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C15 Timing
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Bee175
Felixstowe, Suffolk
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April 12, 2021 - 10:44 pm
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Does anyone happen to know what 1/16" btdc is in degrees btdc for a 1966 C15 side points?

I've fitted a Boyer EI unit and have set the advanced timing at 33.5° btdc, but I want to check if it is correct when fully retarded at low rpm as well.

I think I read somewhere that it is 23° btdc, but I cannot find where I read it to confirm!

I know the B175 is also 1/16" btdc and/or 16.5° btdc, but it will be different for the C15 due to different stroke and crank angle.

Any assistance gratefully received🙂

BSA Bantam B175 1971 and BSA C15 1966

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sunny
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April 13, 2021 - 10:30 am
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C 15  STAR  [ 1959  ] 

valve  timing    26,   70 , 61 1/2 ,  34 1/2 , degres    .,o,oo8 "  clearanance   / cold   

50 W  Lucas  RM 13  alternator     coil ignition  ,  timeing  11/ 32   btdc    

braeker  gap  15thow 

N 5  s/plug      22 thow   gap

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Bee175
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April 13, 2021 - 10:56 am
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Thanks Sunny, but I'm after the figures in degrees for fully retarded timing on the later side points model. 

Fully retarded is 1/16" btdc, but I'd like to know what that is in degrees if anyone knows.

BSA Bantam B175 1971 and BSA C15 1966

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Sponge
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April 13, 2021 - 11:13 am
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I only have data for fully advanced timing but the advance degrees should be written on the back of your Auto Aadvance Unit so you can work it out.   

1966 C15 ignition timing:

Fully advanced crankshaft degrees BTDC = 33.5 degrees or 7.1 mm BTDC 

So if you AAU says 15 degrees then subtract that from 33.5 and you get 18.5 degrees.  Suggest set static timing it to that and see how she runs. 

Sponge

   

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Bee175
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April 13, 2021 - 11:29 am
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Hi Sponge, thanks for that.

AAU says 10 degrees, but I've know way of knowing if it is the correct unit. It was not working at all, hence I've opted for the EI.

The bike is generall running great with EI, but still has this persistent "cough and stall" at low rpm (off tickover) that I'm trying to get rid off.

I have also fitted a brand new carb with all the correct internals which has also improved greatly the general running, but Mr Cough/stall simply won't go away!

Im ordering up a larger pilot and also the next two cutaways up to try those.

BSA Bantam B175 1971 and BSA C15 1966

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Sponge
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April 13, 2021 - 4:30 pm
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The correct AAU for a 66 C15 is 15 degrees but if yrs says 10 degrees then its probably wrong. Anyway you now have a figure of 18.5 degrees for your EI unit static and if you set it to that I do nor think it will be far off the mark. 

Ref the 'cough' ....this is surely a sign of weakness at that rev range so its either yr pilot jet or throttle slide cutaway as you suggest. Failing that check yr float height...especially if it is a monobloc carb....prime suspect is a washer underneath the needle valve seat....worth checking cos that would also produce unpredictable results from low revs. 

Good luck and enjoy your G model C15. Those engines are very strong.

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Bee175
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April 13, 2021 - 5:03 pm
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Thanks for that AAU info Sponge. That's really handy👍I  will aim for 18.5°.

Being a brand new monobloc, everything about the carb is standard. I have ordered a 3.5 slide to replace the standard 4 slide. Hoping that helps.

Was considering getting a 30 pilot to replace the standard 25, but I'm more drawn to the problem being the cutaway rather than pilot.

Float height is pretty much perfect. 1/2mm above the dot on the float chamber cover.

"prime suspect is a washer underneath the needle valve seat"

Are you saying there should or shouldn't be one? Again, being a new carb I assume it is correctly set up. Float height is good.

BSA Bantam B175 1971 and BSA C15 1966

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Sponge
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April 13, 2021 - 9:38 pm
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Sorry ....in my enthusiasm I missed the fact that you have a new carb.

If its a new build by amal it will be correct bit for the record there should be no washer beneath the needle jet housing. Some owners rebuilding a used carb have a tendancy to fit one and wonder why it alters the float height and starts to run wierd. On a monobloc carb that tiny bit of adjustment ( the thickness of a fibre washer), makes a noticable difference..but as yours is a new carb please just ignore my ramblings.

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Bee175
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April 13, 2021 - 10:08 pm
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Now you mention it, you've reminded me of someone that used a washer there to adjust float height on purpose.

Always happy to hear your ramblings!

BSA Bantam B175 1971 and BSA C15 1966

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