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Carb' Issue - ATD's - Ignition timing

cinquecento

Well Known and Active Forum Website User
Non-VOC Member
am i right in thinking that when running flat out in top that if i was to apply the choke slightly and speed picked up , it doesn't, but otherwise this would indicate that a larger main jet could be run.

The exhaust has a little blueing , the motor doesn't pink when laboured or pushed but could a larger main be called for as the pipe colour indicates the motor is running a little lean.

i've not done a plug chop but the plug (B7ES) on the 500 does look sooty generally.
 

Howard

Well Known and Active Forum Website User
VOC Member
Not sure plug chops are very accurate with modern fuel.
If you close the choke when running with the carb wide open, you are richening the mixture, so if it runs better the main jet is too small (all things being equal).
Blueing could be due to too much retard, but may be running too weak, and not necessarily the main jet, because you'll rarely be running on the main jet on the road.

H
 

cinquecento

Well Known and Active Forum Website User
Non-VOC Member
Too much retard !!, so i'll beef up the idling mixture or add a wee bit more weld to the ATD ears?
 

nkt267

Well Known and Active Forum Website User
VOC Member
No..Check your timing.adding weld to the ears on the ATD will only limit the amount of advance you have..What is your timing set at fully advanced??
The standard is 38deg BTDC.I have my Comet set at 34 and it seems ok.I think Martyn has his set at 32 unless he has changed it recently.
What sort of speed is 'Flat out'.
Also some pipes are made of material that is too thin and they can blue badly as well.
If the blueing is only for an inch or two then I personally would'nt worry too much, but check your mixture..What main jet have you got in and what needle position do you have.John
 

Martyn Goodwin

Well Known and Active Forum Website User
VOC Member
No..Check your timing.adding weld to the ears on the ATD will only limit the amount of advance you have..What is your timing set at fully advanced??
The standard is 38deg BTDC. I have my Comet set at 34 and it seems ok. I think Martyn has his set at 32 unless he has changed it recently.
.John
Checked my timing yesterday after a 500 mile trip and was surprised to find it at 28 BTDC fully advanced, clearly I had not been thorough enough when I was last "at it". Could explain why I was starting to get some discoloration or bluing of the exhaust pipe and silencer. Now carefully reset to 34 BTDC fully advanced which gives me 6 BTDC fully retarded. Am planning a short run tomorrow, around 100 miles - and will recheck everything again after then. The other thing I have done is set the exhaust valve just a whisker on the loose side. With unknown amounts of ethanol in the fuel these days (even though I always attempt to use ethanol free fuel) a slightly loose exhaust valve is cheap insurance against valve seat burning and failure caused - in part - by the higher burning temperature of the ethanol additives.

Here is how I set up my ignition timing. Make up a piston stop (ask if you want to know how to) that will stop the piston some where around 40 to 50 degrees BTDC. Remove the oil quill and then mount a timing disk into the quill hole. Remove the spark plug and insert the piston stop. Remove the caps over both the inlet and exhaust adjusters so you can see the movement of the valve rockers. With the gearbox in 4th gear very slowly rotate the rear wheel until the piston JUST makes contact with the piston stop. Now adjust the timing wheel pointer so that it points to TDC on the timing wheel. now slowly rotate the rear wheel in the OPPOSITE direction till again the piston JUST makes contact with the piston stop. Make a note of the reading on the timing disk. The actual Top Dead Center position is EXACTLY half way between the two points where the piston contacted the piston stop. To ensure nothing moves - at this point apply and lock on the rear brake. Now rotate the Timing disk ONLY so that the timing disk pointer is pointing to that mid point position. What you have now achieved is the timing disk correctly positioned to indicate true Top Dead Center.

You can now release the rear brake and remove the piston stop then proceed to set up the timing for whatever advance you want. You should set your timing when fully advanced, so you may consider making up a small wedge that you can use to lock the ATD in its fully advanced position. Remember to set the advance on the compression stroke - that's why you take the caps of the valve adjusters, so you can see that you are setting the timing at the correct cycle of the 4 stroke action.

If you need any more information, just ask.

Martyn
 

Simon Dinsdale

VOC Machine Registrar
VOC Member
VOC Forum Website Moderator
Checked my timing yesterday after a 500 mile trip and was surprised to find it at 28 BTDC fully advanced, clearly I had not been thorough enough when I was last "at it". Could explain why I was starting to get some discoloration or bluing of the exhaust pipe and silencer. Now carefully reset to 34 BTDC fully advanced which gives me 6 BTDC fully retarded. Am planning a short run tomorrow, around 100 miles - and will recheck everything again after then. The other thing I have done is set the exhaust valve just a whisker on the loose side. With unknown amounts of ethanol in the fuel these days (even though I always attempt to use ethanol free fuel) a slightly loose exhaust valve is cheap insurance against valve seat burning and failure caused - in part - by the higher burning temperature of the ethanol additives.

Here is how I set up my ignition timing. Make up a piston stop (ask if you want to know how to) that will stop the piston some where around 40 to 50 degrees BTDC. Remove the oil quill and then mount a timing disk into the quill hole. Remove the spark plug and insert the piston stop. Remove the caps over both the inlet and exhaust adjusters so you can see the movement of the valve rockers. With the gearbox in 4th gear very slowly rotate the rear wheel until the piston JUST makes contact with the piston stop. Now adjust the timing wheel pointer so that it points to TDC on the timing wheel. now slowly rotate the rear wheel in the OPPOSITE direction till again the piston JUST makes contact with the piston stop. Make a note of the reading on the timing disk. The actual Top Dead Center position is EXACTLY half way between the two points where the piston contacted the piston stop. To ensure nothing moves - at this point apply and lock on the rear brake. Now rotate the Timing disk ONLY so that the timing disk pointer is pointing to that mid point position. What you have now achieved is the timing disk correctly positioned to indicate true Top Dead Center.

You can now release the rear brake and remove the piston stop then proceed to set up the timing for whatever advance you want. You should set your timing when fully advanced, so you may consider making up a small wedge that you can use to lock the ATD in its fully advanced position. Remember to set the advance on the compression stroke - that's why you take the caps of the valve adjusters, so you can see that you are setting the timing at the correct cycle of the 4 stroke action.

If you need any more information, just ask.

Martyn
 

cinquecento

Well Known and Active Forum Website User
Non-VOC Member
i'm missing something here, 'too much retard' so it's being suggested i move the full advance forward from 34BTDC a degree or two or whatever would prove to give optimal top end performance / throttle response. But is this not done at the cost of the fully retarded position moving a corresponding amount and leading to difficult starting?

I'm sure trying to address this is the reason my ATD was in the gnarly state i found it when i took over the machine.
 

Simon Dinsdale

VOC Machine Registrar
VOC Member
VOC Forum Website Moderator
Not sure what happened then with my last post so will try again.

I do exactly as what Martyn does except I have set my piston stop which is made from an old spark plug to the actual timing figure I use. That way it can sit in the bikes toolbox and if I need to retime when away from home, you can put the stop in the cylinder, bring the piston up to the stop on the compression stroke and then rotate the mag until the points just open without rotating the engine. You need to lock the atd at full advance ( I use a piece of wood made to fit in the atd). Then tighten the atd nut.
That way you can reset the timing when out on the road without carrying a timing disc. In fact it's the way I do it in the garage as well.

Cheers
Simon
 

Simon Dinsdale

VOC Machine Registrar
VOC Member
VOC Forum Website Moderator
i'm missing something here, 'too much retard' so it's being suggested i move the full advance forward from 34BTDC a degree or two or whatever would prove to give optimal top end performance / throttle response. But is this not done at the cost of the fully retarded position moving a corresponding amount and leading to difficult starting?

I'm sure trying to address this is the reason my ATD was in the gnarly state i found it when i took over the machine.
Originally the atd unit when new had a 17 degree movement giving 34 degrees at the crank. Vincent's set the timing to around 38 degrees fully advanced which gave around 4 degrees fully retarded.
With modern fuel that burns quicker most people now set the fully advanced figure to around 34 degrees. This will set the retarded figure at 0 degrees or even worst if your atd is worn. Most originals are now very worn!
For ideal starting without kickback you want to be 4-6 degrees fully retarded so you need to check the timing of your atd at both fully advanced and fully retarded positions. If it's range is too wide, either replace it or weld up the ears to get the range you want.

In my experience if your fully advanced timing point is less than 30 degrees you will start to get some bluing of the exhaust.

Hope the above is clear to follow.

Simon
 

Martyn Goodwin

Well Known and Active Forum Website User
VOC Member
i'm missing something here, 'too much retard' so it's being suggested i move the full advance forward from 34BTDC a degree or two or whatever would prove to give optimal top end performance / throttle response. But is this not done at the cost of the fully retarded position moving a corresponding amount and leading to difficult starting?

I'm sure trying to address this is the reason my ATD was in the gnarly state i found it when i took over the machine.

Too much retard is actually saying that at the fully advanced position, the ignition is too retarded - BUT your comments on the ATD are a cause to stop and fix that before you do anything else.

A while back I destroyed a piston in my Comet thru detonation - caused mainly by too much advance. And that excessive advance was caused by a faulty ATD. I did not realise it at the time, but the pivot pins in my ATD had worn/broken loose from the backing plate. I set my advance to be 36 BTDC with the ATD locked in what I thought was the full advance position but subsequently found that by applying a bit of brute force I could actually force the advance up to around 43 degrees - remove the brute force and it seemed like the advance was just 36.

I completely rebuilt my ATD, welded up the pivot pins, fitted new springs and also added some weld to the ATD ears so I could set its range to suit what I was after at the time , being 32 Degrees at full advance and 4 degrees at full retard. As I said earlier, I am about to try 34 degrees of advance with 6 degrees retarded. I can already tell you with 6 degrees retarded there are no starting issues.

SO FIX THE ATD FIRST.

Based on my personal experience with detonation, without the validation from a dyno run, I would NEVER set the advance on my Comet to any greater than 36 degrees - just my opinion.

Martyn
 
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