Can anyone help? I have a problem with my recently finished Armstrong. It has a Mk2 Amal concentric on it, goes great for half a trial then tick over gradually increases and the engine starts to run lean, knocks & rattles etc. Any attempt to adjust the pilot screw & tick over are fruitless once this happens. It is very like what happens when you are running low on fuel ie the engine speed increases, but in this case it does not stop running. If I go to the bike now it will run sweetly until it has been going a while.

If asked I would say it has an air leak but the engine has new crank seals and new base gasket. Changing the spark plug when its knocking doesn't improve it, and I've raised the needle to its top groove ie richest setting, no improvement.

The tank breather is clear and the fuel is visible in the fuel pipe, but I can't get low fuel level in the float bowl out of my mind as a possible cause(like when it's running out of fuel). Has anyone seen this on other bikes with the Mk2 Amal? Does anyone know where I go for an alternative carb (Delorto?) All the info I have says the carb is 27mm but the Amal info says 26 or 28mm! The slide in the carb is worn, I'm prepared to replace the carb but I thought I'd ask first, it may be something simple I'm missing!

If this is a Hiro engined bike; is there someone at Sammy Miller's that could help as the same engine was in the SWM?

SWM used rotax, and a Tau in the last model.

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I stand corrected I was sure Sammy Miller had a connection with Hiro.

If your problem happens when the bike is hot and its not a question of carburation and leakages , then its possibly got a lot to do with heat.

Does it improve when cooled down ?. If so ......

Is there a possibilty that the engine of the bike is getting so hot that the supply of petrol is being restricted to your carb as it evaporates in the pipe or float bowl causing the bike to run lean.

Is your carb or fuel pipe very near to the exhaust?

I recently saw this happen to a four stroke Sherco trials bike where fuel was shooting out of the breather pipe... it does happen.

Admittedly this bike is likely to run hotter than your 2 stroke bike , but it may be worth considering.

Please keep us advised

The Hiro motor was used in Sammy's "Miller Hiro", the first Aprilias also used the same engine along with Armstrong CCM.

As for the problem, what plug are you running? have you tried a plug one size hotter???

Also with your worn slide.....this will allow more air into the motor which will lean out the mixture and up the revs. If the carb is warming up it will make the problem worse.

Thanks for the help guys,

I suspect it is only partially heat related because the carb is cold to the touch when running and it was bl**dy cold when we rode on Saturday, the fuel pipe is on the opposite side to the exhaust so it doesn't vapourise or get heated up here.

Have changed plugs but haven't checked heat ranges so could do that.

I did suspect the worn carb slide and I would like to change to a more modern carb but I need one that is jetted ready to go because if the bike has a fault other than the carb it will drive me spare trying to get the new carb working and coping with the fault.

So this is where I am, if I have an air leak it will be on the induction (sucking) cycle of the engine, there are 4 or 5 areas to look at leaving the worn carb aside, these are the carb inlet rubber (looks ok), Inlet pipe to head (Cr*p method of fixing and tiny O ring), crank seals (replaced) and cylinder barrel base gasket (renewed but need to re-check torque on retaining studs). I'm going to silicon the inlet pipe to the head in addition to the O ring and re-torque the barrel. With respect to the carb has anyone got contact details for Delorto or Keihin suppliers?

Really appreciate the help, keep it coming.

Solution to the problem was leaking O ring on the base of the carb inlet pipe. Cleaned the burrs off the casting and set a spare O ring in silicon sealant, problem solved. Thanks again for everone's help.