Carb Probs

[jamestee]

Looking to pick the brains of 2 stroke guru's. Have a Villiers 250 fitted with a Marcel barrel (trials) and am having carb problems. The engine seems to run rich, and to get a 'normal' plug reading, and no oil in the exhaust, it needs a 100/110 main jet. With that fitted, the needle in the top ring, and a No4 slide fitted the engine runs ok at all settings above 1/8th open, ie just a little rich looking at the plug (GKN BP5ES) and 2 stroking as it should. Now the difficult bit! With a 25 idle jet I can get it to tick-over, but at low throttle openings, ie less than1'8th, the engine 4 strokes. If I open out the air screw to where it will 2 stroke, it will not tick over, except at a very high and a little erratic rpm. Trying to get an acceptable low rpm and a 2 stroking engine so far seems impossible. I've tried dropping the pilot jet down to, 22 then 20, even 15 without being able to get it to run right. Running it with the tick-over setting and at low revs with 4 stroking the plug is wet as you would expect. Running at low revs with 2 stroking the plug looks dry but a bit black. But can I get it to tick-over and 2 stroke at low revs, can I heck. I've even tried using a hotter plug BP4ES, but with out success, and it is difficult to start as you would expect. The carb sits vertical on the engine, and the float level has been dropped from the Amal reccommendation in order to get a leaner running engine and reduce the richness.

So does anyone have any suggestions?? All will be welcome, and be aware I've got so jets and slides I could open a shop, so give me a setting and I'll be able to set it up. Many combinations of jets, slides and needle positions I have tried without success. Oh, and it is a 2 stroke carb with a 2 stroke needle, confirmed by Amal who supplied it. Yes it is new!

[cvgmmartin]

It sounds as though you have the right pilot jet with the 25 jet, but as soon as you open the throttle it runs rich (and the needle is already in its lowest position), so perhaps you need to try some other needles that have a slightly larger tip diameter to lean it out at small openings.

[jamestee]

Hi CVGM, that did not occur to me. Problem is that Amal provide only one 2 stroke needle, a 4 stroke needle, a special 4 stroke needle, and an alcohol needle. I most certainly have the 2 stroke needle, however I will talk to the Amal tech people in relation to you suggestion and see what they say.

Thank you very much for your input, much appreciated.

Regards jt.

[oldmonty]

I had big problems with richness on my Montesa and the first thing I checked was the enrichener (choke). As it looked OK, I changed virtually everything else without success, finally going back to the enrichener. I changed the neoprene seal in the end of the plunger and it made a huge difference. I had to change everything back to the original settings and the bike now runs great! When an old bike sits for a long time with the enrichener closed, ie. choke off, the seal apparently takes a "set" and and allows fuel to leak past, so now I always park it with the choke "on".

Works for me!

[jamestee]

Hi Monty, thanks for the input. Though I don't suppose my prob is exactly the same as yours it will be something similar, the job is to find it!!

[cvgmmartin]

It seems a little curious that you have the needle in the leanest position (top groove) and you have a large #4 cutaway on the slde which should also make it lean yet it runs rich at 1/8 to 1/4 opening. It makes you think there is something odd happening. Could your needle jet be worn? You say you have a number of spare jets and slides so perhaps try a few substitutions. It would be very helpful if some member with the same engine/carb combination could tell you what they are using.

[old trials fanatic]

Not running a Marcelle barrel just a Greeves alloy square one but if it's any help i am using an Amal Mk 1.5 27mm bore and use a 2 slide and a 120 main jet. Cant remember if i changed the pilot or not sorry and the bike is at the back of the shed buried at the moment so i cant strip the carb to check.

It does sound as if the float may be at fault as i dont know anybody else who is running a 4 slide and still finding it rich ? Weird.

If i was you i would borrow another carb off somebody and try it with that on.

[pierre66]

is the air filter blocked?

i had a two stroke a few years ago that would run ok but if you neglected to keep the air filter spotlessly clean it would four stroke, unlike yours it would play up at higher revs and when the engine had a bit of load on it, clean the filter and it would be ok again.

looking back there were probably other issues at work here but cleaning the filter always seemed to cure it so.........

[jamestee]

Hi Guys, and thanks for all your input which is much appreciated. It is completely weird. The carb is new from Amal, so the needle should be OK. Similarly the filter is new, and should not be a problem. The float is the latest adjustable from Amal which allows the float level to be altered with out having to move the float needle seat. I've had to drop the float to give a lower fuel level to weaken the mixture off. At present the float is about 2mms below the Amal reccommended position. Any higher and it just runs rich all the way throughout, 4 strokes and pushes a lot, and I mean a lot, of oil out the exhaust. I've tried changing all jets except the needle jet, which is a 106. I shall try a 105 as a last resort before putting someone else's carb on, which I guess is now the only way to go. Thanks again for all your input. jt