Carb'd 350 timing question

[jim_in_dorris]

It's a friend's 67 C30 with a carb'd 350 in it. I just got done putting plugs, filters, and a performer intake on it. After setting the points, my son and I set the timing. What we found was that where it would rev without stumbling on throttle snap, it wouldn't start. Where we could get it to start easily, there was a huge stumble, almost to the point of dying when you snapped the throttle open. We compromised on the timing to minimize the stumble but still start. After going out in the woods yesterday for our first load of firewood of the year, I noticed that when the engine is hot, it doesn't want to crank easily, like the timing is still to far advanced. If I change the timing a little more, what can be done about the stumble on throttle snap? It is completely stock except for the manifold.

[EagleMark]

Points?

If the dwell is 30 degrees and doesn't change when reved up and down indicating slop in shaft... then I would look at vacuum advance canister not working properly, plumbed to wrong vacuum port or timing set with hose connected...

Is mechanical advance working, not stuck?

[jim_in_dorris]

Points are OK, however, I don't have a dwell meter to look at that, will try to work on that. The mechanical advance was sticky, but an application of WD40 seems to have fixed that problem. I am not sure how much vacuum it should take to cause an advance to occur, it seemed like it wanted to try to advance, but it took a lot of vacuum for not very much advance. The vacuum port used is the front port on the quadrajet. Should it be manifold vacuum instead? I still have the fitting to do that, but it was ported to the quadrajet when I got it.

[EagleMark]

It's been so long Jim, IIRC it gets full vacuum, not ported vacuum, to reduce timing, as vacuum drops WOT it retards timing, but to set timing it must be disconnected and set to like 6 degrees, when hooked up it's off the timing marks... I can't even find my old manuals to look it up? I may be exactly backwards or another make car is backwards?

If someone really really old looks at this thread they may remember?

[EagleMark]

I was just looking and pretty sure it is like I said above direct manifold vacuum, disconnected to set timing. This may be differant on later model engines with emmissions to ported vacuum, so there was my memory failure of why some are differant.

OK here is anceint technoligy to add to this thread by Mr Sinister I have read and seems to be what I remembered from high school auto shop, same year electronic ignition was starting to appear!

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.

[gregs78cam]

Double check your condensor.

My Dad just went through this last fall with his Datsun. Replaced condensor, set timing and dwell, and all was well.

[EagleMark]

Double check your condensor.

My first thought was A/C?

Yeah that could cause a can of timing worms. IIRC there is also a wire to starter to supply 12 volt to ignition while cranking, then back to resistor and lower voltage when running. Not sure what no 12 volt would cause a starting timeing issue, it was to supply bigger spark during cranking without constant 12 volt burning up points during run...

[jim_in_dorris]

woodtruck.jpgThis is a picture of the truck with my buddy during a wood cutting trip. As is evident, this isn't a race truck LOL. A/C We are lucky it has a heater. I think that we need to time it without the vacuum plugged in, and probably change the plug wires. I am not sure if the vacuum port its plugged into on the carb is ported or not, will check it next week when I get the truck back. This is a 69 350 we think, no smog whatsoever. The only reason I changed the manifold is because it was leaking gas from the base gasket. I went to replace the gasket, and it wasn't even the correct gasket. The reason being that that particular manifold used a 3 piece gasket with the center part being a stainless gasket which is NLA. Everybody said it was easier to just put a modern manifold in, so that's what I did. (I already had the manifold)

[EagleMark]

I think that we need to time it without the vacuum plugged in

Yup! Problem solved...

[jim_in_dorris]

I will post after we do the re-time, probably next weekend.

[jameslleary]

6BTDC, in drive, 550 rpm. Ported vacuum...usually the pass side lower port on the carb.

[EagleMark]

D'oh!

Half solved...

[1project2many]

I just renewed my license and had to take the damn eye test twice because the first girl missed that I took my glasses off when I passed. I stood up there squinting at those letters forever in front of a bunch of upset people because I wasn't about to have a corrective lenses restriction on my license. What's wrong with corrective lenses? Translation: "This guy's old." Now if you think I'm going to help out after reading this

If someone really really old looks at this thread they may remember?

, well, you've got another thing coming.

Anything I'm going to post is either a damn good guess or something I read on the internet. I wasn't going to help because I'm nowhere near "old" and I have never worked on this stuff but I'm just not the kind of guy to walk away.

By 69 GM had switched most vehicles to ported vacuum. That three piece carb gasket has caused plenty of issues over the years as it directed hot exhaust gas under the carb to heat the fuel. The exhaust ports would plug, the gasket burns out, and there's a ton of trouble with fuel vapors going where they shouldn't when they shouldn't. It's a good idea to get rid of the manifold but what you sometimes find is that someone who knows a bit about carbs has changed the jets and / or readjusted that carb to try and account for unmetered hot fuel vapor being drawn out through the bowl vent. Where the heat made the engine rich, someone knew enough to lean it out. Changing the manifold and putting the same carb back on means no additional heat, so carb is lean.

Set timing with vacuum line disconnected, then check for advance function by starting engine and connecting hose to manifold vacuum source. Engine rpm should increase. Don't leave vacuum connected to manifold vacuum if it was originally connected to ported as you'll experience hard starting. A vacuum can designed for ported vacuum can respond at lower vacuum levels and advance spark during cranking.

If your carb is leaking from the bowl (very common and very easy fix) it will drip some fuel into the manifold while the engine is sitting. If you try and restart it after a 20-40 minute wait you'll draw a hot, combustible mixture of air and fuel into a hot cylinder during cranking. As soon as the plug fires BTDC that mixture burns and applies force to the piston against the starter's motion. Playing with timing can change this by applying force ATDC however, you've incorrectly biased the entire timing curve to account for the fuel leak and you'll have poor power, hesitation, poor mileage, maybe even backfiring. The only real fix is to seal the carb bowl.

Of course there's always a TBI swap.

[EagleMark]

I just renewed my license and had to take the damn eye test twice because the first girl missed that I took my glasses off when I passed. I stood up there squinting at those letters forever in front of a bunch of upset people because I wasn't about to have a corrective lenses restriction on my license. What's wrong with corrective lenses? Translation: "This guy's old." Now if you think I'm going to help out after reading this , well, you've got another thing coming.

Anything I'm going to post is either a damn good guess or something I read on the internet. I wasn't going to help because I'm nowhere near "old" and I have never worked on this stuff but I'm just not the kind of guy to walk away.

Sorry. Didn't mean to hit a soft spot, if it helps this is the first year I could not pass license renewal without glasses. Was worried what would happen next time so I got the ten year license... couple years ago I had to get reading glasses to solder...

[1project2many]

LOL... it doesn't hurt my feelings to be called old. I know I'm no spring chicken. On my days off I take naps when the kids do. On work days I end up sleeping through lunch sometimes. Besides... I started losing my hair at 20 so I've looked older than I am for a long time. Should have put a smiley at the end of that paragraph.

I know those carb intakes really well. Spent a lot of time fighting a couple of the crossover types trying to prove they could run right with the right guy tuning them but long story short, they usually don't.