Jump to content

Ignition Advance / Retard ?


slogger
 Share

Recommended Posts

The last few years have been a great experience for me as I have learnt to maintain / restore a number of T/S bikes following me getting back into trials. I am slowly learning a number of skills which I would have previously not had the confidence to tackle without the help of forums such as this, so firstly thanks to all!

So my next learning curve is around a Bultaco 175 which had its first shakedown ride but i have put this in the general section as it is around tuning...

My symptoms of poor starting and irratic pick up from tickover in sections points to the slow idle jet still being blocked (the bike was stood up for a long time when I bought it and opening up the carb, it was not a pretty site). I thought I had done a reasonable job of cleaning it but it is now having a dip in an ultrasonic cleaning bath!

My questions is around the fitting of a new electronic timing / stator and I have set it up based on the limited A4 sheet that came with it. I am trying to understand the basics of advance / retard and how this will effect running. Any knowledge would be appreciated....

thanks in advance

Rob

Link to comment
Share on other sites

 

advance (turning against flywheel) (sparks sooner ) direction = quicker response more power higher reving poorer starting possible kicking back odd back fire ,,retard turning in flywheel direction easier starting ,flatter smoother power possibly more torquey ,

  • Like 1
Link to comment
Share on other sites

 

No different from points in principle, if it's advanced the firing point is brought forward so that it fires further down the cylinder - this makes the engine respond quicker. If it's retarded it fires later, or, closer to top dead centre, which makes the engine pick up slower.

With points, the timing stays at the same setting throughout the entire rev range, therefore if you set the timing retarded to make it pick up off idle slower, you may find that it is too retarded at full revs to rev right out. Similarly, if you set it advanced to make the engine pick up quicker, it may knock or detonate at higher revs if it is too advanced.

The advance curve on an electronic ignition means that after the first (say 1000rpm for arguments sake) the CDI processor automatically begins to advance the ignition as the revs rise. This means you can set the backplate to a slightly retarded setting to get a slower throttle response off idle, but unlike points, the engine performance isn't then compromised at higher revs due to the ignition being too retarded, as the CDI will advance it as the revs increase.

That's about it in a nutshell. The timing for most trials bike can be set between 1.5mm BTDC to 3.5mm BTDC

You don't say which ignition you have, if it's Electrex I've no experience of them. If it's the type from Rex Caunt or Bultaco UK. You may find that the default setting is retarded a bit too much, especially on a 175. The 3 small allen bolts that secure the stator to the backplate have more than one mounting hole to screw into in the backplate, which enables the ignition to be fine tuned to suit your purpose by repositioning the stator as required. Turn it clockwise to increase timing, turn it anti-clockwise to retard it.

It's just trial and error until you find a setting you're happy with. I'd begin with how it comes.

  • Like 1
Link to comment
Share on other sites

 

Seems Woody has a good grasp on this timing thing which would apply to most any 2T motor as far as base timing. The only thing I might add here based upon my experience might be when venturing below 2mm BTDC, there could be other adverse effects such as elevated exhaust temps on high rpm running, and with a point system, the closer one gets to TDC, the closer one gets toward the point of potential reversal of engine operation. Points do not know which way things are turning! Thus, if one lugs a motor too much, it could reverse operation and send you travling backwards quickly!

I am not sure this exact point, yet I would use caution. There have been deaths due to this issue of a 2T running backwards when too close to TDC.

M2C

  • Like 1
Link to comment
Share on other sites

 

I must disagree with the timing running back to close to TDC Bultacos will kick back and run when set 3mm or more BTDC 2t engines run both ways Villiers fitted to Bond mini car did this as they only had forward gear so to reveres the engine was started backwards using the starter I have a BSA B33 with manual advance retard If the lever is not set right and is to far advanced and kicks back you can have at least a sore ankle or knee (hitting the bars ) or really bad a broken ankle after you come out of orbit Fortunately 4t do not run backwards

Link to comment
Share on other sites

 

I think naichuff is saying that a Bultaco will run backwards even with the timing set quite a way before TDC. I agree with this and add that advancing the timing can increase the tendency for the motor to run backwards, because at very low RPM (when kicking over, or when motor on verge of stalling), the motor is more easily be stopped and reversed because it fires with the piston further before the top of the stroke.

A different factor that can muddy the waters in discussions is that the motor will run better in reverse the more retarded the timing is.

Some bikes have a diode in the primary circuit to prevent ignition when the motor is rotating backwards

  • Like 1
Link to comment
Share on other sites

 
 
  • 2 years later...

has anyone with a more modern 2T any idea what the timing advance is at idle using a timing strobe light?  

I have mine at 6d BTDC at tickover at present and am about to experiment with 4d but based upon the simple fact that more modern trials bike already have this sussed I'd like to know?  anyone tried this trivial and silly game on a bike that doesn't even need it?  

 

so lads: ignition advance at idle on any modern 2T trials bike please?

 

my bike

Bultaco 250cc with electrex ignition.

 

regards

 

Taffy

Link to comment
Share on other sites

 

Depends what RPM idle is set at. Quite a few 2t ignitions rise from 0 advance at 400 RPM to say 7 degrees at 800 RPM.

 

 

Edit - Electrex should be able to supply and ignition map

Edited by dadof2
Link to comment
Share on other sites

 

Some bikes have a diode in the primary circuit to prevent ignition when the motor is rotating backwards

 

 That sounds incredibly simple, and effective. I wonder why all the old piston port bikes don't use it.

 Do you have more details?

Link to comment
Share on other sites

 

Depends what RPM idle is set at. Quite a few 2t ignitions rise from 0 advance at 400 RPM to say 7 degrees at 800 RPM.

 

 

Edit - Electrex should be able to supply and ignition map

they aren't replying to myself or the man that bought two at the same time.  

 

I would have thought 400rpm idle was a bit low for tickover?  I'll have to experiment I reckon. to me 6d should be over advanced but the proof of the pudding will be in the eating.  

 

thanks anyway

 

Taffy

Link to comment
Share on other sites

 

400 to 450 RPM is often cranking / starting RPM. Some ignitions fire at TDC or even after TDC at these starting RPMs to prevent kickbacks. Husqvarna chain saws used to fire after TDC below 480 RPM so they did not kickback and injure you hand with the starter cord.

Link to comment
Share on other sites

 
 
 

Depends what RPM idle is set at. Quite a few 2t ignitions rise from 0 advance at 400 RPM to say 7 degrees at 800 RPM.

 

 

Edit - Electrex should be able to supply and ignition map

 

 

I was always under the impression that 2 strokes had more advance at lower RPM (due to poor cylinder scavenging) and the curve retarded with higher rpm. The opposite to 4T engines?

Link to comment
Share on other sites

 

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

Loading...
 Share

×
  • Create New...