1985 corvette - it back fires like pops through the exhaust and we did a tune up . it had split fire spark plugs and we put ngk's

If nothing else was removed, did you put the correct spark plugs in it?
Sometimes an off brand of spark plug will cause a problem. I would recommend NGK spark plugs.

Start off by checking inlet manifold gasket , change spark plugs, s NGK.BRAND good tune up, fuel filter replaced , check leads coil pack for cracks etc.

The firing order for the Jeep 2.8l engine is:1-2-3-4-5-6
and the locations are:
on the drivers side front to rear 2-4-6
passenger side front to rear is 1-3-5
If you look straight back from the water pump to the distributor splitting the capcylinder #1 is the first one to the left.

several things to check for this one....since its happening under a load.

- timing
- ignition (spark plugs, distributor, cap/rotor)
- Idle controller
- exhaust system

Tune Up

Spark Plug

AWSF-42C

Spark Plug Gap

0.044

Ignition Timing

10 degrees BTDC

Firing Order

1-4-2-5-3-6

Distributor Rotation

Clockwise

Valve Clearance Intake

0.014

Valve Clearance Exhaust

0.016

did you change one plug wire at a time? you may have two crossed , you should double
check the firing order.

What brand of plugs did you use? Nissan engines are very picky, they only like NGK Spark Plugs, if you used a different brand than that your engine will run poorly and misfire.

Well a new set of spark plugs is in order.The fuel most likely has fouled the spark plugs.It is fireing is it not?Fuel octane boost with tune up(ngk brand spark plugs)air filter and a gas dryer(HEET).will help.But do the tune up,if the engine has ignition spark.If you are using anything but the oil specified for the beetle,then change the oil and use the weight the manufacture specifies for the vehicle.I feel,if the engine ignition is fireing,that if you do all these things,and use the octane boost,and the heet additives,the car will perform.

You have the NAPS Z (nissan anti performance system haha) emissions engine. the dual plugs provide a hot spark for exhaust gas reburn and lowered tailpipe emissions. both plugs fire at the same time.
It is not really a good engine platform for high performance modification as its limited by the NAPS Z cross flow cylinder head design, its best to leave them mostly stock. I have owned several of the 720 king cab pu's and can attest to their durability, fuel economy, and reliability when properly tuned and maintained.