New Amal Premier 900 series Carb for Comets
- Thread starter craig
- Start date
After extensive road trials conducted over a number of weeks the following is the current suggested configuration for a Amal Mk1 Premier 930 carb fitted to a Vincent Comet. Also I have had the failed (melted) piston examined by an expert aeronautical engineer who specialises in combustion chamber physics. It was his VERY STRONG opinion that it was NOT a lean mix that caused the 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. So here is the conservative tuning configuration
#3 Slide,
Norton Commando style spray tube
Main Jet 240,
Needle Jet 105,
needle in centre groove,
RJ19 pilot jet
idle mixture screw 1 1/4 turns out.
Timing set to no more than 32 degrees BTDC at full advance
my original post (#7 in this thread) has been updated as well.
Martyn
#3 Slide,
Norton Commando style spray tube
Main Jet 240,
Needle Jet 105,
needle in centre groove,
RJ19 pilot jet
idle mixture screw 1 1/4 turns out.
Timing set to no more than 32 degrees BTDC at full advance
my original post (#7 in this thread) has been updated as well.
Martyn
Martyn,
Very nice work! If you think about the kind of things that cause detonation it becomes easy to see why the singles are so susceptible:
Lean fuel mixture
Advanced timing
High compression ratio
Incorrect octane
Lugging the engine
It is not difficult to get a few of these occurring at the same time, particularly in a single. Detonation can cause wildly high pressures in the combustion chamber as we know. What we are shooting for with the timing is to get the peak pressure in the combustion chamber to occur at 12-14 degrees ATDC. This would mean all of the pressure is pushing down on the piston at exactly the right time. If you think about the dyno, I started at 36 degrees and did a run every time I retarded the ignition a few degrees. I ended up at 19 degrees because the dyno was telling me I was making more and more power (you stop and go back to the previous setting when the power drops.) So you are really measuring where this peak pressure is occurring. So, every time you change the ignition, or the number of spark plugs or the porting or the piston, etc., you need to find out when that peak pressure is occurring.
When you look at a hot rodded engine with twin plugs, squish band and 11.3:1 pistons, the squish band and the twin plugs are not as much "hot rod" parts, but they are detonation preventers. They allow you to run the higher pressures in the combustion chamber without the deleterious effects of detonation. These changes often do not add more power, but they allow you to do the other changes that add more power, like the 11.3:1 piston.
If you think of peak pressure occurring at 12 degrees ATDC you can see that as you advance the ignition you are also advancing the place where that peak power occurs. In fact, you can advance it so much that it is occurring BTDC and actually slowing the piston down on its rise to TDC. This is not only robbing power, but it will wear everything out at an accelerated rate, particularly the big end. So, I hope you can eventually find a dyno as it will give a truly modern timing figure.
David
Very nice work! If you think about the kind of things that cause detonation it becomes easy to see why the singles are so susceptible:
Lean fuel mixture
Advanced timing
High compression ratio
Incorrect octane
Lugging the engine
It is not difficult to get a few of these occurring at the same time, particularly in a single. Detonation can cause wildly high pressures in the combustion chamber as we know. What we are shooting for with the timing is to get the peak pressure in the combustion chamber to occur at 12-14 degrees ATDC. This would mean all of the pressure is pushing down on the piston at exactly the right time. If you think about the dyno, I started at 36 degrees and did a run every time I retarded the ignition a few degrees. I ended up at 19 degrees because the dyno was telling me I was making more and more power (you stop and go back to the previous setting when the power drops.) So you are really measuring where this peak pressure is occurring. So, every time you change the ignition, or the number of spark plugs or the porting or the piston, etc., you need to find out when that peak pressure is occurring.
When you look at a hot rodded engine with twin plugs, squish band and 11.3:1 pistons, the squish band and the twin plugs are not as much "hot rod" parts, but they are detonation preventers. They allow you to run the higher pressures in the combustion chamber without the deleterious effects of detonation. These changes often do not add more power, but they allow you to do the other changes that add more power, like the 11.3:1 piston.
If you think of peak pressure occurring at 12 degrees ATDC you can see that as you advance the ignition you are also advancing the place where that peak power occurs. In fact, you can advance it so much that it is occurring BTDC and actually slowing the piston down on its rise to TDC. This is not only robbing power, but it will wear everything out at an accelerated rate, particularly the big end. So, I hope you can eventually find a dyno as it will give a truly modern timing figure.
David
Update Jan 8, 2014.
Given that the advise I received in December was that the earlier piston failure was due to detonation and not running lean I have been experimenting - at first with ignition timing and them with main jet size. Still no luck in getting dyno time so its still seat of the pants testing.
I changed the timing from 32 BTDC down to 30 BTDC (fully advanced) and found a improvement. So I did it again, this time stepping down to 28 BTDC, which BTW seems to be the full advance setting that Pazon use in their fully electronic system for a Comet. At 28 BTDC there seemed to be a further performance improvement.
It must be noted that I am using the Lucas Magneto and a mechanical ATD - the ATD would normally provide a range of 34 degrees from full advance to full retard. With my ATD I have bent its "ears" reducing its range to 30 degrees. What this means is with 28 BTDC fully advanced, retarded its at 2 ATDC when starting - it also means the when at low speed or at idle the engine is running around 6 degrees more retarded that the original design spec. of 4 BTDC fully retarded.
So with 28 BTDC fully advanced and 2 ATDC fully retarded I commenced further experiments , this time with the main jet. I changed down just one jet size at a time then did an extended run of around 200 miles. With a 230 main performance seemed better than with the 240 - especially high speed performance and pulling power on hills. With the 230 in place there was no noticeable "blueing" of the exhaust system.
I then changed over to a 220 main followed by another 200 mile day ride. Top end performance seems just a whisker better as did the performance on hills, though this time I did notice a slight blueing of the exhaust within the first 4 to 6 inches from the head. I am thinking (feedback is encouraged chaps!) that this Bluing may be happening because these the flame front is travelling down the exhaust pipe when the exhaust valve is open - so I have now reset my ignition to 31 BTDC fully advanced (it was 28 BTDC) which equates to 1 BTDC fully retarded.
Further road testing is scheduled for tomorrow.
Martyn
Given that the advise I received in December was that the earlier piston failure was due to detonation and not running lean I have been experimenting - at first with ignition timing and them with main jet size. Still no luck in getting dyno time so its still seat of the pants testing.
I changed the timing from 32 BTDC down to 30 BTDC (fully advanced) and found a improvement. So I did it again, this time stepping down to 28 BTDC, which BTW seems to be the full advance setting that Pazon use in their fully electronic system for a Comet. At 28 BTDC there seemed to be a further performance improvement.
It must be noted that I am using the Lucas Magneto and a mechanical ATD - the ATD would normally provide a range of 34 degrees from full advance to full retard. With my ATD I have bent its "ears" reducing its range to 30 degrees. What this means is with 28 BTDC fully advanced, retarded its at 2 ATDC when starting - it also means the when at low speed or at idle the engine is running around 6 degrees more retarded that the original design spec. of 4 BTDC fully retarded.
So with 28 BTDC fully advanced and 2 ATDC fully retarded I commenced further experiments , this time with the main jet. I changed down just one jet size at a time then did an extended run of around 200 miles. With a 230 main performance seemed better than with the 240 - especially high speed performance and pulling power on hills. With the 230 in place there was no noticeable "blueing" of the exhaust system.
I then changed over to a 220 main followed by another 200 mile day ride. Top end performance seems just a whisker better as did the performance on hills, though this time I did notice a slight blueing of the exhaust within the first 4 to 6 inches from the head. I am thinking (feedback is encouraged chaps!) that this Bluing may be happening because these the flame front is travelling down the exhaust pipe when the exhaust valve is open - so I have now reset my ignition to 31 BTDC fully advanced (it was 28 BTDC) which equates to 1 BTDC fully retarded.
Further road testing is scheduled for tomorrow.
Martyn
Just back from a 180 mile run. I can report that the change I made yesterday - moving the ignition from 28 to 31 BTDC fully advanced has indeed reduced the tendency of the exhaust to "Blue" (I polished all the blue out of the pipe before todays ride) I can also report that the bike felt like it had increased torque - more pulling power on hills - than on prior rides. Twice I pulled the spark plug, once after around 5 miles at a steady 70 mph with around 20% throttle and once after a long uphill of around 2 miles at 60 mph and a wide open throttle. In both instances the spark plug looked just fine - the electrodes were clean, the centre ceramic dark brown but free from black carbon and the outer metal body had a dusting of black carbon.
I figure for now I an going to call the tuning effort done. All that remains is to fix the timing when the ignition is fully retarded. As noted in my last post I now have fully retarded at 1 BTDC and fully advanced at 31 BTDC. I will add metal (by welding) to the ears of the ATD to yeild a 25 to 26 degree range. That will allow me to then set the ignition to be fully retarded at 4 to 5 BTDC and fully advanced at 30 to 31 BTDC. BUT I do not see this as urgent work.
Regards,
Martyn
I figure for now I an going to call the tuning effort done. All that remains is to fix the timing when the ignition is fully retarded. As noted in my last post I now have fully retarded at 1 BTDC and fully advanced at 31 BTDC. I will add metal (by welding) to the ears of the ATD to yeild a 25 to 26 degree range. That will allow me to then set the ignition to be fully retarded at 4 to 5 BTDC and fully advanced at 30 to 31 BTDC. BUT I do not see this as urgent work.
Regards,
Martyn
Just removed and stripped the ATD (a Roy Price unit that was installed new around 10,000 miles back) in preparation for modifications to reduce its operating range only to discover that the two pins that the bob weights pivot on had become very loose in their mounting. So once I have finished the repairs to the ATD and reduced its operating range (see post#75) I will be conducting additional road tests to validate my earlier findings. I expect to have all back together win the next 4 days.
Martyn
Martyn
ATD and bike all back together today - but with the temp around 43 C its just too hot to ride. Did get a first kick start in the workshop with retard 4 BTDC and advance 31 BTDC.
Martyn
Martyn
I have been following this thread, Martyn, with great interest.
I fitted my newly refurbished mag today and set the ignition at 35 BTDC as a first try.The bike went great till it got hot and the new mag failed
..
When all is ok again I will try 32 BTDC to see if you settings suit my Comet..John
I fitted my newly refurbished mag today and set the ignition at 35 BTDC as a first try.The bike went great till it got hot and the new mag failed
..When all is ok again I will try 32 BTDC to see if you settings suit my Comet..John
Your new "refurbished " magneto should not have failed due to heat unless it had a faulty, old or incorrect capacitor in it. Who did the work and what work was done? "Magnetoman" has published an edifying, nay dazzling series of articles on this subject on Britbike Forum.
I was put on to this repairer by another member who has had a few mags done by him,at the moment I have no reason to suspect that the repairer is not reliable.
The armature was rewound and the slip ring replaced ( yes I have followed all the posts by magnetoman and have noted his suspicions of 'new' slip rings).
I am certain the problem is heat as I had only done 21 miles and after 1 hour after the problem and having been returned home on a trailer the BLOODY THING STARTED.
I am waiting to hear from the repairer, so I will not name him till then. I have heard from a few people who have had problems like this and all from different repairers..John
The armature was rewound and the slip ring replaced ( yes I have followed all the posts by magnetoman and have noted his suspicions of 'new' slip rings).
I am certain the problem is heat as I had only done 21 miles and after 1 hour after the problem and having been returned home on a trailer the BLOODY THING STARTED.
I am waiting to hear from the repairer, so I will not name him till then. I have heard from a few people who have had problems like this and all from different repairers..John