Electrical Issue

[Bob A.]

I have a restored 63 Roadster - 327/340. At a stand still, i.e. traffic light...the rpm's decrease from 800 to +/- 650 when normal electrical options are engaged, ie. foot on brake at a traffic light... The rpm's drop enough for the engine to cough then stall.
Turning on the headlights while the car is idling at 900 on flat ground, i.e. in the garage, make the rpm's drop 150-200.

Maintaining the idle at 700-900 rpm's as specified, while engaging any incremental electrical demands causes rpm's to drop. The battery and the alternator have been load tested and seem to be fine. Not sure what to do next? Voltage regulator, coil... Any guidance in addressing this issue is much appreciated.

Much Appreciated,
Bob #3246
919-619-1339

[Edward J.]

Re: Electrical Issue

Bob, take a voltage test at the ballast resistor at top and bottom when idling with no load, and then turn on lights and see what the voltage drop is at resistor. you should see near battery voltage at ballast at pink wire coming through the firewall to resistor. the lower one to the coil will be less voltage due to resistor. 63 shp engines have a black dot on resistor, its a different resistor the than the 250/300 engines.

[Duke W.]

Re: Electrical Issue

It may not be an electrical issue. If the engine has a Duntov cam or something with about the same effective overlap and the original 15.5" 201 15 VAC (or an even worse B1 replacement), it will have an idle stability problem because those VACs do not pass the Two-Inch Rule. When new my 340 HP SWC had the same issue, and the dealer was clueless.

I finally figured it out and bought a 8" 236 16 VAC that was used on the 365/375HP engines. This problem affected all '63 340 HP engines, and I'm not aware that GM ever wrote a TSB on the problem.

Measure your manifold vacuum at idle speed of 900 and check total idle advance (VAC connected), which should be close to 30 degrees. lf the engine loses timing it will also lose idle speed, which will lose more timing due to less vacuum and eventually stall.

If total idle advance and idle speed are stable with minimum electrical load, add load and see what happens.

The current replacement for the 236 16 VAC is B28 (stamped on the mounting bracket), available as Airtex 4V1053. This VAC is necessary for all small blocks with any OE mechanical lifter cam if you want a stable idle and no stalling.

What's the number stamped on the currently installed VAC? You may have to loosen the distributor cap to see it.

Duke

[James W.]

[Duke W.]

Re: Electrical Issue

The OE VAC for all L-79s was the same 8" 236 16 as the 30-30 cam engines, but it's really more aggressive than necessary since typical L-79 idle behavior is 750 @ 14" versus 900 @ 10" for the 30-30 cam engines. Meanwhile L-79s had the same lazy centrifugal curve at the '66-'67 300 HP engine, 30 @ 5000 max. It was not a very optimized spark advance map.

With 14" idle vacuum L-79s need a 12" VAC to met the Two-Inch Rule. This VAC is stamped B26. The NAPA part number is VC-1765, but the same part is available in other brands, which can be cross referenced from the NAPA number.

Over the years many engines ended up with a sub-optimal VACs. Sometimes it was just speced wrong by the engineers and sometimes a replacement is wrong due to installer error and/or part number consolidations, which were not well thought out. Camshaft changes that alter idle behavior need a VAC based on idle vacuum and the Two-Inch Rule, but rarely does anyone consider this. Also, some are dead and leaky, but are rarely checked for proper function. This simple ten dollar part is critical to good overall engine performance and fuel economy in normal road driving, but few understand what it really does, and how to select a properly speced part.

A few years ago I did a "blueprint overhaul" on a '65 L-79 distributor. We installed the less aggressive 12" B26 VAC and lighter springs in the centrifugal to bring it all in at 3500. The curve is non-linear and most is in by 3000 with 2-3 more at 3500. We set total WOT advance in the 38-39 range and the engine ran great - very linear from off idle to the 6K redline where it was still pulling hard. A less aggressive VAC and more aggressive centrifugal curve really woke up the engine, and it didn't detonate.

I wrote this up, and a search should find it here and also at the Corvette Forum C2 section in a thread started by me.

So you need to verify what VAC is installed on your engine and check that is has the proper specs and that it meets specs. You don't need to completely remove the dist. cap, but probably just loosen it to see the stamped number on the mounting bracket. You should also use a vacuum pump to pump it down to check the start and stop points and verify that it doesn't leak.

Duke

[James W.]

Re: Electrical Issue

Duke,

So I finally had time tonight to go out and loosen the distributor cap to see what my '65 L79 equipped distributor had for a vacuum advance canister. Per the picture, it is a B1 vacuum canister. I purchase a replacement B26 vacuum canister from NAPA per the VC1765 part number you recommended. Next step is to put vacuum pump/gauge on the original B1 can and the B26 replacement to see what vacuum they hold. Would you recommend installing the B26 vacuum canister and seeing if the idle is improved? I'd like perform the same distributor blueprinting that you mentioned. I've always thought the car's performance was sluggish, my '64 with a 327/300 has way better throttle response on the street than the '65.


Thanks,

James West

The OE VAC for all L-79s was the same 8" 236 16 as the 30-30 cam engines, but it's really more aggressive than necessary since typical L-79 idle behavior is 750 @ 14" versus 900 @ 10" for the 30-30 cam engines. Meanwhile L-79s had the same lazy centrifugal curve at the '66-'67 300 HP engine, 30 @ 5000 max. It was not a very optimized spark advance map.

With 14" idle vacuum L-79s need a 12" VAC to met the Two-Inch Rule. This VAC is stamped B26. The NAPA part number is VC-1765, but the same part is available in other brands, which can be cross referenced from the NAPA number.

Over the years many engines ended up with a sub-optimal VACs. Sometimes it was just speced wrong by the engineers and sometimes a replacement is wrong due to installer error and/or part number consolidations, which were not well thought out. Camshaft changes that alter idle behavior need a VAC based on idle vacuum and the Two-Inch Rule, but rarely does anyone consider this. Also, some are dead and leaky, but are rarely checked for proper function. This simple ten dollar part is critical to good overall engine performance and fuel economy in normal road driving, but few understand what it really does, and how to select a properly speced part.

A few years ago I did a "blueprint overhaul" on a '65 L-79 distributor. We installed the less aggressive 12" B26 VAC and lighter springs in the centrifugal to bring it all in at 3500. The curve is non-linear and most is in by 3000 with 2-3 more at 3500. We set total WOT advance in the 38-39 range and the engine ran great - very linear from off idle to the 6K redline where it was still pulling hard. A less aggressive VAC and more aggressive centrifugal curve really woke up the engine, and it didn't detonate.

I wrote this up, and a search should find it here and also at the Corvette Forum C2 section in a thread started by me.

So you need to verify what VAC is installed on your engine and check that is has the proper specs and that it meets specs. You don't need to completely remove the dist. cap, but probably just loosen it to see the stamped number on the mounting bracket. You should also use a vacuum pump to pump it down to check the start and stop points and verify that it doesn't leak.

Duke

[Duke W.]

Re: Electrical Issue

The B1 is a boat anchor. It may take as much at 19" Hg to pull to the limit so it doesn't even pass the Two-Inch Rule for a base engine.

Of course you should replace the B1 with the new B26. You can do it without removing the dist. by pumping the VAC down, then removing the screws and carefully wiggling the pin out of the breaker plate.

Doing a blueprint/overhaul is an even better idea. The disassembly/assembly procedure is in your '63 Corvette Shop Manual and follow the tips in my "L-79 distributor blueprint overhaul article". You'll need a Mr. Gasket 928G spring kit and one or two shim kits to tighten up the end play.

Be sure than when you install the gear that the dimple in the side is pointed the same direction as the rotor tip. This is not mentioned in the service literature, but is critical to a good outcome.

If you do it right you'll notice a definite performance improvement, especially low end torque.

Duke

[Ed S.]

Re: Electrical Issue

Regarding Duke's L-79 distributor overhaul "article". I have searched for an actual "article" a number of times and never could find it but..... there is a thread in this forum on this issue and it contains a lot of commentary by Duke. A link to that thread is at the end of this comment.

I also copied the important technical discussions from that thread and produced a PDF document of it. That document is attached for your and other's convenience.

For Duke, I assume this is what you are talking about when you refer to an "article". If there is an actual article that appeared in some publication could you kindly post it or post a link to it - thanks - if it doesn't exist can you confirm that the info posted in this thread is what you are referring to - thanks again. https://www.forums.ncrs.org/showthread.php?97360-L-79-distributor-blueprint-overhaul&highlight=L-79+dist.+blueprint+overhaul

[Bob A.]

Re: Electrical Issue

Ed,

I appreciate your and Duke's guidance on this issue. My skill base on certain things, electrical in particular, is lacking. Given that you are in the Mid-Atlantic chapter, might you be able to connect me with a good engine guy that can help me address this issue? I'm in Chapel Hill, NC, so travel time is not an issue. Any guidance is appreciated.

Bob

[Bob A.]

Re: Electrical Issue

Ed,

Good thought. Ballast is good as it battery, alternator, coil... This motor drinks fuel. Any other thoughts on corrective action are appreciated.

Bob

[Bob A.]

Re: Electrical Issue

Duke,

I checked the vacuum canister. It has a "201" stamped on it vs. B1 or B236

Bob

[Duke W.]

Re: Electrical Issue

You need a B28... See my prior post. Don't toss out the 201 15. They are rare and are okay from some engines.

Duke

[Duke W.]

Re: Electrical Issue

Regarding Duke's L-79 distributor overhaul "article". I have searched for an actual "article" a number of times and never could find it but..... there is a thread in this forum on this issue and it contains a lot of commentary by Duke. A link to that thread is at the end of this comment.

I also copied the important technical discussions from that thread and produced a PDF document of it. That document is attached for your and other's convenience.

For Duke, I assume this is what you are talking about when you refer to an "article". If there is an actual article that appeared in some publication could you kindly post it or post a link to it - thanks - if it doesn't exist can you confirm that the info posted in this thread is what you are referring to - thanks again. https://www.forums.ncrs.org/showthread.php?97360-L-79-distributor-blueprint-overhaul&highlight=L-79+dist.+blueprint+overhaul

That's the thread. I also posted it on the CF.

Duke

[Ed S.]

Re: Electrical Issue

Ed,

I appreciate your and Duke's guidance on this issue. My skill base on certain things, electrical in particular, is lacking. Given that you are in the Mid-Atlantic chapter, might you be able to connect me with a good engine guy that can help me address this issue? I'm in Chapel Hill, NC, so travel time is not an issue. Any guidance is appreciated.

Bob

Bob, I wish I could refer you to a "good engine guy" but unfortunately, I don't know of any. Since you are in NC, you are actually in the heart of the Carolinas Chapter more so than the Mid-Atlantic chapter. We have a few members that live in NC but most of our members are from VA & MD. Perhaps a Carolinas chapter member that is following this thread can give you a good recommendation.

[Bob A.]

Re: Electrical Issue

Thx Duke.

In the March 10th posting, you referenced getting a B26 VAC or the NAPA equivalent VC 1765 for the 327/340. In the last posting, you suggested a B28. Was that a typo? Is the part you are suggesting part number B28 or B26? I guess NAPA will have the equivalent part.

Much Appreciated,
Bob