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Old 05-14-2012, 02:56 AM
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Low rpm misfire.. carbed 306 large cam

I recently purchased a 90 mustang lx with a carbed 306. It has a larger cam in it.. this one to be exact:

http://www.summitracing.com/parts/SUM-3602/

The problem is this, at low rpm's say under 2500 rpms. The engine feels like Im hitting a rev limiter at part throttle until about 3500 rpms. If I floor it, it screams till 6500rpms. Ive never owned a car with a cam this wild so Im not sure if its "normal".

One of buddies did have a cammed 5.0 in his 91 mustang gt and it drove like stock, it just didnt have any power till 3000 rpms. It just felt sluggish till it got into the cams operating range.

As always you guys give great info, looking forward to hearing your responses.

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Old 05-14-2012, 03:47 AM
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It's difficult to diagnose these things w/o being there, but suffice to say in your small displacement engine that cam is going to be down on power below it's operating range. So that's normal. But there may be more there than what you're getting.

Be sure the plugs and ignition in general is in good shape. Most importantly the timing needs to be optimized for the cam- especially giving it enough initial timing, and the carb may need fine tuning, as well.

Get the ignition sorted out first. I'd use about 20 degrees BTDC initial timing and be sure the total timing doesn't get too high. If it does, the distributor will need work to limit the amount of mechanical advance so you can run enough initial advance.

You might find using an adjustable vacuum advance set to give you 10 degrees additional advance off idle (use ported vacuum in this case) will help drivability.

After the ignition is sorted out, use how the plugs look and performance in general to point you in the right direction for tuning the carb, if it needs it.
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Old 05-14-2012, 09:02 AM
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Quote:
Originally Posted by F-BIRD'88
Is this a 5peed LX or a auto tans
This is with a t5. I have 3.55 gears in the rear... probably should go with 4.10's, but the PO had just installed the gears a couple weeks before I got the car.

I called the guy who I got the car from and he told me it was all in the nature of the cam. Could having a single plane intake have something to do with the "loading up" that seems to be occuring? A guy at work told me that depending on the duration of the cam, It could actually be misfiring at a certain rpm range due to the fact that with so much overlap and not enough air/fuel mix at part throttle its loading up.

any truth to this?
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