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Another Ignition Timing Question

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mylesw

Jedi Hopeful
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Hello

My BT7 has started to backfire on deceleration. I would like to check the timing as this is most likely the cause. I have a timing light with no advance dial etc. I have located the notch on the pulley and used some chalk to highlight it and removed the vacuum pipe. When I check the timing the notch is about two inches or so to the left of the pointer (looking from the front) at idle. Now my question is - do I need to adjust the distributor so the notch and the pointer line up at idle?

Thanks for your help.

Myles
 
Hi Myles,

You do not want to line up the marks at idle. That would set your timing to 0 degrees at idle when it should be 15 degrees BTDC running. 2 inches sounds a little too much. I use a timing light which allows me to dial in the advance. Somewhere around 1 inch is a little closer. Others will chime in with the correct measurement. If your timing made a radical change, either the dizzy moved or your points are having problems (if you still have them).

Backfiring on deceleration also indicates a lean idle mixture. This can be partly due to over advance, but you might also want to check your carb mixture adjustjments after you get the timing and dwell right.

Mark
 
Check your coil if it is original or an unknown age.
Put an ohm meter across the primary, it should be close to three ohms. This is in addition to what the others have and will advise.
 
popping and backfiring on deceleration usually indicates a weak spark or incorrect fuel misture. I'd run through the coil, condensor, etc etc first, then do a complete tune up.
 
Exhaust backfires also indicate exhaust leaks. You don't have to have a big leak for this to happen either. As you're checking other things, be sure to include this as something to look for.

Regarding the pulley and the timing light, if you can get to the pulley and measure its diameter you can make your own timing marks. Measure the pulley OD and multiply by PI (3.14159) to determine the pulley circumference. Record that value and cut a strip of paper that long. Determine WHERE your timing marks need to be: If your manual says 15 BTDC as Mark said, take 15/360 and multiply that by the circumference. Take the resulting distance value and make a mark THAT FAR from one end of your paper strip. Take the paper strip and wrap it around the pulley aligning the cut end of the strip with the timing mark on the pulley. Use paint to transfer the 15 degree mark from the strip to the pulley. This will give you your timing mark so you can use a fixed-flash timing light. BTW, wrap the strip around the pulley in the direction the pulley rotates. Remember the mark should be before TDC.
 
Actually, an excess of fuel will cause backfiring on deceleration. (look at the NASCAR guys as they lift in the corners).
Backfiring on acceleration is a lean mixture.
Check your float levels and needle and seat assemblies and choke operation.
Dave
 
Included in the pile of papers when I bought my BJ7 was a BMC technical service bulletin dated September 1, 1964. (I added the conversions in the parentheses and for 15 degree line)
-----------------
Re: Ignition Timing for Austin Healey 3000 Mark I, II and III.

We have received many requests asking for the actual crankshaft pulley measurements necessary for adjusting and checking the ignition timing. We would like to advise you that the corresponding distance from the T.D.C marking on the crankshaft pulley is as follows:



5 degrees - 17/64" (.265625 or 6.747 mm)
10 degrees - 17/32" (.53125 or 13.494 mm)
12 degrees - 21/32" (.65625 or 16.669 mm)
15 degrees - 51/64" (.796875 or 20.241 mm)
 
Thanks for all your advice. Think I'll go down the route of a full tune up but also not forgetting to check the exhaust for leaks also etc. I'll let you know how I get on.

Thanks

Myles
 
Hi Myles,
I had exactly the same issue with my BN7. Reason were worn out carburators. They could not be adjusted correctly anymore because they got too much air in low trottle. When adjusted to propper low trottle the gas mixture (hope that is the right wording.......) was too rich in normal driving speed. Replacement with refurbishd HD8s gave me more power and solved the problem. Goos luck, regards
Alex
 
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