Hi , I hope some of you Tiger Cub Guru`s can help here. From running on idle,when asking for that initial burst of power, my friends Cub just hesitates, & has a serious flat spot. He stripped the carb to find out what jets were in there ;

the carb is a amal 622

needle jet 106

main jet 100

slide cutaway 3

needle has no numbers but has two small rings round the top if that means anything

Has anyone any idea`s or experience of this problem?

I've a British carb on my cub, made near Whitby ( about 6000 miles off shore in J***n) for just this reason the amal is renowned for the flat spot off the bottom and is going to take some sorting, unless the clubs you ride at absolutely insist on British go to that other famous coastal resort on the south coast and get a 20 or 22 phbl Dellorto, now they do work very well

Amal Concentric carbs are different for 2-stroke & 4-stroke though they can be easily changed.

The needle you have with the two rings around the top is definitely for a 2-stroke, the tapers differ on 4-stroke needles.

It is widely accepted that 199cc Cubs do not run well on the Amal carb & this is why many clubs Yorks Classic etc do not insist on them being used.

When a Cub is overbored 230cc/250cc etc then they seem to run well on the Concentric carb.

Contact Surrey Cycles or Hitchcocks for advice on carb parts either one will be really helpful.

There are three essential components to an engine running properly to respond to throttle demands thru the rev range, viz, sound mechanical Engine condition, correct ignition and clean carburation - pretty much in that order.

Remember Triumph Works riders rode Cubs with Amal carburettors, as did hundreds of us who were Clubmen - I would personally recommend the following checks before swapping carburettors.

1. Compression is good, know what camshaft is fitted, correct valve clearances, sound mechanical engine ?

2. Ignition timing is correct BTDC, and ignition source reliable, viz, solid state, battery, or energy transfer, correct spark plug ?

3. Carburettor fitted correctly with no air leaks, correct needle and needle jet not worn, needle clip set in middle groove, mixture adjustment 1 1/2 turns out, good slide and aircleaner ?.

The first task is to ensure the above list checkout OK, as they all add up to provide the good throttle control required to get the best from a four stroke Trials engine.

You can experiment with a smaller throttle slide cutaway, and also remove the springs in the auto ignition advance unit (if you have points on the RH side cover) Cubs experienced flat spots and spit backs also due to wear in the auto advance assembly, the fix was to lock it up which produced good results.

Hope this helps you get it sorted Brian H

Do not set your idle too low.

I set mine to a fast tickover and use the clutch when necessary (not often).

This then masks the flatspot and make the bike less likly to spit and stop.

I also use fixed ignition advance for the reasons above. (PVL is best)

Edited March 9, 200719 yr by mattylad

Like Totalshell my cub ran best on a TL125 carb. These are hard to find, but with all these pit bikes about now - small carbs are everywhere now (see Eblag).

I understand there's a Villiers carb that works & is still within the rules. However - rules differ & it may be worth having different carbs or different series.

Memo to self .....must resurrect that cub.....

Yes, i use a 19mm Villiers carb on my 250 Cub- modified to suit by Mick Grant. It runs faultlessly, better than the 20mm Dellorto it replaced,

Thanks very much for all your help guys, I thought this might be quite an involved job,my mate is planning to do the scottish 2 day so I guess the Delorto is out of the question, so does anyone know what carbs would be permitted or failing that, has anyone got a similar carb on their cub & is running ok? if so what jetting/needles etc etc do you use, the ignition is a new pvl from Millers so I would at least think that that should be OK,although I know even new products can give problems, but unlikely I would think.

Take a look at 'Bushman Carb Tuning Secrets'. It explains the inner workings of a 22mm Concentric. Some people drill out the fixed pilot jet and replace it with the old screw in type, but in my 200cc Cub leaving the fixed jet in place AND fitting the screw in type improved my slow running and pick up. It all takes a bit of trial and error but does get a result eventually.

Spoke

just for the record a mate of mine just put a dellorto on his cub, the cosidered norm would seem to be 22phbl but as his was fitted by Martin Adams I thought I would include the details here

PHBL 20

40 slide

D40 needle

B2260 atom

45 choke

35 pilot

118 main

these 'foreign' carbs are specifically ACCEPTED by ACU classic series, Red Rose Classic, Yorkshire Classic

they are specifically NOT ALLOWED by Miller series or in the Pre 65 2 day Scottish