Hi, I have just joined this site and looking for some help sorting out my recently acquired Bultaco198A, 250 Sherpa as the bike is running weak.

I have tried another Amal type 2 carburettor, I replaced the following: all the jets to original spec, throttle slide, crank case seals, intake rubber, air filter, spark plug and the engine will not run unless the choke is on.

It has electronic ignition and starts fine. 

I appreciate any ideas or experiences that you may have. 

Thanking you in advance

Graham 

 

My quick thoughts to your matter:

A mounted electronic ignition is a very good way to improve the engine performance. Are you sure the timing is set right too?

The jets are also the right ones?

How old is the air filter and is the filter clean and proper oiled (not too much oil used).

I would too check the piston/slide to wear as too loose fit can worsen the engine performance quite a bit.

Hi, thank you for your reply.

Yes the ignition timing is correct and all the jetting and the carburettor slide and needle has been replaced, carb body seems good, did try another carb, just cant understand why it needs the air screw almost screwed right in and will not run without choke.

 

Is it possible that the crank case seals have been fitted the wrong way round, an easy mistake to make, could the exhaust be blocked or restricted.

Edited July 15, 201610 yr by trialsrfun

Possibles:

piston in backwards

blocked exhaust

worn rings

pilot jet and starting circuit jet in each other's locations (they are the same type of jet but have different size holes)

I purchased the crankcase seals from Bultaco UK and as the original wide seals are no longer available they supply 2 seals per side.

They supplied instructions to fit them back to back, one with the wire spring side to the crank shaft and the other wire side to the opposite side.

I am not sure of the exhaust or silencer but does not appear to be blocked.

 

Thanks feetupfun, compression seems fine but  I have not removed the head or barrel,  is it possible the piston fitted back to front would cause this ?

Your description of the problem is not very clear, but if the piston is in backwards in a Sherpa T or any other bike with piston port induction timing and a piston with uneven length skirts, it greatly affects the inlet timing which can make a bike quite gutless and greatly affect the jetting requirements. If it hasn't been pulled apart since it last ran properly then it won't be this

 

Please don't take offense but I hope you are using the "choke" lever the right way. I'm pretty sure with AMAL Mk2 the "choke" lever operates in the opposite direction to the "choke" lever on a Mikuni.

AMAL Mk2 lever up = on, lever down = off. An easy way to check is "choke" plunger in = off, "choke" plunger out = on

 

If it is not that, the consider that the "choke" on an AMAL Mk2 only has a significant effect when the throttle is in the closed position. It is not really a choke, but is an extra fuel circuit that only allows fuel to flow when the throttle slide is at the bottom. If the bike runs normally with the "choke" on, then the problem is in the pilot circuit of the carby. It is not unusual for the pilot circuit to be blocked in a used carby and it makes no difference what you do with the pilot jet if any of the pilot passages are blocked. I suggest if you can, to test the bike with the carby from a similar engine that is known to run properly.

_{Has it ever ran properly since the electronic ignition was fitted, what system is it. They can be tested it may have developed a fault.}

Thanks again feetupfun & trialsrfun,

I have never ridden the bike as it has never ran properly since I bought it.

First I thought it was either a blocked fuel filter from the tank or in the carb fuel inlet, or a blocked jet, just like fuel starvation.

Then replaced all the parts mentioned at the start of my post.

When I am running the engine with choke on it seems to run for a few minuets its not to bad to start with, but then becomes sluggish, when I turn off the choke the engine revs increase and then appears to run out of fuel, as soon as I turn the choke lever back on and the engine picks up again, its very difficult to explain properly, but very frustrating.

The electronic ignition was on the bike when I purchased it.

 

feetupfun, thank you for your info,  sorry but can you please confirm AMAL Mk 2 choke lever operation:

 

You stated choke Lever up = ON,  this places the plunger down "IN" which you also say is OFF, I really appreciate your help but now I am now very confused !

 

Sounds to me you did all things right.

There is one thing I would like to be checked as it is an old bike.

How is fuel supply. I never had a used bike where the petcock wasn't somehow clogged. It would too explain the "uneven" running.

Edited July 15, 201610 yr by pschrauber

Yes, thanks for that but had the petrol cock apart and cleaned it as well as cleaning the filter in the tank and the filter in the carb inlet banjo, as I said just don't know where to go next

Hi oldmxer

Welcome to the forum.

Perhaps the backward piston theory is a possibility.

I recall a 250 Pursang piston been fitted by mistake to a Metralla 250 GTS, the Pursang piston had a small port In the inlet side of the piston skirt (pre reed valve) and it caused blowback with fuel been blown back through the carb. But it did run "slightly" better with the choke on.

You've seem to have tried the obvious and still no fix. At this stage I'd lift the head and barrell and see whats happening in there.

Edited July 15, 201610 yr by model80

Thanks Model80, I think this is the next step, just a silly question, is there an arrow on the top of the piston ?