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Atlas popping in one exhaust.

Submitted by robert_tuck on Tue, 30/05/2023
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Heavy Twins

The atlas ( single carb)  has been making an unusual popping in one of the exhausts on tickover. I haven't ridden it for some time. . The plug  is also sooty .  I changed the plug and no difference . My suspects are, an air leak into the exhaust at the head ,  the thread is a bit rough where the pipe flange got loose sometime last century and the nut gets tight before it fully grips the pipe. A tight exhaust tappet allowing some mixture to squeeze past into exhaust and getting lit up there. A failing (early type) Boyer that has been ignored for 20 years or so. My question is , if I swap the plug leads , will it run ?, this may transfer the issue and  point to the Boyer, what do you think?.

Submitted by howard_robinson on Tue, 30/05/2023

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hello again Bob, It is possible for the Boyer to be wired up in the conventional way with each ignition coil and pickup coil for each cylinder.  It usually depends on whether your Atlas was a twin coil, with capacitor discharge system, from new. If so it would have already had the extra wiring for the individual coils, ignition switch etc.  Normally if the Boyer replaced a magneto set up or single coil system with a distributor,  people tend to put two 6V coils in series with a simple wiring addition. Was the Boyer fitted by a previous owner or you ?  When i converted my Mercury to Boyer I kept the 12V coils and wired the pickup coils to each coil separately.  Unfortunately when the coils are in position between the battery box and oil tank it's hard to tell if they are 12V or 6V as the number is stamped underneath.  Like the Atlas and 650SS from 1967 a Mercury used two 12V coils. The extra current in the 6V coil will give an increased spark plug voltage but with a wasted spark a lot of the energy is wasted.  The only thing wrong with the original system was the advance and retard mechanism which regularly flew apart - hence the need for the Boyer or similar.  Good luck, Howard

Submitted by george_farenden on Tue, 30/05/2023

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Hi Bob,

in addition to what you are trying, can you check that the inlet manifold is sealing properly on all joints? Could be a small enough leak to allow air in at the offending side, but 'too far' from the other cylinder to affect it? Is compression same both sides?

Otherwise, as you suggest, tight tappets, or the exhaust seal at the head should be at the top of your work list.

Regards, 

George 

 

Submitted by mikael_ridderstad on Tue, 30/05/2023

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My B40 was popping when rolling back throttle, engine braking. Normal BSA problem with leak between head and pipe. My early flathead Harley did pop a lot when rolling back throttle and ignition at the same time, engine braking.

But you say your bike pops on idle. Sounds more like ignition to me. Worst thing that could happen swapping plug leads is that it don´t start.

Usually when you think it's an ignition problem it's a carb issue.

 


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