81 timing and ignition box

[onefour]

Hey all,

Had a timing light on the old betch the other night... Spark. Checkmark. At the notch on the wheel. Checkmark. Revving it and the mark wasnt moving.... Didnt figure that out until tonight... lol

Anyways, does anybody know the advance curve of an 81? Reference points and RPMS would be superb!

Also, anybody have any luck ovaling the mount holes for the pickups to advance/retard timing?

Thank you!

[DGXSER]

The 81 model has the timing curve built into the TCI. The vacuum advance is still functional, and you may want to ensure it is operating properly. But no mech advance.

[petejw]

suck on the end of the vac advance and hold ur tongue
to block it, it shouldnt move, if it does its no good.

dont go elongating the holes in the pick up,
elongate the holes that hold the timing plate in,
i did it so i could get 10 degrees of timing instead of the 5
thats fixed with the timing plate.

[onefour]

Ok thanks...

So what you guys are telling me is the ignition is locked (no mechanical spark curve for rpms) except for vacuum advance in the 81 model only?


And sorry, my bad on the wording as far as ovalling the holes, you said what I meant


Thanks

[crazy steve]

So what you guys are telling me is the ignition is locked (no mechanical spark curve for rpms) except for vacuum advance in the 81 model only?

No, you still have 'mechanical' advance, but it's done electronically in the TCI on the '81 models (assuming you still have the 4R0 TCI). The advance starts at 1800 rpm and adds 30 degrees of timing, being all in at 3900 rpm. Total timing should be 35 degrees.

The vacuum advance may not be working; if you have a true 'blow-through' turbo system with pressurized carbs, you won't get a vacuum signal under boost. If it's a 'suck-through' system (carb or carbs ahead of the turbo, i.e. connected to the inlet side of the turbo) you'll get advance.

As a general rule, vacuum timing advance is not a good idea on a suck-through turbo system...

[onefour]

No, you still have 'mechanical' advance, but it's done electronically in the TCI on the '81 models (assuming you still have the 4R0 TCI). The advance starts at 1800 rpm and adds 30 degrees of timing, being all in at 3900 rpm. Total timing should be 35 degrees.

The vacuum advance may not be working; if you have a true 'blow-through' turbo system with pressurized carbs, you won't get a vacuum signal under boost. If it's a 'suck-through' system (carb or carbs ahead of the turbo, i.e. connected to the inlet side of the turbo) you'll get advance.

As a general rule, vacuum timing advance is not a good idea on a suck-through turbo system...

Thanks for the ignition box advance specs!

The way you are thinking about vacuum advance is partially correct. It still works fine with a blowthru application. When you are running at part throttle high vacuum situations it still activates the advance. The boost comes in when the throttle blades are opened and once the throat sees boost, then it will slam the canister shut, just like no vacuum, but it pushes harder because there is boost and spring pressure on the diaphragm.

I was originally thinking about pulling apart the vacuum canister and adding a spacer and a spring on the top side of the spring, so it floats at 0 vacuum/boost, and returns it to the stockish location. Boost would push it a couple degrees past, retarding the timing on boost only.

This is the best way of doing it mechanically, but spare parts for these bikes are a little harder to come by than small block chevy junk, so I'm gonna attempt pulling mechanical timing, because in essence I wont be running much in between vacuum and boost.

[crazy steve]

If you don't mind removing the vacuum advance, that can give you ignition timing adjustment without hogging out the backing plate holes. Simply fab a small 'arm' to replace the vacuum can arm, find a place to anchor the other end, then rotate the timing plate to where you want it...

[onefour]

Thanks bud, ill do that!