Good carbs, bad carbs, the glycemic index? I don’t think so! No, not an article on weight loss, although
get it wrong and you may lose weight through stress and frustration, this is about the liquid and
gaseous bit of the go faster equation.
Going back through the years your trials machine came most likely fitted with an Amal or perhaps an
Amal or in some rare cases an Amal. It was made of zinc alloy with a host of brass fittings and for the
most part was functional when new. A few had choke mechanisms but for the most part they did not
and the sequence to start was to flood the float bowl until fuel squirted out and went all over your nicely
polished cases. A couple of kicks and the motor would fire but would sometimes need the “tickler”
held down until the cold engine could cope with it all. The “tickler” as they were always called was of
course no such thing but actually a means of depressing the float in the bowl to allow even more fuel
in until it escaped through the overflow. I have seen several frustrated riders punch the tickler up and
down furiously and in the worse case I have known some who actually punctured the float itself.
The Amal “tickler”
Looking at some basics we need a super enriched mixture, and I’ll talk about the “other mixture” later,
of fuel and air to get the machine started and then as it warms up we can reduce the amount of fuel as
the engine will now fire on a vapor produced by Venturi and Mister Bernoulli in the bowels of our
The following is offered in pure layman’s terms so that at least the “shadetree mechanic” has an
understanding of what is happening and where to go if the machine doesn’t ! Back to some real basics
then in the fuel system. We have our fuel tank, a petcock, hopefully some sort of other in line filter, the
carb itself and some sort of breathing device of air cleaner or filter.
In order for good performance all the ancillary parts also need to be up to snuff to have that fun day
out so lets look at some common problems starting with the tank. Is it clean? was this machine a
“barn find” with a steel tank ? A quick inspection and a rattle or two may have a whole handful of rust
deposits in your hand and of course if you fill up this unit the rust flakes have only one place to go
and that will be through the petcock ! So a good cleaning of the tank is a prerequisite and while working
on the tank we may as well undo the petcock and check that over for functionality and leaks. Most
petcocks are of a 3 position system with ON, OFF and RES. Upstairs the petcock will have its own filter
of gauze often soldered together and of course through the years this may have got bent, ruptured or
have been removed altogether. The fuel cock allows the fuel in down to a set level and when you are
getting low it will let you know and you switch over to RES to get some more from the lower levels. The
cock often has a rubber washer with holes drilled in it to allow the fuel to pass as the lever is moved.
The rubber often becomes hard or the holes get “pinched” and fuel now dribbles everywhere and can
be annoying. Sometimes all that is needed is to let everything warm up and let the fuel penetrate the
rubber which will then swell and the leaks will go away. Reminds me of flying the F4 phantom after the
Christmas break and on day one the flight line was a mass of leaks from all systems until they all
magically sealed themselves after some prolonged running.
If you experience poor flow from the petcock, or none at all, then most likely our little piece of rubber
with the holes in it is all gummed up. You might be lucky and get an airline on it and be able to blow it
through but the gummy resin can often set rock solid and needs a little more persuasion to get it out.
Some old dentists tools are useful for this operation.
Fuel lines and filters are inexpensive so I always junk what is on the bike and start afresh. You may
find that some of the modern tubing needs a little stretching to get over the carb fuel inlet which is often
bigger than the petcock delivery side. Here I use a suitable Phillips screwdriver and put it in the pipe
overnight so that it will ease over the carb union when I’m ready.
Don’t forget the fuel cap and the vent system. Cork sealing washers may have shrunk or dried out or
may have been lost and never replaced and like the fuel cock the vent system may benefit from some
high pressure air to blow out any grot and prevent vacuums in the tank.
…… and finally here we are at the source……. THE CARBURETTOR ! Now I know a lot of people like
Amals and if you are riding the Pre 65 Scottish then you are more or less forced to use them………… but
if there is a choice then depending on machine, in my opinion, the only carbs that give the performance
that I want are the Mikuni and the Del Orto.
The Mikuni VM 26 idle screw side
Strangely, and perhaps we should mention it to the Pre 65 Scottish purists, Mikuni used to make Amal
under license for some time post WWII, so they too made Pre 65 carbs! Anyhow, rather than mess
around trying to rebuild your Amal, which is generally lifed at 5 years why not save yourself a lot of time
and trouble and buy a Mikuni correctly jetted for your bike and application. If I’m rebuilding, then unless
the carb works first time after extensive cleaning I fit a new VM26 jetted with a 150 main and a 30 pilot as
my datum for just about all 250 motors of the the 1970 vintage. Of course my bikes have all been
predominately Yamahas and I’m riding between 2000′ and 10,000′. Go higher in altitude, go lower in jet
The effects of altitude and temperature, wherever you are on the graph
multiply the result with your jets to get the correct one ie; 150 X .92 = 138
so fit a 140 main and from a 30 pilot come down to 27.5
While dealing with just the main jet if you are using this VM 26 on anything other than a 250 motor it will
need to be increased on a 2T if you increase engine size and decreased if you are using it on a bigger
4 stroke. To give you an idea the Yamaha Majesty 320 needed a 290 main whereas the 350 Enfield
takes a 90.
Now before entering the world of “fine” tuning lets take our brand new VM26 which was purchased
prejetted for trials. It will have the 150 or 160 main fitted and a 30 or 35 pilot. The needle clip will be in
the central position and the slide will most likely be a 2.5 cut off. The mixture screw will be around 1 to
1.5 turns out on this carb as it controls AIR. If the carb you are using has this screw located towards the
front then it controls FUEL and works in the opposite sense.
The needle and clip positions
Assuming all other factors are good, such as ignition having been correctly set and all units fully
functional, a new or newish plug correctly gapped and some fresh fuel mixed at 40:1 we can begin
tuning once the engine is warmed up. With experience you can do this by “feel” but another way is to
adjust the IDLE screw to a faster than normal setting and now screw the AIR screw in until it bottoms
out lightly. Then back it out to the datum of 1.5 turns, so note where the screw head is when it bottoms
and come out in 180 degree portions. Again allow it to idle and then adjust until you get peak revs with
the airscrew. You can now reduce the IDLE screw to your preferred setting which in my case is zero.
Now you can snap or flick the throttle open and check for that critical response off idle, if it isn’t instant
and perfect you may need to change the PILOT jet. If you have opened the airscrew more than the 1-1.5
turns then your pilot jet is too big and you can’t get enough air in the system so come down 5 on the
pilot and try again.
A simple diagram showing the airscrew and pilot jet
For the main jet or WFO this really only comes into play at settings above 3/4 throttle and is not often a
player in trials but if you are having problems in this area you can take off the air filter and then mark a
datum on your twistgrip for closed, 1/4, 1/2, 3/4 and full. Put the filter back, take her out and see where
the problems occur. If the problems are in the 3/4 region change the main, if somewhere else it will
most likely be a combination of needle jet and clip position.
Now for the most common trials related problems:
Bike refuses to start = Blocked choke or fuel enrichment circuit, remedy clean and blow through with
Bike starts but will not run cleanly without choke = blocked pilot jet, remedy clean and blow through with
compressed air or change the jet if it is still
Poor pick up off idle = bad airscrew setting, adjust
Poor performance at altitude = incorrect jets and airscrew, change and adjust, or lower the needle by
raising the clip position
Poor performance = old fuel! the newer gasoline mixes from the pump seem to go “off” after about 2
months so start afresh.
Erratic performance = water in the float bowl, remove the bowl and see if there is the globule of water
running round like a ball bearing in the fuel caused by over zealous washing!