TIMING- don't understand it

[aussiebug]

<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>Originally posted by kah:
i have 1600cc dual port with a really early style bosch single vacuum distributor, what does it mean when it says 7.5 btdc. i thought you set the timing at tdc. i don't have a timing gun , just a test light.correct me if i'm wrong but do i put engine at tdc then unscrew the dizzy bolt so i can turn it, then ground test light while hooking it to the negative post on the coil, then turn dizzy until light comes on.rigth or do i put engine at 7.5btdc and do the same thing as above . please help<HR></BLOCKQUOTE>


The whole idea of timing advance is that it takes a little time (a few milliseconds) for the flame front to race across the cylinder and burn all the fuel. You want all the fuel burned just as the piston starts back down (for maximum power), and this means you have to light the fire BEFORE the piston reaches the top.

And the faster the engine is spinning, the more advance you need, since in the same few milliseconds the crank will trun further.

There are added complications to the timing issue, but the vacuum distributors look after that for you.

For a 1600dp with single vacuum distributor, you should time it to 7.5BTDC.

There are at least 4 pulley tyrpes used by VW, with different marks.

The "three notch" pulley is TDC, 7.5 and 10BTDC (left to right).

A "two notch" pulley (close together) is 7.5 and 10BTDC (no TDC notch).

The "dimple and notch" (dimple on the reamost rim) is TDC and 7.5BTDC.

The single notch is usually 5ATDC (that's AFTER) for the dual vac 71-73 distributor.

You set the 7.5BTDC mark opposite the split in the crank case, and loosen the distributor so you can turn it bakc and forth until the test light winks on-off. Turn the distributor anticlockwise first, then slowly clockwise til the light goes out. THAT's 7.5BTDC.

I disagree with the other poster about how essential a timing light is - it's only absolutely essential with the double vacuum and 009 distributors. The single vacs can be set very well with a test light - I haven't used a timing light in 31 years of driving the VWs.

Regards,

------------------
Rob
Rob and Dave's aircooled VW pages
Repair and Maintenance for the home mechanic
http://www.geocities.com/aussiebug1970/index.html

[kah]

i have 1600cc dual port with a really early style bosch single vacuum distributor, what does it mean when it says 7.5 btdc. i thought you set the timing at tdc. i don't have a timing gun , just a test light.correct me if i'm wrong but do i put engine at tdc then unscrew the dizzy bolt so i can turn it, then ground test light while hooking it to the negative post on the coil, then turn dizzy until light comes on.rigth or do i put engine at 7.5btdc and do the same thing as above . please help

[TomsBug]

Whoa there,

I have heard that trying to set the timing without a timing light is somewhat like trying to herd cats.

Buy one, rent one, borrow one, but find one, please locate one before you start putting your foot in the gas pedal to far.

Turn the crank pulley by hand (wrench)(engine off, duh) until the TDC notch (dimple) is at the top directly on the split in the engine case. Verify dizzy rotor is pointing at #1 plug wire. Mark a spot (a mark that can be seen when the engine is running) on the crank Pulley about 7/16" to the RIGHT of the TDC mark. With the engine warm, running and off-choke with idle set about 800-900RPM, use a timing light to align the 7.5BTDC mark with the centerline of the engine case.

This will get you going till you find out the real way of timing on these boards. I just don't remember the addy right now.

Happy timing

TomsBug and Molly, the 73SB daily driver