Ignition Timing and Valve Adjustment

By: Bob Goodson
March 29, 2020
Author: Michael Yount
Once I got the air filters on the car, I thought I’d set about checking the balance on the carbs.  It occurred to me in pretty short order there were some other pieces of business to get down to.  It’s imperative that some other things are done first such as being sure the car is in good tune – timing, plugs, valve adjustment, etc.
I borrowed a good friend’s (thx Brad!) fancy digital timing light.  This thing has a built in tach, dwell meter, and lets you select timing increments of one degree so you can check how the advance is working.  When I first hooked the light up and disconnected/plugged the vacuum advance canister, timing was set around 8 degrees BTDC.  But I also noticed that the tach feature didn’t seem to be consistent and no matter how I played around with the initial timing, the advance seemed to be going crazy – 40-50-60 degrees of advance as the engine was revved.  Time to dig in and see what’s going on.
Back when I raced these engines, one of the first things we did was to recurve the distributors.  My 74 was built in the heart of the emissions-era when engineers were still trying to figure out how to meet tightening emissions regs.  Typically initial timing settings were in the 0-5 BTDC range with a bunch (35-40 degrees) of advance built in mechanically to try and get some of the performance back as the engine’s rpm increased.  Vacuum canisters used ported vacuum in this era – so no additional advance at idle — but a bit more as the throttle began to open.  That’s how this was set up.  So I knew I’d need to re-engineer the timing curve.
Started by pulling the distributor.  Hall effect pickup, no points (’81 Corolla 3TC stock dizzy I believe) w/vacuum advance.

A couple of issues right up front.  First, the vacuum canister is kaput.  So if I want that, a replacement will be necessary.  Second, I couldn’t adjust the timing any more than it was because on one end that slotted adjustment tab was limiting; and on the other end, the backside of the vacuum advance mechanism was interfering with the front-most carb/intake flange.  So I started off by removing that tab and fashioning a clamping mechanism using the piece I cut off and a fender washer.  This will enable me to alter the distributor “stab” and rotate the housing by 90 degrees or so eliminating the interference with the intake manifold flange.  I also temporarily blocked off the port vacuum source on the front-most carb — this to get ready to hook up the vacuum advance canister to full manifold vacuum.

If you look closely at the picture above, you’ll see where someone ground off the backside of the vacuum advance canister shaft housing for clearance at the intake flange.  Un-necessary now that I’d cut the slotted tab off and rotated the distributor.
Next, I created a little paper template to measure the number of degrees of mechanical advance the distributor was adding.  19 degrees at the distributor = 38 degrees at the crank!  My target for timing is to have initial advance at about 18 degrees BTDC with vacuum disconnected; have the distributor add another 16 degrees mechanical for a total of 34 degrees advance, have it all in by 3000 rpm and have vacuum advance add another 15 degrees or so under conditions where vacuum is available.  So, I’ll have to find a way to limit the centrifugal advance and likely to also limit the amount of vacuum advance.  Fortunately, it’s not difficult to do either.

In the meantime, I discovered I could get a vacuum canister from Rock Auto for about $60; but I could get the whole new distributor for about $85 – so I ordered one.  Figured I could experiment with the one in the car and when I had it working like I wanted – modify the new one in the same manner.  When it arrived I promptly cut off the adjustment slot.

Mechanical advance is added when centrifugal weights swing out against the springs and the pin you see moves from one side of the slot to the other.  To limit it, I have to limit movement of the pin in the slot.

I did that by fitting a small metal sleeve over the pin and attaching a nylon tie-wrap to one side of the slot.  By removing the heavier of the two springs, I knew total advance would come in at a much lower rpm — to be confirmed with the timing light.

I discovered that the rotor had quite a bit of play in it – so I fashioned a paper “shim” that wedged it much tighter on the distributor shaft.

Last, I used calipers to mark off some more advance marks on the crank pulley so I could see what the timing was doing at rpm with an old-school analog timing light.

My friend Brad discovered that MSD (multiple spark discharge) ignitions don’t play nice with digital timing lights.  My friend John loaned me his analog light — and when I put it all together I noticed two things.  Number one, I was getting too much vacuum advance at idle — I just wanted about 15 degrees but was getting 25 or so.  Luckily, the shaft inside the dizzy is threaded – so it was easy to adjust the stop for less advance.  More problematically, no matter what I did the unit kept adding ridiculous amounts of advance with rpm – still seeing 40-55 degrees of advance.  I even went to the trouble of physically locking out the vacuum and mechanical advance — and still the timing advanced with revs.  So I called the good folks at MSD.  Turns out, if you wire up the Hall effect sensor backwards – the engine will run, but because the MSD is seeing the slope side of a saw-tooth curve generated by the Hall effect sensor  (instead of the flat side) – it keeps advancing the timing!  Whoever put it in wired it backwards.  So I reversed the wires/connectors in the distributor plug (little green plug in previous pictures) and fired it back up.  Voila!  With a couple of adjustments – 18 BTDC initial; 34 BTDC total all in by 2900 rpm; and vacuum advance adding 15 degrees at closed/light throttle.  Perfect!!! 
Next – check valve adjustment as improper adjustment can affect vacuum at idle which affects ignition timing.  The little hemi has a rocker shaft on each side – one for intakes, one for exhaust.  Each rocker has a set screw (flat blade screw driver) and a lock nut.  VERY easy to adjust valves.  Typical guidance for these engines is .008-.009″ hot intake and .012″-.013″ hot exhaust.  I made some allowances for “cold” and checked clearances.  The valves were quite a bit off spec — gap was .003″-.004″ too big across the board.  Set them cold, cranked it up and let it come up to operating temp and then checked/reset.  Everything came right in at .008″ exhaust and .012″ exhaust.  REALLY quieted the engine down as you might imagine.  Also put a new set of Denso wires and iridium plugs in while at it.  The MSD ignition box with a hot coil allowed the new plugs to be opened up to .055″ gap.

The vacuum accessories (dizzy canister and brake booster) were all being run off of the vacuum of just one intake runner — there is no common plenum to pull from.  So I was able to get fittings to connect all 4 runners to a common vacuum hose and run the booster and canister off of that line.  Much more stable vacuum signal for the dizzy and the booster.

Running MUCH better – now on to balancing the carbs.

1 Comment

  1. Michael Rosypal

    This is on point, I’ve read it many times as it has helps me tune my 3tc like a top..thank you so much for publishing this for us young guys to learn


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