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Old 05-14-2012, 01:56 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, 02:47 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, 07:30 AM
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Is this a 5peed LX or a auto tans
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Old 05-14-2012, 08:02 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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Old 05-14-2012, 08:15 AM
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Thats a big cam for a 302. Does not like a low compression ratio.

10:1+ yes need at least 4.10's. The motor wants to rev especaily with a simgle plane. 4.30 4.56 is not too much.

This will help the idle and low speed throttle response some.

recurve the distributor for 24 to 26deg base at idle and 34deg at max advance.
shorten the mech adv curve from 20 to around 10deg to allow this.
The big cam needs the extra timing at idle to idle clean.
The carb may need work.

Why not just change the cam to work with the 3.55's

212 to 224@.050 and a rpm manifold would be a lot better for how you use the car.
Nothing worse than being over cammed with a small motor and modest gears.

I'd change the cam to a summit 3601 or Howards 210031-12

Last edited by F-BIRD'88; 05-14-2012 at 08:23 AM.
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