Help with tuning issue SBC
04-23-2018, 05:29 PM
#1
Burning Brakes
Thread Starter
Help with tuning issue SBC
I have a 350 out of a ‘72 Corvette. It’s in a ‘72 Nova SS but that’s not important...
Engine was rebuilt, balanced and blueprinted about 4 years ago. Other than an hydraulic RV cam, performer manifold and headers, the motor is stock. Heads have hardened valve seats. Engine was broken in by the builder on his test stand and the engine ran perfectly. Distributor was stock with Pertronix module.
After all this time, the Nova is just getting finished and we can’t get the motor to run properly. It will start and idle and it will rev up. The problem is that the engine does not idle smoothly and there is an intermittent backfire in the passenger side exhaust. It is totally random and does it whether the engine is cold or hot. I changed to an American Autowire Hall Effect module in the distributor but no change in the symptoms. 10 degree advance at idle. I got my infrared gun and found that there is a wide variation in the temps of the header pipes. 1,3,5,7 were all around 250 degrees. 2 was around 250 degrees. 4 was 215 degrees and 6&8 were 350 degrees. It seems to be very rich as the plugs are black and the smell of gas is very strong. The gas tank and lines are new and the gas is fresh.
The Q-Jet is off the ‘72 Vette and was sent out for rebuild to a specialist recommended by the engine builder but I can’t recall his name anymore. The engine ran perfect on the test stand with the same carb, ignition and headers.
Any ideas where the problem is? I can’t believe the problem is internal given the intermittent nature of the backfire.
Engine was rebuilt, balanced and blueprinted about 4 years ago. Other than an hydraulic RV cam, performer manifold and headers, the motor is stock. Heads have hardened valve seats. Engine was broken in by the builder on his test stand and the engine ran perfectly. Distributor was stock with Pertronix module.
After all this time, the Nova is just getting finished and we can’t get the motor to run properly. It will start and idle and it will rev up. The problem is that the engine does not idle smoothly and there is an intermittent backfire in the passenger side exhaust. It is totally random and does it whether the engine is cold or hot. I changed to an American Autowire Hall Effect module in the distributor but no change in the symptoms. 10 degree advance at idle. I got my infrared gun and found that there is a wide variation in the temps of the header pipes. 1,3,5,7 were all around 250 degrees. 2 was around 250 degrees. 4 was 215 degrees and 6&8 were 350 degrees. It seems to be very rich as the plugs are black and the smell of gas is very strong. The gas tank and lines are new and the gas is fresh.
The Q-Jet is off the ‘72 Vette and was sent out for rebuild to a specialist recommended by the engine builder but I can’t recall his name anymore. The engine ran perfect on the test stand with the same carb, ignition and headers.
Any ideas where the problem is? I can’t believe the problem is internal given the intermittent nature of the backfire.
04-23-2018, 09:05 PM
#2
Le Mans Master
If motor was rebuilt 4 years ago then it sat .carb could have build up of the old gas.the back firing and the different temp readings on the headers could be from the plugs be ing fouled out.I would look at the carb.JMO
04-23-2018, 10:28 PM
#5
Burning Brakes
Thread Starter
04-23-2018, 10:28 PM
#6
Burning Brakes
Thread Starter
04-28-2018, 07:37 AM
#8
Burning Brakes
Thread Starter
Well never assume that a mechanic knows what he’s doing. I assumed he had the plug wires plugged into the distributor in the right place... he was one terminal off. Got the plug wires in the right place and she runs good! Not perfect because the plugs are pretty fouled from running poorly, but that’s an easy fix.
He was convinced it was an internal problem with the motor, I was thinking of Lar’s statement that most running issues are traced to the ignition. Happy that Lar’s was right!
He was convinced it was an internal problem with the motor, I was thinking of Lar’s statement that most running issues are traced to the ignition. Happy that Lar’s was right!