Post by Rotax4ever on Jan 28, 2011 0:06:29 GMT -5
Heres a nice write up from another Vintage sled site about timing vintage rotax motors
this is way to valuable to not post!!
The guy who wrote this is named Bones
And i thank you bones!!
Check this out. It mostly pertains to twins but the singles are basically same. Check it out then post your questions.
Timing BK PT Rotax engines.
There is timing marks on all engines, one on fly wheel & one on fan shroud ( or area).
BUT…..Some are stamped to line up in the advance mode & some are stamped to line up in the retarded mode. If you have all the old Doo/Rotax specs you would know what you have by serial #s. But many fan shrouds have been replaced over the years, or the fly wheel may have been replaced ?, thus the marks may or may not be “true”, how doo you know if the marks are stamped advanced or retarded ?? don`t believe you can be sure. The best way to time the engine is to use a TDC gauge if you have one or can rent/borrow one. A major problem is many of old Rotax have a “angled” spark plug hole, thus you really should have a TDC gauge with a tiny wheel on the end of shaft, they seem a little elusive, you can doo it with a “non” wheel gauge, but MUST be very carefull not to bend/break the stem. I have a “wheeled” TDC gauge & would not time a old Rotax any other way.
Next you need some type of signal device ( electronic open/closed ). There is buzz boxes etc you can buy that work fine. I use a head light bulb from a Doo & a 12v power supply ( old battery or small charger), very inexpensive. This unit is to signal when the points open or close. Some alligator clips are very helpful when using this unit.
In a breaker point system the spark plug fires when the points open, when they are closed the generating coil is charging the condenser & high tension coil, when the points open the stored current has no where to go so it jumps the electrode ( plug).
You need to find the BTDC ( before top dead center) timing specs for your engine, all specs are given in thou of inch or mm BTDC in the FULL advance mode, regardless of how the marks were originally made, when using a TDC gauge you totally disregard the timing marks.
If engine is still in sled, unplug the wire harness from engine to sled body.
Breaker point gap spec I believe for all Rotax is .014” to .018”. Set both sets of points as close to .016” as possible. As original the top set of points controls the mag side & lower set controls the pto side, trace wires to ensure this is still correct. Unplug wires from point sets to high tension coils.
Set up the signal device ( here will use a light) on the upper point set. Connect the wire from points ( that was on H.T. coil) to one side of light bulb, connect other side of bulb to positive side of power supply, connect neg of power to grd on engine cowl, now the points will act as a on/off switch for bulb. Slowly rotate engine a couple of times to get a feel for light, it will be dim for part of cycle when points are open & fully bright for part of cycle when points are closed.
Install TDC gauge in mag plug hole, rotate engine & determine TDC, zero gauge. With mag piston at TDC, rotate engine apx ¼ turn counter-clockwise, light should be bright. Now hold advance lever in full advance mode ( toward magneto rim ). Slowly rotate engine clockwise (while holding lever), when light dims, the points have opened ( spark occurs). Take note of TDC gauge at this instant. If timing is too early, slacken the armature retaining screws & rotate armature clockwise, if timing is late, rotate stator counter clock, tighten plate screws & repeat procedure, until timing is within spec for mag side piston. Disconnect light from upper points & connect to bottom set. Install TDC gauge in pto plug hole. Repeat same procedure as previous. The timing now for the pto piston should be very close to correct. At this time you can slightly adjust the breaker points to change timing a little. Decreasing point gap will retard timing, increasing gap will advance timing, but you MUST stay within the specified .014” to .018” gap.
If you find you can not get both pistons within the specified timing specs with the point gap in the spec range, then there is other problems. Possible worn key or key way on crank end, twisted crank shaft etc. It is very uncommon to find these problems.
Now….If you don`t have a TDC gauge you can use the timing marks if `ya wish. The procedure is basically the same as above. You will watch the marks as you slowly rotate engine rather than watching the TDC gauge, but as mentioned previously I can not tell you if you hold the advance lever in advance mode or not. On a engine with timing set with little advance ie:~.060”, the difference from setting in advance mode or none advance mode, there is not much difference in timing, but on a engine with a fairly advanced timing ie: .180” the difference in advance/none is very significant due to piston speed being much greater at .180 than it is at .060”
Here is a little FYI ( point of interest). Taken from a `74/75/76 Ski-Doo shop manual.
For timing procedure it necessary to separate the twin cylinder engines into three groups.
Group 1: This group includes engine type 248 & 294. Standard timing procedures apply. These engines do not incorporate a automatic mechanism. The engine timing is set at the full advance mark stamped on the mag ring.
Group 11: This group includes the following: All engine type 338, all type 343 engines prior to engine serial # 2,670,920, all 401 type engines, all 434 type engines, all 440 type engines prior to engine serial # 2,748,146, and all 640 type engines prior to engine # 2,637,301. Standard procedure applies for this group. The timing is set at the no advance mark stamped on the mag ring. ( these engines do have a advance mechanism ).
Group 111: This group includes the following: All 305 type engines, all 343 type engines from serial # 2,670,920, all 440 type engines from # 2,748,146, all 640 type engines starting from # 2,637,301. On these engines the timing mark on the mag ring is stamped at the full advance position. Therefore when using the timing marks to set the timing always hold the centrifugal at the full advance position.
This bit of info is only for a few years, there is other changes to mark stamping over the years, so it is difficult to know what marks a engine has. As above the 248 & 294 have no advance, there is also a few others that way, with those engines, you can`t go wrong with the marks ( its fool proof ) . On others, if possible to use TDC procedure it is much better, all the printed timing specs ie: BTDC are listed for the full advance mode. So if the engine has a advance lever you will always hold it outward when setting timing.
Single cylinder engines are basically same procedure except you only have one set of points to worry about .
Good luck with it Bones
this is way to valuable to not post!!
The guy who wrote this is named Bones
And i thank you bones!!
Check this out. It mostly pertains to twins but the singles are basically same. Check it out then post your questions.
Timing BK PT Rotax engines.
There is timing marks on all engines, one on fly wheel & one on fan shroud ( or area).
BUT…..Some are stamped to line up in the advance mode & some are stamped to line up in the retarded mode. If you have all the old Doo/Rotax specs you would know what you have by serial #s. But many fan shrouds have been replaced over the years, or the fly wheel may have been replaced ?, thus the marks may or may not be “true”, how doo you know if the marks are stamped advanced or retarded ?? don`t believe you can be sure. The best way to time the engine is to use a TDC gauge if you have one or can rent/borrow one. A major problem is many of old Rotax have a “angled” spark plug hole, thus you really should have a TDC gauge with a tiny wheel on the end of shaft, they seem a little elusive, you can doo it with a “non” wheel gauge, but MUST be very carefull not to bend/break the stem. I have a “wheeled” TDC gauge & would not time a old Rotax any other way.
Next you need some type of signal device ( electronic open/closed ). There is buzz boxes etc you can buy that work fine. I use a head light bulb from a Doo & a 12v power supply ( old battery or small charger), very inexpensive. This unit is to signal when the points open or close. Some alligator clips are very helpful when using this unit.
In a breaker point system the spark plug fires when the points open, when they are closed the generating coil is charging the condenser & high tension coil, when the points open the stored current has no where to go so it jumps the electrode ( plug).
You need to find the BTDC ( before top dead center) timing specs for your engine, all specs are given in thou of inch or mm BTDC in the FULL advance mode, regardless of how the marks were originally made, when using a TDC gauge you totally disregard the timing marks.
If engine is still in sled, unplug the wire harness from engine to sled body.
Breaker point gap spec I believe for all Rotax is .014” to .018”. Set both sets of points as close to .016” as possible. As original the top set of points controls the mag side & lower set controls the pto side, trace wires to ensure this is still correct. Unplug wires from point sets to high tension coils.
Set up the signal device ( here will use a light) on the upper point set. Connect the wire from points ( that was on H.T. coil) to one side of light bulb, connect other side of bulb to positive side of power supply, connect neg of power to grd on engine cowl, now the points will act as a on/off switch for bulb. Slowly rotate engine a couple of times to get a feel for light, it will be dim for part of cycle when points are open & fully bright for part of cycle when points are closed.
Install TDC gauge in mag plug hole, rotate engine & determine TDC, zero gauge. With mag piston at TDC, rotate engine apx ¼ turn counter-clockwise, light should be bright. Now hold advance lever in full advance mode ( toward magneto rim ). Slowly rotate engine clockwise (while holding lever), when light dims, the points have opened ( spark occurs). Take note of TDC gauge at this instant. If timing is too early, slacken the armature retaining screws & rotate armature clockwise, if timing is late, rotate stator counter clock, tighten plate screws & repeat procedure, until timing is within spec for mag side piston. Disconnect light from upper points & connect to bottom set. Install TDC gauge in pto plug hole. Repeat same procedure as previous. The timing now for the pto piston should be very close to correct. At this time you can slightly adjust the breaker points to change timing a little. Decreasing point gap will retard timing, increasing gap will advance timing, but you MUST stay within the specified .014” to .018” gap.
If you find you can not get both pistons within the specified timing specs with the point gap in the spec range, then there is other problems. Possible worn key or key way on crank end, twisted crank shaft etc. It is very uncommon to find these problems.
Now….If you don`t have a TDC gauge you can use the timing marks if `ya wish. The procedure is basically the same as above. You will watch the marks as you slowly rotate engine rather than watching the TDC gauge, but as mentioned previously I can not tell you if you hold the advance lever in advance mode or not. On a engine with timing set with little advance ie:~.060”, the difference from setting in advance mode or none advance mode, there is not much difference in timing, but on a engine with a fairly advanced timing ie: .180” the difference in advance/none is very significant due to piston speed being much greater at .180 than it is at .060”
Here is a little FYI ( point of interest). Taken from a `74/75/76 Ski-Doo shop manual.
For timing procedure it necessary to separate the twin cylinder engines into three groups.
Group 1: This group includes engine type 248 & 294. Standard timing procedures apply. These engines do not incorporate a automatic mechanism. The engine timing is set at the full advance mark stamped on the mag ring.
Group 11: This group includes the following: All engine type 338, all type 343 engines prior to engine serial # 2,670,920, all 401 type engines, all 434 type engines, all 440 type engines prior to engine serial # 2,748,146, and all 640 type engines prior to engine # 2,637,301. Standard procedure applies for this group. The timing is set at the no advance mark stamped on the mag ring. ( these engines do have a advance mechanism ).
Group 111: This group includes the following: All 305 type engines, all 343 type engines from serial # 2,670,920, all 440 type engines from # 2,748,146, all 640 type engines starting from # 2,637,301. On these engines the timing mark on the mag ring is stamped at the full advance position. Therefore when using the timing marks to set the timing always hold the centrifugal at the full advance position.
This bit of info is only for a few years, there is other changes to mark stamping over the years, so it is difficult to know what marks a engine has. As above the 248 & 294 have no advance, there is also a few others that way, with those engines, you can`t go wrong with the marks ( its fool proof ) . On others, if possible to use TDC procedure it is much better, all the printed timing specs ie: BTDC are listed for the full advance mode. So if the engine has a advance lever you will always hold it outward when setting timing.
Single cylinder engines are basically same procedure except you only have one set of points to worry about .
Good luck with it Bones
