ignition timing

Bikes or not. Whatever
keith.hodgenia
Holder of a Nylon Anorak
Holder of a Nylon Anorak
Posts: 250
Joined: Sun Feb 27, 2011 11:53 am
Location: Nantwich. Cheshire East
Contact:

ignition timing

Postby keith.hodgenia » Sun May 25, 2014 5:01 pm

Does anybody out there know the formula for converting distance BTDC into degrees BTDC. Thanks. Keith.

User avatar
charles.bulloch
Holder of a Nylon Anorak
Holder of a Nylon Anorak
Posts: 167
Joined: Sun Mar 06, 2011 8:58 pm
Contact:

Re: ignition timing

Postby charles.bulloch » Sun May 25, 2014 6:24 pm

Hi, Keith,

since x = rcos A + sqrt{l^2 - r^2sin^2 A} you can't get the angle in a simple manner. I once wrote a program in basic which got the angle by iteration, but probably I could not do it anymore. The program did it in ten steps. l is the con-rod length and r is half the stroke.

http://en.wikipedia.org/wiki/Piston_mot ... ght_angled

Charlie

keith.hodgenia
Holder of a Nylon Anorak
Holder of a Nylon Anorak
Posts: 250
Joined: Sun Feb 27, 2011 11:53 am
Location: Nantwich. Cheshire East
Contact:

Postby keith.hodgenia » Sun May 25, 2014 6:52 pm

Thanks for that Charles, I think! I'm working on an old blind head two stroke with easy access to the crank shaft end. I had hoped doing it in degrees would be easier and more accurate than a pencil down the plug hole Perhaps not! Keith.

John.Cheetham
Posts: 20
Joined: Mon Oct 14, 2013 9:04 pm
Contact:

Re: ignition timing

Postby John.Cheetham » Sun May 25, 2014 8:25 pm

Keith, You could try this website

http://www.torqsoft.net/piston-position.html

As in any calculation you will require the length of the con rod between centres.

John C

User avatar
PeterW
Holder of a Waxed Cotton Anorak
Holder of a Waxed Cotton Anorak
Posts: 327
Joined: Fri Oct 07, 2011 10:17 pm
Location: Oxfordshire
Contact:

Re: ignition timing

Postby PeterW » Sun May 25, 2014 9:05 pm

My father used to irritate me. I always liked to set ignition timing by the book, he'd set it by trial & error. He'd set it, take it for a spin, advance it, see how it felt, was it pinking etc., repeat a few times and settle on whatever he felt ran best.
His argument was that although the manufacturers settings were best when new, by the time he bought whatever it was it was old and worn. tolerances had changed, compression might be lower, adv/retard springs might be weaker and petrol might burn faster/slower/hotter/cooler. I don't think I'll ever have his feel for mechanics but I still try setting it a degree or two either side of correct just to see where it runs best.

So here's two questions.
1. what matters with timing - the travel of the piston or the time it takes to do it?
2. does modern petrol burn faster or slower?

keith.hodgenia
Holder of a Nylon Anorak
Holder of a Nylon Anorak
Posts: 250
Joined: Sun Feb 27, 2011 11:53 am
Location: Nantwich. Cheshire East
Contact:

Postby keith.hodgenia » Sun May 25, 2014 9:22 pm

Thanks John, that's just what I need. Peter, your Dad was right on the money, this bike is new to me so I am not too sure what" feels about right" is just yet. I took it for a spin today and it was running hot and pinking on the hills. So retard the ignition or richen the mixture or look at the none standard exhaust. Banbury is in 2 weeks so I need to sort it out. Many thanks Keith.

User avatar
charles.bulloch
Holder of a Nylon Anorak
Holder of a Nylon Anorak
Posts: 167
Joined: Sun Mar 06, 2011 8:58 pm
Contact:

Re: ignition timing

Postby charles.bulloch » Mon May 26, 2014 8:47 pm

There was a question on this very subject in this years Advanced Higher Physics paper. Unfortunately nobody at the exam board has an Ariel because the question ignored the effect of the con-rod length. Because the crank and piston is actually a linked system. So the x = rcosA equation is not valid.

Getting out more in future

Charlie

leon.mitchell
Holder of a Nylon Anorak
Holder of a Nylon Anorak
Posts: 279
Joined: Wed Aug 29, 2012 1:47 am
Contact:

Postby leon.mitchell » Mon May 26, 2014 11:08 pm

"I'm working on an old blind head two stroke with easy access to the crank shaft end."

35 degrees BTDC at full advance is probably fine.

Leon

keith.hodgenia
Holder of a Nylon Anorak
Holder of a Nylon Anorak
Posts: 250
Joined: Sun Feb 27, 2011 11:53 am
Location: Nantwich. Cheshire East
Contact:

Postby keith.hodgenia » Tue May 27, 2014 9:42 am

Thanks to everyone for their input. I seem to be making progress. As I mentioned I had some doubts about the exhaust system and whether it produced enough back pressure. It has the "coffee tin" expansion chamber often seen in the twenties, with a very short tail pipe. Period drawings show a much longer tail pipe back to around the rear wheel spindle. I have lengthened it a bit and reduced its diameter. The engine now revs well, Hopefully a bit more tweaking to the timing and mixture will see it running in an acceptable manner. My obsession with the ignition timing has been more about repeatability than ultimate accuracy, with very little room above the engine, measuring the advance by degrees could be a lot easier.

User avatar
Bob.Murphy
Holder of a Silver Anorak
Holder of a Silver Anorak
Posts: 941
Joined: Thu Nov 29, 2012 10:32 pm
Location: Kirkliston, on the West side of Edinburgh.
Contact:

Re:

Postby Bob.Murphy » Tue May 27, 2014 11:03 am

keith.hodgenia wrote: This bike is new to me so I am not too sure what" feels about right" is just yet. I took it for a spin today and it was running hot and pinking on the hills. So retard the ignition or richen the mixture or look at the none standard exhaust. Banbury is in 2 weeks so I need to sort it out. Many thanks Keith.


Aaaagggh . . I can't stand the suspense . . . WHAT IS IT :?: :?: :?:

Show us some pictures and put me out of my misery :twisted: .

:lol: :lol:

Bob.
My avatar shows the late Len Rich in 1970 with the bike I now have - a 1958 Ariel VH


Return to “Main Chat Forum”

Who is online

Users browsing this forum: No registered users and 10 guests