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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 !
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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.
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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?
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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?
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Re: Carbed 5.3 won’t start please help!
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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.
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Re: Carbed 5.3 won’t start please help!
Cam or crank sensor bad?
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Re: Carbed 5.3 won’t start please help!
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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.
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[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 Posted via Mobile Device |
Re: Carbed 5.3 won’t start please help!
where are you getting 12v from? new wire or existing wire from truck
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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.
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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?
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Re: Carbed 5.3 won’t start please help!
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Re: Carbed 5.3 won’t start please help!
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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?
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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?
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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).
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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!
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Re: Carbed 5.3 won’t start please help!
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Could be too far open. Or high idle on choke needs adjusted. |
Re: Carbed 5.3 won’t start please help!
Good to hear you have it running. Glad it was a simple fix. Now you can get into dialing in the timing and carburetor The fun stuff!
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Re: Carbed 5.3 won’t start please help!
Isn't a 600cfm a bit small?
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Re: Carbed 5.3 won’t start please help!
Yes, dialing it in will be challenging but fun. I tried to research a good bit on a carb. I hope the 600 is enough. It’s pretty tunable.
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Re: Carbed 5.3 won’t start please help!
Try removing the accelerator cable from the carb lever, leave the return springs on...then start the engine...I have seen cables hold the throttle lever open...
Also look for vacuum leaks as these will cause high idle levels... Cam sensor fault is most likely a bad sensor... Great that you got it going... :chevy::chevy::chevy: |
Re: Carbed 5.3 won’t start please help!
I don’t have the cable hooked up, I’m reading up on the carb. I’ll have to mess with it some. I ran it about 15 minutes today and cranked it a few times. A couple times shutting it off it wanted to keep running with the key off, like it was running out of fuel. Never had a this happen before, another fault in the wiring somewhere? At least it’s running now thanks to everyone’s direction!
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Re: Carbed 5.3 won’t start please help!
It’s not in your wiring.
Running on is known as dieseling. If your primary throttle is open enough to allow a bit too much air into the motor, a hot spot in your intake will ignite the air and a bit of fuel to make it run without the ignition on. It continues to try to run because the air going into the intake draws a little fuel from the carb. Cure is to close the throttle almost completely. There’s a section in this link which deals with the transfer slot on a Holley. Your carb is the same. If you get your transfer slot set you’ll likely cure your high idle and your run on. http://www.crankshaftcoalition.com/w...EI_distributor |
Re: Carbed 5.3 won’t start please help!
Or the alternator is back feeding power thru the MSD box. Needs a diode in the charge wire.
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Re: Carbed 5.3 won’t start please help!
You guys are very informative and I’m greatful for the help. I’ll look into the article and see what I can find out. Thank you and I’ll update soon.
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Re: Carbed 5.3 won’t start please help!
OK, I went through a similar issue with mine. If you're getting a cam fault code, you will have starting issues. Make sure you have genuine cam and crank sensors!! I honestly believe my issue was using aftermarket cam sensors. I finally went to a front timing cover and timing gear setup from a GTO, with GM sensors. No more starting issues.
As for the 600 cfm carb, it'll be fine for that 5.3. I have a +.010 5.7 with AFR heads making 531hp and it only pulled 580cfm at 6000rpm on the dyno. |
Re: Carbed 5.3 won’t start please help!
I've tried to look but it seems the primaries are not adjustable on the Summit 600 to mu knowledge. The secondaries appear to be adjustable. Summit tech line told me not to mess with any transfer slot adjustment but I may have too much fuel pressure? I have an inline gauge that reads 4 1/2 when running but it could be off. it is also dieseling on occasion when i shut the key off. You guys seem to know much more than the Summit tech. I'll adjust primary and secondary if possible. Anyone else ever adjust this on a Summit carb?
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Re: Carbed 5.3 won’t start please help!
Here are a few pics. Seems my transfer slot is way out of spec.
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Re: Carbed 5.3 won’t start please help!
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Re: Carbed 5.3 won’t start please help!
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Did you pull the carb off to find the secondary screw adjuster? Give us a better pic of the complete bottom of the carb. |
Re: Carbed 5.3 won’t start please help!
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