FINALLY - Vacuum Advance Dilemma Solved

[stallion112]

Installed a Crane vacuum advance kit into a stock Ford electronic distributor. 6 degrees base timing, 16 inches Hg of manifold vacuum. Canister backed off all the way CCW and it is pulling 29 degrees of vacuum advance for a total of 35 degrees at idle. This is way too much. Is there a possibility that the centrifugul springs are too light?

[Silverback51]

Figured I better hurry up and post this before Jim comes on and says, "I told you so".

As already stated I had changed back to vacuum secondary carbs, and was thinking about going back to the vacuum advance distributor.

Well come to find out the mechanical advance distributor was sticking. As soon as it went above 2000RPM's or so, it would stick at full advance, which was 32 degrees.

So I put the vacuum advance distributor back in and the driveability is much improved.

Now my car fully sucks and does run better. I have to admit I miss the mechanical secondary carbs, but the car runs better without them.

And it was great to see you today Jim. Even if you do call my car the "Banana Mobile".

[Wbulk]

Satallion112, yes it's possible. You need to resolved the centrifical advance and get it set right first. On the vacuum advance you want it adjusted to around 10-15 deg. advance. You have to watch your idle; if you are hooking it to manifold vacuum it raises the idle which raises the centrifical advance. You think you have more vacuum advance that you really do because the centrifical has kicked in.

Wayne

[SuperHart]

Note to Stallion, not to begin beating this issue another time, but 29* of advance (vacuum plus initial) at idle is too much. The Crane vacuum cannister is usually adjustable with a small allen wrench that came with the cannister kit (3/32" maybe?). To adjust your total advance at idle, use the timing light with the vacuum advance plugged in, not blocked off. Blocked off will measure only your initial advance (that which you dial in by turning the distributor). The advance with the vacuum plugged in will be your total advance at idle. This should be around 18-20* . As previously noted, as your total advance at idle increases so does your idle speed. The centrifugal advance usually doesn't begin to kick in until above 1500 RPM so keep that in mind. Repeatedly disconnect and plug the vacuum line at the cannister and adjust the vacuum advance with the allen wrench until your total advance at idle is reduced to the 18-20* mark. You may have to increase the idle speed screw on the carb before you achieve the correct advance setting (as you decrease vacuum advance, the idle speed may drop). To recap, check your initial advance at idle with the vacuum cannister disconnected (usually 6-10* for Ford engines) and then check your total advance at idle with the vacuum cannister connected. Always make sure when adjusting the vacuum advance that the vacuum hose from the cannister is plugged (a golf tee is excellent for this purpose) and repeatedly check after each adjustment until the desired advance is achieved. Then make a final idle speed adjustment on the carb and re-adjust your idle mix screws if needed to get a smooth idle.

[jwd]

Originally Posted by SuperHart

Note to Stallion, not to begin beating this issue another time, but 29* of advance (vacuum plus initial) at idle is too much.

WRONG. I suggest you re-read the article you posted by John Hinckley and pay attention to the third paragraph.

[jwd]

Originally Posted by Silverback51

Figured I better hurry up and post this before Jim comes on and says, "I told you so".

As already stated I had changed back to vacuum secondary carbs, and was thinking about going back to the vacuum advance distributor.

Well come to find out the mechanical advance distributor was sticking. As soon as it went above 2000RPM's or so, it would stick at full advance, which was 32 degrees.

So I put the vacuum advance distributor back in and the driveability is much improved.

Now my car fully sucks and does run better. I have to admit I miss the mechanical secondary carbs, but the car runs better without them.

And it was great to see you today Jim. Even if you do call my car the "Banana Mobile".

Nice to see you again John, glad you could make it and to hear your car is running great.

Jim

[DanEC]

Originally Posted by jwd

WRONG. I suggest you re-read the article you posted by John Hinckley and pay attention to the third paragraph.

It shouldn't pose any problems at idle but where you do have to be careful is in a highway cruise situation with a fast centrifugal advance curve. Cruising at 2500 rpm with 36 to 38 deg inital and centrifugal advance all in, and 18 to 20 deg vacuum advance can cause some motors to develop a miss or repetitive light stumble from basically firing too early before TDC. My old, aluminum head BB Chevy developed this problem and I had to modify the vacuum can to limit total vacuum advance to about 10 eg which fixed the problem.

[jwd]

Originally Posted by DanEC

It shouldn't pose any problems at idle but where you do have to be careful is in a highway cruise situation with a fast centrifugal advance curve. Cruising at 2500 rpm with 36 to 38 deg inital and centrifugal advance all in, and 18 to 20 deg vacuum advance can cause some motors to develop a miss or repetitive light stumble from basically firing too early before TDC. My old, aluminum head BB Chevy developed this problem and I had to modify the vacuum can to limit total vacuum advance to about 10 eg which fixed the problem.

I hope you meant 6-8 initial. The only problem I've ever seen from having too much advance at cruise speed is a surging situation. Initial timing, mechanical curve/total timing and the characteristics of the vacuum can (" of vacuum that it starts and at full and the total amount of advance) all have to work together. After you've tuned a few hundred, you get the hang of it.

Jim

[DanEC]

Originally Posted by jwd

I hope you meant 6-8 initial. The only problem I've ever seen from having too much advance at cruise speed is a surging situation. Initial timing, mechanical curve/total timing and the characteristics of the vacuum can (" of vacuum that it starts and at full and the total amount of advance) all have to work together. After you've tuned a few hundred, you get the hang of it.

Jim

No - total 36 to 38 initial and centrifugal advance - please read again. That's what my old 427 Chev runs when all the advance is in. Keith Craft ran my stroked 428 up to 40 degrees total advance (initial and centrifugal) on the dyno. That surging sensation is firing the ignition too far in advace of TDC.

[mreid]

Great article on vacuum advance, but several misinformed posts with blanket statements like "running mechanical secondaries on the street is wrong". Just demonstrates a lack of complete understanding. Power to weight ratio is a bigger determinant than street vs. strip. Also, some street cars also run on the track and need mechanical secondaries. Properly tuned mechanical secondaries on a high power to weight ratio vehicle will always perform better than vacuum secondaries through the range of driving conditions. Add vacuum advance and you have a killer combination.

[jwd]

Originally Posted by DanEC

No - total 36 to 38 initial and centrifugal advance - please read again. That's what my old 427 Chev runs when all the advance is in. Keith Craft ran my stroked 428 up to 40 degrees total advance (initial and centrifugal) on the dyno. That surging sensation is firing the ignition too far in advace of TDC.

Okay, I was reading it as 36-38 initial plus centrifugal advance. Now it makes sense. I know what causes the surging. When I tune a car and encounter it, I change the vacuum can to one that had a little less total advance. Usually only a reduction of a few degrees is needed.

Jim

[jwd]

Originally Posted by mreid

Great article on vacuum advance, but several misinformed posts with blanket statements like "running mechanical secondaries on the street is wrong". Just demonstrates a lack of complete understanding. Power to weight ratio is a bigger determinant than street vs. strip. Also, some street cars also run on the track and need mechanical secondaries. Properly tuned mechanical secondaries on a high power to weight ratio vehicle will always perform better than vacuum secondaries through the range of driving conditions. Add vacuum advance and you have a killer combination.

You are the one who doesn't understand carbs. The second quoted statement above proves it.There's more to a DP carbs. than just the mechanical secondaries. I've changed a lot of cars over to vacuum secondaries and the owners couldn't believe how much better the car ran. Maybe that's why NONE of the big 3 car makers ever used DP's. DP's are designed for the track, not the street.
Run whatever you like, I'm outta here. I've been building and tuning for 40+ years. My knowledge comes from actual experience (on the track and the street) not from a Summit/Jegs catalog.

Jim

[mreid]

Wow, Jim, try not to be such a dick. You don't know me, my experience (Summit/Jegs? Really??), or even the detail behind my statement. Sorry to hear that you are "outta here". I always thought the purpose of a forum was to discuss perspectives and learn something from each other. I didn't realize it was to present yourself as a know it all, spout how much more qualified you must be than who ever challenges you, and then run away. That's real punk behavior in my book.

[DanEC]

Originally Posted by jwd

Okay, I was reading it as 36-38 initial plus centrifugal advance. Now it makes sense. I know what causes the surging. When I tune a car and encounter it, I change the vacuum can to one that had a little less total advance. Usually only a reduction of a few degrees is needed.

Jim

Oh - yeah, I can see now how it could be read that way. I understand.

My BB Chev runs low vacuum due to the big, old-school cam and I had to purchase a special vacuum canister that starts to pull in at 6 inches vacuum. There might be one out there like it with less total vacuum advance but it was easier to make a little bracket that mounted over the arm and had a tab that stops the rod from advancing the entire way. That way I reduced the advance that was kicking in while cruising on the road and stopped the surging. Cruising down the road that surging/missing can be really annoying even if you understand what is going on.

[RET_COP]

I agree with mreid. The mechanical secondaries with a light stick car and good power is always better. The rpm's rise quicker and the car action is very responsive up and down the rpm range. I went from a VS to MS in my 500hp SBF and the difference was very noticable. I like the control I have. The VS no doubt was smooth as silk but it was not as fast and responsive under my foot. This is my experience anyway. Maybe that is why detroit used them from the showroom floor.
If I'm not mistaken (sorry to bring this up on this forum but...) the L-88 vette came with a Mechanical Sec. 850 Holley. This was not for the average joe driver.
Lou

[jwd]

Originally Posted by RET_COP

If I'm not mistaken (sorry to bring this up on this forum but...) the L-88 vette came with a Mechanical Sec. 850 Holley. This was not for the average joe driver.
Lou

Yes it was an 850 CFM Holley but it had vacuum secondaries. Maybe the engineers at GM should have asked mreid what to use since their years of racing experience and dyno. testing must have been wrong.
Like I posted earlier, NO manufacturer used mechanical secondary carbs. on a production car, even limited production ones destined for the track. Not Ford, GM, Chrysler or even Shelby. There is a reason for that.
Jim

[RET_COP]

Here is a quote from a corvette forum,
"68 and 69 l88's used a holey 4150 p/n R4054-A OR A 4296-A (ALSO USED ON THE ZL1) GM P/N 3925519 AND 3955205 respectively.

Here is the link---http://forums.corvetteforum.com/c3-tech-performance/684325-what-carb-did-the-real-68-69-l-88-have.html

Argue with them
I stand by my experiences, I am not an expert.
I am sure your customers liked the VS carb, I did too! But the MS suited my driving style better.
Good debate though.
Lou

[mreid]

I wish we could have a debate or discussion as opposed to ridicule and grandstanding.

By the way, I believe the ZL-1 used a mechanical secondary carb in both the Camero and Corvette (GM p/n 3955205, Holley List #4296).

[Silverback51]

I guess I better qualify my earlier statements.

I really like the control that you have with mechanical secondaries.

My problem was I went with the 660 center squirters. No provisions for power valves.

These carbs were designed for one thing. Drag racing. Either idle up to the line, or full throttle for 10+ seconds.

If there was a viable mechanical secondary carb option I would probably go with it. But to my knowledge it does not exist.

[DanEC]

The 1967 L-88 used Holley R3418A P/N 3886091, which was a 850 cfm, and most definately a vacuum secondary carb.

However, I don't know what carb the 68 and 69 L-88 used off hand. I very much doubt they ever used a mechanical secondary carb in those years but I don't know that for sure.