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ATD Advance Timing Curve?

cinquecento

Well Known and Active Forum Website User
Non-VOC Member
Just want to check i understand this correctly.

Rebuilt mag on Comet and replacing the ATD that has obviously had 'attention' previously. One of the 'ears' on the ATD has been bent to limit the travel of the weights. From reviewing posts this is not an unknown bit of tweaking, my question is could this have been over done by PO to negate the possibility of 'pinking'.

I'm setting the points set at 0.012 opening at 34 deg BTDC full advance. Prior to mag work the motor struck me as slightly wheezy topping out at just below 70mph.

By judicial adjustment of the 'ear' on the ATD and resetting ATD at new BTDC full advance position would it be reasonable expect a littlt more top end 'whumph'. I'm used to making throttle / gear adjustments to counter 'death rattle' on my other bikes and i understand that competative riders would rather suffer the potential of a little pre detonation to ensure they were reaching the limits of out put.

Comments?
 

chankly bore

Well Known and Active Forum Website User
Non-VOC Member
As noted in other threads, Martyn's main problem was small main jet. It is still best to bring the full advance back to about 35 degrees BTDC because of the faster burn rate of modern jungle juice. Use only 95 or 98 R.O.N. fuels. My nip-up was on a long uphill after filling up with ethanol crap.
 

Martyn Goodwin

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VOC Member
As noted in other threads, Martyn's main problem was small main jet. It is still best to bring the full advance back to about 35 degrees BTDC because of the faster burn rate of modern jungle juice. Use only 95 or 98 R.O.N. fuels. My nip-up was on a long uphill after filling up with ethanol crap.
I have had my failed (melted) piston examined by an expert aeronautical engineer who specializes in combustion chamber physics. It was his VERY STRONG opinion that it was NOT a lean mix that caused the piston and ring failure, but rather detonation. He recommended using the highest octane fuel available and also reducing the ignition advance to NO MORE then 32 degrees BTDC.

Martyn
 

redbloke1956

Well Known and Active Forum Website User
Non-VOC Member
Just want to check i understand this correctly.

Rebuilt mag on Comet and replacing the ATD that has obviously had 'attention' previously. One of the 'ears' on the ATD has been bent to limit the travel of the weights. From reviewing posts this is not an unknown bit of tweaking, my question is could this have been over done by PO to negate the possibility of 'pinking'.

I'm setting the points set at 0.012 opening at 34 deg BTDC full advance. Prior to mag work the motor struck me as slightly wheezy topping out at just below 70mph.

By judicial adjustment of the 'ear' on the ATD and resetting ATD at new BTDC full advance position would it be reasonable expect a littlt more top end 'whumph'. I'm used to making throttle / gear adjustments to counter 'death rattle' on my other bikes and i understand that competative riders would rather suffer the potential of a little pre detonation to ensure they were reaching the limits of out put.

Comments?
Hi Cinquecento, I would be interested to hear what advance you settled on and the results of the change.

Regards
Kevin
 

greg brillus

Well Known and Active Forum Website User
VOC Member
Hi Martyn, interesting comments from your aeronautical friend.....There are indeed lots of reasons for detonation, and an old fashioned air cooled engine is very prone to it. Especially now with our modern fuels we have to put up with. Even though your main issue stemmed form an overly advanced ignition, too high a compression ratio will also do it, and without any doubt, a lean mixture will as well. It is a fine balance of getting all you can as correct as you can to overcome this most damaging condition.....As you now, well know. A single which is working hard is more at risk, whereas a twin will still lazy along even if all is not as good as it should be. My Comet used to blatt along happily all day at 70 MPH with it's Mk 2 cam, it's exhaust note used to really sing...A great sound for sure......Greg.
 

cinquecento

Well Known and Active Forum Website User
Non-VOC Member
Hi Cinquecento, I would be interested to hear what advance you settled on and the results of the change.

Regards
Kevin
settled at 34 full avance and 5 fully retarded. I would have to say that these figures are +- 2 degrees as there is so much lash in the gear train, is rotating the engine using top gear and the rear wheel the best method? maybe I will try using the crank pinion next time.

The motor pulls cleanly up to 70mph and I feel there is more there, I've tried closing choke slightly to seeif that improved things ie indicated a larger main could be called for but no power increase, as i have stated elsewhere under extreme load i cannot provoke the least hint of pinking, so will advance by opening up points 2 thou'. However this can wait until i'vedone further shake down run to establish what next will come loose!

I'm new to all this Vincent malarky and if not done already I'd suggest reading through MartynG''s post's on the subject, very orderly and instructive.
 

Martyn Goodwin

Well Known and Active Forum Website User
VOC Member
Hi Greg,

Your observations are spot on. I am (now) running a CP piston reputed to be 7.5:1 and using 98 octane (RON) fuel. I have my Mk1 Amal carb jetted with a 105 needle jet (106 was way too rich at partial throttle openings) and a 220 main (a 210 was used earlier). I am using a Mk1 cam. Biggest difference is the ignition settings - earlier I had 36 BTDC full advance with 4 BTDC when retarded. I also discovered that the pivot pins in the ATD were loose so its very possible that I was actually running a full advance a tad greater then the static set of 36. By repairing and tweaking the ATD (with some kind welding assistance from Ken Phelps) I an now running 32 BTDC advanced and 4 BTDC retarded - and I am using the original Lucas Magneto, rebuilt almost 14,000 miles back by Peter Scott.

Of passing interest - over the last few days I fitted a Pazon system - you beaut modern electronics - that provided 28 BTDC advanced and 2 BTDC retarded; my impression was an overall reduction in performance (of the bike) compared to my magneto setup. I then "adjusted" the Pazon to give 32 BTDC advanced and 6 BTDC retarded but was still not fully happy with overall performance, when "seat of the pants" compared to the earlier magneto setup.

I refitted the original Lucas magneto earlier today and then did a quick 60 miles test run just to confirm my conclusions on performance. The magneto will be staying in the bike!

I know what you are saying about the exhaust note. One of my favorite "tricks" is to take my Comet through the Mullum Mullum and Melba road tunnels at a moderate speed then open up the throttle and revel in the booming noise echoing around - seems to wake up any car drivers in the immediate area! As to open road speeds - I get a small amount of vibration around 92 to 95 kph but it smooths out if I go a bit faster. Happy all day running speed seems to be around 105 kph (about 65 mph).

Martyn
 

TouringComet

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VOC Member
VOC Forum Website Administrator
VOC Forum Website Moderator
as there is so much lash in the gear train, is rotating the engine using top gear and the rear wheel the best method?

When using the rear wheel with top gear engaged, I was told to bump the engine forward slightly past your chosen full advance position, then bump backwards towards your desired setting, to put the lash in the state it would be when the engine is pushing the drive train, then tighten up the ATD on the magneto taper. Is that correct?
 

greg brillus

Well Known and Active Forum Website User
VOC Member
Martyn your experiments with the ignition are impressive if nearly exhaustive......Good on you for putting in so much effort. You have pointed out something interesting re the vibrations.....They differ from machine to machine, and obviously reflect the variance in changes we make when rebuilding these things. What components the crank assembly are built up from, piston weights, etc. I have an early Rapide sitting in my shed, that when running has a terrible vibration at a fast idle, and definitely in the engine, not tank mounts and so on. Can't wait to find out what gremlin is causing that......Greg.
 
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