you a few extra horse power. To check if the rings are the problem, pour a teaspoon of oil through the spark plug and check compression again, (if compression rises it is the rings- TH).
LEAKING THROUGH VALVEs
(to check the valves, move the piston to TDC and pressurise the cylinder. Listen carefully for air leaking into the exhaust or intake track.-TH)

2. FUEL System

Make sure the bike is completely flat first. The fuel level is checked by finding a clear flexible plastic pipe and fitting it over the drain pipes of the carburetter you are checking. Hang the other end over the handle bars and mark with a pencil the level that the fuel is at when you open the drain screw and allow it to settle. Switch the ignition on, and this allows the fuel pump to fill the carb. Mark the position on again. Draw a diagonal cross between the bolts that hold the diphragm cover on. The level should be between 15mm and 17mm (0.59 to 0.67 inches).

Back to Top

3. CARB PISTONS AND DIAPHRAGMS

The pistons and diaphrams are a system that moves depending on how much air is going through the carb. They are attatched to a needle that moves in and out of a hole. The needle is tapered so that the further out it is, the more the fuel can get out of the little hole. It is essential that the piston moves freely and accurately, else not enough fuel will pass (or too much).

CHECK
Remove the airbox and run the engine. You will be able to see the pistons move in and out. They should be exactly equal to each other.

REPAIR
On the suspect carb (If one of the pistons moves slower than the rest or not at all- TH),

• remove the 4 bolts that you can see on the plate on the outside of the carb.
• Carefully remove the components and the spring. The apparatus is exceedingly delicate. You should be able to move the piston in and out without any significant resistance.
• If there is, clean thoroughly with WD40 and do again. If there is still a problem, get some P1200 wet&dry and smooth off the piston until it does move freely. (note, inexperienced mechanics should buff the piston with toothpaste and talcum powder. Sandpaper can take too much off of the piston and ruin the seal -TH)
  
• Also check the condition of the diaphragm. The diaphragm rots with petrol (well done Yamaha - a diaphragm in a carb that rots with petrol! :-)). If you can see light through it, then it needs repair. Smear some Blue instant gasket over the offending part of the diaphragm and reinstall. NOTE: they cost £60 to replace. The front left carb dictates the air pressure that the black box ignition uses to know how far you have openned the throttle, and so is more critical than the others. You may wish to replace this one. You will find that the left hand side ones are more likely to have rotted as the bike leans over this way when it is on it's side stand.

Back to Top

4. IGNITION TIMING

I will not go into details, but a correctly working Vmax should show the following data:

• Spark Timing BTDC = 3 degrees at 1000rpm
  
• Spark Timing BTDC = 43 degrees at 9000rpm
  
• The timing angle does not change smoothly from 1000rpm to 9000rpm. It remains at 3 degrees from 1000rpm to 1500rpm, then rises rapidly to 30 degrees at 2900 rpm, and then smoothly to 43 degrees at 9000rpm.

If you do not have access to a strobe, then get some-one else to do it. The black box is a sealed unit, and like any computer, it either works or it doesn't. (Jim Furbur at LF Racing can repair -TH) However, the input conditions and what happens to the output are changeable and repairable. The input is controlled by the hose that runs from the front left carb to the black box. Check for leaks.

Check ALL the electrical connections you can find and clean them up. Corrosion at the terminals is the most common cause. Check the plugs, coils and condensors by removing each plug in turn, putting it into the HT lead and resting the outside of the plug on the engine (for a good earth connection). Beware of shock - hold it in a rag. Run the engine, and you should get a good spark across the gap, visible in any condition except strong sun.

Back to Top

5. CARB BALANCE

This is easy if you have the tool. A 4 cylinder vacuum guage costs £40. Instructions for use are enclosed with the vacuum guage. It is an easy job, and a garage will do it for you cheap unless they have a minimum charge.

FITTING A DYNOJET

It is always preferable to get a good (and therefore expensive) person with a dyno machine to do this. However, installation can cost in excess of £200 if they have problems. You can buy a dynojet kit yourself and fit it with patience and only take it to a shop if you f@&k up.

There are 4 things that you can change with a dynojet kit. The pilot settings, the needle position and the main jet size.

• PILOT SETTING: controls richness from 1000rpm to 3500rpm
  NEEDLE POSITION: controls richness from 3000prm to 7000rpm
  MAIN JET SIZE: controls richness from 6000rpm to 10000rpm.
  PISTON SPRING STRENGTH: controls the force that pushes the piston back

(before starting check out carb tuning tips at: FactoryPro

SETTING THE PILOT: (5 minutes)

It should be two to two and a half full turns from fully in. The pilot screw is accessible from the outside of the carb. It is a small screw in a deep horizontal hole below the diaphragm cover. (you may have to remove the brass tamper proof plugs -TH)

SETTING THE NEEDLE: (90 minutes for all carbs)
(work on one slide at a time or carefully lable as the diaphragms MUST go back into the same carb -TH)

• Remove the carb diaphragm cover. If you still have the anti-tamperproof screws on it from the factory either buy an anti-tamperproof socket set or chisel the f@&kers off (or cut a slot in it with a dremel and use a regular screwdriver - TH) and replace with something easier.
• Take out the piston, and put the spring aside.
• Unscrew the assembly from the bottom of the piston. It will separate into several pieces, and do not lose any of them.
• The needle itself has several slots on it that a circlip goes on. The position of this circlip is what controls the richness. The needle position is at it's most lean when the circlip is all the way to the end, and richest at the other end. There is also a smaller washer for half positions. If this washer is placed nearer the tip of the needle, then it makes it half a position richer.

  Reassemble the kit. For reference, it goes: little plastic washer with the teat on it, small washer for half position, needle + circlip, bigger washer, spring, plastic screw. I found that holding enerything together with grease made life easier.

• Repeat for each carb.

SETTING THE MAIN JET: (60 minutes for all of them if no special tool)

•  
• Remove carbs. The best way to detatch them from the rest of the bike is to remove the side electrical covers and the left hand side electrical board and unscrew the plastic cable connector, and separate the carbs from the bike there.
• Turn upside down. (Petrol will flood everywhere) If you have the tool (US dynojet kit), then undo the hexagonal bolt. This gives access to the main jet. Unscrew & replace.
• Unscrew the main jet and replace. (or use a spare needle or toothpick to remove the jet - TH) NOTE: the main jet is the one with writing on it. It will have a number between 150 and 170. 170 is richest, 150 is leanest.

About the " Installing a Dynojet kit author:

I have been working on my 1985 US import Vmax. It is a process that has cost me a lot of money because I take great pleasure in doing it myself. I have f@&ked up a lot, and consequently spoken to a lot of people about my bike. This is a collective summary of what I have learnt either from garages that have helped me or from my personal experience. I am a physics teacher with a degree in engineering, aged 25, in the suburbs of London. (Watford).

CONTACTING JUSTIN:

If you have found this useful, please thank me. If it is factually incorrect, please let me know. I would also like suggestions or alternative ways or looking after a Vmax. I check my e-mail about twice a week, and rarely check the newsgroups.
Justin

Back to Top

Home