Hi all

I've been out trying to fine tune the new 350. I have a 69 Corvette q-jet from Advance installed. I adjusted the timing to where it had the best idle, which seems to be around 10-12 degrees initial, then adjusted the idle mixture to where I saw a noticeable drop in idle, then backed it out until the idle came back up (about half a turn or so), then I adjusted the idle screw. It idles fairly well now, but it seems to have a little miss every few seconds (sounds kind of like rumblerumblerumblerumble -chuff- rumblerumblerumblerumble -chuffchuff- rumblerumblerumblerumblerumblerumble -chuff- etc). This gets better if I increase the idle speed.

The other problem I've noticed is it has a pretty good detonation problem under moderate to hard acceleration. This is with the timing set as above. The engine builder recommends regular pump gas, so I have not tried premium yet. Based on past experience, I think the problem is bad enough that this would not fix the problem entirely - only reduce it. I have a generic HEI from Ebay (the made in Australia one). Do I need to mess with the weights/springs or is there some lower hanging fruit besides the gas I can fiddle with?

Cheers,
Andy

The problems may be similar but two different things. From the discription of the idle you may have a lean missfire from too much advance as a result of running manifold vacumm to the distributor. Try it with ported. The pinging under acceleration is from too much advance, or the wrong fuel.

It is generally a good idea to 'know' what the advance curve of a distributor is before installation. All of the responses you get are educated "guesses".

You might be getting too much mechanical advance..do you know how much you have for total timing?

The problems may be similar but two different things. From the discription of the idle you may have a lean missfire from too much advance as a result of running manifold vacumm to the distributor. Try it with ported. The pinging under acceleration is from too much advance, or the wrong fuel.

It is generally a good idea to 'know' what the advance curve of a distributor is before installation. All of the responses you get are educated "guesses".

Thanks for the responses, guys.

Pardon if I seem ignorant on a few points..

Now, ported vs manifold vaccuum - I've heard different responses as to one or the other being better. Manifold vacuum I would think would be stronger in general and specifically at idle, right? Ported vaccuum is vacuum from the carb, I'm assuming, but if the port is below the throttle plates wouldn't it be the same as manifold vacuum? A vacuum source from above the throttle plates would have almost zero vacuum at idle, so I guess if I were getting too much advance at idle that would fix that problem. Right now it is connected to a port on the carb, but I'd have to look to see if it's above or below the throttle plates. It does seem to suck pretty hard when I plug the hose wih my finger, though.

Might a good vacuum guage help with diagnosis?

As far as the advance curve goes, I have no idea what it is. I'm still learning a lot of this stuff (suprising that there is so much to learn even after messing with cars for 20 years), and it never ocurred to me to try to match a curve to the engine. Are there ready made kits available with weights and springs or do I need to start playing around inside the distributor? I'd hate to do that and completely fubar it.

You might be getting too much mechanical advance..do you know how much you have for total timing?

Would that be timing at cruising RPM? I don't know what it is, but my hunch is that it is too advanced at speed which why I was getting the knock. Can I check that with a timing light in park by just reving the engine or is there a different way to check it?

Cheers,
Andy

A ported vacuum source will have no vacuum at idle. So, you want to use ported vaccuum for the vacuum advance because you do not want your timing advanced at idle.

It's easy to check base timing, but to check total timing you need to have a balancer with marks on it to show the higher timing marks. Or you can use a "dial" back timing light in which you dial it to the correct overall timing you want and it will flash on TDC when you have the correct timing.

EDIT: Oh, and usually total timing is all in by 2500 or 3000 RPM's.

Some engines are fine with manifold vacumm at idle, many are not and although they run, the owners talk about low speed surge, the misfire you've noted, having to have high idle speed in neutral to have it idle in gear, stinky exhaust when idling to name some of the issue with manifold vacumm.

Too much advance at idle means that the piston is farther down the hole when you try to light the fire. The farther down the hole, the lower the compression in the cylinder, the less dense the charge, and the harder to fire it becomes. So sometimes one or more, don't.

Many who have installed MSD ignitions and noticed an improved idle probably didn't think about the 20 degrees of multi strike the MSD delivers at idle, and no multi strike after 3000 rpm. Isn't it interesting that the 20 degrees is about the same as most vacumm advances? And the multi strike quits at 3000, about where your advance should be all in? I don't think the guys that decided to market the MSD would be surprised.

Anyway, since the difference is only at idle when you are using the ported source for distributor vacumm, there can be no disadvantage if it cures the issue you are complaining about.

I don't about yours but most HEIs have way to much vacuum advance in them for any type of performance engine. Crane makes a very good limiter that will get it down to usuable levels. It is possible that the carb is too lean on the pri. side or the power piston is not working correctly. Try disconnecting the vacuum advance and see if it changes much if it doesn't you are going to have to get into the carb. One last thing you don't have Vortec heads right?

I don't about yours but most HEIs have way to much vacuum advance in them for any type of performance engine. Crane makes a very good limiter that will get it down to usuable levels. It is possible that the carb is too lean on the pri. side or the power piston is not working correctly. Try disconnecting the vacuum advance and see if it changes much if it doesn't you are going to have to get into the carb. One last thing you don't have Vortec heads right?

No, no Vortec heads. The idle drops considerably when I disconnect the vacuum advance and plug the hose, and gets rougher, too. What does the kit include? Would simply changing the vacuum advance unit help? I would think I can get a different one at AutoStone for a few bucks.

[/QUOTE] The idle drops considerably when I disconnect the vacuum advance and plug the hose, and gets rougher, too. [/QUOTE]

That the idle drops when you pull the hose off tells me that you are using the manifold vacumm port for advance, as I suspected in my first post. Plug the port, raise the idle speed back to where you are now, and re-adjust the carb. Reset the idle speed again to where you want it. Hook-up the vacumm advance hose to to the port on the carb with low vacumm that comes up when you start to open the throttle.

It amazes me the absolute misinformation that comes out about vacuum advance and vacuum ports...

Andy, your ignition system absolutely needs to be tailored to your engine. While every GM distributor looks the same there are in fact many many different versions. There would not be if there were not a need for tailoring.

For setup first start out with no vacuum advance. This simplifies the picture so all you have to deal with is the mechanical advance. While most every shop manual in the world gives you a technique to set timing using initial (this is timing at low idle without vacuum advance), it only works if you have the right factory original setup. I recommend instead setting it for total timing, or with full mechanical advance in. This should be by 3000 RPM but your mileage will vary. There are tuning kits to change the RPM range for mechanical advance. They always include weights and center plate along with springs. Unless your weights are trashed, use the springs and toss the rest. Set your total timing to around 36* (this is the most common GM engine setup for classic engines). You will probably find that the mechanical advance gives you about 20* of advance at the crankshaft, so your initial will be around 16*.

Now you need to work the vacuum advance side of your setup. You will need a vacuum gauge for this and a hand squeeze vacuum pump is also very handy. Many vacuum cans provide around 20* of advance at the crankshaft. This will very likely be too much, you probably want something closer to 12*. Further you want the full vacuum advance to be in about 1-2" lower than your idle vacuum. So you have to measure your idle vacuum and possibly measure/characterize your vacuum can to get this right. Crane makes a kit that includes an adjustable vacuum can and advance limitter that may help here. Or you can search out the previously posted Napa VC-nnnn part numbers and buy a specific vacuum can. This setup means that cruise advance will be close to 50* between mechanical and vacuum. This is an important number as in general you do not want to exceed this ever (the 20* vacuum cans worked at cruise with less than total mechanical advance so the max was still 50* or less).

Now as to where the vacuum advance should be hooked up... carb ports can be exactly the same as manifold vacuum. There are some carb ports that are located *slightly* above the throttle butterfly and are thus called "ported vacuum". These turn off at idle but are otherwise exactly the same as any other vacuum port. You do not want to use these if you do not have to. Your engine needs a bit of extra advance at idle because of the lean mixture and low cylinder pressure slowing down the fuel burn. It gets this from the vacuum advance. I found that I picked up 2" of vacuum when I moved from ported to manifold and eliminated a bunch of idle speed setup problems. On ported I basically had to open the throttles enough to open the vacuum port, which puts you too far into the transfer slots in the carb.

Not sure if we have had the full posting on TC or not, but a google search on "ignition 101" may turn up some very interesting reading for you.

It amazes me the absolute misinformation that comes out about vacuum advance and vacuum ports...

Andy, your ignition system absolutely needs to be tailored to your engine. While every GM distributor looks the same there are in fact many many different versions. There would not be if there were not a need for tailoring.

For setup first start out with no vacuum advance. This simplifies the picture so all you have to deal with is the mechanical advance. While most every shop manual in the world gives you a technique to set timing using initial (this is timing at low idle without vacuum advance), it only works if you have the right factory original setup. I recommend instead setting it for total timing, or with full mechanical advance in. This should be by 3000 RPM but your mileage will vary. There are tuning kits to change the RPM range for mechanical advance. They always include weights and center plate along with springs. Unless your weights are trashed, use the springs and toss the rest. Set your total timing to around 36* (this is the most common GM engine setup for classic engines). You will probably find that the mechanical advance gives you about 20* of advance at the crankshaft, so your initial will be around 16*.

Now you need to work the vacuum advance side of your setup. You will need a vacuum gauge for this and a hand squeeze vacuum pump is also very handy. Many vacuum cans provide around 20* of advance at the crankshaft. This will very likely be too much, you probably want something closer to 12*. Further you want the full vacuum advance to be in about 1-2" lower than your idle vacuum. So you have to measure your idle vacuum and possibly measure/characterize your vacuum can to get this right. Crane makes a kit that includes an adjustable vacuum can and advance limitter that may help here. Or you can search out the previously posted Napa VC-nnnn part numbers and buy a specific vacuum can. This setup means that cruise advance will be close to 50* between mechanical and vacuum. This is an important number as in general you do not want to exceed this ever (the 20* vacuum cans worked at cruise with less than total mechanical advance so the max was still 50* or less).

Now as to where the vacuum advance should be hooked up... carb ports can be exactly the same as manifold vacuum. There are some carb ports that are located *slightly* above the throttle butterfly and are thus called "ported vacuum". These turn off at idle but are otherwise exactly the same as any other vacuum port. You do not want to use these if you do not have to. Your engine needs a bit of extra advance at idle because of the lean mixture and low cylinder pressure slowing down the fuel burn. It gets this from the vacuum advance. I found that I picked up 2" of vacuum when I moved from ported to manifold and eliminated a bunch of idle speed setup problems. On ported I basically had to open the throttles enough to open the vacuum port, which puts you too far into the transfer slots in the carb.

Not sure if we have had the full posting on TC or not, but a google search on "ignition 101" may turn up some very interesting reading for you.

Interesting reading - very helpful. Thanks, Steve.

Cheers,
Andy

Andy,

People should be careful about hanging their hat on that article. Although very well written, and the author claims knowledge.......he hung the whole premise that the use of ported vacumm for the distributor came about because of emmision controls. If that part is not true, one needs to question his conclusions. In reality, the ported vacumm source came about in the '60s, before pcv, while they still vented the crankcase under the car with a road draft tube, to solve drivability problems in high performance OEM engines. Before it was introduced, most, and I've had quite a few, high performance cars had no vacumm advance. Those of us that were there in that day would unhook the vacumm advance when hopping up a stock engine. I bought a Sun distributor machine in the early '60s that I still have and use. I believe having a properly curved ditributor was partially responsible for my ability to have set some national records back then. I used it on the distributor in my hydro which became a two time APBA Word record holder.

Steve said he was amazed at the misinformation available , and then referred you to a source of a great deal of it. Correct me if I'm wrong here. Did not the author claim to be an engineer working for GM. I also have engineering degrees. However, that isn't why I could reconize from 4000 miles away, while reading a computor screen, what the probable cause of the idle issue you decribed. It's because I've spent over 40 years solving drivability issues for hundreds of people.

Greybeard,

I am a seeker of information, I love to understand *how* things work and not just what works. I very often see particular setups thrown out as general examples without the background to evaluate if they are applicable to the application at hand. This bothers me.

The description of lean/rich and high/low pressure burn rate affects made a great deal of sense to me. Those affects also very logically support manifold vacuum, which you are saying is incorrect. Can you tell us why we would want the ignition retarded for idle?

And I believe we are talking only about right at idle, the ported becomes manifold pretty much just as soon as you move the throttle.

Greybeard,

I am a seeker of information, I love to understand *how* things work and not just what works. I very often see particular setups thrown out as general examples without the background to evaluate if they are applicable to the application at hand. This bothers me.

Steve,

I salute you in your quest to seek information, and to question why. Question everybody, and then check it out if you have the resources. I must admit, that I as well as thousands of others, have fallen into the trap that if you make a single change, and the idle speed goes up, that must be better. As I said above, some engines do quite well will manifold vacumm, some don't. Nearly all will respond with more RPM and vacumm. However, drivability is an issue that rears it's head in many cases.

Some of the issues:
Have you ever see the stick car running around town @ around 3000rpm all the time. If you ask the owner why, he say something like "I got such a big cam, it doesn't want to run under 2500rpm" or something like that. "What's it do?" "It starts bucking or surging". If you check the distributor, the vacumm advance is probably hooked to manifold sourcing. Anecdotal: I built a nice little 283 for my shoebox/4sp. It idled at 1100 @ 11 inches with a lot of chop. Driving in town with manifold vacumm it would surge, making you want to shift down, yet with the ported vacumm, you could leave it in gear down to 1000rpm and not have it happen. You see, I do it both ways and decide.

The complaint that the car stinks up the garage and the owner's rebuilt the carb and tried two others. We've seen this posted. The car is probably to far advanced at idle, and some cylinders just aren't firing. This is an issue that MSD are often credited with fixing.

"When i put my car in gear, it stalls. Or I have too have the idle so high it bangs when I put it in gear. People will start advising new torque convertor. What if it's not? What if we just unhook the vacumm advance, reset the carb, and the problem goes away?

When the engine is at idle, it is very inefficient unless we are using a tiny carb, manifold ports, valves, cam, etc. The carb doesn't meter well, and mixture varies is quality, even if it's accurate for ratio. This mixture is travelling slowly in the manifold, and some of the mixture wants to "unmix". What gets into the cylinder is a poor quality mixture traveling slowly. and it goes into a larger space, slowing down even more. As the piston rises, compression starts to impart energy into the mixture and excite some movement. When we get half way up the cylinder, it's been compressed about 1.8-1 on a 9-1 350. As the piston nears the 3/4 mark we compressed the mixture to about 3-1 and @ 30 degrees before TDC we're up to 3.8-1 or so. So in the last 30 degrees, compression rises from about 4-1 tp 9-1. Sometimes, at that approximately 4-1 compression ratio we just don't have the mixture "energy' yet to promote a good flame and we don't get a burn. That's a nice round number for intial plus vacumm as used today. My compression numbers are very "rounded" as well.

So the closer we get to TDC, the more the compression we have, the more the compression imparts energy to the mixture, the better chance we have to light it off. We certainly have a comprimise between TDC and the lower compression, but there is a point where things don't work right. The "sweet spot" for initial advance is around 14 degrees on most of our engines. The engine will start, idle decently, and have good response. Then you confuse thing by throwing in another 20 degrees? In a situation where zero work is done? And for what purpose? An MSD will cover up the problem. A new torque convertor will mask it as well, and may have been a good investment anyway. Maybe, just maybe, we could have fixed the problem for nothing by changing where we hooked up a little rubber hose.

One can not throw out everything in "ignition 101" because the author started with a false information. There is good information there, but poor as well. Totally ruling out the ported vacumm will find someone in a spot where needless money and time will be spent trying to find a solution to a problem so easily fixed in some cases.

The worst part of this to me, Steve, is we spend so much time on this subject that only pertains to an engine @ idle. In my lifetime around this, I've gone from hating vacumm advance and tossing it, as most of my peers did as well, to acceptance that it might have some merit when gas got above $.30/gal. So keep asking, and lay awake at night thinking about it, and if you'd like have a spirited debate, my email works.

Great debate and conversation here!

I subscribe to the notion that not every engine combo can be catagorized into black and white. Tuning is the key and the forementioned advice is good stuff.

Andy, I hope you are able to learn from the info here and get your Vette to running properly!

Good Luck :D

Thanks for the info Graybeard, I could take the discussion to email but think there is more benefit to all by keeping the discussion both public and polite. No need to get too spirited on TC :)

We agree on the poor quality of the mixture and conditions in the cylinders at idle. And you make a good point about the need for sufficient compression of the mixture to get a light-off from the ignition. But if we fail to light don't we lose the energy from that cycle and drop efficiency and vacuum?

For less than full power, is there ever a time when more vacuum does not mean a better running engine (or at least more efficient)? After all we will keep RPM the same by adjusting it up/down as needed. So vacuum is just an indicator of how much we are regulating engine output down.

We also agree that there is a lot of good information in the article. And I think that an important aspect of what was written gets lost in the anectdotal examples. That is the system aspect that the parts all have to work together. For example if the vacuum advance can is mismatched you may get too much advance at idle. By running ported you may be able to mask the problem rather than fix the true cause. I suspect that some OEM designs used a slow mechanical advance and compensated with more vacuum advance. But everybody sells mechanical advance kits to "fix" this, leading to a bunch of distributors with confused/conflicting advance curves.

I was reading today that what we want to achieve is maximum cylinder pressure at 20* ATDC, putting that together with your observed typical idle timing gives a burn time around 34*. Works out to about 8ms at 800 RPM. While I am having some math fun, at 3000 RPM and 36* BTDC we get about 3ms. Further interesting is the cruise timing of 50* BTDC mentioned works out to 4ms. So now we have some idea just how much the burn rate changes and how bad the conditions are at idle. On my 402 BB I am running 30* BTDC at idle for a burn time of 10ms.

PS for those interested the math works something like this: at 800 RPM * 1/60 sec/min * 360 degrees = degrees of rotation per second or 4800. If we ignite at 14* BTDC we burn for 34* to complete burn at 20 ATDC. So 34* *1/4800 sec/degree = 0.007 seconds.

Hey thanks for all the thought-provoking discussion guys. I've really learned a bunch from it. I spent some more time yesterday fiddling with the engine. I switched the vacuum advance hose to ported, and the idle dropped way down, and as much as I tried, I could not get it to idle as well as with it on manifold vacuum, so I switched it back. I think I had said before I set the timing where it idled best. That turned out to be way to much inital advance (something around 20°) so I backed it down to 12°. I also checked the total mechanical advance, and it sits around 36°. Surprisingly now it idles much more smoothly, and I was able to drop the idle speed down. The detonation problem of course went away.

It is much more drivable now, but there are still a few issues, like a slight off-idle bog that curiously shows up with the car in park, and only occasionally so far while driving. It doesn't show up all the time, and it wasn't there before yesterday. I wonder if I uncovered something that was there but masked, or created a new problem somehow. Well, time to go learn some more about qjets, I guess.

cheers,
Andy

A lot of times, carb problems or a vacuum leak get blamed on ignition timing.

What does vortec heads play in this vacuum and total advance??

Vortec heads are so much more effcient than the old style that anything more than 30deg total without the vacuum advance will cause detonation problems. On mine I had to limit the vacuum advance to 8deg. or I had part throttle problems.

The trick with the Vortec heads is that the flame front travels faster across the cylinder. This shortens up the burn time required to hit maximum cylinder pressure and thus less advance is needed. As I understand it, this reduces the ignition advance numbers across the entire top of the scale by about the same amount. Thus if total drops from 36* to 30*, cruise timing with vacuum advance drops from 52* to 46*.

The same type of thing occurs with 4 valve heads for example.

Some things that can cause this: substantially smaller bore sizes, more cylinder mixture turbulence, and centralized spark locations.

This is the BEST explanation I've ever seen on how to properly curve your distributor and how everything works. I HIGHLY recomend these procedures. I'm convinced that these steps added 50 HP to my 66 396 Chevelle.

http://www.stevesnovasite.com/forums/printthread.php?t=11689&pp=40