I need help with my chevy II! Its a 62 4 cylinder car, I rebuilt the engine and it acts like it has a miss/vacuum leak. Ive been trying to figure out for a year now, taking the intake and exhaust for cracks, spraying carb cleaner all over. lastnight I figured out that my plug wires were leaking and arcing all over including my heater hose. I replaced them and my coil now when i pull plug wires it it slows down more but problem still there.

engine details:
short block rebuilt
head surfaced and a valve job
new cam
new carb
new wires and coil

while the engine is running ive had my hand under the exhaust and when it appears to "miss" it feels like more air comes out. spraying carb cleaner in small amounts down the carb makes it rev up. blocking air doesnt seem to do anything. Sorry for the long read but i wanted to be thorough, if i cant fix it ill probably sell it as its my daily driver and an engine swap isnt as straight forward as I want.

 

What are you using for a distributor and carb? What are your tune up settings?

 

I have a stock distributor with a new cap, rotor, vacuum advance and pertronix. Carb is a weber 32/36 and idk my timing, the tab only goes to A which i assume is 4 degrees because that is the stock timing. It runs even worse at that so its bumped up to an approx. 8-12 degrees advance. There is a small gap of timing where it actually runs any good, a few degrees up or down and it runs worse.

 

I think your timing is a bit advanced. Are you running full battery voltage on the distributor (no resistor wire)? Is your carb running lean or rich? Valves not too tight? Is there fuel dripping in the carb as its running?

 

I can't see any fuel dripping, does seem a little rich at times. Channging the timing doesn't affect it. I don't think there is a resister wire I have the wire from the starter going straight up to the coil like it was when I got the car. I also can't get it to idle below 1000, with the carbs I've had on previously I could idle it down until it died but if I tried going below 1000 I wouldn't trust it cause it idle so bad. Now with the weber I have the idle screw all the way out and it's about 1000 at idle

 

Idk about the valves, I'm not good with adjusting valves, by I've had my dad do it before and it still did it

 

The wire going to the starter is the resister wire. It should be connected to the "I or R" terminal and the other wire connected there goes to the coil. It's very stiff with a cloth insulation. The way the starter is wired the bypass happens in the start mode.


Either your choke is not pulling off or you have issues with your new carb. Possible throttle linkage issues? With the rpm your running you are out of the idle circuit so the air/fuel mixture screws are not going to effect anything.


Who adjusted your valves when you put the engine back together?

 

I guess I have a resistor wire then. I have a manual choke and it does go off all the way. The linkage could be my high idle problem with new carb as it is a bell crank type deal, from the factory. its ran the best it ever has with the webber on it, and the idle mixture screw actually does affect it some when screw in or out and i have it adjusted for best idle. I always have my dad adjust the valves for me, I havent had him adjust them in a while.

 

Does the car actually drive?

If so, how does it perform on the road at idle, in transition, and under partial and full throttle?
How about when you let off the gas?

- Eric

 

Disconnect the throttle linkage and see if you can get the idle down, set your timing at the lower idle with the vacuum disconnected and plugged. Make sure your spark plug gap is at .035 and all your wires are pushed on correctly. Set your carb with a vacuum gauge for the highest vacuum.

 

The car has been my daily driver for a little over a year, I drove it a few months before the rebuild. It actually preforms very well while driving, no hesitation from idle and drives in town and on the freeway great. No problems letting off the gas and full throttle pulls strong, for a 4 cylinder. Its a manual trans car. Other than the idle the engine seems to be great. While driving the car does not feel like it misses. Idk if my vacuum gauge works, it got dropped once, i just do the idle/ear thing.

 

Okay, now we're getting somewhere. It's exclusively an idle problem.

Do you have the original BV carburetor? Webbers I've had have been great carbs, but have not been known for their idle finesse.

What do you get in that thing, about 30mpg?

- Eric

 

Before I rebuilt the engine, with the 181 stock marine cam and the rochester b I was getting 27mpg, With the new cam and carb I havent checked. I had the stock rochester b on for a long time but The engine started running weird, dont remember if it started doing what it was doing now or not, so I messed around with it and changed over to another rochester that my dad had from A 65 nova 6 cylinder and it didnt work. I have about 8 model b carbs and I couldnt get anything to run right. I bought an adapter and put a varajet on it but i think it was a crap carb cause it wouldnt run with the choke off, so when a webber came up locally I rebuilt it and after tuning its been the best carb. When i had the original 153 in the car with the original carb and same head and intake Im running now I dont remember it ever running weird the only reason i changed it is because it developed a wrist pin knock.

 

I would like to add, although it doesnt jerk or feel like power is coming on and off "miss" while driving, while parked if I rev it up a little you can still here the puffs in the exhaust. My mufflers blew up a few weeks ago so I have a straight pipe now, maybe if I roll my window down and drive it I can check to see if it still does it.

 

I know they all look the same but heres a picture if your interested.
53B31D9D-5E4A-4F6B-B6CF-2AB8009DBED1-5777-0000093B540ACDA2_zps0877525f.jpg
I really want to get it running right, my dad got it for me for my 18th birthday and I dont want to see it go but its my daily driver and I need something that runs right that I feel comfortable in.

 

Have you gotten the idle speed down and checked your tuneup settings?


Nice car BTW.

 

I havent messed with it yet today, ill disconnect the throttle and see what it does. What could cause a change in timing to cause the idle to be affected so much? Ive never adjusted the timing on anything and had it act like this, whether I advance it or retard it get extra shaky and sounds different, like when your getting into negative timing.

 

It will sound and act different with wide adjustment variations in timing. To get your timing correct it needs to be idled down so that would be my first task.

 

I checked my linkage and the throttle does seem to be closing all the way. I dropped my timing down to the A on the timing tab and adjusted the air/fuel screw and got the idle down to about 700. Im now thinking my carb needs some more tuning to better fit my engine, the carb came off a luv and I had to jet it up a decent amount to get my off idle/acceleration correct. I have done a little reading on webber 32/36 carbs and they say if the idle mixture screw needs turned more than 2 turns out you need to go bigger on something, dont remember what. I shut the engine off and turned the screw all the way in then backed out 2 turns and the engine wouldnt start, ended up have to go to 4.5 turns to get it to run ok.

 

idk if im just being optimistic but the "pops" in the exhaust do appear to be less noticable.

 

Ah so it is probably lean. The other thing is you said you were running a straight pipe. That may also be contributing to a little popping as there is minimal back pressure.

 

Ive only recently been running without a muffler, you could here it while the muffler was on. Ive jetted the carb up but havent done anything to the idle circuit or air bleed jets. should i do anything with them?

 

I'm not a great authority on carbs, like you I just experiment until I get it.

 

Definitely. The Webers probably have changeable idle air bleeds and idle jets. It is vitally important to get these right. I can't tell you which ones you need, but there may be a Weber help line where they can at least point you in th eright direction.

- Eric

 

I messed with it a little more, my dad has a drill bit index that goes by nearly .001 so i adjust the idle jet and got it so it runs the best at about 1-3/4 turns out on the mixture screw. now it is a little stumbley off idle and my idle went up to about 850-900. Im going to look for a weber forum to try and get more help. But for now with my timing down to what looks like just above the A mark and my mixture screw/ idle it seems to be running and sounding better than before. thanks for the help.

 

You're welcome. Glad to help.

My money's on some minor changes to that carb, and it'll be great.

I love those cars, by the way. Great that you're driving it every day.

- Eric

 

Im leaning towards the same thing, im going to mess with it more tomorrow after work and see if i can get it straightened out. My dads always had at least one nova around my whole life so im partial to them.

 

Set your timing to around 4-6*BTDC and see how it runs. You also may check to see if your ignition needs 12v. Glad it's doing better.

 

That is where the timing is at now, ill be driving it to work in the morning so ill see how it does. Ill check to see what kind of voltage my ignition is supposed to have. One thing I hove not had any luck with is the idle, my screw is still all the way out, no linkage hang ups and looking down the carb it appears to be closed. Maybe they just idle high, On one of the tune up things i read it said to set idle from 850-1000.

 

Looked at my instruction paper for the ignitor, it says its designed for a 12 volt system but it also says if you have a resistor dont remove it.
1DF988B7-495A-44F7-B1CD-5AA176F55F1A-6025-000009981D0D8360_zpsb328411d.jpg

 

It can be wired with or without a ballast resister. You might try it with the resister wire bypassed.


http://www.jegs.com/i/Pertronix/751/1144A/10002/-1

 

Yes, BUT, if he has an original-type coil, not using a resistor might overheat it.

My question would be, If he's only got something like 3.5 bolts at the coil, is he inadvertently running TWO resistors in series?

- Eric

 

This is not the thread with the 3.5 volts at the coil. I read the Pertronix instructions and you can wire it either way. I was just wondering if a bit more voltage would eliminate the minor misfire he has.

 

Oh.

Still, the resistor is to protect the coil (if it is a low-resistance OEM type), so running it without one for an extended period could be bad.

No harm in trying it, though.

- Eric

 

The resister wire was to protect the points.

 

Jumping a hot wire from the battery to the coil didn't do anything. I have a surging spot at low throttle, I'm not sure which way to go with jets now. I'm wonderng if it's a lean miss, if that's a thing, before I started adjusting anything a few days ago I had my plugs out and they were a more of a white than brown, and my understanding is that means it's lean. It does seem to be getting better at idle each time.

 

Au contraire.

In order to explain this, I have to briefly discuss the function of LR circuits.

An LR circuit is a circuit containing an inductor ("L") and a resistor ("R").
In the real world, LR circuits are also actually any circuit containing an inductor, since all inductors have some resistance, and thus can be philosophically divided into a separate inductor and resistor.

A Resistor is anything that conducts electricity, but is resistant to its flow.
An Inductor is essentially anything that electric current flows through, as all electric current induces some magnetism, but practically it's an electric coil of some sort.

The ignition coil in your car is an inductor. It also has resistance, as seen in this table:



The key to understanding the LR circuit is that the magnetic fields produced by the windings of the coil interfere with each other, and this produces a reverse voltage, which resists the forward voltage that you are applying from the battery or the alternator.

What this means is that when you turn on the power to a coil, the current doesn't flow all at once, but takes some time to reach full force. While it is ramping up, the magnetic field is expanding, which, in a transformer (which the coil is) will induce a current in the other set of windings.
When you turn off the power, the flow doesn't stop all at once then, either, but takes a bit of time, as the magnetic field collapses.

I will include some pictures from here, which has an excellent description.

A Standard LR Series Circuit:



SO, as with all science-ey stuff, saying "it takes time" isn't good enough - we want to know how much time it takes.
Naturally, there's a formula for that. It's:

Where:

• I = Current in Amps
• I(t) = Current in Amps at a given number of seconds
• V = Potential Difference in Volts
• R = Resistance in Ohms
• t = Time in Seconds
• e = log natural base (a mathematical concept = 2.17828)
• [(A) doesn't mean anything - it's a label from an illustration]

This can be refined into the simpler:

Where:
τ = the "Time Constant"
L = Inductance in Henries
R = Resistance in Ohms

The way that the Time Constant works is that it is the time it takes for the voltage or current to rise from zero to ⅔ of its maximum, OR to fall from its full level to ⅓ of its maximum, AND the Time Constant is also ⅕ of the time required for the voltage or current to go from zero to maximum (or from maximum to zero), which is to say that the Time Constant x5 is how long to get from no current flow to full current flow in an LR circuit.

Here's a picture:


SO, using the L (Inductance) value that I have found for standard automotive coils, which is about 7-8mH (that is, about 0.007 to 0.008 Henries), and the total R value of our system from the above table (about 3.5Ω with the resistor plus the coil), we have 0.007H ÷ 3.5Ω = 0.002 seconds, which is to say that it takes 2 milliseconds or two thousandths of a second, for the coil to get up to ⅔ of its full current flow, and 0.002 x 5, or 0.010 seconds for it to be fully saturated, and start to pass current straight through without resisting its flow.

If we do this calculation again without the ballast resistor in the picture, or 0.007H ÷ 2Ω = 0.0035 seconds to get to ⅔ saturation, and 0.0175 seconds to get to full flow.

You will notice that the coil becomes saturated in less time when there is more resistance.
Specifically, it is close to its maximum (after a period of 2 Time Constants) in 4ms with the resistor, but 7ms with no resistor.

Now, let's do some other math:
You have a 4-cycle V8 engine.
That's 8 cylinders, each of which fires once over a period of 2 revolutions of the crankshaft, which is to say that you need to have one cylinder fire every 90° of crank rotation, or you need 4 plugs to fire each full revolution.
So, at 4,000 RPM, you need to fire 4,000 x 4 = 16,000 times per minute, ÷ 60 seconds = 267 times per second, which would require a pulse every 3.7 milliseconds.

Well, we just saw that without a resistor, the coil needs 7ms to get close to a full charge, but with a resistor, it only needs 4ms.

You can see that once you start getting some decent RPMs, you need the resistance there to speed up the charging of the coil's magnetic field, and that if you do not have it, you will begin to cut the field off before it is fully formed, thus reducing your potential spark voltage at a time when it needs to be increased (because of a denser mixture in the cylinders). Also note that the induced current in the spark plug wires is dependent on the speed at which the magnetic field collapses, so the faster-collapsing field afforded by the resisted circuit allows for a higher potential spark voltage.

Also, at lower RPMs the coil with the resistor will reach a full charge faster than the coil without one, and therefore it will begin conducting with minimal resistance, and will be in this fully saturated state longer between ignition pulses, which is to say that it will want to draw more current and potentially get hotter. BUT, since current flow through a resistor creates heat, and resistance increases with heat, as the current flow to the coil goes up, the resistance of the resistor wire also goes up, creating a natural safety mechanism which protects the coil from damage at the lower RPM range, where it might otherwise seem better to run it without a resistor.

So, no, the resistor is not there to protect the points.
That's the condenser's job.

- Eric

Further reading:
ElectronicsTutorials.ws
"The Physics of the Automotive Ignition System", Harvard University [note:Giancoli is a decently written college Physics textbook]
Automotive Ignition Coils
GoFastForLess.com

 

I stand corrected on principle, the reduced load also extends point life. However in this case we are talking about a Pertronix.

 

I checked my plugs after my drive home and they were black. so I guess when I jetted up my idle jet it made everything else rich so I drop my main jets down and it helped a bunch. It also now sounds the best it ever has and hardly pops through the exhaust. I think with a real jet kit I can fine tune it and get it even better, wish I had an air/fuel meter. I want to thank both of you for the help, I didnt realise my tune was so off. Without the fresh minds to the subject I would probly still be thinking I had a hidden vacuum leak somewhere.

 

You're welcome.

I would add, though, that your idle jets should not affect the mixture while running, at least not enough to matter.
It's more likely that your mains are a bit large (though less popping usually means less lean, so you don't want to go to much smaller). I believe that the Webers don;t have any sort of needle mixture valves, so your mid-throttle running mixture is probably determined mostly by your mains and some sort of air bleed.
To check, just clean the plugs, drive the car at a decent, fairly steady speed for a while, then quickly push in the clutch and kill the engine, without slowing down. Pull over with the engine off and check your plugs again and you'll see what they look like while running.

You can do the same thing to check your full throttle mixture, which is most closely associated with main jet size: floor the pedal, accelerate at full throttle, kill the engine and hit the clutch, coast to a stop and check.

I would strongly recommend, though, that you either get in touch with Weber, or get onto a Weber forum and see exactly which parts of your carb adjust the mixture at which points of operation, so that you can change the parts you need to, in the right direction, and not waste time with the ones you don't need to change.

- Eric