'65 timing troubles

[Art A.]

Re: In the old days ... Art

Thanks Jim, I passed the information along to him.

Art

[William Dillon]

Re: '65 timing troubles

Ok, I set the total wot with the vac can plugged at 38 deg and checked the timing at idle, it is now reading about 8 deg. With the can plugged in the total idle timing is about 24 deg. Off idle response is very poor but the car runs well as soon as the revs build up. It seems the distributor is putting in to much advance, is there a way to limit that? Or should I just stay on the safe side with 38 deg and not expect much more in the way of low-end power?

Bill

[Duke W.]

Re: '65 timing troubles

Refresh my memory on what engine configuration you have.

When you say "poor off-idle response" do you mean lack of torque, or a hesitiation or stumble.

Duke

[William Dillon]

Re: '65 timing troubles

Duke,
I have a '65 327 shp with a Pertronix igntion and Accel coil, 4 speed with 3:70 gears. The carb is a rebuilt stock 2818 that was done last year,along with the distributor, which contains all new stock components. I'm checking total timing with Mr.Gasket degree tape. The car idles around 900 rpms with 10.5" of manifold vacuum. The car has no power (torque) off idle.

Bill

[Duke W.]

Re: '65 timing troubles

One of the potential problems is NOT ENOUGH low rev timing. The 30-30 cam overlap creates a lot of low speed exhaust gas dilution of the fresh charge, and the late closing inlet valve yields a rather low dynamic compression ratio. These characteristics combine to reduce low rev volumetric efficiency and slow flame propogation speed, which are the reasons these engines need a lot of timing coming in quickly.

This is reflected in the aggressive OE centrifugal curve, which starts out at 700 (below idle speed) and is all in at 2350.

The OE spec for max centrifugal is 24 degrees, but if you are seeing 38 total with only 8 degrees initial it means your centrifugal is above spec. One possible reason could be deterioration of the rubber bushing on the advance stop pin. Remove the rotor, weights and springs, and take a look at this bushing, especially by manually advancing the bushing to the end of the slot. This bushing is both a common wear item and commonly overlooked.

Another alternative is that someone may have ground out the slot to increase the amount of centrifugal advance.

If the centrifugal is limited to 24 you should be able to run 14 initial, which will help the bottom end, but the 30-30 cam is inherently soggy, and I always recommend that it be replaced with the LT-1 cam whenever a 30-30 cam engine is rebuilt.

Idle of 900@10.5" is in the ballpark for the 30-30 cam. The Duntov and LT-1 cams pull about 12" at the same speed. The L-79 will pull about 14" at 800 and a 300 HP engine will pull 18" at 500. Even the L-72 is relatively mild as it will pull about 14" at 800-900. The low vacuum of the 30-30 cam is a reflection of its relative overlap, which is too much for a good street engine.

Duke

[William Dillon]

Re: '65 timing troubles

I love the scientific approach on how these things work - you must have an engineering backround! Anyhow, I replaced that rubber bushing with a brass one, so I know that part is good. Is there any other way I can limit the centrifugal advance so I can put in more initial? I wish I would of known about the LT-1 cam before I rebuilt my engine 2 years ago, I replaced the original cam with a Comp Cam of the same spec. I still have the original with the lifters on my shelf.
Thanks,
Bill

[Duke W.]

Re: '65 timing troubles

Someone must have ground the slot. It's a trick you can use if the engine exhibits detonation at low revs - you increase the total centrifugal and reduce initial. This reduces the timing at low revs, but maintains the correct total WOT timing at high revs. Other than brazing up the slot and grinding it out the correct amount I can't think of another way to fix it.

Maybe Clem has a trick.

I have a MSME from the Engine Reseach Center at the University of Wisconsin, so I understand IC engines at a very low level of detail.

Duke

[Chuck S.]

Re: In the old days ...

Sun Machine..."Don't know if speed shops still do this, but I'm sure someone does."

Gene, yer right on...I happened to be in a store that caters to dirt track racers, and lo and behold, there wuz a Sun machine right at the counter. I was amazed to see one again. Note that this was a REAL SPEED shop, and not one of those places that specializes in shiny Moroso air cleaners, valve covers with tee handled bolts, chrome-plated wire looms with big yellow wire, etc.

[Michael H.]

Re: '65 timing troubles

Here's something from a Purdue engineer you should also check. If the harmonic balancer is old the outer ring may have slipped and the timing mark may no longer be at true top dead center (TDC). Set the engine about 30 degrees past TDC. Pull the number 1 spark plug and insert a TDC locator (can be purchased at most auto supply stores). SLOWLY rotate the engine BY HAND clockwise until you feel the number 1 piston come in contact with the TDC locator and put a mark on the balancer across from the 0 degree mark. Now SLOWLY rotate the engine BY HAND counterclockwise until you feel the number 1 piston contact the TDC locator and put another mark on the balancer across from the 0 degree mark. Halfway between the two marks is exact TDC. If this doesn't correspond to the factory mark, you should replace the balancer.