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I'm building up a 250cc grass kart, and I've bought a Honda NX 250 engine for it including a ignition system and carburetor. It is a single cylinder 4-stroke DOHC, simple ignition with 1 magnet on a rotor for ignition timing, and a really good carburetor.

Everything seems to be okay, very great compression, managed the wirings to get a spark, plugged a tube for a fuel, crank it, crank it, crank it, choke on, choke off, throttle on, throttle off.... It does suck the fuel into the combustion chamber, and blows it out through exhaust. The carburetor has a little injector which injects a good dose of fuel straight into a inlet port, and it has 3 hose pipe connectors - 1 under neethe, and 1 on each side. I don't know about them, I want to believe that 2 of them are just for overflow, but if I can't start the engine, I want to blame them. Maybe it doesn't mix the fuel with air properly.. Also 1 thing that is different - I don't have a charging rectifier. I build it to run straight from battery, no charge. So the power for a spark comes straight from the battery to CDI instead of from battery through rectifier. I doubt that it will make any difference. The valve timing is "spot on", and clearance is also great. Sometimes I think it doesn't spark it under a compression....

  • You are seeing spark? If so, take the plug out and heat it up until you cannot stand it. Then quickly put the plug back into the cylinder head and immediately crank the engine. Sometimes the little extra heat on the spark plug will allow it to start. Your engine may be flooded is what I'm thinking, though, which may have caused the spark plug to be wet. Heating the spark plug tip up with a lighter would help that as well. – Pᴀᴜʟsᴛᴇʀ2 Sep 4 '16 at 18:47
  • +Paulster2 The spark plug is always dry. It seems to be blowing out some steam (air/fuel mix) through exhaust, but won't combust it. I checked carburetor, it's spotless clean inside (outside everything looks scrap compare to age), flooded it with fuel, left the engine with a plug out to dry it out. Put the plug in, cranked, all nice and dry, so not flooded. Turn the ignition on, let the fuel in, and the same again - takes the fuel and no combustion. My only concern right now is if the ignition coil could fail because I took it from 125cc chineese 4-stroker. – Arturs Bolsunovskis Sep 4 '16 at 19:33
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    Try injecting some carb cleaner or brake cleaner into the cylinder then cranking (if you have starter fluid, use that: it's better). Does it turn over a few times? – anonymous2 Sep 5 '16 at 1:48
  • +anonymous2 it turns over lovely, the spark is lovely, everything seems to be perfect. I think it is because it needs a rectifier. I believe it has stronger spark from that. I'll make one rectifier, I have diodes, and put the wire back where it is supposed to be, that should make it run. And if it runs, I'll just have to redesign a whole ignition, because I want to save power and run on battery.. – Arturs Bolsunovskis Sep 5 '16 at 18:18

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