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Vaccum Lines-BB 396-chevy 69


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02-22-02, 01:25 PM   #1  
crick
Vaccum Lines-BB 396-chevy 69

i have a couple questions regarding proper vaccum line connections,first there is a port on the intake behind the carb and there is a large port on the carb,rear bottom,its a holley 750 double pumper,i was wondering which of these connects to the brake booster and which to the pcv valve?my next question is the vaccum advance on the distributor,there are two small ports on the carb,one on the front bottom and one on the left side (facing the carb in front) up near the top,which would be the correct to run vac line to or does it matter?

 
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02-22-02, 05:13 PM   #2  
mooser1
crick, most guys use the large port in the manifold for brake booster and the one at carb base for pcv, either way will work.....

use the smaller vacuum port on the bottom of carb in front for vacuum advance, this is manifold (constant) vacuum source....the other port up top is venturi vacuum, only works with throttle hammered open.....

hope this helps....

 
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02-22-02, 06:30 PM   #3  
crick
Thanks mooser1 for your help.

 
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02-22-02, 08:56 PM   #4  
knuckles
vacuum sources & the purpose of vacuum advance

I disagree with mooser1.

The vacuum advance mechanism does not effect peak power output. Leave it disconnected and your engine will make just as much power at WOT as it would with it connected. The purpose of vacuum advance is to improve part-throttle driveability, increase fuel economy & decrease exhaust emissions.

There's no reason to run the vacuum advance line to a manifold vacuum source. Doing so causes the vacuum advance mechanism to provide full advance at idle and part throttle. This can cause the engine to ping, especially if it's a high compression beast like a 396/375.

Use the higher of the 2 vacuum nipples for your vacuum advance source. This is a ported vacuum source, not a venturi vacuum source as mooser1 stated. The ported vacuum source will reach peak vacuum at part throttle cruise, which is where you want vacuum advance to be active.

Venturi vacuum reaches its peak value at high engine speed. Vacuum secondary Holley carbs use primary bore venturi vacuum to open the secondary throttle plates.

I don't think I've ever seen a Holley carb of any kind with an external venturi vacuum nipple.

.

 
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02-23-02, 07:46 AM   #5  
mooser1
sorry knuckles but the vacuum advance mechanism does not aply full advance at idle, because of the centrifugal advance weights and springs inside the distributor cap.....read any manual about setting base timing on Chevy engines with vacuum advance and the very first step is removing and plugging the vacuum line to the advance mechanism, this would be an unneccessary move if the line were attached to a ported source, wouldn't it?

leaving the vacuum advance unhooked will cause severe ping (spark knock) at hard acceleration and under power as when climbing hills.....vacuum advance is actually a misnomer because the device actually is a spring-loaded timing retardation device designed to retard timing under hard engine load to prevent detonation.....

full timing advance on these engines is achieved with centrifugal force on the counterweights, the vacuum part operates against a spring to retard, not advance, timing......

 
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02-23-02, 10:29 AM   #6  
Joe_F
Holley carburetors rate up there with glasspacks with me, but I'll take a shot.

Mooser:

Most times, the timing procedure tells you to cap off the vacuum advance line wso the advance doesn't come into play. Then you set your timing. I know this is how you do it on a Pontiac 400 like my 79 T/A. So, the timng procedure has nothing to do with it. Also, many books would say "do it with the engine at operating temperature", so I would think at that point the ported vacuum switch/source is open anyhow .

How I would proceed:

E-mail Holley's tech line and ask. They will know. Is this Holley carburetor original for this application or not? As I recall, some were on these big block Chevy engines.

If you have a stock manifold on there, get your factory manual out and they will have the vacuum diagram and routing. That's how I square away every car (used or otherwise) that I have owned. My 79 T/A looked like a ghetto box will all those lines capped when I bought it. I put them all back to stock INCLUDING the small and hard to find vacuum delay valve (was missing) which is ONLY used on a 79 400, not a 78 . If it's there from the factory, it belongs. Period.

If you have an aftermarket manifold, consult with that company's tech line.

Put the two pieces of information together and you've got your answer . Done.

 
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02-23-02, 10:46 AM   #7  
mooser1
you're right Joe, it says block off the vacuum advance for a very simple reason, it is connected to constant (manifold) vacuum as it should be.....after setting the timing with the distributor and re-connecting the line, timing will advance approx. 4 degrees at idle, this is normal....full advance will not occur until opening of the throttle actuates centrifugal advance....

if vacuum advance unit were supposed to be hooked to ported source, there would be NO reason to disconnect and plug it when setting timing...

mr. crick, if you want good advice from very knowledgeable folks concerning strictly Chevy's go to www.chevytalk.com forums, these folks deal with nothing but Chevrolet issues and know from whence they speak......

 
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02-23-02, 12:29 PM   #8  
mooser1
Joe, cut this from an article specifically about vacuum advance distributors FYI.....

2. The second major type usually started to advance at around 7-10 Hg/in of vacuum, giving a max. of around 5-10 degrees of vacuum advance. Most Smog equipped engines (later years) used this advance, where vacuum was applied at idle and reduced when accelerating. The vacuum pickup was tapped directly from the intake manifold. Manifold controlled vac. units were usually used on distributors which gave a much greater total mechanical advance, then those using ported vacuum.
3. The third was an intermediate type. Kind of a mix between the first two. The manifold fed the vacuum advance unit, with the advance curve only giving around 5-7 degree maximum.
and didn't start to have any effect until you let off the peddle a bit or were idling. These units were usually used with a distributor that mechanically started to advance at a much lower rpm range.

as stated by the author of this piece, late model HEI or points type distributors use DIRECT manifold vacuum as the source and it is applied at idle, hence the need to remove and plug the vacuum line when setting base timing......

very early models of vacuum advance did use ported vacuum, but we're talking about engines built back in the 40's

 
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02-23-02, 01:42 PM   #9  
mooser1
heeeeeey Joe.....I know you like keeping up on what's what and all so you can go to this link, click on the "garage" button, you will find some very informative and definitive articles, scroll down to the GM distributor articles to learn more than you ever wanted to about GM distributors with vacuum advance.....you'll find the "Ignition Timing Tutorial" interesting.....

www.tugeckos.com

The vacuum advance canister should be plugged into a "manifold" vacuum source on the carb or intake manifold. This is a vacuum port on the carb that provides full manifold vacuum at curb idle. This is best for cruising/part throttle and will give the best performance overall.
enjoy all and I hope this helps someone learn a bit.......

 
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02-23-02, 09:34 PM   #10  
0patience
Knuckles is corect, the upper vacuum port is the one that is normally used.
We used to disconnect the vacuum advance when drag racing, set the timing to a base timing where we had what we wanted at the upper rpms.

The upper port of the carb is the correct port for the vacuum advance and came from the factory that way.

Mooser,
I didn't find the article you were talking about at that site.
Definitive?? Could be, couldn't tell. Couldn't find them.

Not only that, that is a hobbyist site, so how credible is the info?
You see, I worked on those types of vehicles when some of them were new.
And I know knuckles and his reputation precedes him.

as stated by the author of this piece, late model HEI or points type distributors use DIRECT manifold vacuum as the source and it is applied at idle, hence the need to remove and plug the vacuum line when setting base timing......

Very early models of vacuum advance did use ported vacuum, but we're talking about engines built back in the 40's
NOT!!
The ported vacuum was used into the 80's. They came off the assembly line ported to the UPPER port on the carb. Where did they get this info?? Points were ALWAYS ported to the upper vacuum port on GMs.
If what you are saying is true, which it isn't, then I know alot of mechanics, incluuding myself, who have been doing it wrong for the last 25 years. Not likely.

mr. crick, if you want good advice from very knowledgeable folks concerning strictly Chevy's go to www.chevytalk.com forums, these folks deal with nothing but Chevrolet issues and know from whence they speak......
Do WHAT??
Now look, I agree with Knuckles on the info provided and have worked on GM engines and built performance engines for 25 years. Are you insinuating that Knuckles nor I know anything about what we are saying? Really?

 
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02-23-02, 09:53 PM   #11  
mooser1
I'm not trying to insult or insinuate anything....I'm stating that GM distributors with vacuum advance mechanism's use constant vacuum sources, even at idle, and it doesn't matter where it comes from.....if you want to argue that point, talk to someone else.....

 
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02-23-02, 10:10 PM   #12  
knuckles
horse puckey

Sorry mooser1, but the author of that article is wrong on amost every single point.

The second major type usually started to advance at around 7-10 Hg/in of vacuum, giving a max. of around 5-10 degrees of vacuum advance. Most Smog equipped engines (later years) used this advance, where vacuum was applied at idle and reduced when accelerating.

Bull feathers.

Just think about manifold vacuum for a second. Manifold vacuum is at its highest at or just off idle, typically around 18-20" Hg on a stock V-8 engine. As engine load increases (wider throttle opening) manifold vacuum decreases. Manifold vacuum @ wide open throttle should be less than 1"Hg, zero preferred. A reading of greater than 1"Hg would indicate a restricted exhaust system or intake tract (undersized carb or throttle body).

Ported vacuum, however, increases from 0"Hg @ idle to 10" Hg or more at cruise speed. As throttle openings increase further, ported & manifold vacuum values come closer to one another, eventually equalizing at or near WOT.

Using the author's specs, full vacuum advance would be applied at idle speed & decrease as engine load increased. This would accomplish nothing. Additionally, most smog era Chevy vacuum advance cans provided far more than 5-10 of advance.


Try this link for a more accurate definition of the vacuum advance system & how it works when installed properly.

Also, I mis-typed in my earlier post. I said "... the vacuum advance mechanism to provide full advance at idle and part throttle." when I should have said "full vacuum advance at idle..." My mistake.

A vacuum advance unit can add as little as 5 or as much as 20 advance with full vacuum applied. Each part # is rated for the point at which (vacuum) advance begins and the point at which full (vacuum) advance is realized, as well as the amount of total vacuum advance the unit will add. Crane Cams, Mr. Gasket, and others sell adjustable advance units that let you tailor at least 2 of the 3 variables. Specs for OEM and stock aftermarket units can be found in the back of the parts books.

I can only think of a few modern era GM cars & trucks that used manifold vacuum as a source for distributor vacuum advance. Offhand I can remember some 260 CID Olds engines & possibly some smog era 231 V-6 Buicks, as well as late 1960s & early 1970s medium duty Chevy & GMC trucks with a 427 truck engine (tall deck). The 260 & 231 used numerous thermal vacuum valves & vacuum bleed or delay devices in the vacuum line between the manifold and the vacuum advance canister. The 427 truck engines used a small copper line plumbed directly from the dist. with no controls whatsoever.

You said "if vacuum advance unit were supposed to be hooked to ported source, there would be NO reason to disconnect and plug it when setting timing... "

That's true as far as it goes, but the manual writers are not stupid. They take in to account that the vacuum line could be mis-routed or the carb could be mis-adjusted (very common on Q-jet equipped engines with worn throttle shaft bores). Vacuum could be present in these circumstances & the service manual writers don't want to take chances.

You also said "leaving the vacuum advance unhooked will cause severe ping (spark knock) at hard acceleration and under power as when climbing hills.....vacuum advance is actually a misnomer because the device actually is a spring-loaded timing retardation device designed to retard timing under hard engine load to prevent detonation..... "

You got that one exactly backwards.

The vacuum advance unit connects to the breaker plate or pickup coil (on HEI systems). As vacuum (be it manifold or ported) is applied to the diaphragm, the advance unit advances timing by moving the breaker plate or pickup coil in relation to the distributor shaft. When the unit is disconnected, there is no vacuum applied and no vacuum advance occurs.

At wide open throttle, manifold vacuum is 0 or near zero (below the point at which the vacuum advance diaphragm can overcome spring pressure & move the breaker plate or pickup coil), so no vacuum advance occurs.


I checked the www.tugeckos.com link you posted & that author is just as mis-informed as the first author you cited.

I also checked out the chevytalk.com site. It seemed to be much like ford-trucks.com....mostly hobbyists providing other hobbyists with incorrect and/or incomplete information. Many of the articles & postings on the site were incomplete at best & flat wrong at worst. I found the Torque Convertor article especially hilarious. The author made a few valid points, but he never bothered to identify the 3 elements of the typical torque convertor & never mentioned their effect on convertor operation. This is basic stuff that can be found in any decent repair manual. And these are the people who "know from whence they speak"??? Please.

I have been working on cars since I was old enough to see over the fenders of grandmom's '64 Impala. Professionally since 1986, ASE certified, licensed by the Commonwealth of PA as both Emission Inspection Tech & Emission Repair tech, blah blah blah. I know from "whence I speak".

But hey, don't take my work for it. See for yourself!

I suggest you check out a few unmolested smog-era GM cars & trucks. Check the emissions decal for proper vacuum hose routing. Then start the engine, tee a vacuum gauge in to the vacuum advance hose & note the reading at idle. It'll be zero or near zero. Slowly open the throttle & watch the vacuum readings rise. Open the throttle further (preferably with a load on the engine) & watch the vacuum reading fall. Welcome to the wonderful world of ported vacuum.

 
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02-23-02, 10:12 PM   #13  
0patience
I'm stating that GM distributors with vacuum advance mechanism's use constant vacuum sources, even at idle, and it doesn't matter where it comes from.
WRONG!
Look, you have no clue what you are talking about.
I shouldn't need to argue that point, cause it is inaccurate.

You know, I am usually a pretty nice guy, in fact, I use to moderate these forums, but I have been looking thru the posts lately and some of your info has been less than accurate and this one, you clearly do not have the experience to be stating things you don't know about as fact.

Now, if you'd like to question my credentials, go ahead. I like to see quality, accurate informatin and when someone isn't providing that, I have no problem telling them and correcting the information. See, this is what I do for a living, engines.
So, you're telling us, because some site prints an article, that makes it fact? HA HA HA!! Too funny.

 
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02-24-02, 06:05 AM   #14  
Joe_F
Whoa Nelley.....

Fellas:

The answer about whether the vacuum advance uses ported or manifold vacuum is IT DEPENDS ON THE APPLICATION.

I'm going to prove it, as I have a 1979 Pontiac service manual supplement in front of me as I type .

1979 400 vin code Z, MT......clearly shows manifold vacuum going to the advance. In fact, in 1979 only, there is a delay valve before the advance which I believe slowly retards the timing when you decelerate so as to avoid backfire and make the emissions go up. I'm pretty sure this was needed by Pontiac in '79 to make the 400 pass the EPA regs...1978 400's on the F bodies don't have it.

Now go to the Oldsmobile V8, the 350 vin R and 403 vin K for California in that year...ported vacuum source to the advance. Also on the 231 of that year. It is pretty consistent with 0Patience's posts...

Most of the time it's by engine family. The 301's listed in this manual also used manifold vacuum, as did the 400.

Moral of the story: It depends and always consult your service manual and start there.

Carry on.

Hey 0Patience...where in PA are you a ASE tech? My 79 10th Anniversary T/A was from Reading/Sinking Spring (the people I bought it from) and originally from St. Thomas, PA (first owner lived there). It was sold new through Brake Pontiac in Waynesboro.

I finally located the original owner and have been in sporadic contact with him. He talked about getting me all the original items and things he STILL has for the car...even though he has not owned it since 1987.

 
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02-24-02, 08:58 AM   #15  
mooser1
you're right Joe, it can and probably does work differently in different applications....people seem to forget that crick, the originator of this thread, is asking about a 1969 L78/L89 396/375 HP Chevy...Mr.crick, here's the info in an easy to understand, layman's explanation of what makes your particular ignition system work, how and why:



Ignition Timing 101
What Exactly Is Ignition Timing?






In this article I will try and explain to the best of my knowledge what ignition timing is. I have had a few questions about this. A lot of people know how to set their initial timing with a timing light and what this does, but to some that's about as far as their knowledge goes on the subject. I hope this will help to shed some light (no pun intended) on a sometimes confusing part of auto mechanics.


Well let's see, where do I begin. The timing process isn't really all that complicated. In order for the internal combustion engine to run it has to have two things, a air/fuel mixture and a spark to ignite it. I won't go into the fuel part of it here except to use it for examples in the ignition process. A four cycle engine has well, four cycles. The intake, compression, power, and exhaust strokes. Just to make this as simple as I can, I will use the following example. I know it is not exactly the way it functions, because we have valve timing and other things to consider but I will forgo the valve timing for the moment. When the intake valve opens as the piston is on it's way down on the intake stroke, the vacuum created by the piston draws in the air/fuel mixture. When the piston reaches BDC (Bottom Dead Center, or the lowest point it can go before it starts back up), the valve will close and the piston will start on it's way up on the compression stroke. When the piston gets close to TDC and compresses the fuel charge, the spark plug fires and ignites the mixture. When the fuel charge ignites it explodes and forces the piston down in the cylinder. This is the power stroke. As the piston reaches BDC, the exhaust valve will open and the rising piston will force the burnt gases out through the open exhaust valve. When the piston reaches TDC, the exhaust valve will close, the intake opens and the process starts all over again with the intake stroke. For a real nice explanation of this process, and a whole lot better than I could ever do, check out this valve timing tutorial on Comp Cams site.

Now on to the ignition timing part. As all the above happens, it has to have the spark advanced in order to be able to keep up with the speed of the running engine. This spark is supplied by the distributor and related parts. The distributor tells the spark when to go to the plugs. The coil supplies the voltage to create the spark. If the distributor sent the spark to the plugs at the same time the piston reached TDC, it would be too late because by the time the plugs fired, the piston would already be on it's way down, which would be retarded spark timing. This is the reason if you have ever tried to crank an engine with retarded timing you get the infamous backfiring through the carb. It is trying to fire the mixture as the piston is going down, way too late in the cycle.


Initial Advance:

Initial advance is just that, initial timing. This is the initial amount you have the ignition timing advanced. You set this by loosening the hold down bolt that is used to keep the distributor from moving after you get it set. You use a timing light which is hooked to the #1 spark plug wire and the battery. You aim the light at the timing mark on the balancer and turn the distributor body to either raise or lower the amount. There is a timing pointer attached to the engine in a fixed location that will have degree marks on it, usually in 2 degree increments. Every time the #1 wire fires it will cause the timing light to flash. When it flashes it shows the timing mark position at that instant. You always set the initial advance using #1 plug wire, because the cam and crank are timed using #1 cylinder at TDC. Once you have it at what you want, you tighten the hold down at the distributor. All small and big block Chevy's with an vacuum advance distributor will require you to remove the vacuum hose at the distributor and plug it. More on vacuum advance later.

I have my initial advance set at 14 degrees BTDC. This means that the distributor will send a spark to the plug when the piston is 14 degrees before top dead center (BTDC) on the compression stroke. By doing this it can fire the mixture before the piston starts back down on the power stroke. If you have too much initial advance, it will cause detonation or spark knock. This is because it is firing too soon or as the piston is still on it's way up. What you are after is trying to get the piston and spark to coincide at TDC. Too little initial advance and you are wasting energy. The most power comes when the gases are in the most compressed state, sort of like dynamite. The tighter it is packed the more power it will make. Every engine and combination requires a different setting. There is no one setting for all cars and trucks. It requires a lot of trial and error to properly tune the initial setting.


Centrifugal (Mechanical) Advance:

Centrifugal advance and mechanicl advance mean the same thing, the amount of timing advance built into the distributor. This is the total amount that the distributor can rotate internally and thus increase the ignition timing. This amount is controlled by springs and weights mounted on the top of the distributor shaft, under the rotor. As the speed of the distributor increases the weights have a tendency to fly out and when this happens the other side of the weights push on a center cam and turn the distributor shaft. The total amount it can increase the timing is controlled by the shape of the weights. Usually it is around 10-11 degrees, or 20 to 22 degrees at the crankshaft. Why is it doubled at the crank? Because the distributor, which is driven by the engines cam, turns at 1/2 the speed of the crankshaft because of the difference in the size of the timing gears. If they turned the same speed the amount of advance would be the same at the distributor and crank. But because the crank rotates twice as fast as the cam it is doubled. A simple way to explain this is, if you turn the cam one complete revolution, the crank will rotate 1/2 revolution. So for one complete revolution of the crank, the cam will rotate 2 full revolutions.

In order to keep the weights from flying out all the time by themselves they are held closed by two springs. You can change the amount and speed at which this timing can happen by changing the weights and springs. On HEI distributor's, it is best to leave the original weights and just change springs. The amount of advance on these weights is as much as you will need for the street. Just make sure the old weights are not worn where they fit on the pins they rotate on. They have a tendency to wear the hole into an oval shape, which will in effect will increase the amount of timing they have, not good. For some real good reading on how to set up your mechanical advance for performance, check out this article by Damon Nickles.

What does all this mean? It simple terms it means that the faster the engine turns, the more timing advance you need in order to light the mixture sooner. If you didn't have this built in advance in the distributor the timing would be so late that the piston would already be down at the bottom on the power stroke when the plugs fired.



Vacuum Advance:

Vacuum advance is for economy and to help engines run cleaner on the street and help reduce emissions. Drag cars usually won't have any vacuum advance because they are at wide open throttle (WOT) and won't have any manifold vacuum anyway. The vacuum canister is hooked to the pickup coil in an HEI distributor and a hose is hooked to a manifold vacuum source. When manifold vacuum is applied to the canister it will rotate the pickup coil and increase the timing. The amount of vacuum advance you have is built in the canister. There are different canisters with different amounts of advance built into them. Also you can buy adjustable canisters so you will be able to adjust when and how much vacuum advance you get. Again, I recommend reading the above article by Damon for more on this subject.


In ending this article, I hope you find it helpful. I have tried to explain it so it is easy to understand. If you are knowledgeable on this subject you will probably disagree with some of the things, and some of the ways I explained them. Again, let me state, I wrote this for readers who do not fully understand the basics of ignition timing. I think that this article and the other articles I recommend to read will fulfill the requirements.


Happy Motoring,

Mike

 
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02-24-02, 10:37 AM   #16  
0patience
Hi Joe,
Knuckles is in PA.
I'm in OR. LOL!

 
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02-24-02, 03:20 PM   #17  
Joe_F
Whoops .

OK Knuckles, where in PA are you? Recognize the towns where my car has been ?

0patience: I have a customer that is into old cars and has a good shop out in your neck of the woods in Portland. He is one hell of a nice guy.

Steve Rothenbucker Auto and Truck Repair. Wonder if you know him?

 
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02-24-02, 04:31 PM   #18  
crick
thanks you guys for all the help,i was beginning to think a fight was about to break out!right now with this new cam ,i dont have any vaccum(very little) i do have the adjustable canister on my distributor,its at the factory settings right now,how does that work by the way,can it be adjusted to work off of a lower amount of vaccum? and as far as where the vac advance line,im going with the lower port,on this particular engine,when its removed while engine is running,the engine immediately shuts off.

 
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02-24-02, 09:04 PM   #19  
0patience
Joe, don't know that shop.
Thought I knew most of the truck repair shops in Portland.
I live 100 miles west of Portland. Can't go any further west without swimming in the Pacific. LOL!

 
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02-25-02, 03:44 AM   #20  
Joe_F
Lol.

He is a good guy that seems like an old time mechanic. Very well spoken and a helluva intersting guy to chat with.

We guff all the time on the phone and shoot the breeze. He's into old cars like me and likes to work on old stuff. He takes his time and goes through a car soup to nuts.

 
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