Carb Balancing ...

[PeterM]

Most of the engines I've ever fiddled with have only had one carb - so simple stuff,

But the Yams of course have three...

Just to clarify when you 'balance' them - is it just a question of getting the throttle butterflys set to the correct angle/opening to allow each carb the same 'suck' as its mates ? (Allowing for the one that has a 'hole' in it - Edit - think thats the choke one anyhow - Maybe to allow one cylinder to start easier ? I've no idea !)

Current thinking is it cant be much else ? Mixture screws to be set at 1.5 turns out on each and idle speed screw set of the middle carb adjuster ?

Pete

[Locozodiac]

Most engines have fixed throttle rods, theoretically all carbs must have same mixture screws turns, if wanting state of the art accuracy, connect one vacuum gauge to each cylinder manifold to adjust all 3 carb mixtures same, finally adjust idle rpm screw to factory working parameters. There are vacuum gauges in digital versions on the market with led lights readings for banks of 3-6-8 individual cylinders.

Happy Boating

[jwalker]

Originally Posted by Locozodiac

..... connect one vacuum gauge to each cylinder manifold to adjust all 3 carb mixtures same, ....

Erm, unless something has changed, the balance is achieved by adjusting the throttle stop screws individually then locking the throttle spindles together.

For info, you can do a pretty good job by sticking one end of a tube into yer lug and placing the other end at the carb mouth. Listen for the hiss frequency to be the same.

[nugent]

Simple enough with one of these I swear by it

[PeterM]

Originally Posted by nugent

Simple enough with one of these I swear by it

Handy - but at £35 ish I'd probably only use it once every four years....

Wonder if I could borrow one ..from someone ...hmmm

But basically am I right ? you are just checking and correcting the suck on each carb by tweaking the butterfly position ?

[mister p]

use a rubber tube and get the suck noise to be the same

[nugent]

Gunson G4053 Carbalancer

Internal combustion engines use a mixture of air and petrol to generate their power. The performance therefore is influenced by the air flowing into it and the petrol that is metered out by the carburettor. Unless the air flow in Twin or Multiple carburettors is correctly set, the correct mixture adjustment is unlikely to be achieved. Suitable for use on SU, Solex, Auto-Lite (Formoco) Stromberg, Weber and Zenith and can be adjusted to fit most carburettors including side-draught, down-draught with air intakes (Venurei) from 25mm to 67mm (1”-2”) diameter
Easy read scale indicates the air flow and allows
Provides smoother and better acceleration
Carbalancer can be adjusted to fit most carburettor
Easy to follow instructions
£14.00

[nugent]

Cheap enuff?

[Portholme]

I posted on this topic earlier this year on a yamaha forum and someone responded with a description and shots of a home made vacuum gauge. In the end I got a mate who's in to four carb race cars to lend me his. It's a very easy job, but stripping, cleaning and replacing gaskets on four carbs took about eight hours. on my f100 you just balance the top three to whatever output the bottom one gives. I can't begin to tell you what difference it made to the boats performance.

The home made balancer used rubber tube nailed to a piece of wood in a u shape with water in. will try and dig it out for you

[mister p]

well it's only half of £35,
I reckon a tube at £0.75p is better.

[uncle al]

portholme, it is called a manometer I used to use them for running in Cummins diesel engines after o/haul in the 1960s, a plank of wood, a clear plastic tube with a measure both fixed to the wood put a food colouring dye in the water so you can see it easilly, there you are a manometer

[Peter_C]

Carb tuning is an art. It is a balance between mixture, and the idle speed adjustments. Add in multiple carbs and now you have to balance them too. Plus get the throttle rods connected back up after adjustment. Be gentle on the throttle disconnects. It is not a matter of all of the mixture screws needing to be 1.5 turns. One may be 1.7, another 1.9, and the other 2.0. Wear, over tightening, and some design differences account for some differential in the number of turns, so don't get hung up on it. I usually prefer to start on the rich side, and turn them leaner (IE: start at 2 turns, before ever starting the engine). You will hear the engine RPM's increase as you get leaner, until it quickly falls off as it gets too lean. (A good tach is very nice to have too). Being ever so slightly rich at idle is better than too lean. Trying to hit the stoichiometric balance at 14.7:1 is ideal, but next to impossible without a wide band O2. Plus carbs just aren't that accurate.

A tool like the top one shown sucks. Trust me I own something similar. A manometer is the best tool for the job. Total cost should be less than $5. I could walk out in my garage and build one for free, the parts are that simple. I built one a long time ago for doing motorcycles, and they work great. Google "Building manometer" and all the information you will need is available.

Some carbs do not have the fittings already installed and you might need to install some. Just plug them with a rubber plug when finished. There should be a flat spot to drill out if there are not any. I keep brass tubing (Along with stainless) in my tool box for doing just such adventures, and get them from old broken car antennas. I have bought a few different sizes too. You just never know when you are going to need to sleeve a fitting or make something. Part of fixing stuff, is some times you break stuff I have seen folks use welding tips too, which create a nice restriction. Use your imagination.

[camelgas]

+1 for the gunson. cheap and effective, mine has done V8's, bikes and outboards, and must be 25 yrs old by now and still works fine.

[Bushrider]

Originally Posted by Portholme

I posted on this topic earlier this year on a yamaha forum and someone responded with a description and shots of a home made vacuum gauge.
The home made balancer used rubber tube nailed to a piece of wood in a u shape with water in. will try and dig it out for you

Hi ,
In post 5 I showed the selfemade tool I use for carb synchr- but there´s no water but ATF oil in .
Works just great for me .

Yamaha 4 stroke F80 carb synchronization - Yamaha Outboard Parts Forum

[Peter_C]

Originally Posted by Bushrider

Hi ,
In post 5 I showed the selfemade tool I use for carb synchr- but there´s no water but ATF oil in .
Works just great for me .

Both water and ATF work. Both will pass thru a motor should they be accidentally sucked in, in small amounts. Both will clean the carbon out if they are sucked in. The benefit of water is, it tends to not creep, which ATF is designed to do, and water will settle out quicker. If spilled water doesn't make as much of a mess. I prefer water with food coloring myself, but too each their own

[Seabird]

Try the old method,

If the idle mixture screws have been tampered with, start from basics.

Clean the carbs and fit a new set of plugs

1 first check how many rotations you can turn each mixture screw in to the stop (mark each screw with a dot of felt tip marker so that you can count the rotations and if they are all the same then they may be OK)
2 If the idle screws are all different then reset them to 2.5 turns out (all the same)
3 One of the carbs will have a throttle stop screw (use this carb as your set up carb)
4 Start the engine and increase idle speed to about 1200 rpm
5 whilst the engine is running, remove the plug caps on the two cylinders not running on the cylinder with the throttle stop screw and check the rpm when the engine is running on the one cylinder
6 screw the throttle stop screw in to maintain this rpm and lock the position of this carb
7 then connect another plug cap to the next cylinder and remove the plug cap from the first cylinder
8 adjust the carb link arm until you get the same rpm as on the first cylinder
9 then repeat process on the third cylinder

The carbs are now balanced, connect all spark plugs and adjust the throttle stop screw down to the tick over rpm. (Note if there are throttle stop screws on all three carbs then back two of them right off so they are not functional, you only want one throttle screw to control all carbs)

When the rpm is set to tick over on the throttle stop screw, if the engine is lumpy, the adjust the mixture screws by 1/8 - 1/4 turn at time trying both in and out to see if you can smooth the tick over (ADJUST ALL SCREWS AT THE SAME TIME BY EXACTLY THE SAME AMOUNT)

This will get you into the right place and the engine will be fine.

Should take 20 minutes to complete