Converting ported to full time vacuum advance
I have seen this discussed as a good idea, but I have not found how to do it. How do you do this? I am ready to do it on my '67 L71. Thanks fr any advice Duke.
-
Big Tanks In the High Mountains of New Mexico -
Re: Converting ported to full time vacuum advance
Mike -
I'm not Duke, but here's how you do it. The vacuum advance line from your distributor is currently connected to a small nipple on the side of the base of your center carburetor. Pull it off, and cap the nipple. Get a plastic hose tee, and insert it into the existing rubber hose from the front of the carb base to the choke pull-off diaphragm. Connect the vacuum advance line to the tee you just installed.
Your idle speed will increase somewhat due to the vacuum advance being active at idle; readjust your idle speed screw to set the idle back to your preferred level. -
Re: Converting ported to full time vacuum advance
Thanks John, it was Duke who gave me that advice when I asked about things to do with my new '67 L71 earlier this week. I will do it and hope it cures the overheating at idle issue. I can run this car all day on the road at 25-70 mph and it sticks at 180, but when I stop and let her idle for about 5 minute it keeps going to the yellow zone and further if I don't shut it down. I am also going to flush the radiator, as when I looked inside it had considerable white crusts on the tube ends and insides. Thanks again.
Big Tanks In the High Mountains of New MexicoComment
-
Re: Converting ported to full time vacuum advance
That's the quick and easy way to do it. If the carb still has the original choke vacuum break hose, remove it and save it. Get a foot or two of 1/4" vacuum tubing and a tee. Fabricate a new choke vacuum break hose with the tee and run a line to the VAC off the tee.
If you're handy and understand carburetors, you can convert the existing VAC nipple to full time.
It involves plugging a passage with a piece of lead shot or epoxy, then drillling a small hole to place the VAC signal port below the throttle valves.
It's impossible to explain, but, like I said, if one is handy and understands carbs, they'll figure it out.
Converting to full time vacuum advance will likely solve your idle/low speed driving hot running condition.
"Flushing" radiators is a waste of time. If you can see significant deposit buildup at the end of the tubes and the engine runs hot, it's time for a rodout. Since you have a brass radiator it's easy for a decent radiator shop to do. If the rad. still has the OE part number tag, remove it before taking it to a shop because they will always throw it away. They will place a tag on the rad. for warranty purposes. Have them place it in a place where it can't be seen on the car and reinstall the origina radiator tag. No one will ever know...
Use a low silicate antifreeze - Zerex G05 - and the radiator won't build up deposits anywhere near as fast as they did with the high-silicate "green" antifreeze.
DukeLast edited by Duke W.; December 13, 2009, 12:05 AM.Comment
-
Re: Converting ported to full time vacuum advance
Thanks for the tips. Actually, the plumbing (hoses with a "T") were already in place, so all I did was take the plug from the "T" and put it on the ported vacuum on the center carb and hook the vacuum advance tube to the "T". I have reset the idle jets and idle speed to 900 rpm, 14 lbs. vacuum. I will try this setup out and see if it solves the heating at idle/low speed problems before I tackle the radiator. Good tip on the antifreeze, never knew there was a big difference in them. Thanks again.Big Tanks In the High Mountains of New MexicoComment
-
Re: Converting ported to full time vacuum advance
The vacuum advance unit and the vacuum unit for the choke pull off can fail and become a vacuum leak source. Not something that has happened often in my use, but it has occured.
Full time vacuum will reduce engine heating at idle and in stop and go traffic. One of the first modifications I made to my new 70 was hooking up full time vacuum advance. Better gas mileage in city driving since vacuum to the distributor was now available with my change in first and second gear. My 70's TCS unit was bypassed and no longer controlled when vacuum was allowed to the distributor's vacuum advance unit. Have enjoyed the benefits for more than 39 years.Last edited by Jim T.; December 13, 2009, 09:25 AM.Comment
-
Re: Converting ported to full time vacuum advance
Not being to hip about the 67 engine you are referring to, pardon my dumb question; Is your carb a Holley or a Rochester Q-Jet? Reason I ask is that, as I recall, the QuadraJet has a chamber in the casting that can be drilled open to either side of the butterfly. The existing port is drilled at or slightly above the butterfly position at idle, but you can drill into the port chamber from a position below the butterfly to give you the same full time vacuum source. But of course, once modified, it is not easily reversed like a vasectomy. You never want to use any screw, pin, or other metal plug that can possibly work loose and be sucked onto the engine. Don't ask me how I know - oh well, I'll tell anyway - I once locked a hard pin in place in the vacuum butterfly spacer of a a 4GC Rochester. It loosened up and dropped into the engine wiping out every ring land in #8 cylinder. It jammed into the top of the piston and each revolution drove it deeper into the piston as well as doing a nice scratch in the cylinder wall. Not pretty.
Stu FoxComment
-
Re: Converting ported to full time vacuum advance
You mean 14" of mercury vacuum; 900@14" is right on the nominal idle characterstic for SHP big blocks with the OE mechanical lifter cam.
In order to take full advantage of the conversion you need to install a B20 VAC (NAPA VC1765 or equivalent). The OE ...201 VAC does not provide full advance until about 15.5", so there is not enough idle vacuum to keep the plunger locked at full advance at idle.
The B20 VAC starts to pull at 6" and is fully deployed at 12" with 16* advance, so you have 2" of margin, and it meets the "Two Inch Rule".
It sounds like your radiator needs a rod out. This is one area where big blocks are actually less expensive to maintain that small blocks. The BB brass radiator can be repaired. Once the alumimum SB radiator gets clogged tubes, it's toast, and the only fix is to by a new DeWitts OE replacment aluminum radiator.
DukeComment
-
Re: Converting ported to full time vacuum advance
Don't ask me why, but I had put a manual switch on my TCS of my 72 Nova SS. I still have the pieces today. I think I used it to control the engine better in the snow, i.e. so it wouldn't rev up so quickly or deaden the throttle response. The same theory existed about a chevy six was better in the snow than a V8. We lived at the bottom of a mile long hill that we had to climb every morning in order to get to work. The trick to making it to the top entailed good throttle control to maintain traction even to the point of gradually reducing throttle as you approached the top of the hill. If you allowed the wheels to break loose (lose traction), you were done. you would continue to spin as you turned the snow under your wheels to ice. Had to back all the way down and start over, or spin over to the side and get stuck. My 57 post with posi and oversize snow tires was terrible in the snow as that darn Duntov just would not allow you to idle down for control.Comment
-
Re: Converting ported to full time vacuum advance
Right, I meant 14". After a cruise this morning of about 20 minutes I returned to the garage and made the final carb idle screw adjusts and idle speed adjusts, but after about 4 minutes idling the temp gauge went from about 190 to near the yellow and thus it is not fixed yet. I will get the new VAC unit and see if that helps, and then do the radiator.
Stuart, the carbs on the L71 (all three) are Holleys.Big Tanks In the High Mountains of New MexicoComment
-
Re: Converting ported to full time vacuum advance
Also check the fan clutch. Shut it down after idling up to high temp and turn the fan with you hand. Let it cool and then turn by hand again. There should be a big difference.
If the fan clutch is a current replacement, it's setup for 195 thermostats, but should not let the idle temp get much over 200-210 except in extremely hot weather.
DukeComment
-
Re: Converting ported to full time vacuum advance
The clutch seems the same hot or cold, just a bit of resistance to turning. I also rechecked the vacuum at idle, I had read the gauge wrong before, it only has 10 inches of vacuum at 900 rpm. I also checked the timing, it is about 6 degrees on the "A" side (above the timing mark) of the crank grid (BTDC?) at 900 rpm. I don't know what the specs are, but I seem to remember maybe 8 degrees BTDC for the L72. Maybe I should advance the timing to get more vacuum?Big Tanks In the High Mountains of New MexicoComment
-
Re: Converting ported to full time vacuum advance
Your fan clutch is probably toast. It probably has an aftermarket cam, and you need to get a CSM before you start messing...
Search "Two Inch Rule".
DukeComment
Comment