Author Topic: Super rocket how can I tell which cylinder is on compression stroke ?  (Read 901 times)

Offline Brianh

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 i am about to refit my cylinder head on my 1960 super rocket following replacement of loose valve guide and springs   . but before I do I want to check the timing I have previously done this using the pencil down the plug hole approach .  But I think I can get a more accurate reading with the head off .  My question is     how can I tell which cylinder is on the compression stroke ?  I am thinking the push rod position will tell me but I am not sure what to look for   Any help would be greatly appreciated.

Brian H

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Put one pair of pushrods in place. Turn the engine over until the pushrods are rocking, (one coming up, other going down) when a piston is at top of stroke. That cylinder is not at the firing stroke so use the other one for timing.

Online RichardL

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I don't think it matters what cylinder you use when timing with the head off. I say, use the left, since it's convenient while tugging your slice of fag paper out from between the points.

Richard L.
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I made up a tool that has an adjustable bolt that acts as a stop for the piston at exactly the correct BTDC. Having set this I used my multimeter to detect when the magneto points opened before tightening the points taper bolt.

Offline Brianh

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Thanks Greybeard just the info I needed . I have made up a similar tool to the one you mention and have a good quality digital multimeter but am having trouble making any sence of the readings can you explain where to make the connections so I can try again   or maybe I will stick to the tried and tested Rizla tool .

Richard  thanks for your comments which I understand .   but I wanted to know what pot was on compression for timing so I can reuse my existing HT Leads etc  in the same configureation as they were before I took the bike to bits.

What a Great Forum this is, I doubt you would have had been able to get more information from BSA themselves

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...digital multimeter but am having trouble making any sence of the readings can you explain where to make the connections so I can try again...
It's not a clear closed/open change it's a just a difference in resistance. I clipped the meter leads onto points arm and mag body and tried different resistance ranges until I could see a change from points closed to points open. I know cigarette papers are popular for this job but I thought I'd try using my meter and it worked. Anyway, I don't smoke these days.
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What a Great Forum this is, I doubt you would have been able to get more information from BSA themselves
Yup; I love it!  I feel these folks from all over the world are my friends *loveit*

Online RichardL

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BrianH:

Just look for the brass on the slip-ring passing by the pickup hole.

As for using rolling papers, I tear off a thin strip because it's easy to get between the points. Sometimes I tug on it by hand and sometimes I attach a clip lead draped over the brake pedal as a tell-tale weight. When the points open the lead and paper fall to the floor.

Richard L.
Plan on signing up for the world-wide 2020 DISTINGUISHED GENTLEMAN'S RIDEon September 27, 2020. This year it's a solo or pillion ride in dapper attire. Watch website at https://www.gentlemansride.com for details.


Online terryg

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Richard, I do something similar.  A strip of aluminium foil with a heavy nut/bolt taped to it - large enough to make a good 'clunk' as it hits the floor.
Terry
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Offline a101960

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I bought one of these about 10 years ago it makes setting up the mag an easy job. I do not worry to much about which cylinder I am setting up the timing on, after all it is a simple task to swap the HT leads over. By the way if you are want to make certain that you are 100% accurate then you will need to make yourself a piston stop. Turn the engine over carefully untill the piston touches the stop. Mark your timing disc, then rotate in the opposite direction untill the piston again touches the stop, and mark your timing disc again. Then mark the point halfway between the two marks and you then have absolute T.D.C.

Online RichardL

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I bought one of these about 10 years ago it makes setting up the mag an easy job.


Really? I can't see the socket it uses for tightening the mag pinion bolt without changing the points position. ;) *smile*

Richard L.
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I bought one of these about 10 years ago...
Is that device a continuity tester or is there more magic to it?

Offline a101960

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Is that device a continuity tester or is there more magic to it?
Not sure exactley how it works. It is powered by a 9v battery and connects across the points. L E D lights up and buzzer comes on when the points open. It was advertised and sold as being a dedicated tool for detecting when the magneto points were just opening. It was sold by Idependent Ignition Supplies (No long trading). The description in the instruction book says this: "This unit has been designed to accuratley determine the contact breaker opening point on any type of rotary high tension magneto, including flywheel types, and takes the guess work out of ignition timing" The book goes on to say that it is not suitable for use with coil ignition. It can also be used to set the "E" gap what ever that is.

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Think it'll be a v sensitive ohmmeter, designed to detect the 1/2 ohm odd difference between points shut (in theory if all is well, zero ohms) and points open (when you're measuring the R of the low tension winding).

One day someone will probably make a 'points setting special cb centre screw'  of tough non-conducting material, strong enough to hold the backplate where it needs to be . . .
. . . and then a battery and bulb would do the job. 
When done, replace screw with steel one.


Bill

Offline kiwipom

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Hi guys, My electronic Ign uses the wasted spark system so I don't have to worry about which cylinder is firing, cheers
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Offline Rocket Racer

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its also worth checking the timing on consecutive TDC's as each cylinders timing may differ a few degrees due to points cam variation, to find the best compromise between the two... Quite a few ignition variables out there. Neither of my twins run a lucas mag, just my single which I only check the one side for  *whistle*
A good rider periodically checks all nuts and bolts with a spanner to see that they are tight - Instruction Manual for BSA B series, p46, para 2.
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