850 Mk3 Set up with 230 main Amal jets?

acotrel · Oct 15, 2018

watson rider said:
I agree 100% with Fast Eddie, go read the tuning brochure that comes with any new AMAL carb, the reason why they [AMAL] tell you to tune from main jet down is that main jet size DOES have a marked affect on all other settings, these are the folk who designed and manufacture the things, they should know what they are talking about. Possibly the reason why you chaps from the US have a bit of an issue dealing with these critters, is the popularity of Mikuni,s and Mikuni conversions over there, which are tuned from the bottom [idle-circuit] up, which works a treat on them but does not work/ apply to AMALS.

I suggest the main jet only has a marked effect on the other jets, if it is TOO LEAN - NOT if it is TOO RICH. If it is too rich, the other jets do the metering and when you use full throttle you get TOO RICH which often does not matter unless the main jets are so large, that they are silly. Perhaps you are suggesting there is an increase in flow through the other metering jets, if the mains are large ?

Fast Eddie · Oct 15, 2018

If it’s as simple as that, why do Amal instruct to set the main jet first?

Frankie17 · Oct 16, 2018

SIMPLE TO AVOID LITIGATION too small a main jet equals burn pistons

carburettors can be supplied jetted for specific machines or just by bore size so by reccomending start with main jet they place responsibility on the installer

watson rider · Oct 16, 2018

Yes and too big mains equals fouled plugs poor running/frustrated /annoyed customer who has just spent their hard earned on a new set of carbs.Any one is not prepared to or capable of doing what Amal instruct,that is setting the main jet size first, either by the time honored " plug- chop" or by checking the fuel/air ratio with the correct meters/ equipment,is only going to waste a lot of time getting no result.Also the need to check/ correct float level is vital to the correct set-up of these carbs, something that is now very simple thanks to the "Stay-up" adjustable floats.

Tinkerbell · Oct 17, 2018

Hi Everybody,
I have returned to Amal OEM 260 mains and the plugs (new) are not sooting--but are running towards a
chocolate color and should be more tan. I have put the needle groove at the top of the needle to run leaner and will see what things look like. BTW , I would not buy Wassell EVO carbs again. They work, but there are too many dodgy things about them .

Regarding Wassell EVO: When I received them new (1) one pilot air jet was plugged with swarf. (2) one of the choke springs had so much flaky plating on it , that I used the old Amal spring. (3) at 1-1/2 turns out on the pilot mix screw, one screw is flush with the carb casting body and the other is about .030" proud. On my old beaten Amal 932s , at 1-1/2 turns, both screws were proud of the casting by the same amount. It does not
mean that the Wassells are broken, its just that their QC is non existant. They do not inspire confidence that they will deliver the best from my 850, and it was the dumbest $100 I ever saved.
Sincerely,
Kara

acotrel · Oct 19, 2018

Fast Eddie said:
If it’s as simple as that, why do Amal instruct to set the main jet first?

Normal approach is to fit the biggest specified main jets for the particular bike, then adjust the needles and needle jets - finally get the main jets right by doing plug chops. As long as the main jets are too rich while you are setting the needles and needle jets, you won't have a problem. However if you are doing plug chops and increasing the main jet sizes does not richen the top end, you are metering off the needles and needle jets - which is wrong. At full throttle the metering must be done by the main jets. Metering off the needles and needle jets at full throttle is too unreliable. There is a risk of burning a piston or valve, when you allow that to happen.
You will notice that there is a wide range of needle tapers being used in a variety of four-stroke motors - that is a reflection of the different vacuum conditions which exist. In some motors the advance curve is adjusted to provide some compensation, so that constant combustion conditions are maintained. It is possible to get better acceleration up through the gears, by playing with the various needle tapers and the advance curve. But in the end, if you are using full throttle - the use of the main jets and plug chops is necessary, particularly if you are going to ride at full blast for any length of time.

acotrel · Oct 19, 2018

Because you guys are playing with petrol, and errors you have in jet sizes are twice as critical compared with what I encounter when I use methanol. Half a thou of an inch error in a needle jet size when using methanol, is enough to make the difference between fast and slow. So when you use petrol, it is extremely important that the needles are needle jets do not exhibit any wear and float levels are critical.

Tinkerbell · Oct 22, 2018

Hi All,
I did a plug chop off the freeway and the 260 Amal jets certainly were not looking overly rich . The chocolate appearance (around town) disappeared in 8 miles of freeway. The needles are in the leanest setting. I may want to set them to the middle--but I feel better with 260 jets considering the Peashooters and K&N filter.
The riding season is coming to a close here in Vancouver WA and the next adventure begins:
Pull out the gearbox and go through it. I already have the phenolic layshaft bearing from Mick Hemmings.
Thanks to all for the responses on the jetting!

acotrel · Oct 23, 2018

When you do a plug chop, the part of the plug which is important is the porcelain, right down inside where it meets the metal. There should always be about a 2mm wide black ring there which indicates the level to which the soot has been burned off. The heat range of the plug affects where it will occur, so it is wise to use a hotter plug when tuning. That way you stay safe.

ewgoforth · Oct 23, 2018

Tinkerbell said:
Hi All,
I did a plug chop off the freeway and the 260 Amal jets certainly were not looking overly rich . The chocolate appearance (around town) disappeared in 8 miles of freeway. The needles are in the leanest setting. I may want to set them to the middle--but I feel better with 260 jets considering the Peashooters and K&N filter.
The riding season is coming to a close here in Vancouver WA and the next adventure begins:
Pull out the gearbox and go through it. I already have the phenolic layshaft bearing from Mick Hemmings.
Thanks to all for the responses on the jetting!

I found that 260s were too lean on my 1973 850 with twin 932s. I saw this when holding WFO two up on a steep hill about twelve years ago. Bike ran out of power above 3/4 throttle feeling like it did when i run out of gas and have to turn on reserve.

I went with 280s and the bike ran better up the hill the next weekend. I did a dyno run a month ago and I was around 12 to 12.5:1 at WFO on a 90 degree F day. I understand that 12.7:1 is ideal for an aircooled engine. This seems to confirm that 280s are still a good choice for my bike.

By the way my dyno run was with one fuel tap open. It seemed to flow plenty of gas at least for as long as my pull lasted.

Eric

acotrel · Oct 23, 2018

Have you checked that the breather hole in the fuel cap is clear from obstruction ?