Gunz_Guntaku

Hello everyone. I have a 1983 Z28 that I've put a brand new crate carb 350 in.

My z28 was a carb 305 not a cross fire injection as it was ordered that way. Vacuum issues have all been fixed but the shop is now stuck on the timings being off.

Engine idles just fine but is doggy down the road they say. Very weird to say the least.

I'm being told because of the computer that might be the issue but they tell me they can't use a 350 ECM because the ones they could put in there are for TDI which this engine is not.

Can anyone help me find the right answer here? The car is running but the timings seem to be all off.

Or is it as simple as getting a carb 350 ECM from that 1983 year good enough to fix this issue?

naf

iTrader: (7)

no need for a 350 carb ECM

assuming everything is hooked up correctly and functioning, set the timing with the est connector to the dist (connected down by the body in the rear) disconnected.

set it to 4-6 degrees advanced and re-connect the est. you should then be able to observe the timing advance to around 20-25 degrees advanced at idle. from there you can fine tune to your motor's taste and preference.

Gunz_Guntaku

So basically your saying disconnect the computer from the engine completely while adjusting it normally with Vacuum?

I'm hearing from my mechanic that I probably should switch all EGR parts out of the vehicle at this point now because of this but seems extreme. We already took out the intake with a none EGR intake because of leakage.

Gunz_Guntaku

Well that didn't work from what I understand.

I was told now that It Idles fine in Park and Neutral but upon shifting to Drive is when the problems happen because it's advancing.

So sounds like a total computer problem at this point because mechanicaly everything is ok. But my mechanic is stumped and has no idea what to do now....god this is so frustrating.

Gunz_Guntaku

Here are the engine specs I was just told. Not sure if this helps

290HP 350 19421178

sofakingdom

iTrader: (1)

Somebody is feeding you a line of horse plop.

It's somewhat true that a 350 "tune" will run better in a 350 than a 305 "tune". However, the difference is minimal.

It helps, to understand what you're doing and why, to step back for a minute and THIMK about what's REALLY going on inside the engine.

Consider briefly, a one-lung lawn mower or pump or generator engine. Generator is probably the best, because those are designed to run at a speed such that their output is 60 Hz; and to hold that steady, the engine has to run at a constant speed. Often 3600 RPM. Imagine that it's a 2000 watt generator. Now close your eyes and imagine the load on such an engine: if nothing is plugged into it, then it's basically idling, at 3600 RPM; it has to produce a certain amount of power to spin the generator armature, but not very much, because there's nothing extracting power from it. Now, imagine that somebody came along and plugged in a 15 amp heater and turned it on. 120 V × 15 A = 1800 W; nearly the full rated load. What's going to happen? Well, all of a sudden, there will be a TREMENDOUS amount of drag on the motor from the armature, which is now suddenly generating 1800 watts or so. What's the motor doing? Well, in order to keep running at 3600 RPM with all this load on it, its governor is going to open the throttle, right? In fact, since this is a 2000W generator and we're making it produce 1800W, it's gonna have to open the throttle damn near WIDE OPEN, eh?? Remember, a governor is a spinning thing with weights and a spring such that at some particular RPM, the weights can overcome the spring tension, and fly outward; their motion of doing that, operates some sort of control action, such as closing the throttle down... when the RPM is below the set point, the throttle is opened, but when the RPM rises above that point, it closes the throttle; ideally it settles in at some sort of balance where the throttles are opened just enough to maintain the desired RPM, but no more. Opening the throttle allows in more air, which as it flows through the carb draws in more fuel, thereby making more power.

Now: this particular engine is designed to ALWAYS run at 3600 RPM, right? The only time it's running at any other RPM, is when it's turned off, or when it's being started. So, if you look at the typical ignition system for a thing like this, it has no variation in the ignition timing whatsoever; the "advance" is always the same. Let's think for a minute then: how would we determine what that "advance" should be? Well, we'd most likely shoot for some combination of best power, best fuel economy, cool running temps, and engine longevity; in other words, EFFICIENCY, in a broad and general way. Would it make any difference to that "ideal" timing, whether this engine was a 3 HP, 5 HP, 15 HP, or any other power level, as determined by its cylinder size? Not really, right? In reality, the amount of required "advance" is determined by THE FUEL above all else: ideally, you'd want to light the mixture off at just the right point in the engine's mechanical cycle, that depending on how long it took for the fuel to burn, it would be just about completely burned - maybe not quite completely, but close - just as the piston reaches TDC, such that the maximum possible pressure would be developed in the cylinder at just the right time to exert the maximum force pushing the piston back down. IOW, "advance" aka "timing" depends on THE FUEL BEING USED, and only to a very small degree on ANYTHING else.

Now let's consider an engine like is in a car, that instead of operating at one fixed RPM at all times, has to work over a wide range of RPMs. At lower RPMs, i.e. below the engine's max HP RPM, the piston is moving MUCH more slowly toward the top; and if you lit off the mixture at the same point in its motion as you would at the max power RPM, combustion would be LONG SINCE complete before the piston reached the top, and it would have to be attempting to compress gases that are already at a very high pressure. Highly inefficient to say the least, and possibly damaging to parts, because of the extreme forces that are involved. Thus, at lower RPMs, a car engine would need LESS "advance" than it does at its max power RPM.

Make sense so far?

Butt: there's another consideration. At high RPMs and low load (we call that "cruising") the amount of air/Fuel mix reaching the cylinders, is MUCH less than at max power, right? IOW, the overall pressures in the cyl are MUCH lower, because there's less fuel & air, because the throttles are just barely cracked open. In effect, the engine has less "compression" than it does at max power, because the material in the cyls is so much thinner. Under these conditions, the mixture burns more slowly, therefore needing more "advance". We call this "vacuum advance": high vacuum in the intake means that the cylinders are drawing all of the material out of the intake at a faster rate than the throttle is letting it back in, and therefore is an indication that the engine is at high speed and low load. aka "cruising". That's when you need vac adv. Does this sound like it would be materially different between a 305 and a 350? Not really, except to the extent that a 305 would have lower vacuum at all times than a 350 if their loads are the same, because the smaller motor would have to have its throttle opened more to let it make the same power that the larger motor would make with its throttle not opened as much.

The problem is, it's impossible to design a mechanical distributor that RETARDS the timing when the engine slows down. We have to settle for a design that starts out with some "zero RPM" timing, which we call "static" (as in, standing still), and then as RPMs increase, more "advance" is added. A mechanism rather like the governor discussed above is usually used: a set of weights, opposed by a set of springs, moves something in the distributor, to control the spark timing. In an electronic system the possibilities are much more wide-open. Does THIS differ materially between a smaller and a larger motor??? Not really, eh?

So, we have just established that the "timing" requirements of a 350 aren't really all that different from a 305. Virtually ANY engine is going to want about 36° of advance at its max torque / max HP RPM, if it's running on typical pump gasoline (with small variation due to head design and its effect on combustion rate); some considerably lesser amount at lower RPMs, typically 15 - 18° at idle and gradually increasing starting at just off idle, maybe 1200 RPM or so, and reaching the "max HP" amount by 2500 or so; and when the throttle is opened somewhat and engine RPM is high but vacuum is low ("cruising"), some added amount, maybe 12 - 15° or so, to keep it running at max efficiency even when the cyl fill is very thin. Every individual combination of engine (compression, heads, cam, exhaust), torque converter, transmission, car weight, gearing, and driver preferences, is slightly different; but in general, a "hot street" timing curve will almost always look something just about like that. Doesn't matter if it's a 6-cyl or a big block; doesn't matter what mfr cast the castings; doesn't matter what year it was made; it's gonna be ... just about like that.

To adjust your computer-controlled system's ignition timing, you disconnect the electronic advance control, such that the spark occurs at its minimum value, aka "static" timing. The process looks and feels alot like adjusting the timing on a motor with mechanical timing control. Then when you hook the electronic control back up, the ECM does whatever it thinks it ought to with the "timing"; which in the real world, means it adds around 22° or so of "advance" at idle, under most circumstances.

In the electronic system, the ONLY ways to change the "timing" are: change the values in the lookup table in the "chip"; or, manually twiddle the dist body to find the best possible setting that it can have, within the limits that reasonable operation is produced under all operating conditions. Problem of course is, you have VIRTUALLY NO WAY of changing those lookup tables, so the only tool you're left with is, dinking with the "static" setting.

To find this spot, put down your "light" and "book", forget about "spec", and don't bother even looking at "mark" or "tab". Wherever your dist is right now, advance it (rotate the body counter-clockwise) and go test drive the car. Keep advancing it until you find the point at which it begins to "ping". Back it off until it just quits that, and then back it off a little bit more. Maybe not exactly "ideal", maybe not under ALL conditions, but you're at least somewhere close. You are now fairly close to "ideal" for your max HP conditions, which will be, if you had a way to measure it accurately, pretty close to 36° BTDC. From there, adjust it in small increments, to find the point that it runs "the best" overall. Great thing about this is, YOU get to decide what's "best" TO YOU, for the way YOU drive, using the fuel YOU can buy, on the roads in the place YOU live. Altogether better than being a slave to "spec" in "book", which is optimized for emissions, CAFE, emissions, high- and low-altitude locations, emissions, the fuel that was available in 1982 or whenever, emissions, hot & dry vs very cold vs very damp locations, emissions, driver preferences all the way from grandma going to church on Sunday to construction workers hauling dirt in their truck to maniacs like us having fun, emissions, noise vibration and harshness, emissions, ease of starting (too much "advance" can make an engine very hard to start when hot), emissions, and ... oh, I forgot to mention the most important thing of all that the factory has to contend with, EMISSIONS. If those things aren't the same as YOUR priorities, then "spec" in "book" isn't for YOU anyway. By using the method above, called "tuning", YOU get to balance the relative merits of max power, best throttle response, smooth running, cool temps, gas mileage, whether it's unacceptably hard to start when hot, maybe even emissions it they check that wherever you live, and so on; in short, YOU get to pick what mix of those properties is important to YOU.

Once you find that spot, pick "light" back up, and use it to observe "mark" and "tab". Write down what you see so that you can PUT IT BACK in case some yutz (aka "mechanic") decides that he knows better than your engine does, what its "ideal" timing should be.

dmccain

iTrader: (2)

Ive seen faulty distributors and or modules make engines run really stupid more than once

Gunz_Guntaku

I'm sorry that's alot to read and I'm not the mechanic. I don't know how to translate this for him. Is there any documentation I can read on this?

Note that I had a computer controlled Carb in the 305. It was special ordered without cross fire injection from the factory on this 1983 Z28. When using the same type carb on the new 350 the computer is not functioning correctly.

The options we are talking about now are to just eliminate the computer entirely at this point because my mechanic has no idea what he's doing at this point.

T.L.

The computer has nothing to do with it. UN-PLUG the connector to the distributor. Set initial timing to 6° before top dead center. PLUG the distributor BACK IN.
If it doesn't run properly, you have some other problem...

T.L.

NO. Disconnect the distributor. Set the initial timing with a timing light. Then re-connect it.



The EGR has nothing to do with the ignition timing, however removing it may cause the Check Engine light to come on...

Gunz_Guntaku

I've been told by the Mechanic he's done this but of course I can't verify I'm trusting him at this point. I don't know what's going on at this point. I know the car was running fine a few years ago as it was garage kept with just the 305. This should just work I would imagine but I'm thinking the ECM or something with this Computer Controlled Carb is faulty.

T.L.

The ECM will tell you what it's not liking by throwing codes.
When this happens, the Check Engine light comes on (unless the bulb is burnt out)...

Gunz_Guntaku

Check engine light is not coming on and from what I understand it's not throwing codes. The engine is running as far as I know it's just not running beyond idle once it's taken into drive.

previously however when it was experiencing vacuum issues and with a bad idle the car once it gets going down the road runs just fine but would die once it comes back down to idle.

Gunz_Guntaku

So I should state when I talk to my mechanic tomorrow that he needs to

A. Disconnect the distributor
B. Re Time with a timing light
C. Start her up in idle and drive her....if still dogdy then I have a totally separate other problem.

Did I read correct?

T.L.

Yes, but any real mechanic should know this. And don't forget to re-connect the distributor...