Author Topic: popping and banging  (Read 388 times)

Offline edboy

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popping and banging
« on: 01.09. 2019 22:59 »
anywhere near the top end of the carb circuit the engine dies away with popping and banging in the exhaust. went to a manual mag thinking my ignition timing was at fault . then tried carb settings. i have gone from a 380 , 330 and now 280 main jet and its almost perfect. 260 is probably on the money. very near to book settings for road rockets and early s/rockets. i have an ally head with bellmouth . 1 1/8 mono ,2 into 2 exhaust.
where have these big main jet sizes have come from ? 420 main jet is massive.
now , i only need to set up motorway cruise jetting.

Offline Rocket Racer

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Re: popping and banging
« Reply #1 on: 05.09. 2019 06:07 »
which engine? what specs? what model carb? which fuel
spitting is a carb symptom, popping and banging typically air leaks in the exhaust.
More info please, with modern fuels jets required are typically smaller than the books originally specified
near the top end or at full throttle?
A good rider periodically checks all nuts and bolts with a spanner to see that they are tight - Instruction Manual for BSA B series, p46, para 2.
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Offline edboy

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Re: popping and banging
« Reply #2 on: 05.09. 2019 19:19 »
hi rocket racer,
i m still having problems with this one. new valves, ally head, now on 260 main jet and perfect except for full throttle in any gear. pops and banging and no power. less with smaller main jet.
the exhaust might be partly to blame but i will try to find a 240 m.j. next

Online RDfella

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Re: popping and banging
« Reply #3 on: 05.09. 2019 19:29 »
Have to say it sounds like ignition. Popping in the exhaust is usually on the overrun and too rich a main jet should make it run hard with sooty exhaust, but not misfire. Remember under full throttle the plugs have got the most work to do, so are the plugs the correct heat range, with .018" gap (as opposed to the usual .025 they come with). Plug caps non-resistor type?
'49 B31, '49 M21, '53 DOT, '58 Flash, '00 Firestorm, Weslake sprint bike.

Offline edboy

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Re: popping and banging
« Reply #4 on: 05.09. 2019 22:01 »
you could be right as i ve had a run tonight and i have no advance left on the mag lever. only its bouncing down the road [ feels advanced ] on acceleration as it is.
cant work on the s/r until monday now.

Online RDfella

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Re: popping and banging
« Reply #5 on: 06.09. 2019 09:26 »
edboy - I'm confused by your last post. With the ignition lever fully advanced, is the ignition set at the right place, ie 3/8" (or whatever is specified for your engine) BTDC?
'49 B31, '49 M21, '53 DOT, '58 Flash, '00 Firestorm, Weslake sprint bike.

Offline edboy

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Re: popping and banging
« Reply #6 on: 12.09. 2019 00:24 »
hi rd fella,
my ignition timing was slightly out as i m not used to manual mags and the lever which likes to move about on its own. with the super rocket tune the mag setting seems to require more accuracy than i usually  bother with. what i m aiming for is 4 to 5,000 rpm cruising settup for long distance.

Offline muskrat

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Re: popping and banging
« Reply #7 on: 12.09. 2019 12:32 »
G'day Ed.
Keep in mind that at 4-5000rpm on the flat the throttle would only be at about 1/8 to 1/4. A slight hill will be at 1/4 to half and a good hill about 3/4. So for the same revs in different situations you will be in three different throttle stages, slide, needle and main.
You can stiffen the a/r lever by the big screwdriver slot on the top. Is the cam ring and adjusting lug a nice sliding fit?
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Offline edboy

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Re: popping and banging
« Reply #8 on: 12.09. 2019 22:33 »
hi muscrat,
i m getting there.slowly.
the timing feels now its spot on. engine runs cooler also. it did like a bit more advance setting with the ally head. popping and banging now gone at top - ish speed. it goes up to 50 mph no problem but hit 60 and it starts jerking and missing a beat. i m on a 260 main jet with guaze bellmouth fitted 1 1/8 monobloc.
bp7 plugs are fairly new  but could be ignition. i will try a 240 m.j. next when i find one.
i m using a cable tie to hold the mag lever in the best possition. as slips.

Offline edboy

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Re: popping and banging
« Reply #9 on: 22.09. 2019 22:57 »
no popping and banging now. tried a 105 needle jet and now takes off upto top. main jet only now. but no time or clear roads.thanks for suggesting needle muskat.