Valve and Ignition Timing

[frankfast]

Years ago I replaced worn out cam and rockers on my 350 Scrambler with a cam and rockers from a 250 Scrambler. Understanding that performance would be different, the bike ran well. I lined up the dots both at the cam and crank. The valve timing would obviously be different but should I have moved the ignition timing gear? If not, would the static timing be different than on the 350? Anybody done anything like this?

[Duccout]

I don't think that changing the cam will affect the ignition timing, so no, I don't think that you should move the timing gear.

[frankfast]

Thanks. Also, since I was told the cam was from a 250 Scrambler (not sure) but since the rockers have screw type adjusters (not shims), they may have come from a Monza or GT. When adjusting the valves what would you suggest as clearances since bikes with screw type adjusters have smaller clearances than those with shim type? Are clearances determined by the type of adjusters used?

[Duccout]

That I wouldn't know, although I would guess that the valve clearances are determined by the cam grind and not the type of adjusters, but you would need Eldert or Nigel to advise.

Colin

[frankfast]

Thanks Duccout for the suggestions. I, obviously have a bastard engine with a 350 WC bottom, a new high compression piston, a cam from a 250 Scrambler I'm told, although not positive (WC or NC?) and screw type adjusters. I wonder whether the screw type adjusters would be subject to more damage at a larger tappet clearance than the shim type. Tappet noise is noticeable. I haven't checked whether the valves would impact the high compression piston at smaller clearances but I doubt it since the piston was made for a 350 Scrambler with the higher lift green and white cam.The pockets in the piston are much deeper than the original. Just looking for some advice concerning tappet adjustment and static ignition timing for this hybrid.
In a discussion with Dewcati Bob a long time ago, he said he preferred the 259 Scrambler cam rather than the 350 green and white cam that was original. More bottom end at the expense of high end.

[themoudie]

I would agree Colin, but I'm a woodsman, not an engineer!

Good health, Bill

[frankfast]

I'm a woodsman too trying to engineer.

[themoudie]

Aye frankfast,

I believe that the 250 scrambler cam (White), if indeed that is what it is, is a 'softer' cam than the green and white cam.

My understanding is that the more valve overlap timing and lift the cam profile gives, the more stress that you are going to put on the interface between the rockers and the valve stem. Hence, whilst the threaded and locknut adjustable tappets are easier to maintain, they are more likely to succumb to higher stress input, than the shim.

Regardless, of cam, I believe that the shimmed rockers are going to be more stable and less likely to move out of adjustment than the threaded and nut locked adjusters. If all you have are the adjustable tappets and you want reliability, don't go mad with the cam timing and lift. If you have a lumpy cam and only adjustable tappets, you can still use them, but be prepared for them to require checking and adjustment more frequently than the shimmed version.

After all, 350 Sebrings used to have their necks rung, with green and white, or radical "one off" cams and adjustable tappets.

As for the working clearance between the tappet/shim and the valve stem, I believe that is influenced by the shape of the cam. Stick to the manufacturer's recommendations. Making it larger than recommended, increases the 'hammering' effect of the rocker on the valve stem. Whilst reducing the recommended clearance increases the risk of the valve not closing or even being battered by the piston. Neither of which are benificial for engine longevity!

If any of this is incorrect, I apologise and will be happy to take on board others opinion with more experience.

Good health, Bill

[LaceyDucati]

Hi
I would stick with the original 33 to 36 degrees BTDC ignition timing, I even use that range on my race 250/350 engines on single plug.

The 350 SCR's started with green and whites then quickly went over to white cams for most of the production. I would determine what the cam is (most likely a White cam if it was from a 250SCR). This cam is fairly easily identified from it's side profile and measured lift.

Regarding clearances I never run greater than 6 thou inlet and 8 thou exhaust regardless of profile and it's never been an issue for me. That said some of the milder cams can be run a bit tighter, more like the Ducati spec. However I err a little on the looser side for those rather than the 2 thou specified on some. All the Race cams I've used are all specified between 4 to 6 thou inlet and 6 to 8 thou exhaust, so if you run 5 and 7 thou on everything you won't be far wrong

The danger of running too tight a clearance is that if the valve gets hot under certain conditions and elongates, you will then possibly loose the clearance and risk burning the seat. The odd extra thou however will have little/no effect, but allow an element of safety.

Regards Nigel

[frankfast]

Thank you Nigel. Nothing out of the ordinary then.