Linkert issue on Big Flathead

low popping on M51L

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paddy
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Bikes: 1965 FL panhead
1947 FL knucklehead
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Linkert issue on Big Flathead

#1

Post by paddy »

I have a M51L on a 80 in flathead. This engine starts right up and idles perfectly. Crack throttle while on the kickstand it is very clear no smoke no popping.While riding at high speed 55 to 60mph when I let all off throttle then the revs come down I hear a low popping out of exhaust. Only at high speed...I am running the rocket muffler setup.I have searched for air leaks but can not find any..On occasion gas will overflow out of the small hole above the float bowl. I have had this apart numerous times. Nitrophyl float and steel needle valve are installed.. have not been able to do a real plug chop but the plugs are light brown on electrode and a little soot on outer edge of plugs..Are these 2 things related? Any advice?



Pascal
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Re: Linkert issue on Big Flathead

#2

Post by Pascal »

The popping comes from airleaks between cylinder and exhaust/ down pipes /headers.
Flathead are notorious; the headers are not a tight fit in the exhaustports.
It is harmless..

mark59
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Bikes: 39 UL bobber,78 FX wide glide, 99 FLHT, and 48 U with sidecar project
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Re: Linkert issue on Big Flathead

#3

Post by mark59 »

I have two big flatheads, air leaking into the exhaust causes the popping, my 39 leaks between front down pipe and squish pipe and has an empty hollowed out muffler, it backfires very loudly and at night emits a wonderful blue fire ball, my 48 has a much tighter exhaust and seems to sound very similar to your description...

flatheadmatt
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Re: Linkert issue on Big Flathead

#4

Post by flatheadmatt »

I just fixed this problem on my 37. I just expanded the exhaust pipe (slowly and rechecking each time) with a pry bar around the circumference of each so they were a tight fit . I then put some permatex exhaust sealant (red) in each cylinder around the inside. Worked each pipe in slowly and bolted em up. Not a single pop on a 120 mile ride. Every flathead I have owned has had the same issue.

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