68_newport

Hello all, just joined on here today as I'm having a minor but annoying issue with my car that I can't seem to solve. Engine is a 383, stock 2 barrel internals with an edelbrock intake and Holley 670 street avenger. The problem I'm having is that when I'm cruising at light throttle the engine is hesitating and not running smoothly. However it idles great and when you give it any more than light throttle it pulls strong and smooth through the rpm range. I've figured out that the problem is related to the vacuum advance because when I plug off the vacuum advance hose and go for a drive the problem is gone. I've tried using manifold vacuum but the engine doesn't idle well and even worse once you put it in gear. After hooking up a vacuum gauge to the ported vacuum port on the carb where I've been running the advance from it is evident that the ported vacuum comes on with the lightest touch of the throttle so my theory is that the advance is coming in and out when it's not yet necessary, causing the engine to run rough from too much timing.

Any ideas? I'd like to make this thing run with vacuum advance as it is a street engine, but this issue has been persistent for a while.

Cheers! Ethan.

RacerHog

What are the setting and the increments at the moment?

68_newport

The idle mixture screws are currently set for maximum vacuum, also base timing was set to factory recommendation with vacuum advance disconnected.

RacerHog

Using the Ported Vacuum for the distributor on the upper right hand side of the carb. on the Metering block?

68_newport

Yes that's right, not sure if there's known problems with the ported vacuum on these carbs but I have done some quick searching and haven't found anything.

RacerHog

No.. That port work like it should...

Whats the initial timing and the total time?

68_newport

Just checked the fender decal and it says 7.5 degrees initial timing which is what we used to set it. As for total timing I'd have to check but the distributor was replaced only a few months ago and the mechanical advance was fine at that point so total timing should be up to spec.

TVLynn

It could be that the timing mark is wrong ? Maybe the dampener has slipped ? At cruse it it over advancing ?? I'm just guessing.. You need to verify the timing marks at TDC and check the timing with a light thru the rpm range. IF it was ok before you may have the wrong dist vacuum advance unit

demetri

Are you sure there are no vacuum leaks? If the hesitation goes away at higher speeds, it is very likely a vacuum leak. What are your vacuum readings, say at idle, and at cruising, say 40 mph flat, 60 mph flat.

Coronet 500

Do not diagnose ignition timing with a vacuum gauge. Do exactly what TV said!

Once you have your numbers you can adjust/repair. A vacuum gauge may confuse as it will also be affected by cam timing, bad valves, carb A/F or bad head gasket wasting your time.

Use the proper tool for the job, I'm sure most here would agree.

Iowan

demetri

The timing light will tell you if it is over advancing. At any rate, over advancing will not really cause misfire unless may be its extreme. Vacuum leaks do cause misfire though. The vacuum reading is very useful to know the state of the engine anyway.


The vacuum advance unit may be defective with a broken diaphragm, creating the vacuum leak.

Iowan

68_newport

Thanks everyone! A couple other things I should add; we replaced the distributor a few months ago so that should be okay, the mechanical advance was tested when we installed it. Also we resealed the intake and replaced the carb base gasket and the engine holds 22 inches of vacuum at idle. Perhaps I'll look into the dampener a little closer to see if it has moved at all and then check the timing.

demetri

22 inches sounds healthy. What happens under load with and without the vac advance connected? Even if vacuum leak may be less likely with such a high reading, the symptoms running good under load but not so good under part load are really classic vacuum leak.


Simple test (you may have already done that). Suck or blow through the vacuum advance canister, and see if air goes through. You said its new, so its probably OK, but try it anyway.


Finding TDC on cyl 1 is not simple, and may require a special tool to get though spark plug hole. If you really suspect that you get over advanced under part load, first check w/ timing light and see how much is your vacuum advance adding (if any). If it does advance a lot, try retarding the timing a bunch, like 5 or 8 degrees, and take it for a brief test drive. If that helps low load, has little effect on your idle vacuum or cruise vacuum, and has no effect at high load, may be it is worth the effort to explore your TDC location.


But from my experience, excessive ignition advance did not cause misfire. And to my surprise, it also did not cause pinging. It only made the car hard to start, and caused some performance loss.


By the way, can you give more details on your car? 2 door, 4 door? Color? Condition? Pictures?

68_newport

I tried disconnecting the vacuum advance and taking it for a drive and it runs perfectly under the same conditions where it usually stumbles. Also the vacuum chamber on the distributor was tested with a hand pump and is holding vacuum as it should. I still think that the vacuum advance is just at the rpm where it's coming on and off, causing inconsistent timing and then misfiring but I could be wrong. If so I'm not even sure what could be done to fix this without compromising the way it runs everywhere else? Anyone ever seen a situation like that?

About the car: it's a 2 door hartop, black vinyl roof, gold paint and white interior. I was thinking of posting a few photos in my gallery soon!

Coronet 500

I have had a similar problem that was corrected with an adjustable vacuum pot, accurate TDC mark and timing light.

Essential to do initial, full mechanical advance and at what RPM (no vacuum attached).

Then advance vacuum attached raising RPM slowly and record numbers, Advance/RPM.

This will take any guess work out and either correct the problem or allow confidence to move on to the real problem.

This is the ONLY way to do it.

Iowan

Save your breath Dan, your not telling them what they want to hear.

TVLynn

All said is good but YOU NEED TO START WITH THE BASICS

RacerHog

Does the Car still has points in it?

demetri

68N: I was suggesting to try retarding 5-8 degrees WITH the vacuum advance on. That way, we can explore the theory of potential over-advancing due to incorrect TDC. That is what I was suggesting. Very simple test, that you can reverse after you are done. If the car runs better at light load with no deterioration of high load, then may be the TDC exploration is worth while. If no change at low load, and high load deteriorates, then your TDC location is probably OK.


One thing that you did not answer. How much is the vacuum advance really advancing? May be you can idle the car with the vac advance disconnected, and connect it to your hand vac pump and watch the timing light. Or try with and w/out the vac advance, and have the vac advance on manifold vacuum so that it advances at idle. It may change the rpm some, but try it anyway, see what you get. You may learn something.


BTW, sounds like a great car with an interesting color combo. Too bad these cars do not have much market value. I have two 68 Imperials, which also suffer from poor market value.

RacerHog

Might try putting the heaver spring in the secondary and see if the takes care of the problem...

Iowan

demetri / You can't fix your own car why are you advising others?

demetri

Not particularly accurate.

Iowan

Yes, yes it is. Timing and fuel management are precise needs for a internal combustion engine to run properly, your haphazard ways of adjustments, determinations and assumptions are completely incorrect for proper tuning!

There are known procedures for determining proper timing and fuel management and you defy them all, and now you try spread your ignorance as if it's knowledge. Everyone has tried to help you with more experience than you could atane in a lifetime but for some reason you don't listen and continue with your ways. If you were smart you would take in what your being told and learn a few things.

demetri

I think Iowan is throwing terms out there without fully understanding them. What does the term "precise? mean? Precise does not exist, there are always tolerances around all these parameters. The precision applies to the "as new" condition. Many of these engnes are 50 years old, and the old parameters are no longer relevant. The "precise" parameters that Iwoan is looking for were set for fuels and lubricants of 50 years ago. Yes, there are procedures set for fully equipped shops with inflated budgets. When these cars were new they were not low cost as they are now, and people were willing to put up with the corporate set procedures and pay to keep their warranty going.


Things have changed since then. We want to get the best performance out of these old engines with relatively small budgets, and relatively low effort, if possible. The aftermarket parts available are not of the same consistent quality as the OEM parts were back in the day when the procedures were made (they may be better in some respects, worse in others). Therefore, using your brain to make reasonable tests is worthwhile. You can go ahead and suggest the "right way" all the time, but you have to keep your mind open that there are other ways that can get you where you want to get, and sometimes budgets, time, prior modifications, engine wear condition, or other unknown factors on a 50 year old vehicle may make the "right way" irrelevant.

Iowan

You will always be half assed, at best.

demetri

Better then been complete assed.


Here is another example. I had read an SAE paper (soceity of automotive engineers) from 1958 about the development of the 413. The recommendation of spark timing was set on the conservative side, in order to compensate for the accumulation of deposits and increase of compression ratio. They stated that the power and torque loss compared to the actual optimum timing (about 3 or 5 degrees higher than recommended) was about 2.0-2.5% (can't remember exact numbers). Well, modern fuels have very good anti-deposit additives, and I have a good ear for spark knock. So, I will advance the timing until I hear knock, and get the lost 2.0 %.


Another example is this thread. This fellow has a 2bbl internals with an after market intake manifold, and an after market carburetor that did not exist in 1968. The original numbers are not that meaningful other than may be just an initial baseline.


Iowan neds to ask himself, why is he in the forum? Does he want to learn something, and teach something when applicable? Or does he just want to feel good by feeding his ego?

Iowan

You have been told this by others, but then again you did not listen.
I feel sorry for you because you just don't get it .

Coronet 500

We should listen to Demetri, he read it in a book guys!