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  1. #1
    FEP Super Member mmb617's Avatar
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    Default Can't get the idle lower, Holley carb

    This ones got me scratching my head and it's probably something obvious.

    I can't get my idle speed below 1400! I don't mean that the car won't keep running with the idle set lower, what I mean is even with the idle speed adjustment screw backed all the way out till it's not even touching the linkage the idle doesn't go any slower. The car is running fine other than that problem. I checked to be sure the choke is fully open and made sure nothing is binding.

    This is a Holley 4150 carb, 750 cfm and it's only about a year old. I didn't have this problem last year and I haven't done anything to the motor or carb over the winter other than set the idle mixture adjustment with a vacuum gauge (wound up at 1 3/4 turns out on all four), and reset the float levels, which are if anything, a hair low.

    So lets hear your ideas.
    408/T5/3.73's

    We're not fast racers, we're more what's known as half fast racers.

  2. #2
    FEP Super Member PaceFever79's Avatar
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    If you have no vacuum leaks, It sounds like your secondaries are open.

    I'd try running the primary idle screw in until it opens the blades about .040 (just a crack)

    Then start the car (it will probably be idling very high now)

    Then turn the secondary stop out until the idle starts to come down to the desired level.

    Then fine tune the idle with the primary.

    You may need to reset the idle mixture screws and repeat the process.
    Last edited by PaceFever79; 03-11-2009 at 05:40 PM.

  3. #3

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    the screws on the throttle plates are not for adjusting idle. they are a set adjustment during a rebuild. the grooves cut in the venturis are supposed to be visible the same height as width, with the plate held upside down. the idle should only be adjusted with the idle mixture screws on the metering block. depending on model, some have 4 corner idle control, some have idle control just with the front block. i would recomend resetting your throttle blades back to where they were originally, and start with your idle mixture screws about 1 1/2-1 3/4 turns out, and go from there.
    79 Capri RS
    341 rwhp,348 rwtq 11.20@127


    1939 Pontiac Deluxe 6. 73,xxx original miles. Barn find. Currently the secomd owner.

    2013 Focus ST
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  4. #4
    FEP Super Member PaceFever79's Avatar
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    Quote Originally Posted by varn82 View Post
    the screws on the throttle plates are not for adjusting idle. they are a set adjustment during a rebuild. the grooves cut in the venturis are supposed to be visible the same height as width, with the plate held upside down. the idle should only be adjusted with the idle mixture screws on the metering block. depending on model, some have 4 corner idle control, some have idle control just with the front block. i would recomend resetting your throttle blades back to where they were originally, and start with your idle mixture screws about 1 1/2-1 3/4 turns out, and go from there.
    I was going to say that (really) but I went for the open secondary. Good call

  5. #5

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    i think the original thread was about garage gremlins, and having the timing retarded 10 degrees back from where it was thought to be. if the carb was adjusted and then the timing advanced 10 degrees, the idle will come up as described.
    79 Capri RS
    341 rwhp,348 rwtq 11.20@127


    1939 Pontiac Deluxe 6. 73,xxx original miles. Barn find. Currently the secomd owner.

    2013 Focus ST
    2012 Taurus SEL
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  6. #6
    FEP Super Member mmb617's Avatar
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    Quote Originally Posted by varn82 View Post
    the screws on the throttle plates are not for adjusting idle. they are a set adjustment during a rebuild. the grooves cut in the venturis are supposed to be visible the same height as width, with the plate held upside down. the idle should only be adjusted with the idle mixture screws on the metering block. depending on model, some have 4 corner idle control, some have idle control just with the front block. i would recomend resetting your throttle blades back to where they were originally, and start with your idle mixture screws about 1 1/2-1 3/4 turns out, and go from there.
    I think we're talking about two different screws here. What I'm trying to adjust the idle with is the curb idle adjusting screw:



    The Holley tuning video says "the curb idle adjusting screw sets the idle speed by opening and closing the primary throttle plate". I did not adjust any screws that are internal to the carb, and in fact have not had the carb removed.

    I don't really think the idle mixture screws were meant to adjust idle speed, as the video says they should be set for highest vacuum and makes no reference to idle speed.

    PaceFever79, where is the secondary stop screw located? The only one I see in the video is only accessible from the bottom with the carb removed.

    if the carb was adjusted and then the timing advanced 10 degrees, the idle will come up as described.
    You are correct that I found my timing was retarded and brought it up 10 degrees, and that did raise the idle. But why can't I adjust the idle back down? What I'm trying to say is that no matter how far I back out the curb idle adjusting screw, the idle doesn't drop down. The screw is no longer even contacting the linkage so the primary throttle blade should be closed as far as possible.

    I do think that for some reason either the primary or the secondary throttle blades must not be closing completely. As soon as I get a chance I'll pull the carb and see if I can find out what the problem is.

    I hope that neither of you thinks that I don't appreciate that you're trying to help, but I guess we're just not communicating on the same page.
    408/T5/3.73's

    We're not fast racers, we're more what's known as half fast racers.

  7. #7

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    not a problem at all, just trying to get on the same page as you are. i think holley has possibly tricked you with a play on words. that screw you are talking about is also what i am talking about. i just went through an idle adjustment on my 650 double pumper 2 weeks ago, after an intake manifold change. the curb idle screw is a "basic setting" that is set when the carb is manufactured, and also during a rebuild. this can be adjusted slightly after the car is running for tuning purposes, to take out a stumble, or hesitation off idle. the idle mixture screws are just that. they are used to adjust idle speed depending on what engine combination you are running. alot has to do with where these are set. put it this way. my car idled at 600 rpm. i removed the intake, and did some port work to it. thats the only change i did to the engine. i removed alot of meterial to the tune of 10 hours of grinding and polishing. i reinstalled the intake, and the carb. and the car would not idle. it would slowly drop to 200 rpm, then die. i adjusted my idle mixture screws on the metering block (older 650, no 4-corner idle) and brought the idle back up to the my desired 550-600. just try resetting the throttle blades where they were, and setting your mixture screws at 1.5-1.75 turns out and go from there. i think you will be pleasantly surprised.
    79 Capri RS
    341 rwhp,348 rwtq 11.20@127


    1939 Pontiac Deluxe 6. 73,xxx original miles. Barn find. Currently the secomd owner.

    2013 Focus ST
    2012 Taurus SEL
    1997 F350

  8. #8
    FEP Super Member PaceFever79's Avatar
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    Default

    I think there is a few things going on here.

    1) The idle mixture will have drastically changed from the timing change (it should be re-calibrated post timing change)

    2) With a performance cam, setting the idle mixture by vacuum is a waste of time (because of cam overlap)

    3) The primary and secondary blades should be roughly .040 open by the set screws (this is barely cracked open)

    4) Once the timing and set screws are done, adjust the idle speed via the mixture screws (all of them in concert)

  9. #9
    FEP Super Member PaceFever79's Avatar
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    Default

    PS - Your carb may not have secondary stop adjusters (I can't remember) if not just set the primary to .040 open (like 1.5 turns)

  10. #10

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    Quote Originally Posted by PaceFever79 View Post
    PS - Your carb may not have secondary stop adjusters (I can't remember) if not just set the primary to .040 open (like 1.5 turns)
    i think on my 650, i have to remove the base plate to adjust the secondarys, just a little flat head screw you cant see with the main body screwed on.
    79 Capri RS
    341 rwhp,348 rwtq 11.20@127


    1939 Pontiac Deluxe 6. 73,xxx original miles. Barn find. Currently the secomd owner.

    2013 Focus ST
    2012 Taurus SEL
    1997 F350

  11. #11
    FEP Super Member PaceFever79's Avatar
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    Default

    Btw, your secondary adjustment is probably okay, if you never touched it from factory setting.

    So I wouldn't worry about that unless you are still having trouble.

  12. #12
    FEP Power Member grtskydog's Avatar
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    Default

    The secondary adjustment screw is on the choke side of the carb. It is underneath the base plate and is recessed. You can see it sticking out the top of the throttle plate contacting the little tab on the secondary throttle shaft. It can be adjusted without removing the carb, you just have to have a tool that can get under there (one of those little 90 deg screwdriver ratchets works great).

    The "grooves" in the bores are called transfer slots. When looking from under the carb, the idle stops should be adjusted so that only a little of the transfer slots are showing under the throttle plates...so that they look square. This is just a starting point and you may still have to adjust the curb idle screw as needed.

    Also, if you have your vacuum advance hooked to manifold vac, it will idle higher.
    Ed

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  13. #13
    FEP Super Member mmb617's Avatar
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    Quote Originally Posted by grtskydog View Post
    The secondary adjustment screw is on the choke side of the carb. It is underneath the base plate and is recessed. You can see it sticking out the top of the throttle plate contacting the little tab on the secondary throttle shaft. It can be adjusted without removing the carb, you just have to have a tool that can get under there (one of those little 90 deg screwdriver ratchets works great).

    The "grooves" in the bores are called transfer slots. When looking from under the carb, the idle stops should be adjusted so that only a little of the transfer slots are showing under the throttle plates...so that they look square. This is just a starting point and you may still have to adjust the curb idle screw as needed.

    Also, if you have your vacuum advance hooked to manifold vac, it will idle higher.
    Your mentioning the advance made me think. I wasn't having this idle speed problem last year and I didn't change anything on the motor over the winter, but I did change the advance springs in the distributor.

    I have an MSD with no vacuum advance and I changed it from the slowest, heavy silver advance springs to the light blue springs. This should make the advance all in by 3k, where with the heavier springs it would have been 4k before full advance was in. I did not change the advance stop bushing so it would still give 21* of total advance. I have the initial advance set at 15*. I don't see how this change would affect the ability to set the idle speed down, but it's the only change to the motor since last year, so that makes me suspect maybe there's a connection.

    One more thought I'd like to throw out there. Could my problem be caused by too high of fuel pressure? If so it's kind of odd that I ran the same setup last year without this problem, but it's a thought.

    I have as fuel pressure gauge on the carb feed line and it bounces around between 7-9 lbs. The fuel pump I have says it has a maximum pressure of 8 lbs. I understand that Holleys like 6-7 lbs the best.

    I think I'm going to give the Holley tech line a call and see what they have to say about the whole situation. I'll let everyone know it I find out anything useful.
    408/T5/3.73's

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  14. #14
    FEP Power Member 306gt's Avatar
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    check the mechanical advance Not just the weights & springs. Make sure it is not sticking. My MSD mechanical advance froze up at 25*BTDC. at idle. The weights and springs were free it was the advance plate that froze. It would not advance any more and it would not come down to the 21* I had set the base timing at. The only way I found it was by hooking up a timing light the night before I went racing. The timing would not advance when I goosed the throttle. It had run fine up untill I found it. Only clue was sometimes a hard start-up when the engine was warm. The motor even ran fine with 25* of locked timing. Never back fired or even bogged.
    If the advance plate is frozen your idle can increase depending on where it froze.
    85 G.T. All motor
    337 c.i.d 11.44-120 mph

    1984 1/2 G.T. 350 (13.01-106 mph)

    1984 G.T. (Daughters car)

    1986 G.T. (Son's car) (12.99-105 mph)

  15. #15

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    any luck yet?
    79 Capri RS
    341 rwhp,348 rwtq 11.20@127


    1939 Pontiac Deluxe 6. 73,xxx original miles. Barn find. Currently the secomd owner.

    2013 Focus ST
    2012 Taurus SEL
    1997 F350

  16. #16
    FEP Super Member 82mustang's Avatar
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    i would remove the carb and go over it.. you might find something mechanically wrong...
    86' Coupe Street/Strip Never Ending Project-351w, 4 Wheel Disk, WC T5, etc...

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  17. #17
    FEP Super Member mmb617's Avatar
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    Just an update on where I'm at with this problem. I tried calling the Holley tech line, but after 15 minutes or so on hold without ever speaking to a human, I got tired of listening to the commercial they play in lieu of hold music and hung up.

    The mechanical advance on the distributor is working properly as I've put the light on it and as I rev the engine the timing advances as it should, and when I let off the throttle the timing drops back to it's base setting.

    After some thought I see it this way. There's only three settings I've changed recently on the carb, float levels, idle mixture, and curb idle adjustment. The problem should be one of, or a combination of, these three things. If I take my time and work with these three adjustments, I should be able to fix it.

    The only other possibility that comes to mind is that the fuel pressure is a little higher than Holley recommends, although it seems odd that the problem didn't surface last year if that's it since I had the same setup. Just in case I have a fuel pressure regulator on it's way, should be here Monday. I'll install that before I do anything else.

    I have two weeks left till track opening day and I'm hoping to have this figured out before then. The seat-o-pants-ometer says the car is pulling fine at WOT, although I won't know for sure how well till I see a time slip, so if by chance I don't get this squared away before then it won't keep me from racing. It just bugs me because I want to know the why of it.

    I don't think I'll get a chance to do anything with the car this weekend. We had a big windstorm a few weeks ago that tore up a big piece of the stockade fence we have around the backyard and I've had it temporarily propped up to keep the dog in. So today and tomorrow will be spent replacing 50-100 feet of fencing with a chain link fence. Several of the fence posts are snapped off so this will be a pretty big job.

    I would like to take a moment to thank my friends here at FEP for all the helpful suggestions. I will keep you posted.
    408/T5/3.73's

    We're not fast racers, we're more what's known as half fast racers.

  18. #18
    FEP Super Member PaceFever79's Avatar
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    It is the idle mixture screws.

    Just open the primary throttle screw 1 full turn.

    Then adjust the idle mixture screws in until the idle comes down.
    Last edited by PaceFever79; 03-14-2009 at 07:49 AM.

  19. #19

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    This is a much too simplistic an approach. It presumes that the amount idle air
    opening an engine wants is a constant, and idle speed is a function of mixture.

    The problem with this is, the amount of idle air opening an engine wants is never
    a constant, and idle mixture can only be made so lean before misfiring sets in,
    unless we're working on a Diesel engine here.

    The proper amount of idle speed screw and idle mixture screw is a balancing act.
    One affects the other, and both must be adjusted to arrive at the optimum mixture
    and idle RPM.

    mmb617, from what you've told us, it's pretty evident that you have a source
    of additional idle air that you're not aware of. Engines with larger than stock
    cams usually have an opposite problem from the one you're dealing with.

    That additional idle air could be coming from just about anywhere, including the
    carburetor itself. You said the choke is fully open, but have you made sure that
    the fast idle cam/screw are not still holding the throttle open after the choke
    opens up? The fast idle screw should not contact the fast idle cam at all once
    the choke is open. Don't assume that because the choke is open, you're off
    the fast idle cam. They are somewhat independent of one another.

    Exactly how much transfer slot do you have exposed with the throttle fully closed
    with the choke open?

    Have you tried blocking off all vacuum feeds to accessories, including the brake
    booster? Have you blocked off the line to the PCV valve? Do you still have the
    evaporative emissions stuff hooked up? If you have a vacuum advance, have
    you tried disconnecting it and blocking the hose?

    You're going to have to do some diagnosing here. Anyone who gives you a magic
    bullet is just guessing, no matter how educated. The thing to keep foremost in
    your mind is, an engine cannot idle faster without additional air coming in from
    somewhere. Period. Your job here is to figure out where that air is coming from.

    I'm guessing it's likely as not going to be something simple that's being overlooked,
    so don't take any possibility off the table until proven.
    Last edited by JACook; 03-14-2009 at 02:46 PM.
    Cheers,
    Jeff Cook

    '85 GT Hatch, 5-speed T-Top, Eibachs, Konis, & ARE 5-Spokes ... '85 GT Vert, CFI/AOD, all factory...
    '79 Fairmont StaWag, 5.0, 62K original miles ... '04 Azure Blue 40th Anny Mach 1, 37K original miles...
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  20. #20
    FEP Super Member PaceFever79's Avatar
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    If he cranked those mixture screws way out to get max vacuum (as he stated) that could be
    the source of extra air intake, couldn't it? That's what I'm thinking. It's easy to get tripped up
    tuning a carb for max vacuum with a high overlap cam.

  21. #21

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    I agree it's easy to get tripped up looking for max vacuum with a big cam. But the
    only way tweaking the idle mix screws influences air intake, is a result of the higher
    manifold vacuum. The idle mix screws are adding in a small amount of air, emulsified
    with the idle fuel, but it's not enough by itself to account for the high idle speed.

    More likely is, he needed to open up the mixture screws to compensate for the
    additional air that's coming in. At this point though, it's chicken vs egg with the
    information we have to go on.
    Cheers,
    Jeff Cook

    '85 GT Hatch, 5-speed T-Top, Eibachs, Konis, & ARE 5-Spokes ... '85 GT Vert, CFI/AOD, all factory...
    '79 Fairmont StaWag, 5.0, 62K original miles ... '04 Azure Blue 40th Anny Mach 1, 37K original miles...
    2012 F150 S-Crew 4x4 5.0 "Blue Coyote"... 65 coupe, 289 auto, Pony interior ... '67 coupe 6-cyl 4-speed ...
    '68 Vert, Mexican block 307 4-speed... '71 Datsun 510 ...
    And a 1-of-328 Deep Blue Pearl 2003 Marauder 4.6 DOHC, J-Mod, 4.10s and Lidio tune

  22. #22
    FEP Super Member mmb617's Avatar
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    Quote Originally Posted by JACook View Post

    mmb617, from what you've told us, it's pretty evident that you have a source
    of additional idle air that you're not aware of. Engines with larger than stock
    cams usually have an opposite problem from the one you're dealing with.

    That additional idle air could be coming from just about anywhere, including the
    carburetor itself. You said the choke is fully open, but have you made sure that
    the fast idle cam/screw are not still holding the throttle open after the choke
    opens up? The fast idle screw should not contact the fast idle cam at all once
    the choke is open. Don't assume that because the choke is open, you're off
    the fast idle cam. They are somewhat independent of one another.

    Exactly how much transfer slot do you have exposed with the throttle fully closed
    with the choke open?

    Have you tried blocking off all vacuum feeds to accessories, including the brake
    booster? Have you blocked off the line to the PCV valve? Do you still have the
    evaporative emissions stuff hooked up? If you have a vacuum advance, have
    you tried disconnecting it and blocking the hose?

    You're going to have to do some diagnosing here. Anyone who gives you a magic
    bullet is just guessing, no matter how educated. The thing to keep foremost in
    your mind is, an engine cannot idle faster without additional air coming in from
    somewhere. Period. Your job here is to figure out where that air is coming from.

    I'm guessing it's likely as not going to be something simple that's being overlooked,
    so don't take any possibility off the table until proven.
    Jeff, thank you for your response. I've gone back and checked a couple of the things you mentioned. With the choke open the fast idle screw is not touching the fast idle cam at all.



    I'll admit I don't know how much of the transfer slot should be exposed with the throttle closed so I tried to take a picture. I hope it's clear enough for you to see.



    There are only three vacuum ports on the carburetor. Both the timed port and the small one on the front of the base plate are capped as the car has no vacuum advance, emissions, or pcv (breathers in both valve covers). The large one in the back runs to my brake booster and has a newer hose which looks to be in good shape.






    The cam in the car is not very big at all with 232/236 duration at 0.50, and I ran the car half of last season with the same cam without this problem.

    As for the idle mixture screws, they are not cranked in very far, all four are at 1 3/4 turns out and Holley recommends 1 1/2 turns as a starting point.

    Your comment about extra air has to be entering from somewhere makes sense. Let me ask this. In order for the extra air to cause a high idle wouldn't it have to me coming through the carb somewhere? If I had a vacuum leak somewhere in the intake manifold wouldn't that just cause a lean condition since it wouldn't be pulling extra fuel with it, or is my logic faulty there? I ask this because I did have the intake manifold off this winter to fix a pesky oil leak in the back. It's not leaking oil anymore and to the best of my knowledge isn't leaking vacuum either.

    As always your input is appreciated.
    408/T5/3.73's

    We're not fast racers, we're more what's known as half fast racers.

  23. #23
    FEP Power Member 306gt's Avatar
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    Take some carb cleaner and spray around the intake, see what happens. Pinch off the brake booster hose too, and see what happens.
    85 G.T. All motor
    337 c.i.d 11.44-120 mph

    1984 1/2 G.T. 350 (13.01-106 mph)

    1984 G.T. (Daughters car)

    1986 G.T. (Son's car) (12.99-105 mph)

  24. #24
    FEP Power Member grtskydog's Avatar
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    To look at how much transfer slot is showing, you have to remove the carb and look how much is showing under the throttle plates. That said, I really doubt that's your problem, especially if it was fine before.
    Ed

    "The Dude abides."

  25. #25

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    The additional air does not have to be coming in through the carburetor. At idle,
    the throttle blades aren't much more than a controlled vacuum leak. The carburetor
    just adds in emulsified fuel through the idle passages. It can still do that if you've
    got air getting in from somewhere else. A less brute-force carburetor would have
    a pretty severe off-idle stumble under those conditions, but I can't say for sure
    whether that double-pumper would ever notice.

    Since you have very little connected up to manifold vacuum, it should be pretty
    easy to eliminate the obvious suspects. After that, it gets a bit more interesting.

    If blocking off the brake booster hose doesn't settle down your fast idle, the
    next thing I would do is block off those valve cover breathers, and see if anything
    is trying to pull a vacuum on the crankcase.

    BTW, once you get this sorted out, you really do need to consider running either
    a PCV system, or pan evac to the headers. It's really not healthy for your engine
    not having anything to suck out crankcase vapors.
    Cheers,
    Jeff Cook

    '85 GT Hatch, 5-speed T-Top, Eibachs, Konis, & ARE 5-Spokes ... '85 GT Vert, CFI/AOD, all factory...
    '79 Fairmont StaWag, 5.0, 62K original miles ... '04 Azure Blue 40th Anny Mach 1, 37K original miles...
    2012 F150 S-Crew 4x4 5.0 "Blue Coyote"... 65 coupe, 289 auto, Pony interior ... '67 coupe 6-cyl 4-speed ...
    '68 Vert, Mexican block 307 4-speed... '71 Datsun 510 ...
    And a 1-of-328 Deep Blue Pearl 2003 Marauder 4.6 DOHC, J-Mod, 4.10s and Lidio tune

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