Engine and Transmission Tech

I thought I was a pretty good crab tuner but I am struggling with my new 5.0 Ford swap. The motor has a new 1405 Edelbrock carb on a performer intake. Stock Mustang GT cam with 1.7 rockers. Ignition is a new points style distributor with pertronix Ignitor and 35000 volt coil. The car idles great at 800 RPM with 20 inches of vacuum. It also runs pretty good a WOT. At part throttle, 3000 RPM or so, it runs rough and if you hold it there for a minute or two it will backfire in the exhaust when you let off the throttle. I can feel the secondaries kick in at WOT, which usually means the primary circuit is lean but fattening up the metering rods makes no difference. Leaning it out also doesn't seem to make any difference. I assume the ignition is ok since the motor pulls well at WOT up to about 5700 RPM.

Any sage advice?

Nevermind, wrong reply copied.

Double check your ignition.



Edited 1 time(s). Last edit at 10/06/2014 10:32AM by MGBV8.

Is that something you do before dunking them in a pot of boiling water and then eating them? What'll they think of next?

Some carbs are known for leaning out in the midrange, especially under no load, so if this is sitting still you may want to see what it does when driving.

Jim

I think we all need to go EFI. I'm about sick of these Edelbrock/Weber carbs!

Keith,

have a look at your firing order.
Ford has two separate firing orders depending upon your cam.
I've seen this before where the engine actually has two dead cylinders but appears to run OK.
Your firing order should be 13726548 with the gt cam. The other possibility is 15426378.
Cylinders are numbered differently than most as well. Cyl #1 is the front right cyl (passenger side in NA) followed by 234 down the same side.
Cyl # 5 is the front cyl on the left (drivers) followed by 678.
Hopefully it's just that simple.

Cheers
Fred

OK youll never guess what was wrong. After spending 300 bucks on a new carb that behaved just like the rebuilt one, i went back and looked at my ignition again. I had eliminated it because it ran good at WOT. After looking at pictures of the Pertronix Ignitor kit and getting some info from the MGB forum; I realized my distributor did not comer with a magnetic pick up. its completely missing. It amazes me that the car runs as good as it does....

Unfortunately, I didint need a new carb and Ive had the dizzy so long that the seller probably wont help me out, but I am sure once i get the magnetci pick up from Jegs; all will be well.

I'm amazed that it ran at all.

Something is not making sense. Is it triggering off the points lobes of the distributor?

Any pics we can peruse?



Edited 1 time(s). Last edit at 10/07/2014 01:44PM by MGBV8.

What version of pertronics module are you using Keith?
The P-III doesn't use a magnetic pickup anymore.
Are you using the vacuum advance can? If so you might be experiencing a rotor phasing issue within the distributor.
This causes the rotor to favor the wrong cylinder as the advance can engages.

Live like you mean it.
Fred

Quote:

Is it triggering off the points lobes of the distributor

That's how my old Crane electronic ignition worked - used Hall effect principle as it passed by the cam lobes (no magnetic pickups) - the electronic module is sensitive to clearance gap to the cam lobes, so you might want to check (think it was something like 10-12 thou) .
With the Crane, I eventually got to a condition where the vacuum advance mechanism actuation would distort the clearance gap as it operated.(not sure if the centre pivot plastic ring was wearing or distorting) and I discarded the system.
This past Winter, I switched to an original Duraspark system and it's working like a charm..