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Ironhead Sportster Motorcycle Talk (1957-1985) For all those that wanna talk about Ironhead Sportster Motorcycles

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  #1  
Old 4 Weeks Ago
Andy Hog's Avatar
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Default Help & advice needed

Hi,

81. Yesterday I was out & about on the bike when it just started to back fire & pop, only from the rear cylinder, it was so bad the bike was unrideable, I had to pull over & call for a recovery vehicle to get me back home, which took 4 hours to arrive. The recovery guy said he saw flames coming out of the rear exhaust.

What is the problem & how do I fix it please, until that point the engine was purring along & all was good in the world of Ironhead riding.

TIA,

Andy.
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You should probably start with push rod adjustment just to see what you find. Then move on to ignition timing components and the timing itself. Original ignition? Maybe it’s on its way out.
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I can't do anything till the weekend, I'll check those rods. I have fitted a replacement electronic system but for the life of me can't remember the make.

If it is the electronic ignition why is it only affecting the rear cylinder?

Many thanks,

Andy.
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I'd pull the rear plug and ride it home on one cylinder. A guy I know had to do that one time back in the day and he made it home.
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I've had both PowerArch and MSD V-Series electronic ignitions drop a cylinder on my '78. Current MSD unit is working OK, so far. Never had a problem with the points on my '77!

Eric
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Riding on one cylinder doesn't sound like a good idea, the unburnt petrol would wash away the oil on the cylinder bore which will probably end up welding the piston rings to the bore & the petrol would end up in the crank & then into the oil tank.

Mechanical or electronic, either will fail eventually, just a case of when.

Thanks,

Andy.
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Andy- as a fyi- on a points ignition Norton, the recommended way to tune, is just as mentioned earlier- disconnect one spark plug and run on 1 side. Its also mentioned that is a failure occurs on 1 side, to disconnect that plug, and you can safely ride it home.

So apparently they weren't worked about gas overflow so to speak, at least for short periods.
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The fuel is a bad lubricant, but it does some good. It's a pretty good coolant when it's not burning.
The "live" cylinder will run very hot, use minimal throttle or it will seize.

My guess? Tight exhaust tappet.
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I was riding along just fine once and it started popping. My buddy gave me a fresh
spark plug and I was on my way.
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I don't know why but these bikes sometimes just "eat" a plug.

I do suspect it may have something to do with the fuel additives they are using these days. As they have replaced lead with chemicals things have deteriorated.

I first noticed the problem way back when they introduced alcohol into our fuel. It's not the alcohol, but it happened around the same time. It might just be s deposit on the spark plug that allows it to "track". But if you look at the plug you can't see anything obvious.
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