I'm running a stock '71 350 in my Supreme while I build my '72 350.

Since installing an aftermarket HEI and a rebuilt and rejetted '77 Qjet, the timing has been wonky. Cliff Ruggles of Cliff's High Performance says my timing at idle should be 8-12 degrees, which sounds low to me for a HEI engine. I'm currently at 25 degrees at low idle, vacuum disconnected and plugged. Going any lower seems to make it even more rough, while advancing the timing (turning the distributor counter-clockwise, yes?) seems to make it rev higher and smoother. When I hold the car on the brake in Drive, there's a thumping/jerking/thunking quality to the engine. RPM values are set to CSM specs.

I'm at a loss. My timing light is acting strangely, like the dial-back is broken or loose, as I started my timing test with it showing 60 degrees, then dropping to 24 when I pulled and plugged the vaccum advance. When I attached the vacuum advance again, it was at 31. I'm perplexed.

Defective timing light? Slipping timing mark/balancer/whatever? Incorrectly adjusted idle mixture screws?
In any case, how should I go about setting at least an approximate timing? Can I do it by listening to the engine? I've been told that doing so by vacuum gauge is possible, but I don't know how.

 

With the engine idling at 700 and vacuum advance disconnected and plugged. Set your timing light to 18 with it connected to the #1 spark plug. Point timing light at balancer and slowly turn your distributor till the line on the balancer lines up with "0" on your indicator. Readjust your idle and repeat. Lock everything down reconnect your vacuum advance and reset idle again.

 

I will try again, but doing so today seemed to kill the engine.

 

Have you adjusted the idle mixture screws? Was your carb rebuilt by a reputable place, because there's no telling what sort of a hackjob may have been done. Yes, there could be something wrong with your timing light and dialback lights have been known for being inaccurate at times too, so you may want to borrow a known good one if you can and compare.

Do you know the amount of mechanical advance your HEI gives you? That would determine where your base timing should be set.

 

OldCutlass's and VV's advice is good.
The HEI distributor has less centrifugal advance than your original had, about 12 degrees if I recall, so for 35 degrees of full advance, you'd be at about 23 at idle.

I agree with you that you may have a timing light problem
I would recommend trying a different timing light (maybe one without the dial-back feature) to see whether the readings are the same, or, alternately, try the timing light on a different car and see whether it still gives erratic readings and whether the dial-back feature works smoothly and consistently.

I believe that on that motor, there are punch lines on the front, on each piece, at the edges of the rubber part of the balancer (in line with the TDC groove) that will be out of alignment if the outside of the balancer has walked.

- Eric

 

DoubleV: I rebuilt the carb myself, with the deluxe kit from Cliff Ruggles, his book, and the jets he recommended. I've had the mixture screws out, and the carb wasn't running before I rebuilt it, so I didn't reference any of the screw settings.

I'll see if I can find the make and model of HEI, as it's impossible to know what kind of advance it has without that knowledge, at least for me. The vacuum doesn't look adjustable, at least.

I'll check the timing light and the balancer when it turns light again.

Listening to the engine does nothing at all?

 

If you have an unmodified aftermarket HEI, then I think it should be a known value as to how much mechanical advance it provides. Just give the make/model and I'm sure someone here would know how much advance that distributor provides.

Check to see what your idle screws are set at right now. Hopefully they're way off, which would then be an easy fix to your problem.

As for tuning it 'by ear', personally I've always thought that was old-school BS. Yeah, with alot of experience and a a highly trained ear, I'm sure there are some guys out there who could dial something in pretty close, but a timing light allows anyone to do this and you know exactly what you end up with. No guessing and/or human error mistakes assuming you have an accurate timing light and you know what you're shooting for.

 

I literally don't know the make and model, people are terrible at posting such info here. This looks close, both in price, shape and specs: http://www.summitracing.com/parts/sum-850006

The idle screws are set at 3-3½ turns from bottoming out.

I'll hold off from tuning by ear, then. :P

 

The other way of setting your timing would be to set your light to 35, rev the engine to 3500 rpm line up your mark @ 0. Reset your idle speed and check your timing. If you do it this way it does not matter what your initial is because your total is set almost at max. If you subtract your initial from your 35 total that you set, then the difference is the amount of mechanical timing built into your distributor.

I find sometimes that engines may prefer ported vacuum vs manifold for the advance can. That is up to you.

To set the idle A/F mixture use a vacuum gauge or a tach. Turn them in 1 at a time and then back them out until you get the highest vacuum or rpm.

 

Hrm, I could try that as well.

From what I've been told, ported doesn't add any vacuum at idle, unlike manifold. Ported will still have the same amount of vacuum as manifold once the throttle plates open up, meaning that you can advance further without pinging at idle, and still get the full advance at off-idle operation.

Right now my DVCS has transmission, distributor, manifold and ported plugged into it, as per the CSM.

 

If your oem vacuum system is working correctly then leave it alone. Yes, the difference between ported and manifold is only at idle, ported has no vacuum at idle.

 

How can I tell if my OEM vacuum system is working correctly? :P

 

Not true. At light cruise you will have more vacuum from the manifold than you will from the spark ported side.
And how can you recommend that he set his initial at 18btdc when you don't know how much total centrifigul is in the distributor? He may end up having too much or too little at higher rpm's.

Check it at idle with and without the vacuum advance. Then check your total the same way. You'll want approx 35 or so total at 3000 (no vacuum advance) for a starting point, then go from there.

Thanks

 

So apparently you didn't read my other suggestion.

The other way of setting your timing would be to set your light to 35, rev the engine to 3500 rpm, line up your mark @ 0. Reset your idle speed and check your timing. If you do it this way it does not matter what your initial is because your total is set almost at max. If you subtract your initial from your 35 total that you set, then the difference is the amount of mechanical timing built into your distributor.


Ported and manifold vacuum are the same at light throttle as long as the throttle plates are open.


Seff, Did your car come with an HEI Originally?

 

You have to test each component (the Chassis Service Manual has instructions, or the information needed to figure out what to do), and check all your hoses and steel lines (which can rust and develop pinholes).

You shouldn't have that many components on yours.

- Eric

 

The only way to properly set your carb is to use a vacuum guage on the ported source!
I usually put it where I unplug the vacuum advance, killing two birds with one stone - you don't have to plug it.
Find the highest vacuum attainable, while screwing them in, [so they can't 'back out'] and you're done!

 

Oldscutlass: Nope, it came with a points distributor. When I changed to HEI, I got the appropriate plugs (well, the Bosch equivalent of the S45SZ or whatever it's called) and replaced the resistor wire with a nice normal thick wire.

Mark: I'll do as you and oldscutlass suggested, right after I test my timing light for consistency.

Eric: I don't suspect it, but I'll check it, might as well. :P

Rickman48: So you're telling me that setting idle mixture screws at proper idle RPM is best done with a vacuum gauge to find the highest idle vacuum? Sounds more scientific than turning them until the car complains, then backing them off. I'll try.

 

Are you sure you mean to attach the vacuum gauge to the ported source instead of the manifold source?

Ported vacuum goes to about zero at idle, so won't give a particularly good signal for tuning.

Seff, the reason to use vacuum instead of (or in addition to) RPMs is that the vacuum is a bit more sensitive and can give you more precise adjustments.

- Eric

 

I think he meant manifold, some people don't have a vacuum gauge but have a tach. I've used both but I'm used to doing it either way.

 

I agree, but I don't want anyone else to get confused.

- Eric

 

Alright, timing at 3000 rpm set to 35 without the vacuum, reads 20 at idle. Still choppy at idle, adjusting the mixture screws a turn out didn't make it any better.

I seem to have gotten consistent results out of the timing light by setting it to zero before starting the engine.

 

I suggest setting the carb with a vacuum gauge or a tach. What is your idle rpm?

 

700 in park, 600 in drive. I have a tach, but not a vacuum gauge.

 

Here's a thought. Why not verify that the timing light is good in the first place. I've never used a dial-back light, only a fixed one, so I don't know what possible failure modes the light might have, but try it on a known good car first and see how it reads.

 

I don't have any gas cars on hand, everyone drives diesel. Not that I know the expected timing of modern cars. But I'll try.

 

Connect your tach and adjust your carb airfuel mixture screws 1 at a time in until the engine starts to stumble and the out for the highest rpm. This may take a few tries until they are very close to being the same amount of turns out and having the highest achievable rpm.

 

Seff, the technique of setting the idle is deceptively simple, but takes far longer to explain than it does to do.

After you have warmed up the car, and set the dwell (if you have points) and the timing to your satisfaction, you set the idle speed to spec. with the idle speed screw.

Then, you do a rough adjustment of the mixture by turning the screws in slowly until the idle / RPM / vacuum drops.

Then you reset the speed screw.

Then you set the mixture screws, one at a time, by checking whether the car wants them in or out, then turning them to the right point: you turn in about half a turn, then out about a full turn - one of those ways will increase the idle, the other will decrease it.
Keep going on the mixture screws about half a turn at a time to get a sense of where the idle is best - it'll probably be about 2½ turns out.
If you go out too far, the engine will get sloppy. If you go in too far, it will slow down, then stumble.

So, you get a sense of where the engine wants to be, then reset the speed screw, then start turning the mixture screws in.
Usually, you will find a broad area where turning the mixture screws doesn't seem to do much, before it either gets sloppy when they're too far out, or grumbly when they're too far in.
From the middle of this "neutral" area, you go in with each screw until the engine slows down.
I usually go in ½ turn at a time until I hear / feel / see the idle / RPM / vacuum drop (this may be one single ½-turn twist), then I go back the last amount I went in (in this case, ½ a turn).
Next, I go in ¼ turn at a time until it starts to slow down, then go back ¼ turn from that point.
Next I go in ⅛ turn, and back and forth a bit to find the right spot.
Then reset the speed screw, and repeat fine tuning of the mixture screws.

Note that mixture adjustments do not take place immediately - after each turn increment, you pause for ten to twenty seconds to let the change take effect - that's why you start by taking bigger "bites" and tapering down to ⅛-turns. If you tried to just turn the screws ⅛ turn at a time the whole way, waiting after every movement, you'd be there all day.

Like I said, this looks complicated at first glance, but really you're just turning the screws until the idle is right, in smaller and smaller increments, until you have it zeroed in.

Others may have developed their own personal ways of doing the same thing - there are probably many roads to the same place in this case - but that's how I do it.

Good luck,

- Eric

 

Some motors like closer to 38 total. It sounds like there could be a vacuum leak.

 

Olds207+403: What's your reasoning for thinking that it's a vacuum leak? Just curious, because you might be right.

Eric: Gotcha, I'll try when I get my tach dug out of my dash again. It DOES seem simple.

 

I have a pretty stock build and my engine likes 32 degrees, as determined by dyno testing giving highest integrated torque and BSFC.

see this link on vacuum tech: http://www.secondchancegarage.com/public/186.cfm
Read this on vacuum advance tech: http://www.highperformancepontiac.co...h/viewall.html
Get yourself a vacuum gauge - well worth the money.
Get yourself a new timing light with tach/timing functions - the new ones are fun
Use the CSM tuning procedure (Eric's procedure expands on that). I start with 8 degrees at 1100 rpm - but I have a 1970 TQ engine.
Road Test - cruise, load, adjust

Tuning is fun, Peter

 

Seff - do you have the Chassis Service Manual? You will start at 10 or 12 degrees at 1100 rpm (1971 350 A body)
This link will give you the CSM tune up section at Wild About Cars:
http://wildaboutcarsonline.com/membe...Section_6D.pdf

Good luck, Peter

 

His distributor isn't stock rendering factory timing specs irrelevant.

 

Could the distributor be 1 tooth off?

 

VV - of course - what was I thinking!

Peter

 

That would be the lowest bsfc, not the highest.

 

that makes two duh moments in one post - must be a record.

Peter

 

Mikes65: Anything is possible, but the car runs well and certainly has power - I ran the needle off the speedo range today, so it's not running horribly.

Peter: I have a vacuum gauge at my disposal, but it's 85 miles away, and I'm not going back there until Monday.


May I ask what bsfc is?


My cheapo tach quit, so I shrugged and tuned my idle mixture by tip-in of the choke plate, until my engine just barely increases RPM when I half-close the choke plate. With easter shutting things down, I won't get another tach until Tuesday, so this setting will have to do for now.

 

Usually a vacuum leak causes a rough idle. Especially true if you are border line lean. Get a can of carb cleaner and spray aound your carb and double check your vacuum hoses. I remember when the choke pull off diaphram sprung a leak on my 307, idled like crap. Replaced and it ran glass smooth. Push it in while holding your finger on the hole where the hose goes on. If it releases, it is garbage.

 

Have you taken the HEI apart to look at what's under the coil? Those parts are known for going bad. Especially if you have a cheap bushing that is sandwiched between the coil and the top of the cap. Many of these have high resistance and burn up causing the spark to scatter everywhere. Just remove the coil and inspect the parts under there for burning or carbon tracking. Low resistance models are sold at many places. The bushing is what transfers the spark to the rotor from the coil. Also is the cap flat on the distributor housing? They can be tilted easily and may not be noticed. Vacuum leak is also a possibility.
This is just an example of what I'm talking about. Lower right pic for $13.50
http://store.jetracinginc.com/heicomponents.html

There should also be a rubber gasket in there about the size of a large krone piece, lol. Half dollar if you know American coins.

 

Seff: BSFC refers to Brake Specific Fuel Consumption and is typically shown at various RPM on the dyno test. The units are normally in pounds per horsepower hour (lbs/Hp-hr). This reference is essentially how efficiently liquid fuel is turned into horsepower.
see: http://www.enginebuildermag.com/Arti..._business.aspx

Cheers, Peter