750 91 - 97 Carb Balance Guide.

This guide is for the 750 models and covers the 91-97 model years. There are some differences between the FL-FP and FR -FV models that will be covered.

Firstly, remove the left and right centre panels. If you can't do this then step away now.

Once they are off you need to locate the four vacuum take off points. On the FR-FV they are brass tubes with a rubber cover and look like this.



On the FL-FP models they are a screw and washer. The screw is made of butter and is easily damaged. You need a long philips screwdriver at least 18" long. Insert the screwdriver and give it a couple of sharp taps with a hammer, then remove it. On the FR-FV remove the rubber cover by pulling it off with a pair of long nosed pliers.



They now look like this.



If you have an FL-FP you need to screw in some take off adaptors, which will have been supplied with the vacuum gauges. Do not over tighten them, finger tight is plenty. They should look like this.



If you are planning on keeping your bike it is worth considering changing the screws for the later FR-FV take off. I did this on my FL and seem to think it was not expensive to do. It certainly makes this job miles easier.

Now, on to the vacuum gauges themselves. I am fortunate enough to own both dial gauges and a Carbtune. Of the two the Carbtune is far easier to use, is more accurate and has much less flutter on the scale.



Now connect the gauge. Connect the four rubber tubes to your vacuum gauges and in the case of the Carbtune, hang it vertically from the left handlebar grip. It is much easier if you connect the tubes 1-2-3-4 left to right then you can tell at a glance which cylinder needs adjusting.



Now, start the engine and run it until it will tick over without the choke and adjust the idle to the recommended settings. For FL-FP models that should be 1200rpm +/- 100rpm and on FR-FV models 1000rpm +/- 100. This is for UK spec bikes so check what yours should be if you are from another country.

You will now see something like this.



The idea is to get all four vacuum levels as close to identical as possible. You do this by adjusting three screws underneath the carb bodies. They are an absolute pig to get at and for many years I did it using a very short flat blade screwdriver. If you choose this method then be prepared to burn the back of your hands when the engine casings get hot. On the FR - FV models there is one screw at the rear that adjusts No3 and two at the front that adjust No2 and 4. No1 (Rear left) is the base cylinder and cannot be adjusted. On the FL - FP models there are two adjusters at the rear and one at the front that adjusts No4. No2 (Front left) is the base cylinder and cannot be adjusted.

I have taken a picture of the No3 adjuster that looks like this.



This picture was taken from under the engine looking up and gives some idea of how well hidden they are.

I was lucky enough to find a screwdriver perfect for the job. It's about 18” long and has a 90deg bend at the tip. It makes adjustment a doddle. I have had this for six or seven years now and can't for the life of me remember where I got it from.



Adjust whatever screw requires it a VERY SMALL AMOUNT at a time. Remember that any adjustment on one cylinder will also have an effect on the others. This is where that screwdriver comes in handy. After each small adjustment blip the throttle slightly to reseat the throttle cams on the adjuster screws. Remember to keep checking that the idle speed is correct after each adjustment.



Keep at it until you see this or as near to it as you can get.



That's it job done, put everything back together and give yourself a pat on the back.

It may seem as if you will have to shell out a few bob to get started, well you will, however the cost is recovered by the first two balances that you do yourself.

 

Nice thread, good job. That's why I had the shop do it. Honda has a special tool that does those screws figures huh? . and it's like $70.00

 

Nice write up! Anyone have one of these tools in Ca. ??? I hear they work good to set FI throttle bodies too! We can have a Carb Tune / Sag Setting BBQ !

 

Motion Pro make the screwdriver I have and it is avaliable here for $29.40:

http://www.motionpro.com/motorcycle/tools/pilot_screw_adjusting_tool/

 

This is excellent - many thanks for sharing. Can I just remind people: when taking Keihin carbs apart - The "+"
head screws that Keihin (and Mikuni) use are NOT Phillips or Pozi-drive heads. They are "Japanese Industrial Standard" (JIS) heads. This is why we all come across so many mangled screws! Definitely worth buying a set if you are taking carbs apart.

 

found these screws on a set of VTR 250 carbs sent to me for cleaning ! not quick or easy to get these 4 out. amazing innit ?? somebody put these damaged screws back in instead of replacing them.



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i've developed some of my own ways to bench-synch carbs very closely and scientifically.

 

@OP thanks a million for this awesome write-up. I created a forum account specifically to thank you.

This tutorial (and adjusting idle at hot) helped me fix my issues were the bike would not properly di engine braking when coasting and a much too high idle when hot.

Too additions I have is: get a specific pilit screw / carb adjustment screwdriver for motorcycles. Bought mine online for about 20 euro. Makes this job much easier.
Also get the long philips screwdriver if you you an older vfr model like me. I bought the longest one i could find and ik made removing the screws out of the carbs much easier whilst not destroying the screws.

One thing I did was loosening an allen screw and size 14 nut on the right hand side to remove the rear cilinder carb screw. It is some bracket that seems to hold something like a thermostat or whatever. It was blocking a direct path to the screw.

Again thanks a million and ride safely!

 

Nice write up, I glossed over the pictures, two game changers for me besides working on these bike many years was using the Morgan Carb Tool, having a 90 degree screwdriver for the screws and using an appropriate torch/LED light in the bottom of the V. In addition to all this stuff, you have to have a nice nurse bottle and my Handy air lift table takes any aches out of your body/mind. Last time I did this process, it took literally 5 minutes to get them perfect. Mechanical gawds where on my side. Peace