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Unread 04-17-2013, 04:52 PM   #1
WSS
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350 SBC w/carb or distributor problems....HELP!

I have a 350 SBC in my '72 CJ5. It HAD a Holley that was a little worn and leaky but ran OK (Compared to my new carb). I just installed a Carb Shop Quadrajet. It has float problems from the beginning. The carb was set-up as "off-road" meaning it has a different needle seat affair, not sure how.

I took it by them today and was told to relocate my fuel filter further from the heater hoses and I may have a bad distributor. The vacuum adv was not hooked up on the Holley set-up but I did hook up a hose when installing the Q-jet.

It is acting as though it is starving for fuel under accel and comes back after it drops, kinda a on-off quick thing and is not starting to awfully quick as though it is flooded, it does kick out some nasty smelling dark smoke too when first starting. The engine feels like it is running hot as well but shows normal on the dummy gauge.

Anyone have similar issues? OR any advice? I'm not all that carb savy which is why I chose a carb shop Q-jet.

Thanks for any input!

WSS

PS, I posted the same question in the engine section, not sure were it would be most appropriate. Mods, let me know if I need to delete or move. THX!

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Unread 04-18-2013, 12:43 PM   #2
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My experience was hooking up the vacuum advance line to the wrong port. Where you hook up should have no vacuum at idle and then pick up with engine speed. I accidentally hooked mine up so that it was advancing at idle and then drop back under load. Your problem sounds kinda similar, I would check that first. Have you checked your timing?
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Unread 04-18-2013, 02:53 PM   #3
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That is definitely one thing to check. The vacuum port should be hooked to the top half of the carb (typically), so anything from the top half of the base. Anything below that should be full vacuum at all times, which you do not want.
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Unread 04-18-2013, 03:42 PM   #4
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Quote:
Originally Posted by AFRd_CJ5 View Post
Anything below that should be full vacuum at all times, which you do not want.
I think there are many, here, who disagree with this statement.

WSS, I run my 360's vac advance on manifold vac, with a Q-jet, and I'd highly recommend you do the same.

I would explain why, but that subject has been pummeled to death, here. A search for "manifold vs. ported vac advance" should provide more info than you care to read.

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Unread 04-18-2013, 06:44 PM   #5
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Thanks guys! I do have the correct port, no vac at idle and increases with rpm, it is on the bottom. Typical for a Q-jet.

The timing has not been checked but I do suspect it is close to the problem area (gave my timing light away 10yrs ago!). I opened the distributor and found the weights to be RUSTED tight. The vacuum advance module thingy was not working very well either. I believe the weights were opening fully and not smoothly then letting go. I have put in a brand new accel distributor that is a basic stock replacement and has vacuum adv and no governor. It without question starts better. I had to actually work today and have not had a chance to run it on the street, just start and rev. I'll get it out this evening to see what she does. Exhaust sounds and smells better too. No more black/dark smoke when starting.

WSS

PS, I'll google the manifold vs carb vacuum location too.
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Unread 04-18-2013, 06:52 PM   #6
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X2 on the manifold vacuum.
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Unread 04-18-2013, 07:28 PM   #7
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Oh if hammer saw the rusted tight HEI flyweights.

I'm thinking the rust wasn't really rust but a worn cap and rotor arching through the rotor to the fly weights. And they arched fast.

It was misfiring? To do that it must have ran like crap under load.

IMO of coarse.

The 396 in the Chevelle has been on manifold vacuum since 2006.
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Unread 04-18-2013, 09:22 PM   #8
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OK, problem solved. She runs like a ape of some sort.

What is it with the manifold vacuum that is different than the carb feed? I am very interested.

The rust probably was a combo of water and electrolitic reaction. The stuff was a rich red color and probably induced by some arcing somewhere. It did run "fine", no mis-fires, just sluggish, didn't realize it was sluggish though. Now, tap the throttle and it will hit 3k rpm without hesitation. Actually had to tighten up some belts to stop the squealing from the fast accel.

Headed to silverwood/pilot rock instead of hammers to give it a shake down.

Thanks again!
WSS

Hooked up my tow wiring tonight.Chomping to get out!
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Unread 04-19-2013, 03:28 AM   #9
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When you got on the gas and the motor cut out that was the spark blowing through the rotor insulation and hitting the advance weights. That sparking blows metal away which rusts quite fast.

It's harder for a spark under compression then in open air. That's why sometimes you'll think you have bad plugs and pull then and see good spark. But run bad in engine and new plugs fix the problem.

Keep your old carb and distributor. They may come in handy. You can clean up the dizzy and reuse it.
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Unread 04-19-2013, 05:33 AM   #10
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Interesting...... I did google some, and it seems some people say to use full manifold, and others say not to. I have not had a vacuum advance type distributor in probably 15 years, but that's what I was always taught. Interesting to see others have a different view.
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Unread 04-19-2013, 06:14 AM   #11
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Quote:
Originally Posted by AFRd_CJ5 View Post
Interesting...... I did google some, and it seems some people say to use full manifold, and others say not to. I have not had a vacuum advance type distributor in probably 15 years, but that's what I was always taught. Interesting to see others have a different view.
This was written by a former GM engineer as a response to a similar question on a Camaro board:


"As many of you are aware, timing and vacuum advance is one of my favorite subjects, as I was involved in the development of some of those systems in my GM days and I understand it. Many people don't, as there has been very little written about it anywhere that makes sense, and as a result, a lot of folks are under the misunderstanding that vacuum advance somehow compromises performance. Nothing could be further from the truth. I finally sat down the other day and wrote up a primer on the subject, with the objective of helping more folks to understand vacuum advance and how it works together with initial timing and centrifugal advance to optimize all-around operation and performance. I have this as a Word document if anyone wants it sent to them - I've cut-and-pasted it here; it's long, but hopefully it's also informative.

TIMING AND VACUUM ADVANCE 101

The most important concept to understand is that lean mixtures, such as at idle and steady highway cruise, take longer to burn than rich mixtures; idle in particular, as idle mixture is affected by exhaust gas dilution. This requires that lean mixtures have "the fire lit" earlier in the compression cycle (spark timing advanced), allowing more burn time so that peak cylinder pressure is reached just after TDC for peak efficiency and reduced exhaust gas temperature (wasted combustion energy). Rich mixtures, on the other hand, burn faster than lean mixtures, so they need to have "the fire lit" later in the compression cycle (spark timing retarded slightly) so maximum cylinder pressure is still achieved at the same point after TDC as with the lean mixture, for maximum efficiency.

The centrifugal advance system in a distributor advances spark timing purely as a function of engine rpm (irrespective of engine load or operating conditions), with the amount of advance and the rate at which it comes in determined by the weights and springs on top of the autocam mechanism. The amount of advance added by the distributor, combined with initial static timing, is "total timing" (i.e., the 34-36 degrees at high rpm that most SBC's like). Vacuum advance has absolutely nothing to do with total timing or performance, as when the throttle is opened, manifold vacuum drops essentially to zero, and the vacuum advance drops out entirely; it has no part in the "total timing" equation.

At idle, the engine needs additional spark advance in order to fire that lean, diluted mixture earlier in order to develop maximum cylinder pressure at the proper point, so the vacuum advance can (connected to manifold vacuum, not "ported" vacuum - more on that aberration later) is activated by the high manifold vacuum, and adds about 15 degrees of spark advance, on top of the initial static timing setting (i.e., if your static timing is at 10 degrees, at idle it's actually around 25 degrees with the vacuum advance connected). The same thing occurs at steady-state highway cruise; the mixture is lean, takes longer to burn, the load on the engine is low, the manifold vacuum is high, so the vacuum advance is again deployed, and if you had a timing light set up so you could see the balancer as you were going down the highway, you'd see about 50 degrees advance (10 degrees initial, 20-25 degrees from the centrifugal advance, and 15 degrees from the vacuum advance) at steady-state cruise (it only takes about 40 horsepower to cruise at 50mph).

When you accelerate, the mixture is instantly enriched (by the accelerator pump, power valve, etc.), burns faster, doesn't need the additional spark advance, and when the throttle plates open, manifold vacuum drops, and the vacuum advance can returns to zero, retarding the spark timing back to what is provided by the initial static timing plus the centrifugal advance provided by the distributor at that engine rpm; the vacuum advance doesn't come back into play until you back off the gas and manifold vacuum increases again as you return to steady-state cruise, when the mixture again becomes lean.

The key difference is that centrifugal advance (in the distributor autocam via weights and springs) is purely rpm-sensitive; nothing changes it except changes in rpm. Vacuum advance, on the other hand, responds to engine load and rapidly-changing operating conditions, providing the correct degree of spark advance at any point in time based on engine load, to deal with both lean and rich mixture conditions. By today's terms, this was a relatively crude mechanical system, but it did a good job of optimizing engine efficiency, throttle response, fuel economy, and idle cooling, with absolutely ZERO effect on wide-open throttle performance, as vacuum advance is inoperative under wide-open throttle conditions. In modern cars with computerized engine controllers, all those sensors and the controller change both mixture and spark timing 50 to 100 times per second, and we don't even HAVE a distributor any more - it's all electronic.

Now, to the widely-misunderstood manifold-vs.-ported vacuum aberration. After 30-40 years of controlling vacuum advance with full manifold vacuum, along came emissions requirements, years before catalytic converter technology had been developed, and all manner of crude band-aid systems were developed to try and reduce hydrocarbons and oxides of nitrogen in the exhaust stream. One of these band-aids was "ported spark", which moved the vacuum pickup orifice in the carburetor venturi from below the throttle plate (where it was exposed to full manifold vacuum at idle) to above the throttle plate, where it saw no manifold vacuum at all at idle. This meant the vacuum advance was inoperative at idle (retarding spark timing from its optimum value), and these applications also had VERY low initial static timing (usually 4 degrees or less, and some actually were set at 2 degrees AFTER TDC). This was done in order to increase exhaust gas temperature (due to "lighting the fire late") to improve the effectiveness of the "afterburning" of hydrocarbons by the air injected into the exhaust manifolds by the A.I.R. system; as a result, these engines ran like crap, and an enormous amount of wasted heat energy was transferred through the exhaust port walls into the coolant, causing them to run hot at idle - cylinder pressure fell off, engine temperatures went up, combustion efficiency went down the drain, and fuel economy went down with it.

If you look at the centrifugal advance calibrations for these "ported spark, late-timed" engines, you'll see that instead of having 20 degrees of advance, they had up to 34 degrees of advance in the distributor, in order to get back to the 34-36 degrees "total timing" at high rpm wide-open throttle to get some of the performance back. The vacuum advance still worked at steady-state highway cruise (lean mixture = low emissions), but it was inoperative at idle, which caused all manner of problems - "ported vacuum" was strictly an early, pre-converter crude emissions strategy, and nothing more.

What about the Harry high-school non-vacuum advance polished billet "whizbang" distributors you see in the Summit and Jeg's catalogs? They're JUNK on a street-driven car, but some people keep buying them because they're "race car" parts, so they must be "good for my car" - they're NOT. "Race cars" run at wide-open throttle, rich mixture, full load, and high rpm all the time, so they don't need a system (vacuum advance) to deal with the full range of driving conditions encountered in street operation. Anyone driving a street-driven car without manifold-connected vacuum advance is sacrificing idle cooling, throttle response, engine efficiency, and fuel economy, probably because they don't understand what vacuum advance is, how it works, and what it's for - there are lots of long-time experienced "mechanics" who don't understand the principles and operation of vacuum advance either, so they're not alone.

Vacuum advance calibrations are different between stock engines and modified engines, especially if you have a lot of cam and have relatively low manifold vacuum at idle. Most stock vacuum advance cans aren’t fully-deployed until they see about 15” Hg. Manifold vacuum, so those cans don’t work very well on a modified engine; with less than 15” Hg. at a rough idle, the stock can will “dither” in and out in response to the rapidly-changing manifold vacuum, constantly varying the amount of vacuum advance, which creates an unstable idle. Modified engines with more cam that generate less than 15” Hg. of vacuum at idle need a vacuum advance can that’s fully-deployed at least 1”, preferably 2” of vacuum less than idle vacuum level so idle advance is solid and stable; the Echlin #VC-1810 advance can (about $10 at NAPA) provides the same amount of advance as the stock can (15 degrees), but is fully-deployed at only 8” of vacuum, so there is no variation in idle timing even with a stout cam.


For peak engine performance, driveability, idle cooling and efficiency in a street-driven car, you need vacuum advance, connected to full manifold vacuum. Absolutely. Positively. Don't ask Summit or Jeg's about it – they don’t understand it, they're on commission, and they want to sell "race car" parts."

Matt
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Unread 04-19-2013, 07:04 AM   #12
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Yep, that was one of the ones I read. Very interesting!
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Unread 04-19-2013, 07:13 AM   #13
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AFR,

Once you try manifold vac advance, I almost guarantee you'll never use ported, again. Your engine will run cooler and smoother, especially at idle, and it will cold start easier. Low-end performance will improve across the board.

Expect your idle to increase about 150 RPM when you connect to manifold. This is normal due to the added advance. Just back your idle screw down to whatever RPM you prefer, and go.

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Unread 04-19-2013, 09:16 AM   #14
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AFR has a big damn cam with, I'm guessing, pretty low and unstable manifold vacuum. It'll be interesting to see how the vacuum advance works on his manifold source. It will certainly take a Crane adjustable unit to work. No stock canister will get there. Try to set it up to be fully deployed at 2 inches below normal manifold vacuum and limit the advance to 10*. It may not work at all. Definitely a good test, though.


Shawn
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Unread 04-19-2013, 09:26 AM   #15
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Shawn,

That's a great point.

Question...if the cam is too big to provide enough pressure at idle to activate the advance, wouldn't the advance operate much like it would on ported?

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