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fine tuning with big cam????

6433 Views 6 Replies 6 Participants Last post by  camachinist
I was trying to dial in my Boss 302 today, and the cam has .639 lift, 296 duration, 260 @ .050. Here is my question, when I try to dial it in, I set the timing to 16 deg BTDC then I can't get it to idle, then I advance the timing and I can get an idle, but then when I adjust the fuel mixture the thing is thumping around so much I can't really identify if I am improving the tune or not. After screwing around with it for a little while it is now blowing smoke out the exhaust and I still can't tell if it is running any better.

Any ideas on how to tune with a large cam? Or do I just keep going on the trial and error method.

Thanks again
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You don't actually think a 302 (Boss or not) with 260 degrees of duration is going to idle well?
Where is the lowest rpm you can get at an idle?

What carb is on it? If its a Holley you might need a lower (much lower) power valve on it.
Bob first check what amount of vacumn the motor is making,then make sure the power valve in the carb is rated 1-2 inch,s lower example 8" of vacumn a 6.5 power valve you do not want the power valve open at idle.Also it can help to crack the rear barrels open a small amount to get a better idle.(there is a adjusting screw on the bottom of the throttle plate)I am running almost the same cam in my Boss and it idles very good. Robert 0F02G133327
The carb your using may not have an idle circuit that is large enough to support the cam. I also have a big cam in a 302 (except mine is a W motor) and had to give up on my 625 Demon and put on the racing version of the 650 Holley which has a large idle circuit just for big cams.

Drilling the throttle shafts on the secondary side may help the carb your using. If your using a Holley, you might consider sending the carb to Bigs and let them set it up for your application.

Don't rule out the possibility of a vacuum leak. Torque the intake again just to be sure.

What carb are you using?
I should have said: drill the plate, not the shaft. But do some reading before you do this. /forums/images/icons/blush.gif
You will most probably have to crack open the secondary throttle blades. If you can't get enough air that way which is probable you will also need to drill holes in the secondary throttle blades. Set your basic idle up using the secondary throttle blades and with not much idle speed screw. Then later if you want more you can use the idle screw to bring it up some.

Where timing is concerned you probably need more idle than normal. This is likely to be causing the timing advance to come in. When it does your idle will go up and you'll be running on the primaries for a bit then it will fall back on the idle mixture screws then up and down and up and down(called hunting). You need a distributor that will let you set your advance so it comes in later like around 1200 rpms. The MSD distributors do this. I'm not sure which all else ones do.

Since you have more air you need more fuel so you may have to turn your idle mixture screws way out. I think mine are 3 or 4 turns out from stop.

The idle vacuum / power valve suggestion needs to be checked also.

Lastly, if you have an auto with a tight converter just forget it. You'll never get it to idle right. Putting it in gear will kill the motor. Also, the suggestion about the 477x series carbs is a good one. They are calibrated for motors such as what you have and adjust to it better.
Later on after you get it to run we will probably be discussing pump cams and squirters too, LOL. Good luck.
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hehehe...gotta love those big cams..

What carb make and model? Single or dual quad? Manifold make and type? carb spacer(s)? Engine static C/R? Ignition system configuration? Entire camshaft specs (including lobe seperation)?

Without knowing the details, from personal experience I can tell you the best way to improve your idle with that cam is to run a single 4V carb set up with a secondary idle circuit; use a plenum divider if running a single plane manifold; run a ton of compression (13:1 comes to mind) and a high quality ignition system (like a crank trigger/MSD billet distributor with a MSD 7 and appropriate coil)

Get us the specific (and I mean very specific) details on your combo and maybe we can help out...
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