Setting distributor timing on a 65, 350 HP small block

[Mark H.]

Hello all.

Although my engine idles and revs up fine in the garage (nice and smooth, no hiccups or sounds) I thought I'd check my ignition timing (Note: It's running very poorly once on the road, even at very little load/gas it is very rough/missing (hence my check on the timing), but that's another story...). The timing is way up there around 12 degrees BTDC. It's off the marking tab, so I'm estimating (BTW: why does the marking tab max out at 8 degrees when the spec timing is (I believe) 8 degrees BTDC @ 700 rpm?). Dwell is fine/steady at 30 degrees. [I'm surprised it runs fine (in the garage with no load) with such a large advance.... Could my poor-running when-loaded motor, as mentioned above, be due to such a high advance? Never noticed any detonation.]

Here's the problem: when I try to reduce the timing by rotating the distributor clockwise (when looking down on the distributor) the vacuum advance hits my coil (coil is mounted right and has not shifted/moved). The lowest I can get the advance is ~12 degrees BTDC. Almost seems as if the distributor is off a tooth (on cam), but this "can't" be since the car was running fine for years after my rebuild. (I recently rebuilt the carb because the car was running poorly at higher rpm. I did this to address the "another story" I mentioned earlier).

Any thoughts are much appreciated !

-Mark

[Don H.]

Re: Setting distributor timing on a 65, 350 HP small block

I would pull the distributor & move it one tooth so you can set the timing to spec. Is the vacuum advance working properly? Good luck, Don H.

[Bill M.]

Re: Setting distributor timing on a 65, 350 HP small block

I had the same problem with my 350 hp 67 after engine rebuild. I moved the dist a tooth and still the engine would run great and then act up. I have the original distributer in the car and it was suggested that maybe it was worn out. I brought it to local speed shop and they rebuilt it. the springs were shot and so was the vacuum advance also replaced the shaft as I was eating tac cables. car runs for hours now without a problem.

[Duke W.]

Re: Setting distributor timing on a 65, 350 HP small block

Sorry, but the above "fixes" aren't that at all, nor does changing the wire indexing do anything but screw it up worse. Chances are the gear on the distributor is indexed 180 degrees out of phase. The dimple should be pointing the same direction as the rotor tip as long as it has a properly manufactured cam, but some aftermarket cams aren't.

So you need to remove the distributor and inspect. Also, there's a lot more you can do to get your L-79 running like it should. Search for a thread started by me in December, 2012 - L-79 distributor blueprint/overhaul. That was a really sweet running L-79 after we were done. I didn't rev it above 6000 on a test drive, but it wanted to keep going.

As a minimum you need to get the gear properly oriented and verify that the plug wires are indexed IAW the '63 Shop Manual, but the blueprint/overhaul and modifications to the spark advance map aren't that much more cost or work.

To remove the dist. set the engine at 10 BTC on the compression stroke and don't move the crank before it's reinstalled. Note how the rotor turns CCW and is pointing at about engine centerline when the gear is free. Upon reinstallation start with the rotor tip in the same orientation and drop it in. If it won't fully seat inspect the oil pump drive slot and tweak with a paint mixing stick as required.

Once seated rotate the housing until the points just begin to open. This is called static timing the engine. The initial should be near 10 and the engine will start right up. Then go through either the static timing procedure or maximum WOT advance procedure.

Duke

[Bill M.]

Re: Setting distributor timing on a 65, 350 HP small block

Oh well I wont tell the car that!

[Chris D.]

Re: Setting distributor timing on a 65, 350 HP small block

Mark,

You are disconnecting and plugging the vacuum advance line when performing these checks, yes? In answer to your "BTW" be aware that the timing tab marks are incremented 2 degrees each. If you are "off the tab" with the vacuum advance plugged, then something basic is amiss. Follow Duke's instructions to correct the index of the distributor gear and the index of the wires on the cap. If your rebuild years ago involved a non-stock cam it could push you to a non-stock distributor setup.

Chris

[Mark H.]

Re: Setting distributor timing on a 65, 350 HP small block

Thanks everyone. Great info.

Chris - yes I did disconnect and plug the vacuum line before dwelling and timing. Disconnecting didn't seem to make a difference - bad sign? I am "off the tab" so it sounds like the distributor may be a tooth off as you and Duke mentioned. Very well could be it got put on wrong after the rebuild with a non OEM cam as well. Having said that, while certainly not optimized for HP or mpg, the car's been running well just like it is for years.

Yikes - I never knew that each timing mark increment was 2 degrees. Never saw this in the Shop Manual - guess it's in there some where. In that case my initial/static timing is way up around 24 deg, which seems crazy almost to the point of "no way". Who knows what the total advance is.

My next step is I'm going thru some info that that I got from Lars yesterday. I'm in the process of verifying my total advance is around 36 deg at 2500 rpm, then verify my initial/static timing is around 12 in Hg. Total advance seems the more critical of the two. I'm going to mark my harmonic balancer so that I can read 36 degrees with the tab on "0", at 2800 rpm. I have 10.5 pistons, so hopefully I won't run into detonation issues on today's swill they call gas. If needed (probably) I'll then get into the bigger effort Duke outlined.

Thanks again to all. I'll let you know as things progress. Once the timing is verified I'll get to what I think the real problem is - fuel delivery (after my carb rebuild; see my first post).

-Mark

[Mark H.]

Re: Setting distributor timing on a 65, 350 HP small block

meant to say 12 degrees, not 12 in Hg .....

[Chris D.]

Re: Setting distributor timing on a 65, 350 HP small block

That's a sign of something. No timing change or idle speed drop on vacuum disconnection suggests the vacuum advance is not working. Disabled vacuum advance, all centrifugal advance, with a ton of initial would be an old hot rod approach of little use on the street. Perhaps that was the intent in your rebuild years ago or perhaps the vac can is simply broken.

To be clear, Duke was referring to rotating the drive gear 180 degrees on the distributor shaft. With an odd number tooth count, that changes the rotational position by a half tooth. Getting that backwards typically causes the type of interference you've seen between the can and the manifold or coil.

Check the vacuum advance operation and follow Lars and Duke. They give the best advise there is.

[Duke W.]

Re: Setting distributor timing on a 65, 350 HP small block

" Disconnecting didn't seem to make a difference - bad sign?"

Yes, if engine speed didn't change when you pulled the VAC signal line it could be an indication that it is dead or someone put a BB in the hose. OE is stamped 236 16. You need to check the start/stop points with a vacuum pump... should be 4/8" with 16 degrees max added advance.

If it needs replacement use a B26, NAPA VC-1765 or equivalent in another brand, assuming is a real L-79 cam, but make no assumptions. L-79 idle behavior is 14-15" @ 750. If it's not close to this it's not a real L-79 cam, so you will have to use the Two-Inch Rule to select a functionally correct VAC.

The OE centrifugal curve is 30 @ 5100 Max, so you would have to rev it that high to check total WOT advance, assuming it's still OE. But if you install lighter springs as I explained in the 2012 thread that I referenced it can be checked at 3500-4000.

Do a Web search for my tuning seminar - duke Williams corvette san diego - are the search words.

Duke

[Mark H.]

Re: Setting distributor timing on a 65, 350 HP small block

I'm betting you're right on the distributor needing to be rotated. Thanks for the procedure to do it.

Before I remove it maybe you'll have some thoughts on:
- Chris mentioned that each mark on the timing plate is 2 degrees. Just to verify, true? Hence if I want 8 deg of static timing, I"ll be on the 4th mark (at 750 rpm mine currently is off the chart up at what would be a "12th mark" (24 degrees BTDC static timing!), prob since it may have been installed 180 off. Not sure how the car could idle and rev so nice in the garage with that timing. Maybe it would as long as it has no load on it? (I tested w vac line plugged).
-I have 15" Hg at the manifold while idling. Pretty sure the right cam was used during rebuild. The guy was good and the car has run great for years, though it certainly may not have been optimum. I just take it on short drives anymore - no more long drives or high rpm.
-I put a vacuum gauge btwn the carb and the vacuum advance unit. Looks good with 0" Hg at idle and then steadily climbs as I raise rpm. About 10" Hg at 2000 rpm or so (by ear - rpm not observed).
-I watched my timing at around 1500 rpm with the vacuum advance line attached (btwn carb and vacuum advance unit), then disconnected the line and the timing did retard. Seems as though the vacuum advance unit is functioning. Yet, I also notice that with everything hooked up, the timing does advance off idle up to an rpm around 2500 or so (due to the centrifugal weights) but then stops advancing even though I continue to increase the rpm. That's why I thought the vacuum advance unit was an issue. This is where the vacuum advance should take over and continue the advance.
More figuring out to do...

Thanks again for your time and thoughts,
Mark

[Duke W.]

Re: Setting distributor timing on a 65, 350 HP small block

It sounds like ported vacuum advance, but '65 is full time since the engines were not exhaust emission controlled. Is the carb OE or a "replacement"? It also sounds like the centrifugal may have been modified to come in faster, which is good, but you need to record the stop and start points and the total amount of centrifugal.

What is the number stamped on the VAC bracket?

Spark advance map specs are in your '65 Corvette Shop Manual Supplement and the AMA specs, and a better map is explained in the thread I mentioned in a previous post.

Duke

[Mark H.]

Re: Setting distributor timing on a 65, 350 HP small block

(not sure if pics I tried to attach went thru)

It's a 4150 carb with the right numbers on it (per Holley guy and kit). If it's not the original, pretty sure it's the right one nonetheless. Pics, etc look identical to the 65 shop manual. I bought the car in 88 - no changes since then anyway.

It does, and should, have the vacuum port off of the carb (aka "ported vacuum" I believe). See pic. Definitely not hooked up to the manifold. I did have the weights and springs modified many years ago since I was only able to run near-zero advance to prevent detonation w 11:1 pistons (now I have 10:1). I blew it and did not get the stock weights and springs back. I'm told they're impossible to get again. I'm sure my weights are lighter and/or the springs are stiffer. Having said that, the car has run fine for decades with whatever it has. It certainly is down on power above 4000 rpm (which I rarely hit anymore). It used to kick hard around 4000 but now it's all linear. Again, I'm OK with this, though it could be it's hurting my mpg.

Take a look at the pics. Bracket has the # 446-15 on it. Certainly different than the one you mentioned (OEM: 236-16 or non-OEM B26/VC-1765). Maybe time to go buy the NAPA part. Again, for what's it worth, it's been working.

To get the total timing I guess the first steps/b to take the distrib off which I guess means gutting it first then rotatingit. You had mentioned this: "Pull dist. andverify that drive gear dimple points in the same direction as rotor" What/where is the "dimple"? Any pic you could send mighthelp. Getting #1 at 10 deg BTDC on the compression stroke: what'sthe best way to do this? Any way other than plugging the spark plug holew finger in order to verify this? I guess looking at when the pointsopen/close would work. Wrench on harmonic balancer bolt the best way to get to 10 degBTDC? Easy to do on my motorcycle, toughon a V8 I would think.

So far as verifying total advance (you said 30 deg, Lar said 36 deg?), other than using lighter springs in order having to rev it less (~3500 rpm per you), Lars said in one of his articles that I could just take off one of the weights. True? I did notice that any/all advance seemed to stop at around 2500, which led me to believe the vac adv had issues. Vac advance is not totally non-functional. At ~2500 rpm plugging and unplugging the vac adv does move the marker on the harmonic balancer. Lars had also said that each timing mark was 2 deg, not 1. True??

Lots here....
Thanks

[Mark H.]

Re: Setting distributor timing on a 65, 350 HP small block

see prev msg

[Duke W.]

Re: Setting distributor timing on a 65, 350 HP small block

Right now I don't have the time to write and edit the 500 word essay that is required to answer all your questions and correct your misunderstanding and misconceptions. I invite you to send me an email through the TDB. I'll respond with my phone number and we can get things squared away on the phone.

In the meantime read the thread I referenced earlier and do a Web search -duke Williams san diego corvette - and download and study the pdf of the Powerpoint presentation.

Duke