Dave Cunningham

Thanks to all who posted suggestions on my overheating problem while I am here in California. I have been implementing all of your ideas. I have the car so it is drivable now after installing a 16" electric fan on the back side of the rad, as shown by one of you in my other thread. It now will stay under 200 even in the worst conditions.
But I was still wondering why the idle did not change when I switched over to manifold vacuum. I changed the vacuum supply from the brake booster port to the choke, as suggested. Then I bought one of those mightyvac vacuum pumps, and hooked it to the manifold, and fired the car up and got a reading of 10 in.hg. Then I plugged that line and hooked up the pump to the diaphragm on the distributor, started the car and introduced some vacuum, nothing happened until I hit about 14 on the gauge and then the idle jumped right up. So I gues I need to change the actuator to one that kicks in earlier. I don't know what the specs are so I am not sure if the one I have (basically brand new from TI specialties) is faulty, or my car isn't producing enough vacuum ( the idle was set at 800 by the way)
And my next question is can I change the advance pot without removing the distributor?

Westlotorn

Your vacuum at idle is determined by health of the engine and your camshaft.
Since all is new in your engine it may be 10" is normal for your camshaft.
I don't know specs on your cam. You can change to a vacuum can that starts pulling vacuum at 8-10 to get the vac advance working for you. First verify that you are seeing the correct vacuum for your cam and compression. If you are off of what it should be start looking for your vacuum leaks. A mild cam will have close to 20 inches at idle, a wild cam can be below 10 at idle. Enjoy your visit.

SWCDuke

This has been discussed many times. With the OE cam a L-71 should pull 14-15" at 900, but less if you run a lower idle speed. I recommend 900.

The OE 201-15 VAC does not pass the Two-Inch Rule for full time vacuum advance (search the forum if you don't understand what it is). You need a 12" B26, NAPA VC1765 or equivalent in another brand.

If you have a different than OE cam use the Two-Inch Rule to select a VAC. The next most aggressive is the 8" B28.

You can change the VAC without removing/disassembling the distributor. Pump it down all the way so the pin is pulled as far as possible, then carefully wiggle it out from the hole in the magnetic pickup base. Reverse the procedure to install a new VAC.

The easiest way to tap into a full manifold vacuum source is to tee into the choke vacuum break signal line. Also, the carb base can be easily modified by plugging one hole and drilling another if you want the change to be invisible.

Duke

Dave Cunningham

Thanks for your reply, the cam is the factory GM camshaft for this app, the compression ratio is stock as well at 11:1

Dave Cunningham

Duke thanks, you answered all of my questions ( and I get what you mean by" this issue has been discussed many times" , in other words, "check the archives") I'm not a life long mechanic, but I do understand things when they are explained to me, and I understand what you are saying, thank you!

Westlotorn

Duke, maybe you can share a little more, you mentioned this engine should pull 14" at idle. Is that with the vac advance hooked up and properly advancing the timing or disconnected at idle like when you are setting the timing? Hooked up and working the engine may have an additional 10 degrees timing at idle which would increase vacuum.
I was wondering if his vacuum advance was working properly at idle what the vacuum might measure. Trying to think why his vacuum is at 10 with everything new.
Mark

Dave Cunningham

Mark, I would think that if it is supposed to pull 14" at idle, that is either hooked up to the vacuum advance pot or to a vacuum gauge, in either case, mine should work properly now.so maybe my idle is set too low. I talked with Dave from TI specialties, and I think he told me that the stock unit on the distributor, should start advancing at around 7" and be all in by 10", I am going to try to bump up the idle to 900 tomorrow, and see if it kicks in.

427435

My experience is that an L71 idling at 900 rpm will diesel for a long time when turning the ignition off. If mine isn't at 750 rpm idle, it diesels when hot. The 750 idle is quite possible with properly adjusted carbs.

Dave Cunningham

I normally do run my car at a 750 idle, are you experiencing any over heating issues?

SWCDuke

Idle vacuum should be tested with the engine in the normal configuration at the desired idle speed.

Optimum total idle advance is in the range of low 20s to low 30s, and high overlap cams require the high end. The total is usually provided by the sum of initial and full vacuum advance, except configurations with ported vacuum advance. In those cases cases since idle advance is well retarded from optimum, EGT goes up and thermal efficiency goes down. For emission controlled cars ported vacuum advance is usually implemented so the higher EGT helps oxidize HC and CO.

L-72/71 engines had to meet CA emission requirements, so ported vacuum advance and AIR was installed. Since these were low volume engines GM decided to use the same carb(s), so even 49-state cars that didn't require exhaust emission controls got the same carb set up for ported vacuum advance.

Search for a thread started by me with an attachment "Tuning Vintage Corvette Engines..." It goes into a lot more details on the spark advance map requirements for maximum performance and fuel economy.

There was a recent thread on the NCRS TDB - same issue - L-71 only pulled 10" @ 800. The owner confirmed an OE replacement cam and once he converted to full time vacuum advance with a B26 12" VAC he got the usual 14-15" @ 900.

Ported vacuum advance will increase the tendency of an engine to run-on when the key is turned off because the retarded idle advance heats up the combustion chamber boundaries creating enough hot spots to cause preignition after the ignition is shut off.

I've never heard of any run-on complaints when set up to my recommendations - full time vacuum advance with a B26 VAC - idling at 900.

Duke

DansYellow66

I hadn't thought about the lower cylinder combustion temp with manifold vacuum advance, when pulling in to shut down - but that makes sense. Also, the additional timing from full vacuum advance at idle also usually allows closing down the throttle idle position a bit from where it would be with ported advance for any given idle rpm - which I think helps with controlling run on.

My BB used to have an Accell distributor in it w/o vacuum advance and I used to have to clutch it as I shut it down due to run on. After converting back to a TI and putting in a can that would operate with the low vacuum signal from my aftermarket cam - lower engine temps, better idle, and no more run on. It works.

Dave Cunningham

Duke, I tried looking up those threads but couldn't find either of them, but I did run across lots of other ones. I thought I better try turning the idle up a bit more, so I set it to 1000 and I am still only getting 10" so I obviously need a unit that is all in by 8" . I have seen you guys talking about different part numbers, what unit do I need, and where would I find one. I asked at the Napa store , and they said none of that stuff is available any more. But they did have an adjustable one that I bought, and you can set it to come in that early. But I think you mentioned somewhere that you wouldn't recommend using one of those.

SupremeDeluxe

Mine is stock and idles fine at 750, pulling 15" of vacuum. Distributor is connected to full manifold vac. Increasing the idle to 900R does not cause dieseling on shut-off, but I see no reason to idle at 900 when it's perfectly smooth at 750.

Westlotorn

Dave, if others all get 14" or more and your engine is pulling only 10 even at 1000 RPM you have a leak in your vacuum/intake system or a mechanical issue to address.
I lean towards a leak. The problem is a leak can affect your air fuel ratio and cause other issues. Best to find out what you are fighting early.
Might try another Vacuum gauge first to rule out a gauge problem.

Dave Cunningham

I hear what you are saying but I just can't imagine where I could have a leak, everything is brand new, manifold carbs, hoses, etc getting frustrated, just got back from taking it for about a ten mile drive, started off fine, up the mountain for a few miles running around 200, then had to wait at two long traffic lights and sure enough it starts to climb, by the time I got pulled over it was at 230, had to let it sit for half an hour to cool off. The idle is all over the place, probably because I switched to manifold vacuum and the advance is partially kicking in.

babbah

What type of carb gaskets did you use? Check the carb attaching bolts to make sure the they are tight and the carb bases are true, not warped.
Also check the choke pull off and make sure it's good and holds vacuum.

Dave Cunningham

I had just come in from checking all the attaching bolts when I saw your post, so I went back out and put the gauge on the choke, in about twenty minutes it went from 20" to 15" so I would imagine it is ok. I think I just used the gaskets that came with the carbs.

Westlotorn

Can you advance your timing 10 degrees at idle as a test and see how much vacuum you have a idle with more advance dialed in? Don't drive it like this, just a test to see how it would respond to a working vac advance. If you find this test gives you 14-15 inches at idle you found your problem. Reset timing to spec and find a new vac canister.

Dave Cunningham

I've been reluctant to change the timing, due to the fact that I neglected to bring my timing light with me, and I really didn't want to buy another one, but it looks like I may have to. As I asked before where would one go about buying a new vac canister? I have this adjustable one that I was going to try but I would rather have a stock looking one if I could get it.

Westlotorn

It looks like Duke says the B28 comes in at 8" vacuum. I think that is an AC Delco part.
I would hit the traditional loca Auto Parts store. There is a chain of Warehouses in LA locally owned. Hanson Distributors. Call Hanson, tell them where you are and ask for the number of their closest store to you. They sell most independent stores in your area and deliver parts from the WD to these stores every day giving you access to all parts they sell.
I would think Napa could have offered the same service but your guy chose not to put in the effort.
You might also find the part on Amazon and get it next day delivery.