A STRANGE THING..

[Steve Morris]

I had a cutting out problem with the Comet. Every time it did 'it' there was no spark. I decided to bite the bullet and took the Mag to a chap who overhauled it for me. Now i have to say that since i refitted it 2 things have been quite obvious..

1. The bike performance is way better than it was. I put that down to 2 things, weak spark or ignition timing was a little retarded.
2. This is the strangest thing.. Before, the bike was smoking (white so presumed oil) white a lot. Since i refitted and timed the mag, the smoking is considerably less? It is really a lot less than it was?

Couple of things come to mind. Spark is now stronger so the oil is being burned more efficiently, OR, the timed breather was out of sync somehow, but how??

Can anyone tell me how the timed breather system works please?

Anyway, its running so well i am going out again today...

Cheers!

 

[chankly bore]

Search the Forum and refer to the "Richardson" diagram on the last pages of the "Timing Gear" chapter. The breather position here is shown correctly even if the camshaft isn't. Spindle slot points down. P.E.I. recommended increasing the width of the breather slot to 3/8" and radiusing the outer edges of the slot. Position of the gears shown is 4 degrees B.T.D.C. The other error in this section is that the ET98 thrust washers on the followers must be INSIDE, next the alloy timing chest wall.

 

[Martyn Goodwin]

I had a cutting out problem with the Comet. Every time it did 'it' there was no spark. I decided to bite the bullet and took the Mag to a chap who overhauled it for me. Now i have to say that since i refitted it 2 things have been quite obvious..

1. The bike performance is way better than it was. I put that down to 2 things, weak spark or ignition timing was a little retarded.
2. This is the strangest thing.. Before, the bike was smoking (white so presumed oil) white a lot. Since i refitted and timed the mag, the smoking is considerably less? It is really a lot less than it was?

Couple of things come to mind. Spark is now stronger so the oil is being burned more efficiently, OR, the timed breather was out of sync somehow, but how??

Can anyone tell me how the timed breather system works please?

Anyway, its running so well i am going out again today...

Cheers!

Think of the motor as an air pump. As the piston rises it attempts to create a vacuum in the cases and as it falls it attempts to compress the gases in the cases. Compressed gasses will try to force oil out of whatever openings it can find - and it there are none to find it will try to create them!.

So you want the timed breather closed for the full stroke of the piston rising - thats when its generating a vacuum and you want it open when the piston is going down so the gas in the cases does NOT get compresses but instead passes out thru the timed breather opening - BUT the standard slot in the timed piston sleeve ET141 needs to be widened so the breather is open for more of the down stroke; irrespective the open breather must close at or just before the piston gets to bottom dead centre so you get the maximum vacuum effect as the piston then rises- in my Comet this means I have it starting to open at 24 degrees ATDC and closing at bottom dead centre.

Here is an extract from "Tuning For Speed" where Phil Irving is talking about Vincent motors

Combining information from Irving, Richardson and Stevens I learnt that the breather spindle needed to be installed in my Comet with the breather spindle slot pointing downwards. In twins the slot in the spindle should be pointing forwards directly towards the top magneto stud.

Hope that makes it clear.

M

 

[Steve Morris]

Thanks Martyn, perfect explanation!

The more i work on the Vincent the more i am thinking how over engineered the bike is! I can also see why the bike was so expensive in its day...

I have another problem....

The bike is running well like i said, but....

Carburation. I set the carburettor up using the method i read in the Vincent.com website.

1. Standard pilot jet settings. i.e. screw pilot screw in until lightly seated then screw out 11/4 turns.
2. Then run the engine and adjust the throttle stop until a good idle is achieved, then adjust the pilot until the highest tick over is achieved, then repeat.

Problem i have is this. When i do this, all seems to work as per the instructions, but when i rev the bike instead of the engine returning to normal idle, it hangs up quite high? It is almost like the slider is sticking? So, i slacken the throttle stop nut and lower the slide, then adjust the pilot again etc, etc. I just cannot seem to stop the idle hanging up?

I stripped the carb last night (Amal 229 i think) and it was as clean as a whistle, then carefully put it back together, still the same.

Question is;

When setting a carburettor up from scratch, how do you set the throttle stop? My thoughts are it should be closed and the engine is breathing/fuelling through the pilot?

Any thoughts please!

Best regards,

Steve.

 

[passenger0_0]

Worn jet block and/or slide is the most likely cause.

 

[greg brillus]

This is a pretty common fault.......It is basically a lean condition at low engine speed. If the slide was badly worn you would need to have the mixture screw wound nearly fully in to compensate for the excess air bypassing around the slide. You will need to enrichen the mixture and get the engine to idle quite slowly, so by winding in the mixture screw this in itself will slow the engine. You are best off carrying out this on a warm but not hot engine. Go for a quick run around the block and then adjust.......When you adjust the mixture you need to set it on the rich side rather than where the engine revs at idle are at a higher level, this will make the engine rev like you describe. This can also be caused by weak ATD springs which don't allow the engine to retard at low speed, thus causing the engine to rev more at idle, this often causes quite savage kick backs on the kick start lever as well.

 

[Steve Morris]

Thanks Greg.

I have just had the Mag overhauled and i did check the ATD while it was down, all OK there.

The most annoying thing? It wasn't doing this before i 'played' with the carburettor, but i didn't make a note of the original settings, schoolboy error...

I am glad to hear though that this is a pretty common fault, for me this means i can fix it!

Greg, do you know, is there a 'base' setting for the throttle stop?

 

[greg brillus]

Not really given an old carb will let some air around the slide, sometimes the opening at the front of the slide ends up quite small. On a new one the small crescent shape at the front could be about 1 mm open, or about 10 to 12 mm in width visually. Make sure there is enough slack in the throttle cable and just lower the throttle stop screw for a slow idle, then adjust the mixture outward till the revs pick up to a peak, then screw the mixture screw back in about 1/8 th of a turn perhaps more to enrichen the mixture. A slightly rich mixture will help cool the engine and avoid spit backs as the throttle is opened. It is not uncommon on an old carb for the mixture screw to be only a half turn out from fully in, to achieve a good idle mixture. Only carb's in top condition actually have the supposed setting of around 1 and a 1/4 turns out, this varies from bike to bike and several other factors like cams an so on. Cheers and good luck.............Greg.