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E: Engine Comet Ignition Timing 2018


Bill Thomas

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VOC Member
I should have also mentioned that David T is running a single plug new BT-H, so a very typical ignition. The piston was changed to a 10.5:1 and the subsequent dyno run set the ignition at 28 degrees.

Many racers run at full advance, including me, but I could not tell you that it is hard on the engine from the performance.

I avoided mentioning ATD's because I think it can be a problem with no good answer, but it is interesting to see how owners deal with it.

Bill, you are good on that 64 mph at 4000. Of course, maybe never getting to full advance is a good thing!

David
What I am saying is if you set a small spring to work midway, It will stretch in a short time and the settings will go to pot !.
I don't like ATD s, I prefer the "D" type distributor, But that also has springs, When I was running 12 to 1 s
and wanted 24 degrees, I just put a plastic tube over the shaft to stop the bob weights shutting fully, It worked well. As I have said, My Comet has Manual advance, And I have played with the lever at all speeds, But once over 2000 ish it's best left alone.
I think theses are old bikes and as a rule work well. Cheers Bill.
 

greg brillus

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VOC Member
Most auto advance units usually had two springs of differing tensions. The fact that these seem to advance quickly to me would suggest that at low engine speed pulling up a hill with too much advance would cause detonation. But the extra heat from detonation is very different from a retarded ignition where the flame is burning too late.
 

vibrac

Well Known and Active Website User
VOC Member
Just take the alloy end cap off the BTH and look at the small pole ends of the coils, if there is any signs of rust or similar, buff it off gently with a dremel. Just make sure the rotor arm with the magnet on it is out of the way and be careful not to pinch the wires that pass through the small cutout at the lower rear of the cap when you re-install it. This turned a Shadow from a non starter to first kick again.
"Just take the alloy end cap off the BTH" mmmm
I am faced with one off round head allen screw, off set from the center on a plain alloy surface that I expect is the cap but it does no feel like a securing screw when I turn it anti clock it does not come out nor does it return to its original tension on going clockwise is this the securing screw for the cap? or how else is it secured? as it is supposed to be nonmaintainable is there a step by step guide anywhere I can see its a little delicate its also a model BTH from the early days of production
 

BigEd

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VOC Member
VOC Forum Moderator
Early versions of this single magneto have an extra part on the end cover with the BT-H logo machined in. This made it easy to locate and secure the items/wires inside and then fit the logo item to cover the hole. The Comet version didn't have this extra part because it had to be short enough to go under the magneto cover. The dome head Allen screw at the bottom right hand of your photograph holds an earth wire and a clamp for the CDI unit. It was awkward to fit things in and keep wires clear of the rotor arm. The end cap is held on by the Allen head caps screws, one of which can be seen at the top right of your photograph.
If you are going in there I suggest you go very carefully.
 

vibrac

Well Known and Active Website User
VOC Member
Early versions of this single magneto have an extra part on the end cover with the BT-H logo machined in. This made it easy to locate and secure the items/wires inside and then fit the logo item to cover the hole. The Comet version didn't have this extra part because it had to be short enough to go under the magneto cover. The dome head Allen screw at the bottom right hand of your photograph holds an earth wire and a clamp for the CDI unit. It was awkward to fit things in and keep wires clear of the rotor arm. The end cap is held on by the Allen head caps screws, one of which can be seen at the top right of your photograph.
If you are going in there I suggest you go very carefully.
thanks Ed careful it is, but I think it needs to be done as its spent most of its life on a racer with long periods of just sitting there
 

vibrac

Well Known and Active Website User
VOC Member
Early versions of this single magneto have an extra part on the end cover with the BT-H logo machined in. This made it easy to locate and secure the items/wires inside and then fit the logo item to cover the hole. The Comet version didn't have this extra part because it had to be short enough to go under the magneto cover. The dome head Allen screw at the bottom right hand of your photograph holds an earth wire and a clamp for the CDI unit. It was awkward to fit things in and keep wires clear of the rotor arm. The end cap is held on by the Allen head caps screws, one of which can be seen at the top right of your photograph.
If you are going in there I suggest you go very carefully.
Eddie
(This is a bit like a UXB defuse) I have removed the two screws but the cover wont shift with normal hand pulling. As its off the racer I expect there is a contingent of congealed R pulling the other way :)
so questions questions
would it be OK to inset a thin blade in the fine joint twixt cap and body to ease the cap off? could a smigin of petrol round the crack hurt the contents? is the cap over a long shoulder of the body or a short one? must I take care with the strength of pull and the distance moved because wires attached to the body and cap will be under strain?
 

greg brillus

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VOC Member
Just give the side of the cap a tap with a soft hammer, it possibly has a small bead of sealant on it, but it should come away without too much trouble. They are not that complicated, no de-fusing necessary.
 

vibrac

Well Known and Active Website User
VOC Member
soft taps have no effect and a sharp penknife blade cant penetrate the hairline crack I am lothe to apply a strap twist to the cap but I have applied some penetrating fluid and will tryit all again soon.
its no good me reading the thread about the stuck cap on the amal float bowl I cant use heat here..:oops:
 

vibrac

Well Known and Active Website User
VOC Member
a side ways tap on a projecting ear has opened up a can of worms (literally!)
1530350398031.jpeg
1530350432074.jpeg
referring to the first image I guess the face I need to clean is the edge of the rusted arm (difficult to see in all the rust) pointing at 10 oclock under the white plastic hex nut and the edge to clean is the one in the anular space between arm and inside body is that correct? or is it the tip of the rotor?.
I dont think I need look any further to the cause of my bad starting but as I say once running it worked fine....
 

Oldhaven

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VOC Member
The pictures of the (amazing) rust damage look be a result of condensation rather than leakage. Heating and cooling can pull in moist air that has no way to get out through the grommet or close fitting surfaces. (Think of a double pane window with a bad seal.) I am working on a design of a similar ignition housing and I am considering adding a small 1/16" weep and ventilation hole at the very bottom. This would be in the discolored area in the picture at the bottom of the cover that appears to have had moisture pooling. You mention that this is an early version, but is it necessary for the functioning of the magneto that ferrous metal parts be used? (that disc right under the rotor)
 
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greg brillus

Well Known and Active Website User
VOC Member
Wow Tim you've got a cracker there............... No wonder the bugger would not start. You're going to need a snorkel and flippers for that one.................You might even need to mark the arm and shaft, then remove the arm so you can clean the magneto properly. The smaller the air gap the better say 0.008 to 0.010" and use a brass feeler gauge if you have one. Yes original magnetos have small breather holes to stop this but also to vent the Ozone away from the points...............Now where's that bilge pump..............o_O
 

Pete Appleton

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VOC Member
VOC Forum Administrator
VOC Forum Moderator
Makes me wonder what the rest of it is like, generator portion and bearings. If it were mine I would carefully mark everything and have a look inside.

DISCLAIMER.... I have taken many more things apart than I have ever put back together.
 

vibrac

Well Known and Active Website User
VOC Member
I must admit I took the pictures and walked away so without a closer look, gutted. Thank heavens I remembered that forum post about rust. I will clean it up as best I can without a big strip and see if it kick starts OK as it works without fault once running. I am over my time budget for this bike with bad starting at the core and other jobs are backing up and the annual and the Manx are looming. if starting is still a problem then its away to BTH who are just down the road (but on Holiday at the moment) and perhaps its back to the original mag (which may be dodgy) or coil but that means loosing my independence from the electrical system. Hey Ho
 

chankly bore

Well Known and Active Website User
VOC Member
Two or three days in a 40% vinegar solution will get rust off steel, but keep it away from alloy! It also works well on seized steel bolts in steel holes. As long as yer don't mind yer bike smelling like a flying Chippy.
 

Normski

Well Known and Active Website User
VOC Member
a side ways tap on a projecting ear has opened up a can of worms (literally!)
View attachment 22703
View attachment 22704
referring to the first image I guess the face I need to clean is the edge of the rusted arm (difficult to see in all the rust) pointing at 10 oclock under the white plastic hex nut and the edge to clean is the one in the anular space between arm and inside body is that correct? or is it the tip of the rotor?.
I dont think I need look any further to the cause of my bad starting but as I say once running it worked fine....
Did you get this sorted Tim? I've had a misfire for some time and now not firing on front cylinder and I've changed just about everything in the ignition system and am starting to suspect the BTH electronic mag.
 

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