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Discussion Starter · #1 ·
1971 SL175...bike will not fire on the left hand cylinder with the plug cap completely on but will fire with the boot loosely sitting on the plug not completely pushed down.....although it fires at this point it is a bad misfire and the bike backfires through the exhaust really bad.
The right hand cylinder runs perfect...

Any ideas?????
 

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The plug is fouled, pulling the cap up a little ways creates a gap between the plug terminal and the cap. The extra gap forces the coil to build up enough power to fire through the crud. Change the plug.
 

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Discussion Starter · #4 ·
Placed a set of plug tester caps on the bike and started it up. Immediately noticed that the cylinder will fire at low RPM's but will drop out (lose fire) once the RPM's are raised.
 

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Placed a set of plug tester caps on the bike and started it up. Immediately noticed that the cylinder will fire at low RPM's but will drop out (lose fire) once the RPM's are raised.
I'd still go with the dodgy plug diagnosis. Stick a new set of plugs in, put the tester caps back on and see what happens.

If the budget doesn't stretch to new plugs at this stage..:cool:.. swop the plugs to opposite sides and see does the problem move with them. If it doesn't then it's time to start looking at the leads, coils, and other ignition components.
 

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Discussion Starter · #6 ·
Ok...new electronic ignition (Charlie's place), new coil, new plugs, new plug caps, new battery...and still have the same problem with just the left cylinder. Advance is working correctly and not stuck in any way...just the left side is the problem once the rpm's are raised.
 

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At this stage I'd be trying to confirm whether is it a spark, fuelling, or compression problem.

That's a fairly comprehensive overhaul of the ignition components. Assuming the new parts are good, and were installed correctly, I'd be thinking about checking the valve clearances on both sides, carburettor cleanliness and settings, and perhaps a compression test.

I don't know if your diagnostic plug-checker is the same, but something I found found useful on trying to diagnose fueliing/carb problems was the little glass-window see-through spark plugs:

 

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At this stage I'd be trying to confirm whether is it a spark, fuelling, or compression problem.

That's a fairly comprehensive overhaul of the ignition components. Assuming the new parts are good, and were installed correctly, I'd be thinking about checking the valve clearances on both sides, carburettor cleanliness and settings, and perhaps a compression test.

I don't know if your diagnostic plug-checker is the same, but something I found found useful on trying to diagnose fueliing/carb problems was the little glass-window see-through spark plugs:

I wouldnt assume anythings good just because its new.
The 175's are all wasted spark and the Coils are transformer type (one Ht lead is +, and one is -) so both plugs should fire at once
 
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