Carbed 5.3 won’t start please help!
 

Built from bare block, 5.3, LS series MSD box. The engine fires up with first key turn and runs for a second or less and shuts off. Fresh build and first time cranking was today. I have a ground from battery to block, firewall to block. MSD box is grounded to intake, alternator not wired up currently. Was wired from main post on back of alt to positive battery 10g wire. Jumped exciter wire to main alternator post. Didn’t make any difference. Cranks for a second and shuts off. Engine is a 2001 5.3 from a Tahoe. MSD box is set to Truck on the front dial. If I was not getting fire would it not crank up at all? I had a friend holder open the joke and give it a little throttle when I tried to crank and it shop a little bit of fire out of the top of the carburetor. Any help is greatly appreciated !

Re: Carbed 5.3 won’t start please help!
 

Backfire like that is usually because your motor is lean and retarded.
What carb?
What’s your initial timing?

Re: Carbed 5.3 won’t start please help!
 

Brand new Summit 600 electric choke carb. I see fuel in bows through sights. I’m not sure of my initial timing. I set the MSD LS series box on the “truck”dial, the book says that’s for low octane 4.5, 5.3 and 6.0 motors.

Re: Carbed 5.3 won’t start please help!
 

What other settings does it have on it?
You might need to experiment.

Re: Carbed 5.3 won’t start please help!
 

Does the engine dying coincide with you releasing the key and the starter shutting off?

Voltage issue? Im dealing with a similar problem and its from the distributor not getting the full 12
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Re: Carbed 5.3 won’t start please help!
 

The box has multiple settings, 4 which are custom that I haven’t messed with. I’ll post a pic of the box this morning. The engine dies very quickly after I release the key. Could be losing voltage when I let off of key. I’ll try holding in start position for a few seconds and see what happens. Should I try starting fluid and see if that helps?

Re: Carbed 5.3 won’t start please help!
 

No...
You have fuel or it wouldn't start. You have ignition issues. I am no help on what it might be but it ain't fuel. It does sound like You could be loosing spark after the key goes to run position.

Re: Carbed 5.3 won’t start please help!
 

Very true, it wouldn’t start even briefly without fuel I would think. I must be losing spark or something. I’m going to try and hold key in start position a few seconds and see what happens.

Re: Carbed 5.3 won’t start please help!
 

Cam or crank sensor bad?

Re: Carbed 5.3 won’t start please help!
 

Re: Carbed 5.3 won’t start please help!
 

Just jumper 12v from the battery to the msd box power. Won’t loose the voltage for sure. Easy test and possible confirmation.

[QUOTE=clay68c10;8381368]Just jumper 12v from the battery to the msd box power. Won’t loose the voltage for sure. Easy test and possible confirmation.[/QUOTE

If that works, grab a paddle and hop on it because your in the same boat as me. Which means you need to solve the voltage loss or lack of from ignition to msd box. Mine is ignition to distributor
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Re: Carbed 5.3 won’t start please help!
 

where are you getting 12v from? new wire or existing wire from truck

Re: Carbed 5.3 won’t start please help!
 

12v red wire going from msd box to relay. I’ll try directly to battery when I get home.

Re: Carbed 5.3 won’t start please help!
 

Sounds like you are pulling the relay in with the Start signal from the Ign switch, but are lacking the run circuit voltage to keep it fired...

You sure you are not feeding the relay from the old feed wire from the Starter R terminal on the solenoid?

The R terminal receives power only when the key is in the Start position. Once the key is released, the R terminal loses its power supply.

Feed the relay voltage supply from the Run terminal on the Ignition switch...

It should have 12V when Ign switch is On, and 0 V when Key is off....

If this truck had a Coil and Points style distributor as original, it would have a resistor style wire in the original coil feed circuit...dont use that wire to feed the relay...it will not supply enough voltage to the controller.

Hope this helps...Bill

:chevy::chevy::chevy:

Re: Carbed 5.3 won’t start please help!
 

Bill, thanks for the input. I’m going to look it over again in the morning. I know I have my purple ignition wire going to starter the it does go to #86 on my relay. I think what you’re saying may be happening. I wired it like Msd said to, I’ll attach a couple pics for their setup so you can see it. The Msd box itself is getting power through the relay as well #87. Is the R terminal you’re talking about on the back of the keyed ignition?

Re: Carbed 5.3 won’t start please help!
 

Re: Carbed 5.3 won’t start please help!
 

Re: Carbed 5.3 won’t start please help!
 

So what will give the system continuous volts to run after I release the key? Purple ignition wire from back of key to starter then to relay. Makes sense of it losing power when I release key. So the RUN terminal has 12 volts when key is on position and run position? Zero when off correct?

Re: Carbed 5.3 won’t start please help!
 

Correct on the Run connection...assuming your ign switch isnt faulty...feed the relay direct from the Run terminal on the switch...beware of the firewall connector block, they crap up all the time...

If the Purple wire on your is the Purple wire from the Ign switch, that is your issue...

I believe the correct wire on the ign switch is Pink and originates from the Run terminal on the ign switch...

....you can test this by removing the Purple wire feed from the controller, and temporarily run a 12V positive lead direct to the controller wire where you disconnected the Purple wire...have a 30 amp fuse in that wire as a safeguard...

:chevy::chevy::chevy:

Re: Carbed 5.3 won’t start please help!
 

Yes, purple wire from ignition to starter to relay. Does the picture I posted of what Msd says look correct to you? Did I just wire it incorrect?

Re: Carbed 5.3 won’t start please help!
 

The instructions look ok, but hand-written "purple" is incorrect. Purple is not keyed 12V, it is momentary just to run the starter. I think by keyed 12V they mean 12V when key is in the on position (and should also have voltage while in start position).

Re: Carbed 5.3 won’t start please help!
 

UPDATE***
The truck runs and sounds like a beast! I did have it wired wrong and I spliced into the pink ignition wire for the constant 12v, ran to my relay for the MSD box and she fired right up! Now I have the computer hooked to it and it gave me a cam sensor fault. One other thing is it’s idling about 1100rpm and I have the idle screw all the way out. It’s a summit 600 carb. I can press the throttle forward barley, just enough to get the idle down to about 850rpm or so. Any ideas on the idle? Thank you everyone for your help! I’m just glad to see and hear it run!

Re: Carbed 5.3 won’t start please help!