C15S Cam Timing Mystery

[C15S]

I know, never question Rupert, but the cam timing specs for my C15S don't make any sense.

I installed a degree wheel under the rotor nut, then marked TDC on the stator.  Per the specs in Rupert, the intake valve starts to open at exactly 51 degrees BTDC.  Rupert says, if I read it correctly, the exhaust valve is supposed to open 78 degrees BBDC.  My exhaust valve opens at almost exactly BDC, that is 78 degrees late!  Since the cam is not broken, and intake cam is correct, how can the exhaust valve opening specs be that much different?  Even if someone swapped cams the difference stated in Rupert between the various 250cc singles is only 61.5 - 78 degrees BBDC.

The engine is difficult to start but when I get it started it only runs for about 10-15 seconds before it quits, and it's not from fuel starvation.

What am I missing?

[rhyatt]

Has it ever run with this cam in it before ?

[Rupert]

You're always welcome to question Rupert!
Any valve timing figures I've quoted are BSA's own.
It's always possible that you have a rogue cam, if unlikely.
If you have another cam of any sort to try instead that would determine whether the problem is cam-related or not.
Either way, always check the camwheel tooth to use for timing by -
Crank at exactly tdc. Cam fitted with timing mark at 12o'clock should result in both followers being equally, slightly raised. If not, rotate the cam to achieve that and the tooth to use for timing will then be at 12o'clock.
However, the fact that the engine runs, if only for a short time, suggests that the cam is not at fault.
Definitely check valve clearances and that the valve lifter is clear of the exhaust rocker.
Having verified those, fuel is the next on the list and what you describe sounds like fuel starvation. Before tinkering I'd be looking at choking the intake to richen the mixture. If you don't have a choke fitted cover the intake about half way for starters.
Anything which makes any change in how the engine behaves can be taken as indicative of what to do next.

[scifi]

If you still have the timing wheel fitted, can you supply us with all the timings...

INLET    Opens..BTDC........    Closes ABDC.........

EXHAUST  Opens..BBDC........    Closes ATDC.........

These figures should be (diagonaly) almost identical, but some cam grinders may have different ideas.

By setting the overlap to be equal at TDC, you will be near enough for NASA work.

[C15S]

Sorry for the delay in responding to your excellent advice.  What follows would seem to make no sense.  I took the tappet covers off and the clutch cover off so that I can turn over the motor using the rotor nut.  The data below is the START of a rocker arm going down (open) or coming up (close).

Intake Open = 51 BTDC (per Rupert)
Intake Close = 60 BBDC (not even close to Rupert, should be 68 after)

Exhaust Open = 20 BBDC (not even close to Rupert, should be 78 before)
Exhaust Close = 70 BTDC (not even close to Rupert, should be 37 after)

I have had the trans cover off more than once and the dot on the cam gear lines up with tick mark just peeking out from behind the washer on the drive gear.  There is no V on the cam gear.  As much as I hate to pull off the transmission cover, I guess I I will have to do that to confirm that the cam gear is correctly set.

I should add that I also install a new Lucas RM13 rotor and stator as well as a new coil.  The output more than exceeds the minimum voltage.

The motor has randomly started, but the exhaust is very erratic with a very loud popping out the exhaust, like maybe the motor is firing before the exhaust valve is completely closed.

 

[rhyatt]

Has the woodruff key on the crankshaft / cam pinion sheared.
Or did it ride up and out of its slot when you reassembled the timing pinions onto the crank ?

[Derek996]

I'm not familiar with the unit construction singles, but since all the cam lobes are on the same shaft, if the inlet opening is correct then all the other timings should also be correct with possibly slight variations depending on the accuracy of the cam grinding.

[JulianS]

It can be quite difficult to accurately measure the valve timing.

The tappets need to be set at the checking setting not the running setting. If your engine number has the 51/68 inlet and 78/37 exhaust cam then the checking figure for this cam from the BSA data book is 15 thou. Then you need to start the measurement from when the valve is about to start to opening to when the valve has just closed.

[scifi]

Hi C15S...  Somethings that you have written makes no sense.  The START of the rocker arm going down and then coming up.'

It is not when the rocker arm starts to come up, it is when it has fully returned to UP.

i.e. you should measure when all Tappet Clearance is lost, to when the Tappet Clearance re-appears.

Each cam starts Before and ends After their respective strokes.  Your figures all say Before XXX

.

[V500]

 The quick-and-dirty test is to get the engine on TDC on the EXHAUST stroke, and rocking the crank around this point should see the EX rocker close and IN rocker start to open.
Not exact but close enough for a check.
The one-piece cam should make it very easy too.

[C15S]

Thanks again for all the comments.

V500 - Your suggested test showed the cam timing to be correct. I think all of my confusion about cam timing was due to my own ignorance of the process.

Still, it will only start, and then run for only 20-30 seconds, if the timing is retarded to fire 3-5 degrees AFTER TDC.  If I set it to spec (1/16" BTDC) I just get nothing but a sore leg.  I am really ready to quit this beautiful 1962 Scrambler, completely disassembled and rebuilt, into which I have poured more money than it's worth.

[rhyatt]

when it stops , does it stop due to no spark or no fuel ?
a simple HT neon tester on the plug will show if you are running out of sparks--

https://www.ebay.co.uk/itm/127238419013


and when it does run , is it healthy or flat

[C15S]

Good question.  Just ordered the tester so I will have to wait a bit to find out if its spark.  Hard to say.  Alternator is putting out plenty of voltage, but there also seems to be plenty of fuel.  With the distributer correctly set, I have sprayed in starter fluid with no luck.  I should add that the distributor body has never been out of the motor and appears to be correctly positioned.  As to your other question, when I do get it to start it runs very poorly, will not respond to the throttle, with a very loud popping from the exhaust.  As I said earlier, it is though it is firing before the exhaust valve is completely closed.

This is a very simple motor, and I feel the solution to all of my issues is something very simple I just keep missing.

[rhyatt]

You will only know the alternator output if it has been running and you have a voltmeter across the battery to monitor the charge.
Or , is it running on the 2 MC capacitor system without a battery?

Could it be a dodgy big blue capacitor , not storing enough voltage to give a decent spark and giving you a weak slow burning spark.

I think the Boyer power box is an optional replacement,  but others may know more

You say the distributor body has never been out of the engine and "appears correct" , but this is how you check the ignition timing by getting the engine just before tdc and rotating the distributor just to where the cb points break.

to eliminate the capacitor (if this system is what you are using) just lash up a decent charged battery and hot wire it to the coil and earth (care which polarity to earth) , this will give you a total loss voltage to the ht coil - don't worry about charging at this stage - see if it runs better like that.
another thought , is your rectifier fine ? or suspect if you are running the 2MC system?

if you are running the 2MC, then the capacitor will take a few kicks anyway to "charge up" sufficiently to give enough voltage to spark. it may well just be "tired"
bearing in mind they are 50 years old and they do have a shelf life that is way past its best - new ones are around 25 quid I think

[limeyrob]

I'm going to lay my reputation on the line (what reputation I hear you say) and say its nothing to do with the cam timing. Its a bit if mixture and a lot of ignition.  The mixture is igniting very late and that could be due to very retarded or very advance timing or a "ghost" spark due to alternator pulses.  I have personally seen an A65 with 8 sparks on a strobe, The 2 correct ones and 6 from the alternator magnets.  The cure was a new battery as this was not smoothing the current. I later replaced the battery with a large (about the size of a coke can) capacitor and the bike ran well with no battery.
I would strip the distributor, check the drive gears and auto advance and get that right, then go over the alternator and wiring and pay particular attention to any capacitors.  If possible I would hook up a total loss battery and see if that solves the problem as a bit if fault finding.