Dual 450 Idle Problems
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Dual 450 Idle Problems

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    79 Sanger Picklefork Tittyman's Avatar
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    Default Dual 450 Idle Problems

    Dual 450'S (I know)..I can't get them to idle down. If I set the curb idle while running on the water I can get it to idle fine. But when I get on the throttle for a short time and them let off it wants to idle high. When I set the idle too low so it almost wants to die and then get on the throttle and let then it idles down right away to the too low setting. The throttle plates were drilled out years ago but I've always had this problem. I messed with the secondary throttle plates and it didn't like that. I left the curb idle where it's at and just upped the secondary plates about 1/8 turn. It didn't like that..Has a mag that works great and does not stick..big lope cam..I assume I'm in well into the transition slots..floats and fuel pressure are perfect..can I set the rear carb so it's closed more or what....thanks.
    Last edited by Tittyman; 07-12-2013 at 08:20 AM.
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    Or Seth, either one Budweiser's Avatar
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    Is the engine is warmed up and in gear when you set idle and are you absolutely sure there's no sticking mechanical advance issues in the mag?

    When it's idling the way you like, are you able to manipulate idle quality and speed with the idle mixture screws?

    What is the manifold vacuum reading at idle?

    Can you further explain how the engine responded when you opened the secondary's a bit?

    After a short cruise and it's idling high, are both carbs returning to their full idle position? In other words can you get them to idle back down if you manipulate the linkages by hand?

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    79 Sanger Picklefork Tittyman's Avatar
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    Quote Originally Posted by Budweiser View Post
    Is the engine is warmed up and in gear when you set idle and are you absolutely sure there's no sticking mechanical advance issues in the mag?

    When it's idling the way you like, are you able to manipulate idle quality and speed with the idle mixture screws?

    What is the manifold vacuum reading at idle?

    Can you further explain how the engine responded when you opened the secondary's a bit?

    After a short cruise and it's idling high, are both carbs returning to their full idle position? In other words can you get them to idle back down if you manipulate the linkages by hand?
    I am sure there is not problem with the mechanical advance...I can adjust the idle mixture screws...I have 2.5 pv...after short cruise both carbs are returning back to the original curb idle setting but running fast..the linkage is fine and both carbs return to the original position...after I idle for a couple of minutes it will finally idle down..I know it sounds like a mechanical advance problem but it is not..the engine is warmed up when this happens..I'm thinking I'm into the transition slots and not sure how to handle that problem..thanks..
    "Bill, if your gonna ride my ass that hard, at least you could pull my hair"....Hillary Clinton

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    Or Seth, either one Budweiser's Avatar
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    Quote Originally Posted by Tittyman View Post
    I'm thinking I'm into the transition slots and not sure how to handle that problem..thanks..
    Being able to manipulate idle with the mixture screws is a good indication that you're not too far into the transfer slots, but it could be that it's right there at the edge. One way to find out. Pull the carbs off and take a look. The portion of the transfer slot showing below the throttle blades should be pretty much square, as much uncovered as the slot is wide.

    While you have them off take some pictures of the transfer slot and modified throttle blades and post 'em up.

    What is the manifold vacuum reading at good idle and what is it when it idles high? What is your target idle RPM?

    Can you further explain how the engine responded when you opened the secondary's a bit? Did you also close the primary a bit?

    How did you eliminate mechanical advance as a possibility?

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    79 Sanger Picklefork Tittyman's Avatar
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    Quote Originally Posted by Budweiser View Post
    Being able to manipulate idle with the mixture screws is a good indication that you're not too far into the transfer slots, but it could be that it's right there at the edge. One way to find out. Pull the carbs off and take a look. The portion of the transfer slot showing below the throttle blades should be pretty much square, as much uncovered as the slot is wide.

    While you have them off take some pictures of the transfer slot and modified throttle blades and post 'em up.

    What is the manifold vacuum reading at good idle and what is it when it idles high? What is your target idle RPM?

    Can you further explain how the engine responded when you opened the secondary's a bit? Did you also close the primary a bit?

    How did you eliminate mechanical advance as a possibility?
    I think I'm right on the edge...I know it's not the advance because if I lower the curb idle it will idle right down every time..I just have to use the foot pedal to keep up the idle...When I opened up the secondary's the motor started running rough, but I did not reduce the curb idle to see if it would level out. I'm going out this afternoon and will open the secondarys again and reduce the curb idle..I only opened the secondarys up about 1/8 of a turn. My target rpm is about 1100-1200..I'm no expert but it sounds like when I get off the gas it almost goes too lean for awhile, then after a while it will go rich and idle at a good rpm...unless I'm way off in my thinking..Air bleeds have long been cleaned up but you can see the additional hole in the blade.
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    Last edited by Tittyman; 07-13-2013 at 08:51 AM.
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    Or Seth, either one Budweiser's Avatar
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    Quote Originally Posted by Tittyman View Post
    I know it's not the advance because if I lower the curb idle it will idle right down every time
    I'm not entirely convinced. If you allow less air in, the speed will reduce no matter what the timing is doing. One way to eliminate mechanical advance as the culprit is to put a timing light on it when it wont return to your desired idle and see if it reads the same base/initial timing. I assume you are running the lightest springs in the advance, usually all in by 2000-2500. In that case, your higher idle issue may become pronounced with the light springs. Your 1200 idle could hold it off base timing anyway.

    An even better way to eliminate it, is put some heavy springs in or lock the advance out entirely. Most would recommend you lock it out anyways, as long as the starter is still able to start it.

    While it's important to have the carbs set up right anyway, I'm leaning toward the advance being your issue at this point.
    Last edited by Budweiser; 07-13-2013 at 09:58 AM.

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    Quote Originally Posted by Budweiser View Post
    I'm not entirely convinced. If you allow less air in the speed will reduce no matter what the timing is doing. One way to eliminate mechanical advance as the culprit is to put a timing light on it when it wont return to your desired idle and see if it reads the same base/initial timing. I assume you are running the lightest springs in the advance, usually all in by 2000-2500. In that case, your higher idle issue may become pronounced with the light springs.

    An even better way to eliminate it, is put some heavy springs in or lock the advance out entirely. Most would recommend you lock it out anyways, as long as the starter is still able to start it.

    While it's important to have the carbs set up right anyway, I'm leaning toward the advance being your issue at this point.
    I have a Vertex Mag...it was completly gone through and I have a readout of all the settings...It is all in at 2500 rpm. Like I said when I back the curb idle down hit the throttle to say 3500 rpm and then let right off the throttle it idles way down just fine..but too low. I don't know if you noticed the hole in the throttle blade..Yesterday the Holley clowns said I should just drill a bigger whole until I can reduce the curb setting..not sure about that..
    Last edited by Tittyman; 07-13-2013 at 10:07 AM.
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    Quote Originally Posted by Tittyman View Post
    I have a Vertex Mag...it was completly gone through and I have a readout of all the settings...It is all in at 2500 rpm.
    Looking at the readout, what RPM does it start to advance?

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    79 Sanger Picklefork Tittyman's Avatar
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    Took the boat out yesterday...the secondary throttle plates were almost closed. If I closed them just a little more the motor wanted to idle rough. I opened the secondary plates between 3/4 and 7/8 turn. Motor picked up a lot of RPM so I was able to back down the curb idle. It's still not perfect but now after say 3500 RPM it takes a much shorter time to idle back down. Not sure how much I can open the secondaries before I run into other problems..Looking at my mag sheet I have 20* at the crank and all in by 2500 RPM. It is no an advance issue. Another question, if I opened up the secondaries do that change the issue of the mixture screws and need to adjust those also..motor seems to idle fine.
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    Quote Originally Posted by Tittyman View Post
    Looking at my mag sheet I have 20* at the crank and all in by 2500 RPM.
    Looking at the readout, what RPM does it start to advance?

  13. #11
    steelcomp was here
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    How big is the engine, what are the cam specs?
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    Anytime you make a adjustment to the pri/sec. blades, you gotta get back onto the mix screws. At least to see if you can make it a little better. 450s are awsome carbs and work on just about anything. BTW get blades with no holes in them.

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    Quote Originally Posted by Budweiser View Post
    Looking at the readout, what RPM does it start to advance?
    Doesn't say when it starts, just all in at 2500 RPM
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    Quote Originally Posted by steelcomp View Post
    How big is the engine, what are the cam specs?
    454, 12.5 to 1, don't have the cam specs but it is major over-lapping
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