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Armstrong 250 Hiro Problems.


steve
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Hello all, I am having a problem with my Armstrong CMT 250 (Hiro) and wonder if anyone may be able help.

The bike hadnt been run for many years when I bought it so I wasnt too surprised that when I did get it going it sounded very lean on low revs and no amount of carb cleaning made any difference. Mains were a little noisy too so I stripped the motor and replaced all the bearings and seals thinking it was drawing air through crankcase seals.

I also bought a 26mm OKO with a range of pilot jets to replace the worn Amal Mk2. However on rebuilding the motor the problem still exists. The bike has fair compression, although I havent measured it. It starts easily on choke and the motor is quiet. As soon as I take it off choke all is well on mid / high revs but let it drop and it wants to die, pulling the choke on a little brings the bike back to a better more settled tickover. I thought pilot must be too small so increased from 38 but have gone right through the set to a massive 58 pilot and it makes little difference. Main I tried 110 & 120 but Im not running on main yet anyway so as expected it didnt make any difference

I have tried the Amal back on and its worse, I even grafted a Dellorto from a Beta Zero on to try in case I had 2 faulty carbs with blocked pilot drillings, but this was the same.

Convinced it is lean I made bungs for the inlet rubber and exhaust port with a Schrader valve, then pumped the crankcase up to 4 psi which it held for a good few minutes so Im confident its not leaking from inlet manifold, head or base gasket. (I havent vacuum tested, only pressure)

I have modified the airbox to take a REV3 filter as standard are not available but surely I could leave the filter off totally and it would be somewhere near especially if jetted slightly bigger to compensate for less restriction so doubt that is the cause.

Middle box is a straight through tube with no blockages, tailpipe appears ok although its quite a big thing so could be blocked in the centre but the bike didnt sound muffled, I even took it for a blast down the road!

Im not a beginner when it comes to bikes and tuning (been riding trials for 20+ years) and have a very good ear for setting up carburation having spent years repairing and tuning 2 stroke hire tools! Im also not too proud to ask for help and can only hope Ive overlooked something simple.

So whats next guys? Could ignition influence it? I set the stator plate bang centre as per factory setting.

HELP !!!

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What colour does the plug come out?It should be light brown,if it's white or grey it's too weak.4 psi doesn't sound enough pressure to prove or disprove an air leak.The problem was there before the motor was rebuilt,is there with different carbs,makes me think it's something you haven,t changed.Start it then get your oil can and dribble oil all round the inlet.If there is a leak it will start smoking as it sucks the oil in.I don.t think it's ignition related

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I had a CMT310 with the Hiro motor. I could never get the thing to run properly. Different carb, the only spare cylinder in the US, months of engine noodling then one day when I was setting the ignition timing for the umpteenth time I noticed something odd. As I revved up the engine the timing retarded! This WTF moment was a real head scratcher. After reviewing my options I realized I only had one and swapped the two wires going to the CDI. From then on the thing purred like a kitten. That is a kitten that got traded in for a Rotax powered Can-Am (Armstrong) So try throwing a timing light on it just to make sure your advance is advancing.

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  • 1 month later...

HI Steve, I have been running Armstrong for the past twenty years and that's a new one... Firstly I have been trying different carburetor for some time and cannot get them to run as good as the amal. 1st check when you had the motor down did you check the magnets they can go off and also fall off when fitting. So pull the flywheel off and check them. 2nd remove the brass choke plunger out of the amal and see if the black rubber is still on they come off. As of the ignition never had any problems with them trimming never gone out. You can run them without the rear exhaust and should run ok. Remove the filter in the tank as that can block.

Can you updated on the fix if you have sorted. Simon

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  • 6 years later...

Ok, so this is a bit late, but I did get to the bottom of the problem. It was a carburation problem as I thought all along. 
So the std Amal must have a blocked drilling in the pilot circuit. No amount of cleaning managed to dislodge it. The 26mm OKO turned out to be a Chinese copy. It was after I'd tried the OKO on a TY Mono and had exactly the same issue, the penny dropped. 

I had a Keihin PWK28 flatslide off a Gas Gas 125, so put that on the Armstrong, WOW, bike was suddenly running almost perfect. Just a clip on the needle and a different pilot jet was needed. 

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