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-   -   Pinging on cruise part throttle acceleration? (http://www.hotrodders.com/forum/pinging-cruise-part-throttle-acceleration-180905.html)

kyle86 07-11-2010 11:13 PM

Pinging on cruise part throttle acceleration?
 
I've been trying to get my tune squared away and in the process of leaning the jets, developed a little chatter sometimes when I slightly accelerate after cruising around 2300 rpm. I have a feeling it's the vacuum advance. I'm riding with a vacuum gauge and averaging around 20" vacuum at full cruise.

Will an adjustable vacuum advance can fix this? or what about the vac advance limiter cam? Not sure which one to get. Thanks!!

cobalt327 07-11-2010 11:20 PM

Quote:

Originally Posted by kyle86
I've been trying to get my tune squared away and in the process of leaning the jets, developed a little chatter sometimes when I slightly accelerate after cruising around 2300 rpm. I have a feeling it's the vacuum advance. I'm riding with a vacuum gauge and averaging around 20" vacuum at full cruise.

Will an adjustable vacuum advance can fix this? or what about the vac advance limiter cam? Not sure which one to get. Thanks!!

In your case you need to limit the total of the advance added by the vacuum advance. Changing the point where it tips in won't help your condition.

The Crane part: VACUUM ADVANCE LIMITER PLATE- Crane #99619-1, #99619-1 INSTRUCTIONS.

There is a vacuum can that is supposed to be adjustable for both: The ACCEL #31035 is an adjustable vacuum advance can for GM HEI that allows infinite adjustment to BOTH the amount and rate of advance. Comes w/instructions and tool.

kyle86 07-12-2010 12:55 AM

Great, thanks cobalt327!!!

Just to help clarify, for what conditions would you want to adjust the advance rate, and when do you limit the advance?

cobalt327 07-12-2010 05:12 AM

the total vacuum advance would be set to allow for ping-free part throttle cruise, and limit the tip in to a vacuum amount that would allow for no ping as it was brought in.

Just like w/the mechanical, the basic idea is to supply the engine w/enough advance as possible, w/o causing any detonation. It's all a compromise, as I'm sure you know. But having the adjustability gives you more leeway and options.

oldbogie 07-12-2010 12:09 PM

Quote:

Originally Posted by kyle86
I've been trying to get my tune squared away and in the process of leaning the jets,

The simple solution is to realize the carb is too lean.

Bogie

kyle86 07-12-2010 12:22 PM

Interestng.

This might have something to do with that I had to put a stock vac can on it because my adjustable accel diaphram broke after only 7k miles. I'll get the numbers off the stock can and see what I can find out.

Also is there any way to limit the vac advance without buying something?

kyle86 07-12-2010 12:25 PM

Quote:

Originally Posted by oldbogie
The simple solution is to realize the carb is too lean.

Bogie

I thought about that. I have 72s in it right now which is the factory jet size. I had 74s in it before which turned my plugs a darkish gray color. I cruised for about 2 miles at 45mph (2200rpm) and then cut the ignition and pulled over and then pulled some plugs out to inspect them. The middle portion of the insulator is what I was looking at with a flashlight. I havn't pulled the plugs yet since I put the new jets in.

cobalt327 07-12-2010 12:49 PM

Quote:

Originally Posted by kyle86
Interestng.

This might have something to do with that I had to put a stock vac can on it because my adjustable accel diaphram broke after only 7k miles. I'll get the numbers off the stock can and see what I can find out.

Also is there any way to limit the vac advance without buying something?

Someone had posted this in another thread:

http://www.hotrodders.com/forum/atta...chmentid=44636

http://www.hotrodders.com/forum/atta...chmentid=34464

Almost every OEM can will have in excess of the 10-12 of vacuum advance that's usually wanted. So, if you're using an OEM can now- that could have a lot to do w/the pinging during light throttle cruise.

Obviously, excessively lean primary jetting can get you into detonation, but it would need to be pretty lean- I'd think you'd need smaller jets than the original baseline unless you have an economy-type Edelbrock Performer like a 1406. The cam can also affect this. What cam, carb and intake do you have, a rundown of the whole engine will help.

kyle86 07-12-2010 01:49 PM

The problem isn't that it's pinging at cruise, it's when I accelerate from cruising. Like If i tip into the throttle a little to speed up. It dosn't do it going uphill either, it's got to be a light acceleration like I'm going to pass someone, but not in a hurry. And it's a very light detonation sound that only lasts for a second. If I hammer it off cruise, I might get one ping if even at all. It doesn't ping any at WOT which I think it would do if it was too lean, that's why I'm thinking vac advance.

As far as the engine,
383 stroker
9.8-10:1 compression - zero decked with .041 gasket
67 cc heads pro topline sr/torkers 2.02 1.6
XE284 advanced additional 3*
Holley 300-36 dual plane intake
holley 750 carb, vacuum secondaries
turbo 350, 3.73 gears
Initial timing 15*, total timing 32* ported vacuum

The numbers on the vacuum can are AR-10

Thanks!

kyle86 07-12-2010 01:56 PM

Oh and also regarding those pictures, I've already welded up and ground my advance slots a few times and ended up with 16-17 degrees mechanical advance. What a pain in the butt that was haha.

The first picture you posted looks very interesting. I have the ability to do some small fabrication work. Maybe I can figure out a way to modify the vac advance unit. Thanks for posting these!

kyle86 07-12-2010 02:17 PM

I also found this after some looking

VC1836 AR10 1977 305 All Hi Alt, Exc. Calif. 3-5 9 @ 11-13
from http://www.stevesnovasite.com/forums...ad.php?t=11689

From what I understand this can give 9* of advance starting at 3-5" and all in by 11-13". My cruise is 20" of vacuum. Could my problem be that the vac advance is coming in too soon?

I notice when I accelerate off cruise and get the ping, it's around 10-15" of vacuum which according to that spec, the vac is all in still and doesn't tip out. What's funny though is that before I put the can on, I tested it with a vacuum pump to make sure it worrked, and it looked like the advance started at 10" however I didn't think to notice at the time what it's max advance was.

oldbogie 07-12-2010 02:33 PM

Quote:

Originally Posted by kyle86
I thought about that. I have 72s in it right now which is the factory jet size. I had 74s in it before which turned my plugs a darkish gray color. I cruised for about 2 miles at 45mph (2200rpm) and then cut the ignition and pulled over and then pulled some plugs out to inspect them. The middle portion of the insulator is what I was looking at with a flashlight. I havn't pulled the plugs yet since I put the new jets in.

Dark gray is just fine, excessively rich would be black, to fluffy black, to wet black.

But given the range of different metering circuits, simple plug reading isn't a sure fire indicator till the signs get out on the edges of being black from rich or stark white, to white speckled on the lean side. Between these two extremes is a range of colors influenced by startup on choke which is quite rich, idle which tends to be toward the rich side, transfer which can be rich if the throttle plates are not aligned properly, main metering which only happens, or should only happen after the throttle blades move across the transfer slot, to the power jets which should come on at about 1/2 cruise vacuum and the jets are sized correctly (typical Holley, others use metering rods which are an exercise in and of themselves).

A carb and intake have problems with delivering fuel mixture of the same ratio to each cylinder. The overall mixture has to be rich enough to keep the leanest cylinder rich enough to stay out of detonation. This why fuel injection ALWAYS delivers better economy with more power for otherwise identical engines.

Mixture temperature and engine temperature have a big influence on how the plugs look and even how the mixture shows on a mixture ratio gauge. Cold manifolds where no exhaust heat is applied can cause the plugs and a gauge to read rich while the mixture that burns is actually lean. This is because where the intake is cold and the engine RPMS low, there is a tendency for the fuel to settle out. In that case raw fuel runs into the chamber and doesn't begin to burn till quite late and then incompletely. This will give a false plug reading as rich while what is burning is lean. The same thing happens when the engine is operating under 180 degrees or has a plug whose heat range is too cold. Cold plugs are a common problem with a high cruise vacuum. High cruise vacuum is a sign of a lightly cammed and or low compression engine at low RPMs; this configuration has a hard time keeping the plug tip hot enough to stay clean again leading to a false rich look on plug color.

The cruise detonation you're seeing is an excessively lean mixture, or a hot mixture, or hot engine, or too much timing lead. A common problem with too much vacuum advance as the centrifugal is coming in. High vacuum begets high amounts of vacuum advance. Vacuum advance is there for off idle (part throttle) at low RPMs and low load on the crankshaft. Centrifugal advance is there for high RPM operation at wide throttle opening; the latter causing a low manifold vacuum. Therefore, as the throttle is opened and the RPMs come up, the amount vacuum advance diminishes in proportion to the reducing manifold vacuum. But the centrifugal starts to pick up the advance. It is tricky to time them such that too much vacuum advance and too much centrifugal advance for the engine's operating condition aren't happening at the same time. This is why these systems are adjustable. However, in the case of vacuum, one usually needs to go to the aftermarket for an adjustable vacuum cannister. Certainly and more commonly the centrifugal is thought of as adjustable. But each needs to be played with along with mixture.

Typically the only way to read plug color is to warm the engine up on a set of plugs. Then replace them with new and make a banzai run from which the engine is shut off at red line and the car coasted down in neutral to prevent pulling mixture from an ignition dead but rotating engine as this will give a false rich. Once stopped the plugs are pulled for reading. Like I said before; just pulling plugs after driving around gives some sort of average reading based upon all the carb circuits that operated and how hot the engine/plugs get. A rich look can also be the result of excessive pump pressure or a high float setting or too small an air bleed in the emulsifier circuits.

A blown diaphragm in the vacuum advance would be a vacuum leak into the engine, resulting in a lean "firing" mixture which would make the engine ping prone on very little advance or load. One, also, needs to be careful with emissions capable carbs as some have taps for venturi vacuum. This is the reverse of manifold vacuum, where the higher the air flow thru the boosters and venturies the higher the vacuum on the port. Such a hook up on a conventional engine would apply way too much advance all the time above idle, so be double sure of your vacuum source. This is not to be confused with timed vacuum which is ported below the venturi and above the throttle blade such that blade position controls whether and when the vacuum port is active. This is something you have to tune for against full time manifold vacuum and will greatly affect idle and just off idle operating quality.

This is a lot more complex process than it looks to read plugs as trail to mixture ratios as a lots going on and quite a bit of that has nothing to do with mixture ratios.

Bogie

cobalt327 07-12-2010 02:51 PM

Quote:

Originally Posted by kyle86
The problem isn't that it's pinging at cruise, it's when I accelerate from cruising.

My choice of words was poor, I do realize that there's no pinging as you are going down the road- sorry for the confusion.

There's a fine line as to how much vacuum advance you can use- both total, and how soon it comes in.

The first thing I'd do is limit the total vac. adv. until the transition ping at tip in was cured. Then, see whether you can quicken or need to delay the onset of the vacuum advance. But because you're getting a real good vacuum reading during P-T cruise, the OEM setting for when the vac. adv. begins will be close to what's needed.

If you still have pinging at tip in- even w/the total vac. adv. backed well down, then I'd suspect the main circuit was too lean. But that you do not get any ping at brisk throttle application, that tells you the accelerator pump and PV are close to ideal.

Should you add an appreciable amount of jet to the primary side, you may get by w/ a bit less accelerator pump shot.

kyle86 07-12-2010 03:00 PM

Quote:

Originally Posted by oldbogie
Dark gray is just fine, excessively rich would be black, to fluffy black, to wet black.

But given the range of different metering circuits, simple plug reading isn't a sure fire indicator till the signs get out on the edges of being black from rich or stark white, to white speckled on the lean side. Between these two extremes is a range of colors influenced by startup on choke which is quite rich, idle which tends to be toward the rich side, transfer which can be rich if the throttle plates are not aligned properly, main metering which only happens, or should only happen after the throttle blades move across the transfer slot, to the power jets which should come on at about 1/2 cruise vacuum and the jets are sized correctly (typical Holley, others use metering rods which are an exercise in and of themselves).

A carb and intake have problems with delivering fuel mixture of the same ratio to each cylinder. The overall mixture has to be rich enough to keep the leanest cylinder rich enough to stay out of detonation. This why fuel injection ALWAYS delivers better economy with more power for otherwise identical engines.

Mixture temperature and engine temperature have a big influence on how the plugs look and even how the mixture shows on a mixture ratio gauge. Cold manifolds where no exhaust heat is applied can cause the plugs and a gauge to read rich while the mixture that burns is actually lean. This is because where the intake is cold and the engine RPMS low, there is a tendency for the fuel to settle out. In that case raw fuel runs into the chamber and doesn't begin to burn till quite late and then incompletely. This will give a false plug reading as rich while what is burning is lean. The same thing happens when the engine is operating under 180 degrees or has a plug whose heat range is too cold. Cold plugs are a common problem with a high cruise vacuum. High cruise vacuum is a sign of a lightly cammed and or low compression engine at low RPMs; this configuration has a hard time keeping the plug tip hot enough to stay clean again leading to a false rich look on plug color.

The cruise detonation you're seeing is an excessively lean mixture, or a hot mixture, or hot engine, or too much timing lead. A common problem with too much vacuum advance as the centrifugal is coming in. High vacuum begets high amounts of vacuum advance. Vacuum advance is there for off idle (part throttle) at low RPMs and low load on the crankshaft. Centrifugal advance is there for high RPM operation at wide throttle opening; the latter causing a low manifold vacuum. Therefore, as the throttle is opened and the RPMs come up, the amount vacuum advance diminishes in proportion to the reducing manifold vacuum. But the centrifugal starts to pick up the advance. It is tricky to time them such that too much vacuum advance and too much centrifugal advance for the engine's operating condition aren't happening at the same time. This is why these systems are adjustable. However, in the case of vacuum, one usually needs to go to the aftermarket for an adjustable vacuum cannister. Certainly and more commonly the centrifugal is thought of as adjustable. But each needs to be played with along with mixture.

Typically the only way to read plug color is to warm the engine up on a set of plugs. Then replace them with new and make a banzai run from which the engine is shut off at red line and the car coasted down in neutral to prevent pulling mixture from an ignition dead but rotating engine as this will give a false rich. Once stopped the plugs are pulled for reading. Like I said before; just pulling plugs after driving around gives some sort of average reading based upon all the carb circuits that operated and how hot the engine/plugs get. A rich look can also be the result of excessive pump pressure or a high float setting or too small an air bleed in the emulsifier circuits.

A blown diaphragm in the vacuum advance would be a vacuum leak into the engine, resulting in a lean "firing" mixture which would make the engine ping prone on very little advance or load. One, also, needs to be careful with emissions capable carbs as some have taps for venturi vacuum. This is the reverse of manifold vacuum, where the higher the air flow thru the boosters and venturies the higher the vacuum on the port. Such a hook up on a conventional engine would apply way too much advance all the time above idle, so be double sure of your vacuum source. This is not to be confused with timed vacuum which is ported below the venturi and above the throttle blade such that blade position controls whether and when the vacuum port is active. This is something you have to tune for against full time manifold vacuum and will greatly affect idle and just off idle operating quality.

This is a lot more complex process than it looks to read plugs as trail to mixture ratios as a lots going on and quite a bit of that has nothing to do with mixture ratios.

Bogie

First of all, thanks for the lengthy writeup. That's some interesting stuff to read.

So basically tuning main jets by looking at the plugs is useless. I havn't had much luck with it so far anyways. My plugs take forever to get any color on them. What way do you sugguest is the best for tuning them? I've done the surge method before on a previous engine but that is tough to get dead on because you have to go by feel. I've also tried cruising with the vacuum gauge hooked up. On a flat road going the same rpm with different jets and noting how it effects the vacuum. I don't have a wideband. This is kinda frustrating.

Custom10 07-12-2010 03:27 PM

Your ported vacuum of 20" seems high if you are at a 2300 RPM cruise as you posted, not doubting your info just seems high too me but I run an eddy carb, checked your vac gage? When you give it some light throttle at this speed your vacuum gage should drop off sharply to a point below were your can starts to add advance so basically it is out of the picture. If it is staying at 12-15 then yes it may be adding to much advance, get an adjustable start point can and the crane limiting plate in there would be my advice and see that the mech advance springs are in good shape and let it all in by about 2800, clean the dist throughly. Did you limit the total advance without vac to 32 because of the pinging? you may want to but back about 3 or 4 deg on the mech side once you get things in check or up the base a bit.

Also did the ping get worse after you leaned the carb up?


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