Timing a 650 twin

Helpful information and requests for assitance and advice
SOHC
Posts: 36
Joined: Tue Mar 07, 2017 11:04 pm
Location: Newzealand

Timing a 650 twin

Post by SOHC »

Hello

How meany degrees full advance BTDC should a 1959 AJS 650 twin fire? it has stock pistons, I just want to set the timing with a degree disk.
User avatar
ajscomboman
Member
Posts: 3963
Joined: Mon Jan 01, 1990 12:00 am
Location: HAMPSHIRE UK

Re: Timing a 650 twin

Post by ajscomboman »

39 degrees BTDC
SOHC
Posts: 36
Joined: Tue Mar 07, 2017 11:04 pm
Location: Newzealand

Re: Timing a 650 twin

Post by SOHC »

ajscomboman wrote:39 degrees BTDC

Cool thank you,
SOHC
Posts: 36
Joined: Tue Mar 07, 2017 11:04 pm
Location: Newzealand

Re: Timing a 650 twin

Post by SOHC »

I used a degree disk to set the crank at 39 degrees BTDC but when I measure down the plug hole with a measuring stick its 49mm.

Dose that sound right?
User avatar
ajscomboman
Member
Posts: 3963
Joined: Mon Jan 01, 1990 12:00 am
Location: HAMPSHIRE UK

Re: Timing a 650 twin

Post by ajscomboman »

No sorry my mistake, 39 is for a 500 it is 35 degrees for a 650. The measurement down the plug hole is 11/32 before TDC. Hold the rod vertical and bring the piston up to TDC and mark it. Remove the rod and mark 11/32 higher up and reinsert it. Rotate the wheel backwards so the top mark drops below the point of measurement and then slowly rotate the wheel in the correct direction of travel till the upper mark just appears at the point of measurement. That's about as close as you'll need to be on our tractors especially if you have manual advance/ retard. When setting I set the advance/retard lever just off full advance so it gives a little lea way.
SOHC
Posts: 36
Joined: Tue Mar 07, 2017 11:04 pm
Location: Newzealand

Re: Timing a 650 twin

Post by SOHC »

ajscomboman wrote:No sorry my mistake, 39 is for a 500 it is 35 degrees for a 650. The measurement down the plug hole is 11/32 before TDC. Hold the rod vertical and bring the piston up to TDC and mark it. Remove the rod and mark 11/32 higher up and reinsert it. Rotate the wheel backwards so the top mark drops below the point of measurement and then slowly rotate the wheel in the correct direction of travel till the upper mark just appears at the point of measurement. That's about as close as you'll need to be on our tractors especially if you have manual advance/ retard. When setting I set the advance/retard lever just off full advance so it gives a little lea way.

So 11/32 mark should be at the lower edge of the sparkplug hole when measuring?
User avatar
ajscomboman
Member
Posts: 3963
Joined: Mon Jan 01, 1990 12:00 am
Location: HAMPSHIRE UK

Re: Timing a 650 twin

Post by ajscomboman »

The top 11/32 mark should be at whatever point you chose to mark the rod in relation to the plug hole and the lower mark should be at the same measuring point point when you rotate the wheel to raise the piston to TDC. You must find TDC first and mark the rod then take it out without moving anything and mark the higher point 11/32 above it. Reinset the rod and rotate the wheel backwards whilst still holding the rod and then rotate forwards till the 11/32 mark is where your measuring datum point was.


So 11/32 mark should be at the lower edge of the sparkplug hole when measuring?[/quote]

If that measuring point was the point where you also marked TDC then yes.
ian_5931
Posts: 6
Joined: Tue May 09, 2017 10:30 pm
Location: England

Re: Timing a 650 twin

Post by ian_5931 »

User avatar
ajscomboman
Member
Posts: 3963
Joined: Mon Jan 01, 1990 12:00 am
Location: HAMPSHIRE UK

Re: Timing a 650 twin

Post by ajscomboman »

ian_5931 wrote:3/8" for 1959 models. See http://archives.jampot.dk/book/Owners_m ... Models.pdf page 41
Nope, that's a generic print from earlier manuals for the 500 twins despite it being stated it's for the 650.

The later workshop manuals will all state 11/32 for the 650 equates to 35 degrees BTDC as opposed to 39 degrees BTDC for the 500's which equates to 3/8".

Page 18 of the 57-64 workshop manual shows the different timing settings for 500 and 650's
SOHC
Posts: 36
Joined: Tue Mar 07, 2017 11:04 pm
Location: Newzealand

Re: Timing a 650 twin

Post by SOHC »

I was having brain fade there for a wile, I made a rod and marked off the top fin,

When I measure 11/32 off the piston and then check my degree disk it still comes up at 39 degrees BTDC
Locked