Carb tuning.

[Stevethomas]

Hi experts!
My vh was clearly running out of puff on large throttle openings. It was noticeable that closing down the throttle a bit certainly didn't reduce power and may have increased it a little.
I put an air filter on and saw a small improvement.
Changing the main jet from 200 to 220 improved it greatly.
I didn't have a 240 jet but I had a 260. This nearly got it right. I can now believe that a top speed of 85 is possible although I'm unlikely to go that fast being a lot older than I once was!
BUT. It's supposed to have a 200 jet.
Comments please.

[dave.owen]

Are you sure it's not over fueling,ive just reduced mine to 190 after trying a 220.
Worked for me

Dave

[gerry.cox]

Have you verified that the float fuel height is correct? The number and thickness of fibre washers between the float chamber to jet block on a 276 Amal makes a difference. Similarly the washer being present or not under the float needle seat makes a difference on a monobloc.
There are different ways of checking the level which are dependent on the carb model.

Gerry
1949 500 single.

[will_curry]

At first sight it does seem as if the main jet is too small but the engine does need to be revving
well and the throttle needs to be fully open. If all the rest of the carburettor is in good order the
main jet only affects the last quarter or so of throttle. Another thing to check is that the throttle
really is opening fully when the twistgrip is as far round as it will go.

My experience with main jets is that the carburettor will need a much larger main jet if the
throttle is opened at low revs - so much so that once the revs build up the mixture becomes
too rich. Similarly, fitting a carburettor with a larger than necessary choke size needs bigger
jets. In both cases the airflow is too slow to pick up the petrol efficiently.

[nevhunter]

An oversize Bore Carb becomes insensitive to Main Jet size due to Lack of Velocity of the Airflow it the Motor is Slogging say up a hill in ahigh gear Usually backing off the throttle a bit will make it run Better. Ie Bring it back to where the size of the Cutout becomes effective. Usually enlarge the Main jet about 20 for that.
Too small a Carburetter and as the engine revs out it will richen up. This only applies to the top1/2 of travel of the slide. Dropping the airslide (choke) a bit can confirm the Mixture. If it was Too rich it will run worse. To people familiar with AMALS this is OLD Hat. IF you can Locate the Tuning Diagrams which were common showing what happens with What & when It may be of help. Low float level can Make starting more difficult and Idling erratic. Have the Idling slightly RICH of"Nice" . (slight rev drop with bike vertical). Nev

[tony.barnett]

Putting on an air filter will reduce the air flow, meaning you need smaller jets ands settings. not bigger. I'd go with Dave - are you over fuelling on the main jet?

[SEDoan]

Monoblock or remote float carb? Monoblocks usually need larger mainjets. The listings below are from the AMAL application chart.
1954 Ariel NH = 150 (remote float carb)
1955 Ariel NH = 200 (monoblock)
1954 AJS 18 = 180 (remote float carb)
1955 AJS 18 = 250 with air filter and 260 without (monoblock)
1954 BSA B33 = 200
1955 BSA B33 = 260
1954 Norton ES2 = 160
1955 Norton ES2 = 270

The same chart shows Ariel VH to be 200 for both carbs so maybe it's wrong???

[david.anderson]

The question is does the bike have the original carburettor or has a larger carburettor, or a bored out carburettor been fitted. It was fairly common to bore out the original carb, which always meant a bigger main jet. On one VH I run a 32mm concentric which I have bored to 33mm which is the maximum for the 932 concentric body without a slide problem and I have a side bowl 289 carb which some previous owner has bored to 1 3/16”. It was a fairly common hot up here at least.
The HS ran a 389 1 3/16” monoblock fitted with a 380 main jet for its open exhaust.
The carb type and size needs to be confirmed
David

[MarkO]

Assuming your issues are carb related......

Put it all back to standard using the correct carb, needle jet, needle, main jet, and pilot jet.

I'd be tempted to renew the last 4 items - cheap enough. The difference can be quite noticeable. My 1931 500 Sunbeam now cruises at a happy 55/60-ish rather than the previous 45/50-ish.

Check if the needle valve is passing - replace if it is. Clean out the seat while you are there. Check the float is not leaking.

Set the float level correctly

Check cut-away in the slide is suitable

Check the slide does not bind or is too worn

Make sure there are no leaks on the flange to the engine. Do not over-tighten....

Did it have an air filter originally? - if not, bin it.

Clean it all out - especially all small orifices/jets/tiny holes in the carb body, and set it all up as per the book.

Fiddling about with carbs when there may be simultaneously many variables/unknowns in play is a fruitless exercise.