tuning woes

[popinsmoke]

Gentlemen,

Ok here is my saga of tuning woe, I swapped intake manifold (Performer rpm air gap) Carburetor (Carter Competition AFB 750cfm) and cam (Comp cams xe268H) on my 67 coronet small block car.

At this same time I installed a MP electronic ignition conversion kit with the orange box.

After the install she fired right up didn’t really want to idle but I wasn’t real worried about it as I just needed to break in the cam I would fine tune her after break in.

I started by setting the timing, I can’t really make out the timing marks so I hooked up the timing light and rotated the distributor clockwise and watched the tach gain revs until the engine began to stumble the back it off the other way until the engine stumbled then roughly split the difference and turned the idle down to about 850rpm.

The car idled pretty well, but stumbled off idle and would pop at start up, and sometime off idle transition to cruise.

I bottomed out the mixture screws and backed the out a quarter turn at a time as the car would pick revs and smooth out. I back them out about one and a half turns and the engine began to stumble so I turned them back in a quarter turn.

Engine was now idle real smooth no off idle stumble but now it surges, at idle and a little at cruise.

Throttle response is far from crisp if you floor it from a stop it bogs till it gains some revs then cleans up and accelerates briskly.

My 87 D150 will blow the rear tires off from a stop.

So I know I need to sort the timing first then move on to the carb I think I am on the right track but let me ask some dummy questions that might keep me from chasing my tail anymore then I already have.

My distributor vacuum advance is hooked up to the right hand vacuum port on the carb when looking at it from the front of the car the middle is for pvc and the left is plugged is that correct.

When I set the initial timing I plugged the hose to the distributor is that correct?

Being that I can’t really make out the timing marks is the method of timing by ear correct or am I screwing something up?

The car pulls nice and hard in the upper rpms but is a complete pooch on the bottom end too much timing?

I will be the first to admit I have been spoiled by EFI cars I am needed to get back to basics I know I am close to getting this car to run the way I want but I feel like I am missing something simple stupid.

Thanks in advance for reading or for any advice.

 

[Elvad]

Timing

Your distributor needs to be connected to manifold vacuum, make sure there is vacuum there at idle (NOT ported vacuum, which is negligible at idle and increases with rpm). Next you need to find and mark TDC on the balancer and buy a timing tape. The tape is like $10 from Summit or wherever. There are a few different ways to find TDC with the heads on. To time the engine correctly you want full advance which is at about 2200-2500 rpm with the vacuum line disconnected. This is the full centrifugal advance from the weights in the distributor. The Mopar Performance Engine book explains all these things in detail. If you search P4876825 on Amazon it will show you the big block Chrysler book, I don't have the part number for the small block book but the principals are the same. Good Luck

 

[popinsmoke]

Well I got off duty at 0300hrs so I figured why not fiddle with it some more. I backed off the initial timing, that cured the surging, Still can't see the timing marks but the car isn't pinging under load or WOT and holds a steady 850rpm idle.

The car is still real soft on the bottom end not sure what that is about perhaps I am expecting to much from a grocery getter with highway gears.

Elvad thanks for the advice on the reading material.

 

[1971RR]

I thought the vacuum advance was supposed to be hooked up to the left vacuum port (passenger side) which I assume is the ported vacuum. Might explain why my car doesnt run as well as I thought is should. Seemed the right hand port (driver side) had TOO much vacuumm which I am guessing is the manifold vacuum. I might try an experiment when I get home today.

 

[Photon440]

I thought the vacuum advance was supposed to be hooked up to the left vacuum port (passenger side) which I assume is the ported vacuum. Might explain why my car doesnt run as well as I thought is should. Seemed the right hand port (driver side) had TOO much vacuumm which I am guessing is the manifold vacuum. I might try an experiment when I get home today.

Too much vacuum? Remember, the vacuum goes away when you hit the gas, so it shouldn't cause pre-ignition. Just give you better response and mileage.

-=Photon440=-

 

[snakeoil24]

Ported vacuum is for later model pollution control engines. Manifold vacuum is the correct way to hook up.

 

[1971RR]

I'm running a Carter like Poppinsmoke (625, electric choke). Seems when I hooked up to manifold vacuum, the car didnt like it too much, but then again I didnt really take the time to adjust the idle and air/fuel when I did it either. Hmmm.

 

[Meep-Meep]

What is the engine? A 67 small block may be a 273, as I don't think you can get an air gap for a poly. The 750 carb is way overkill. Also, if you did nothing to the short block that 268 cam surely has a later intake valve closing point over the stock cam so I bet you are bleeding off valuable compression pressure. A 260 might have been a better choice or a dual pattern grind. Whether the vac advance is hooked to the ported or manifold port isn't the main issue here.

 

[popinsmoke]

Short block is a 67 318 compression should be 9 to 1.

I have run the 268 in a 327 chevy and it ran great.

The 750 is overkill I agree but most of the parts I am running on the 318 are going to be swapped over to a 360 stroker next summer.

I have a set of 360X heads (bought for the stroker) that I could run to boost compression a bit if the 268 is bleeding it off.

It will hold a steady idle now, the engine kinda shakes like a paint mixer and still way soft on the bottom end but runs great on the top end.

If you ease in to the secondaries it will pull nice and hard, If you just stomp on it then the engine pops through the carb.

So something is still off somewhere I know it is something that is simple that I am overlooking.

 

[Photon440]

Is the accellerator pump set to give maximum squirt right away? Maybe the pump spring is soft and it's taking too long to get the gas in?

What happens if you hold down the secondary flap and snap open the throttle by hand?

-=Photon440=-

 

[popinsmoke]

Thanks to all that replied.

Got the timing and pump shot issues sorted out and the car runs much better, I now have to move on to the fuel tanks vent,Sending unit and brake job, then car will be all set for now(they are really never done are they?)

 

[hendo0601]

You said you were having a hard time finding/seeing the timing mark on the balancer. Here is a trick I learned years ago... "bump" the starter a few times until you can actually see the timing mark on the balancer. All it is is a "groove" machined into the outer edge. Take a piece of white chalk and pressing kinda hard run the chalk along that groove, then take a rag or even your finger and wipe off the excess. The groove will hold the chalk in place and make the timing mark much more visible when using a timing light. I keep a piece of sidewalk chalk in my toolbox at work just for this purpose and it really comes in handy.

 

[Sixer]

For balancer timing marks I use a white paint pen-doesn't disappear if it gets wet, nice and bright in the light. I also mark the 0 on the timing tab (I have a light that's adjustable for advance/retard) and use that to set timing, not the tab marks. The best timing guide though is a vacumn gauge.

 

[Elvad]

"Vacuum Retard"

Dextron is not Dextron it's Dexron. I have been saying Dextron for years too. Vacuum advance... isn't. The vacuum supplied to the distributor retards the timing so you can start the engine and get a better idle. It is NOT vacuum advance, the advance is by mechanical weights. The vacuum supplied to the distributor MUST be manifold vacuum, which is highest at idle, and holds the distributor timing retarded. As the throttle opens, manifold vacuum goes away and the weights in the distributor take over and advance the timing to the maximum, set by you, around 2200-2600 which is why you need a timing tape on the balancer to see the total advance. You MUST also verify TDC before any of this applies. Old Mopar balancers have a rubber ring that can slip and be WAY off! If you don't know where TDC is YOU CANNOT TIME YOUR ENGINE CORRECTLY, PERIOD! Todays gas really sucks, it is harder than ever to tune these engine to run well on this crap. Chrysler's official solution to our woes is to lower the compression of all street motors to 9:1. They even sell .040 spacer head gaskets for older motors. This means that for the engine to operate and be timed correctly we have to run lower compression.(because the gas today sucks so bad) A low compression engine that you can time correctly will yield "more usable horsepower" than a high compression engine you can't time right. This seems like a compromise, and it is, but the gas today totally SUCKS!! You can go all electronic an use fuel injection and O2 sensors and stuff an run 11 to 1 or more but, as usual, Big Block Chrysler stuff costs at least 3 times what ford and chevy stuff costs. You get what you pay for.... but, Buy the engine book and get the knowledge. Big Block Chrysler engines are easily the strongest of the big three hands down. Just look at the centerline of the crankshaft, ford and chevy, the mating surface for the pan gasket is the centerline of the crank...not a Chrysler Big block, the mating surface for the pan is like 3 inches below that, they are crazy strong. Stock internal engine parts for these engines are FACTORY rated at almost 1.5 horsepower per cubic inch... Read the book(s)!

 

[Photon440]

Hey, I think I've been calling it 'Dextron' as well...

I read the book, Chrysler calls it 'vacuum advance'. It does what it implies - under instances of manifold vacuum, the vacuum pot pull the distributor plate and advances the timing. Vacuum is often not highest at idle, but during light throttle cruising at higher rpms where cylinder filling is minimal, dynamic compression is low, and more advance is possible without danger of pre-ignition. Both the advance weights and vacuum pot work together to determine the optimal timing for a given rpm and throttle position scenario, provided that they're set up correctly.

There were a few distributor dash pots in the 70's that used a combination of advance AND retard, with both ported and non ported vacuum, including timed or temperature actuated ports, but these were for emission reasons and not likely part of popinsmoke's problem.

-=Photon440=-

I agree with the use of timing tapes, unless you're using a dial back timing light of course.

 

[coloradodave]

ELVAD: There is some misinformation in this post. Photon440 got it right, but I have included a great primer on timing and vacuum issues from another site. It's so long that I have to do it in 2 posts, but it's VERY informative and well worth reading. Keep in mind that I am copying this post. I am NOT the author:


"This is a great article!!!!I had my vacuum advance hooked up to the ported vacuum port after reading this article I hooked it up to full manifold vacuum adjusted the idle rpm.The car has so much more power now."



"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."
Continued on the next post.

 

[coloradodave]

Dextron is not Dextron it's Dexron. I have been saying Dextron for years too. Vacuum advance... isn't. The vacuum supplied to the distributor retards the timing so you can start the engine and get a better idle. It is NOT vacuum advance, the advance is by mechanical weights. The vacuum supplied to the distributor MUST be manifold vacuum, which is highest at idle, and holds the distributor timing retarded. As the throttle opens, manifold vacuum goes away and the weights in the distributor take over and advance the timing to the maximum, set by you, around 2200-2600 which is why you need a timing tape on the balancer to see the total advance. You MUST also verify TDC before any of this applies. Old Mopar balancers have a rubber ring that can slip and be WAY off! If you don't know where TDC is YOU CANNOT TIME YOUR ENGINE CORRECTLY, PERIOD! Todays gas really sucks, it is harder than ever to tune these engine to run well on this crap. Chrysler's official solution to our woes is to lower the compression of all street motors to 9:1. They even sell .040 spacer head gaskets for older motors. This means that for the engine to operate and be timed correctly we have to run lower compression.(because the gas today sucks so bad) A low compression engine that you can time correctly will yield "more usable horsepower" than a high compression engine you can't time right. This seems like a compromise, and it is, but the gas today totally SUCKS!! You can go all electronic an use fuel injection and O2 sensors and stuff an run 11 to 1 or more but, as usual, Big Block Chrysler stuff costs at least 3 times what ford and chevy stuff costs. You get what you pay for.... but, Buy the engine book and get the knowledge. Big Block Chrysler engines are easily the strongest of the big three hands down. Just look at the centerline of the crankshaft, ford and chevy, the mating surface for the pan gasket is the centerline of the crank...not a Chrysler Big block, the mating surface for the pan is like 3 inches below that, they are crazy strong. Stock internal engine parts for these engines are FACTORY rated at almost 1.5 horsepower per cubic inch... Read the book(s)!

Continued from my last post:


"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."
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[RJS]

This is one of the best posts I've seen in some time!!!
Ron

 

[Elvad]

Hmmm

Looks like I need to do some reading myself... Good article

 

[oldbee]

Coloradodave has hit it right on the money...I've forgotten half of this stuff as the years went by..GREAT ARTICLE!