402 backfiring hard start

[1972_C20]

So, got it all apart. The chain just has a lot of slack in it. Going to get some parts ordered and tackle it when they get here.

[1972_C20]

So, finally was able to finish this up. Have it all back together. Did cap rotor plugs and wires since everything else was apart. Truck still backfires when the throttle is pressed. Timing is set at 14 and teeth on the timing gears are lined up. I checked them, rotated the engine and checked them again. I did this multiple times because I didn't want to have to pull it apart again.

[geezer#99]

What else did you check when you had it apart?
Did you check #1 tdc being accurate on your balancer/timing tab?
Did you check the crossover in the intake to see it's open?
Is your heat riser working?
Did you check the rockers are all moving the same?
Do you let it warm up totally before you wack the throttle?
What's your initial timing now?

[1972_C20]

I did not have any of the top end of the motor off. Intake, carb, valve covers. None of it. I'm still kind of new to the carburetor game. The truck was warmed up when I set the timing. It's at 14 BTDC right now. The heat riser is just the tube that goes from the exhaust manifold to the air filter correct? And as far as the cross over, I have no idea. I'll double check that the balancer mark and timing marks are accurate.

[70STOVEBOLT]

The heat riser is a valve between the exhaust manifold and the head pipe, not sure if it's on the left side or the right side. It has a butterfly that is controlled by a bimetal spring that opens the valve once the engine reaches a certain temperature.

[demian5]

If its a Q-jet it could be the secondary flapper spring needs to be tightened up to prevent a huge gulp of air (and backfire/bog/stumble) when the throttle opens quickly under load..

[geezer#99]

Originally Posted by 1972_C20

I did not have any of the top end of the motor off. Intake, carb, valve covers. None of it. I'm still kind of new to the carburetor game. The truck was warmed up when I set the timing. It's at 14 BTDC right now. The heat riser is just the tube that goes from the exhaust manifold to the air filter correct? And as far as the cross over, I have no idea. I'll double check that the balancer mark and timing marks are accurate.

Look here for tdc checks.
http://www.crankshaftcoalition.com/w...op_dead_center
It's for a sb but bb is the same. No cheating. Use a piston stop.
To check for heat under the carb just put your finger on the intake down below the carb between the intake bolts. If the heat riser is on pass side check for heat on drivers side and vice versa.

[1972_C20]

I worked on the truck yesterday. Took the timing cover off again just to make 110% sure it was on there right. Question. Before I had it apart I put to TDC with rotor pointing to number one cylinder. Got it apart and the cam gear was pointing up and crank was too. I thought at TDC the marks were supposed to line up like in my picture. But, when they line up it seemed the distributor was 180. So, I 180 the dizzy and truck didn't start when I tried it. So, I put it back. Is this how it's supposed to be?
I'm beginning to think the slack in the chain was never the cause of this. But, yes. It did need fixed. If it was it would have fixed it.
Is it possible the valves just need adjusted? If so, what are the specs? I know a flat cam is possible too. Does anyone know the specs on how much the valves are supposed to open?

One last thing. The engine is factory, so I find it highly unlikely for TDC to be wrong according to the marks on the timing tab and balancer. I had the valve cover off too to double check TDC. Both valves were closed when I set the dizzy. And it runs just like before. Idles good backfires when throttle is pressed.

[geezer#99]

#1 tdc compression is indeed as you found it. Dot to dot is #6 tdc compression. Designed that way so it was easier for the engine guy to line it up.
To check for a flat cam just pull the valve covers off and watch the rockers to see if any aren't moving as much as the others.
What did you set your timing at?

[1972_C20]

Timing is set at 14 BTDC

[geezer#99]

Bump it up to 18 and see if that helps.

[hdpete98]

Originally Posted by custom10nut

x2 check to make sure your distributor is tight. Your symptoms sound like the hold down came loose and it advanced too far

what he said.

[El Dorado Jim]

Originally Posted by cleszkie

Don't forget to check you accelerator pump in the carb. If you are not getting an extra shot of fuel when you try to accelerate or pull a load, the engine will bog down from a lean condition. This could lead to a backfire. A faulty accelerator pump can also make for hard starting as well. Just a thought.

x2

[1972_C20]

Checked the accelerator pump in the carb and it's squirting how it should be. Advanced the timing and it ran better, but still backfires. Gotta be the camshaft. I'm going to pull the valve covers this weekend just to double check. What would be a good cam to install. It's all stock, no intake no aftermarket carb, just an HEI dizzy. I'm new to how they identify the lift and what the different lifts do performance wise. I figure if I'm putting a cam in it, I may as well put a beefier one in. What would be a good mild cam to install?
Thanks for all of the help on this too guys. It's very much appreciated.

[geezer#99]

If you got a flat cam then any new one won't fix it.
It's tear down, clean and hot tank time.
New rings, bearings etc.
All that fine metal goes everywhere in your motor.

[demian5]

Keep throwing parts at it and let us know which one fixed it.

[1972_C20]

That's a good point geezer. I'll find out this weekend. And I don't just throw parts at things. The only thing I've spent money on was the timing chain kit, which had slack in the first place. Wires, cap, and rotor are just basic tune up pieces I already had for the truck. I'll keep you guys posted.

[El Dorado Jim]

you might consider a new gm crate motor, that is what i am going to do....summit has them with free shipping and 3 year /50,000 mile warranty