New Member
May 4, 2020
Hi,i have a 1983 mustang gt with the original 302 H.O i just recently installed a set of gt40x heads a lunati 289/290 cam and i am having trouble getting it to run properly i have my timing set spot on with the timing marks on the crank and cam sprocket and distributor but it seems like it has a misfire and is popping out the carburetor and is hard to start and when it does start it idles at about 2200 rpm i believe its valve related but i adjust the push rod to 0 lash but it takes about 2 turns to get any type of torque reading i used the factory non roller pedestal rockers and factory length pushrods but on the ford performance website said it was perfectly fine if anyone has come across this and knows how to resolve it it would be greatly appreciated
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General karthief

wonder how much it would cost to ship you a pair
Mod Dude
Aug 25, 2016
polk county florida
Take a deep breath and start over.
check firing order, timing starting at tdc with the distributor rotor at #1. If that is right check the push rods, make sure they are not bent and you are adjusting them correctly.
This my help ya:
Same principle with a 83 carb engine as far as installing h/c/i


StangNet's favorite TOOL
SN Certified Technician
Mar 10, 2000
Dublin GA
The 2200 RPM idle is the clue: you either have a vacuum leak or an extremely lean air/fuel mixture.

Backfiring out the intake is either a valve stuck open or a lean mixture or spark plug wire(s) connected to the wrong cylinder(s). Check compression on all cylinders and then look for vacuum hoses loose, cracked, or misconnected. Check the line for the vapor recirculation system – it is easy to knock loose and not see it when you connect the air pump plumbing. If the vacuum line for the EGR valve and the air pump are cross connected, some very strange things can happen. Check the mass air flow electrical connection and see that it is tight, the same goes for the fuel injection wiring harness connectors up on top of the manifold near the firewall.

Sticking valves: If a intake valve is bent, has a bad spring or is misadjusted, the engine will sometimes backfire through the intake. Use a vacuum gauge connected to any convenient spot on the intake manifold. Run the engine at 1000 RPM & look for 18-21 inches of vacuum with a steady needle. A problem intake valve will make the vacuum gauge needle sweep 5-10 inches.

Lean fuel mixture breaks out into several sub categories:
A.). Vacuum leaks
B.) Air entering the intake without passing through the MAF on Mass Air cars (89-95 models).
C.) Failure of the MAF, BAP/MAP (Baro or Manifold Air Pressure, same sensor, different name), ACT (air charge temp), or ECT (engine coolant temp). These should set a code in the computer.
D.) O2 sensor problems: one or both O2 sensors with low output or bad O2 sensor heater ground. This should set codes 41/91. The O2 sensor heater ground is an Orange wire in the engine mounted fuel injector harness. Ground it to the back of the head or intake manifold.
E.) Leaking exhaust gases from EGR valve at WOT or EGR opening when it should not be open.
F.) Poor fuel delivery due to bad fuel pump, clogged filter or bad fuel pump wiring. Look for low pressure or fluctuating pressure. Standard injector pressure is 39 PSI at idle, with the vacuum line disconnected from the regulator and capped.
G.) Clogged fuel injectors.- see the cylinder balance test below
H.) Fuel injector wiring problems causing injector not to deliver rated flow (dirty or stuck shut injectors).
I.) Computer problems: (computer problems are not common like sensor problems)
J.). ROM has bad data in fuel or timing table. This should also set a code in the computer.
K.) Failure of one or more of the computer's driver transistors for the fuel injectors. No code set on this one. Use a noid test light to test the injector wiring & injector drivers,
L.) MAF calibration off or mismatched to injectors.
M.) ACT or ECT bad. Sometimes the sensors will be off calibration, but not bad enough to set a code. If they falsely read too high a temp, the engine will back off fuel delivery.

The HO firing order is 1-3-7-2-6-5-4-8.
Non HO firing order is 1-5-4-2-6-3-7-8

Vacuum leak due to slipped lower intake manifold gasket...

Ask Nicoleb3x3 about the intake gasket that slipped out of place and caused idle and vacuum leak problems that could not be seen or found by external examination. I don't care what you spray with, you won't find the leak when it is sucking air from the lifter valley. It simply isn't possible to spray anything in there with the lower manifold bolted in place.


General karthief

wonder how much it would cost to ship you a pair
Mod Dude
Aug 25, 2016
polk county florida
Jrichker posted that above referring to efi cars but the same principle applies to carbed engines with only minor variations.


StangNet's favorite TOOL
SN Certified Technician
Mar 10, 2000
Dublin GA
It is time to beg, borrow or buy a vacuum gauge to troubleshoot the problem. Most auto parts stores will rent or load one if you have a credit card.

Vacuum Gauge readings
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