motoplat trigger rotor removal

[IanA]

After fighting the bike (1975 Mark 111 350) for ages to get it to start regularly, i bought a brand new PHBH 30 AS dellorto carb and range of jets to replace the ageing Amal, and re-shimmed the valve clearances, but still no joy, although it would "Cough" every now and then. Played with the ignition within the allowance of the slots and the more advance the better the cough but would catch and run.
In desperation i removed one backing plate screw and advanced the ignition even further, and bingo, it now starts relatively easily and runs smoothly.

I have 2 issues now. One is the trigger rotor has a lot of play "in and out" on the crank, and secondly i cannot get the rotor off the crank either. I am suspicious that the rotor has "spun" a limited amount which is why i need to move the backplate beyond normal limits and that the locating slot on the back of the rotor is damaged hence hard to remove.

Has anyone encountered this problem before and any ideas on how to get the rotor off? I am loath to cut the trigger stator wires, but this looks the only solution if i want to remove the complete backplate to give better access to the rotor for attempts to remove using pullers etc
Any assistance greatly appreciated, regards Ian

[Jordan]

Maybe removing the whole front side cover off the engine will help?
Access to the rear of the ignition shaft might provide a way to release the trigger rotor.
The rotor should come away easily once its retaining screw is removed, as it is just slotted at the end with a sliding fit on the shaft, not keyed.
The Motoplat rotor is a solid-looking item, hard to see how it could slip.

[IanA]

Thanks Jordon. Inspecting it again it looks like the wires have been cut and joined before, i wonder if this is an old issue, not properly resolved. Will see if i can get away without removing whole cover but you are right that is a good next option. Once removed ill post photos if i can work out how.
I assume i'm correct there should be zero end float between the rotor and the crankshaft, not the 3-4mm i have??

[Duccout]

Ian, the rotor is not fitted to the crank, the in and out play that you have is play on the timing gear that is geared to the crank, and the play is normal. I doubt the rotor has slipped either, it engages in the timing gear via the slot (in the previous picture from Jordan) and is usually a tight fit. I think that the only way that you will get it off is to remove the timing cover, then you can gently lever the gear off the rotor, but if you haven't done it before, there are things to watch when you remove the timing cover - you must take off the top bevel cover and turn the engine until the dots line up, mark the position of the ignition rotor against the pick-up, so that it goes back in the same place, line up the oil pump tang in its slot, and make sure the timing gear has a shim at the front and back of the shaft. If you have a manual, study it carefully!

I doubt that you need to disturb the timing cover though, it sounds like you need to re-time the engine to find out where the timing actually is, possibly the slots in the back plate may need elongating, but you won't know this without timing the engine properly.

[ducwiz]

Snap 2021-01-21 at 10.58.07.png

Snap 2021-01-21 at 11.03.10.png

The rotor (200, on rhs) is slipped over the thin part of the timing shaft (613). Normally, it has a loose fit, but might be blocked by corrosion. (613) is accessible when the timing cover is removed. If you do this, it will be much easier to separate the rotor from the shaft, after having unfastened the rotors center screw. Now you also can check if a shim (29.120) is present on either side of the gear. By these, the shaft's axial float is controlled. According to the parts catalog, their dimensions are 15.5 x 22 x 1mm. You can add shims here to further reduce the float (i.e. 1/10 - 2/20mm), but check for the axial position of the rotor.

cheers Hans

[IanA]

Thanks Hans, spoke to Nigel Lacey who said movement should be approx 0.5mm - 1mm and that shims were required. Rotor off and timing cover removed to reveal why i couldn't get timing correct without removing a backplate screw.
Being a late 1975 bike the engine was obviously made in spain/Mototrans plant and shipped to Italy, bike bought from Mick Walker of Wisbech. The engine has the fibre timing gear and a tooth has seperated and jammed in the next and caused the gear to split, at which point i guess the timing jumped one or more teeth. The bike had stood for 10 years before i bought it so plenty of time to sit and harden. Replacement steel gear ordered and expect to have it back together by the end of the month. The rotor was in good condition and no damage, will use copperslip when re-assembling to make future removal easier.

[Jon Pegler]

Whilst the timing cover is off it is worth checking the gears inside the oil pump.
Some, but not many, Mototrans bikes had nylon gears in there too.
I would replace them if they are plastic.

Jon

[Ventodue]

Hi Jon,

Are you - or indeed any one else - able to explain just why Mototrans might have decided to fit these plastic gears?

Seems most strange to me ...

Ta.

Craig

[Duccout]

Hi Craig,

I was surprised when Jon said that some Mototrans bikes had nylon gears in the oil pump. I suppose that it is not too bad, at least they were well lubricated! It was just cheaper to make I suppose. Velocettes used a fibre timing wheel on the magneto drive automatic A/R unit which was prone to stripping its teeth, and I don't know why Velos fitted those either.

Colin

[Jon Pegler]

I think Colin is right when he states that the plastic gears are cheaper to produce.
I can't think of any other reason for fitting them.
The problems with these plastic gears is the cracking shown in Ian's photo, and stripping of sections of teeth.
With the idler gear it is not too catastrophic if it fails. As it drives the points gear, the motor just stops before any damage is done.
If the gears in the oil pump fail, and the motor continues in use, considerable damage could occur before the rider was aware of the problem.
Certainly, Mototrans used some of the plastic oil pump gears from the mid-1970s, although I don't think they used them for very long, returning to steel gears after a couple of years.
They continued to use the plastic idler gears right up to the end of production, though.
I think companies like Velocette used fibre timing gears to reduce gear noise as much as any other reason. Those gears were rubbish too, having worked on them in the past.

Jon