1953 BSA B31 top speed and Timing

[briancairns]

Have ypu checked for free play in the cable of the valve lifter.
Does the mag advance the same amout by hand with the cable disconnected?

Hi

I take it there should not be any free play on valve lifter cable?

Apologies ( I am only really starting my own mechanics) but what do you mean by "Does the mag advance the same amout by hand with the cable disconnected?"

Apolgies again
Brian

[cdsdorset]

You need a small zmout of free play on the lifter cable. If its dead tight you could be just lifting the valve  off the seat.
Take the advance cable off the mag and move the advance unit fully to both clockwise and clockwise directions and mark the mag casing at each position.
Then  refit the cable and repeat. See if you get full movement with the cable fitted.

[briancairns]

You need a small zmout of free play on the lifter cable. If its dead tight you could be just lifting the valve  off the seat.
Take the advance cable off the mag and move the advance unit fully to both clockwise and clockwise directions and mark the mag casing at each position.
Then  refit the cable and repeat. See if you get full movement with the cable fitted.

Thanks will check that out

[briancairns]

You need a small zmout of free play on the lifter cable. If its dead tight you could be just lifting the valve  off the seat.
Take the advance cable off the mag and move the advance unit fully to both clockwise and clockwise directions and mark the mag casing at each position.
Then  refit the cable and repeat. See if you get full movement with the cable fitted.

Thanks will check that out

Lifter cable is fine, I will check the advance cable after Christmas.....as i also have to fit a rear hub sprocket and gear sprocket along with a new chain.
Thanks again

Brian

[scifi]

I think you are supposed to lay on the tank to get top speed...!
Can you adjust the timing for a peak in revs, when the engine is idling.  That will be the best setting for starting the engine.
I hope you are timing from when the points start to open, not when they start to close..?

[cdsdorset]

Is the carb slide definetley being pulled all the way to the top.

[briancairns]

I think you are supposed to lay on the tank to get top speed...!
Can you adjust the timing for a peak in revs, when the engine is idling.  That will be the best setting for starting the engine.
I hope you are timing from when the points start to open, not when they start to close..?

Yes starting timing  when ponts opening

[briancairns]

Is the carb slide definetley being pulled all the way to the top.

yes but a bit sticky, need to check cable route

[briancairns]

Thanks all for your help.
I will look at everything again after Christmas and let you know what happened.

Cheers

Brian

[Steve.S]

 Brian,
In my view your engine should probably start on full retard, but most people recommend a little advance for starting.
I agree that it should almost certainly kick back on full advance.
Hopefully, Father Christmas will put a Neon tester in your stocking, and you will then be able to see if the magneto is sparking on full retard, and whether it is much brighter on full advance and very dim on full retard.
I agree with Julian, sounds like a weak spark.
You say the magneto is new, more likely, it's just been overhauled. Unfortunately, I have to tell you that I have had quite a few magnetos rewound by professionals, and had them fail shortly after.
If you want to go further, you could invest in a Spark Gap Tester (as I always say), then you will know that there should be a spark under compression (but don't test it with more than a 5/16" gap).
Assuming your spark is good, it could simply be that the ignition timing is too far retarded. If you're new to all this, are you sure you're following the correct procedure? For example, setting the timing on full retard rather than full advance? I assume you're using a fag paper, and checking it again after you have tightened the drive pinion nut? Are you using a rod down the plug 'ole or a degree disc?
Further to Scifi's suggestion, I understand that if you take all your clothes off while laying prone, you go faster still, but you may want to wear a Jock Strap just in case.

[neil1964]

It has been suggested to me (please correct me if you can show that I am way off track) that if you have a slightly weak magneto (failing magnets, poor slip ring, excessive air gap failing coil insulation, poor capacitor and  dirty contacts etc) that you need to consider the flip point of the rotating coil in the magnetic field where the output of the coil is at its maximum.  This can manifest itself when you retard the ignition fully and you are behind the flip point (ie no longer in the peak phase of the output cycle and you get a weak signal) when the points open.  This can be an issue if a clockwise mag is repurposed for an anticlockwise application (when looked at from the drive end ie timing cover).

The BSA magneto for a b31/3 is an anti-clockwise (you will see and arrow on the A shaped cover plate on the mag, but of course these things have often been rebuilt and repurposed many times over the years but if the builder has not allowed for this you may get a good spark when advanced (ie in flip point) or weak spark retarded (outside flip point).
Once the engine is running you are more likely to get a decent spark but if you are not giving the engine a good swing for 500rpm to start up then a weak and ineffective spark is the result.

[briancairns]

It has been suggested to me (please correct me if you can show that I am way off track) that if you have a slightly weak magneto (failing magnets, poor slip ring, excessive air gap failing coil insulation, poor capacitor and  dirty contacts etc) that you need to consider the flip point of the rotating coil in the magnetic field where the output of the coil is at its maximum.  This can manifest itself when you retard the ignition fully and you are behind the flip point (ie no longer in the peak phase of the output cycle and you get a weak signal) when the points open.  This can be an issue if a clockwise mag is repurposed for an anticlockwise application (when looked at from the drive end ie timing cover).

The BSA magneto for a b31/3 is an anti-clockwise (you will see and arrow on the A shaped cover plate on the mag, but of course these things have often been rebuilt and repurposed many times over the years but if the builder has not allowed for this you may get a good spark when advanced (ie in flip point) or weak spark retarded (outside flip point).
Once the engine is running you are more likely to get a decent spark but if you are not giving the engine a good swing for 500rpm to start up then a weak and ineffective spark is the result.

Apologies Steve, i am only seeing this. My friend who knows alot more about mechanics than I is looking over the bike next week....I will let group know what I'm doing wrong ( no doubt) . thanks for your help
Brian

[briancairns]

It has been suggested to me (please correct me if you can show that I am way off track) that if you have a slightly weak magneto (failing magnets, poor slip ring, excessive air gap failing coil insulation, poor capacitor and  dirty contacts etc) that you need to consider the flip point of the rotating coil in the magnetic field where the output of the coil is at its maximum.  This can manifest itself when you retard the ignition fully and you are behind the flip point (ie no longer in the peak phase of the output cycle and you get a weak signal) when the points open.  This can be an issue if a clockwise mag is repurposed for an anticlockwise application (when looked at from the drive end ie timing cover).

The BSA magneto for a b31/3 is an anti-clockwise (you will see and arrow on the A shaped cover plate on the mag, but of course these things have often been rebuilt and repurposed many times over the years but if the builder has not allowed for this you may get a good spark when advanced (ie in flip point) or weak spark retarded (outside flip point).
Once the engine is running you are more likely to get a decent spark but if you are not giving the engine a good swing for 500rpm to start up then a weak and ineffective spark is the result.

Apologies Neil, i am only seeing this. My friend who knows alot more about mechanics than I is looking over the bike next week....I will let group know what I'm doing wrong ( no doubt) . thanks for your help
Brian

[briancairns]

Hi all,
Finally got bike sorted.
The problem was my own stupidity .
I did not have engine tiiming set up correctly---too retarded, that along with a small tweak to carb .
Bike now going fine, except for  a gear box issue which I will put into a differnt thread.
Thanks so much for all your invaluable advice.

Brian

[Dan_B33]

With whole respect You owe us top speed, right?