'64 327/300 Stalling Issue

[Bruce Boatner]

My '64 327/300 recently developed a stalling issue. The stalling occurs when accelerating from a standing stop after the engine becomes "hot" from running the A/C on a hot day (say 220 degrees--engine temp. not ambient temp. even though I do live in Houston). The problem goes away when the engine cools down closer to 180 degrees.

The stalling started after I installed a freshly rebuilt distributor. Could be just a coincidence, but not sure. I checked the dwell yesterday and found that it was at 26 degrees. I reset the dwell to 30 degrees as suggested in the shop manual, and am hoping this may have solved the problem. Guess I'll find out later today when I drive the car in the heat on the way home from the office.

Could an improper dwell setting create a stalling problem as I described? If not, what else could be creating this problem?

Thanks guys!

[Jim T.]

Re: '64 327/300 Stalling Issue

Reference your freshly rebuilt distributor. Do you know the clearance is on the shaft and drive gear?

[Bruce Boatner]

Re: '64 327/300 Stalling Issue

Jim,

The distributor was rebuilt by a reputable shop who has been mentioned favorably many times on this forum, so I assume the clearance is within proper specifications--still, you never know. I did not measure the clearance specifically, but did notice that it seemed tighter (less movement) than it was before the rebuild. Could too tight of clearance cause the problem?

Aside from the stalling issue, the engine runs reasonably well. I am able to restart the engine within a few seconds (thank God for that) and away I go until the dreaded next red light. When the engine cools down a little the stall problem goes away.

Bruce

[Wayne P.]

Re: '64 327/300 Stalling Issue

Now that you changed the dwell you should reset the timing. What idle speed are you running? How are your mechanical and vacuum advance? If the rebuilt shop missed the dwell, the advance is suspect too.

[Jerry G.]

Re: Maybe fuel boiling causing instability

I have a 65 FI car. All the plumbing exagerates fuel boiling problems. I recently spent a considerable time sorting out idle stability problem and it seems to be solved by going to a fuel with a higher set of boling point curves. 256 degrees vs 195 degrees. Just another point of view. Jerry

[Bruce Boatner]

Re: '64 327/300 Stalling Issue

I did reset the timing to about 8 degrees. Idle in drive is about 600 rpm (I have a PG). I suspect I may have some issues with he vacuum advance because the car is definitely running hotter AT IDLE now with the rebuilt distributor and new vacuum advance can.

How do I know if the advance/timing is accurate on my 327/300 with auto and a/c? I have a dial back timing light and vacuum pump. I know they're have been several posts in the archives on this, but it seems like they all related to higher horsepower engines.

[Jim T.]

Re: '64 327/300 Stalling Issue

Bruce I checked my 64 Corvette service manuals and the clearance is .002 to .007 thousands using the hardened shims available from GM.
The service manual for the 64 has the timing at 8 degreesnominal or 6-12 degrees range. I always ran as much as I could on pump gas without pinging.
Your distributor vacuum advance should be full time vacuum and disconnected and vacuum line plugged for adjusting timing.

[Duke W.]

Re: '64 327/300 Stalling Issue

A dial back timing light and vacuum pump make it very easy to check the operation of the vacuum advance. You also need the OE specs, and what is the number stamped on the vacuum can bracket currently installed?

At idle the total timing (vacuum can connected) should be the sum of initial and full vacuum advance.

You can also check the vacuum can by disconnecting the signal line and connecting your vacuum pump to the can. Start pumping down the can, then check to see how much advance is added for different vacuum values and compare it to spec. In particular, you want to determine the vacuum level required to start pulling the plunger and the maximum vacuum required to achieve full advance and the amount of full advance.

If you need a new vacuum can the NAPA/Echlin VC680, 0@8", 16@16", is suitable for the 300 HP engine.

The basic timing map requirement for medium performance engines is very similar to SHP engines, but varies slightly in the details. Total idle timing should be in the range of 20-26 degrees for MP engines and 26-32 for SHP. This is achieved by the combination of initial plus full vacuum advance.

The vacuum advance specs need to be tailored to the engine's manifold vacuum characteristics, which are primarily determined by valve overlap as indicated by typical idle vacuum. The vacuum can should provide about 16 degrees maximum advance at no less than 2" below typical idle vacuum with about 25 degrees of total idle timing. Since a 300 HP engine pulls about 18", a can that provides 16 deg. @ 16" is suitable. The 30-30 cam only pulls 10" at 900, so it needs an 8" can.

The following three NAPA/Echlin vacuum cans should cover the range of needs depending on the engine's typical idle vacuum with the correct amount of total idle advance. Advance is listed in crankshaft degrees. Some vacuum can specs list advance in distributor degrees, so the values must be doubled to yield crankshaft degrees.

VC680 0@8", 16@16"
VC1765 0@6", 16@12"
VC1810 0@4", 16@8"

The VC680 is suitable for medium performance small blocks (all w/hydraulic lifter cams except L-79/46/82), and all hydraulic lifter BBs. The VC1765 is suitable for SHP big block. The VC1810 should be used with all SB mechanical lifter cams. The OE L-79 can is an eight-incher, but since it pulls about 14" at 800 a 12" can is sufficient. L-72/71 also pull about 14" at idle with full time vacuum advance, but all have ported vacuum advance, which I recommend be converted to full vacuum advance by teeing the vacuum signal line into the choke vacuum break line or other source of full time manifold vacuum.

It's sometimes okay to use a "more aggressive" can than necessary, but this may increase detonation propensity since there will be more advance at many part throttle conditions, so, as a rule, I don't recommend it. An "insufficiently aggressive" can (full vacuum advance at less than 2" below typical idle vacuum) may not keep the vacuum can "locked" at full advance at idle. This can lead to wandering idle timing, which can cause idle instabililty.

Duke

[Bruce Boatner]

Re: '64 327/300 Stalling Issue

Duke,

Thanks for an EXCELLENT explanation. I know you have covered the subject of vacuum advance many times before, but for some reason I have a hard time keeping things straight.

One thing I did remember from your earlier posts is that improper vacuum advance will result in higher engine operating temperatures. Does lack of additional advance at idle create excess engine heat? I am not sure of the science behind this, but I am confident you could provide a good explanation that could take some of the mystery out of the phenomenon.

I will check my vacuum advance can with a vacuum pump as you described tonight. My guess is that either the can is defective or the breaker plate is too tight (new shaft was installed during rebuild) because earlier I noticed no change in advance when I disconnected and reconnected the vacuum can signal line. If I understand you correctly, connecting the vacuum can AT IDLE should increase the timing by 16 degrees, so my timing light dial should read around 24 degrees with the can connected (assuming the "intial" timing (can disconnected) was at 8 degrees).

Thanks again for your help. I think this vacuum advance stuff is finally sinking in. We'll save the counter weights and springs for another day.

Bruce

[Berton E.]

Re: '64 327/300 Stalling Issue

You didn't mention re-setting the timing after you adjusted the dwell. When you change the dwell setting it affects the timing. You may want to confirm timing to see where it is currently set.

[Duke W.]

Re: '64 327/300 Stalling Issue

Correct!

However, the breaker plate fits over a machined boss on the distributor housing and is retained by a wavy ring. It does not bear against the mainshaft. The breaker plate should rotate freely (without the vacuum can screwed into the housing), but it should not wobble. A wobbly breaker plate means the breaker plate bushing is worn or the distributor boss is worn, and the problem must be corrected if you want a reliable and properly functioning ignition system.

For every engine operating condition there is an ideal spark advance that will maximimze torque/power at that operating condition and result in the lowest EGT and fuel consumption.

Unfortunately, most enthusiasts don't understand engines beyond the one-dimensional view of peak power at WOT, but there are an infinite number of operating conditions, which may be expressed uniquely by manifold vacuum and RPM. This is your chance to break away from the crowd and understand the spark advance requirement for all engine operationg conditions!

We can't determine the ideal advance for an infinite number of operatiing conditons, but three will characterize the spark advance requirement quite well - idle, WOT at high revs, and cruise - such as normal manifold vacuum and revs cruising on a level road at 70 MPH.

If the timing is retarded from the ideal for any operating condition, EGT increases. At idle, flame propagation speed is relatively slow because the mixture is low density and is diluted with exhaust gas. A medium performance engine needs about 20-26 degrees of total idle spark advance to minimize idle EGT and idle fuel consumption, and this is provided by the sum of initial and full vacuum advance. SHP engines need more total idle timing, about 26-32 degres because their high overlap camshafts result in increased exhaust gas dilution and slower flame propagation speed at idle and part throttle than a MP engine. SHP engines may have very quick centrifugal advance curves that start below idle speed, so total idle timing may include a few degrees of centrifugal, which will get the total idle timing up over 30 degrees.

If the engine has less than the ideal range of total idle timing for any operating condition, EGT goes up and as the hot exhaust gas travels through the port it heats up the coolant.

This is a primary reason why engines without vacuum advance or engines without a properly designed/configured vacuum advance system will tend to run hot/overheat at idle and low speed stop and go driving, and they may also suffer from idle instability if the vacuum can is not "locked" at full advance at idle. If there is insufficient idle vacuum to lock the can at full advance, any slight loss in manifold vacuum causes a loss of advance which reduces idle speed, which further reduces manifold vacuum/vacuum advance, which causes further loss of idle speed/manifold vacuum/vacuum advance, and so on. In as little as a few seconds of idling the engine speed can be reduced to the point where it is on the verge of stalling.

WOT operation is characterized by rich, dense mixtures, and flame propagation speed is faster than at idle and cruise, so less advance is needed, and the total WOT advance is provided by a combination of initial and centrifugal. As a rule it is best to get the centrifugal in as fast as possible, but detonation is often a limiting factor. The WOT advance characteristics can be characterized by simply disconnecting the vacuum advance and then measuring the timing at increasing engine speed until it stops advancing.

The sum of intial plus full centrifugal is "total WOT advance", and is the amount that the engine needs to make peak power at high revs, but this is not the only "total" advance requirement.

At cruise, advance is determined by both RPM and manifold vacuum and if the centrifugal advance is quick or you are cruising at high revs and low load the
"total cruise advance" can be in the range of 50+ degrees - the sum of initial, full vacuum advance, and full centrifugal advance. Such is the case with the OE 365 and 375 HP 327 spark advance maps.

When talking about "total timing", also referred to as "total advance" you need to specify whether you are talking about "total WOT advance", "total cruise advance" or "total idle advance".

They are not the same and there is a lot more to engines than peak power at high revs.

Duke