What causes a motor to spit through the carb?

[team39763]

A friend told me it could be stuck valves, but I don't know.  I tried the 670 Avenger carb on my truck and it ran like crap, just like my Holley 750DP.  It idled rougher than ever and was rich enough to burn my eyes.  I tried setting the idle mixture, but it made no difference.  Floats were perfect, fuel pressure was normal.  But I did notice it spit.  I don't know what to do with my truck.  I'm thinking of junking the 6.0 and putting in a stock 5.3 with fuel injection.  But I don't understand what went wrong with this motor.

[choptop]

Try another carb or put the fuel injection back on the 6.0, but not replace it with the 5.3. It's too cool as is. Don't know the cure for sure, but the carb sounds like the issue.

[TexasRed]

If by spit through the carb, you mean backfire, it could be valves being too tight, too lean of a mixture (ie, there's a vacuum leak). I'd check to make sure all your vacuum lines are hooked up, including the one in the back for the power brakes.

[HAULIN IT]

Team, Please explain the "spit" in more detail. Does it do this anytime or only when you try to blast from idle? If you want to sort out the inner motor from carb working, a compression or leak-down test will tell you if the intake valve is open. Could just be a lean "pop" if it is as you try to accelerate, meaning you need more fuel (pump cam,squirter) most Holley carbs are rich at idle, but that does not mean it is rich once off idle. Lorne   

[VileZambonie]

Check your firing order first or are you using a coil direct with stand alone cpu? Not really sure what your set up is. If it's backfiring through the carb it's usually an ignition timing issue. If it's popping through the carb check your valve timing and adjustments.

[team39763]

Yeah, it's more of a popping, not really any fire.  I can actually see fuel being spit upwards out of the carb.  I didn't get to drive it, because it would barely idle.  I was too scared to drive it, it sounded like it wanted to puke it's guts out though the intake. 
Vile, I'm running the individual coils controlled by stand alone Cpu.  I believe my initial timing is set to 12*....that's a good place to start right or should I bring the timing down more?
I'm gonna send my 750 off to a carb builder, but I decided to go through it myself one more time and clean everything really good this time.  I can accept the fact that something doesn't work, but I'd like to understand what exactly is going wrong(or what I did wrong).  I'm gonna try the 750 once before I send it away.  I'll let ya'll know if it's doing the same thing.

[Blue 82]

I can actually see fuel being spit upwards out of the carb.

That's called reversion


How much cam are you running?
How much vacuum do you have at idle?
If your vacuum is too low, the power circuit(s) are gonna kick in, thus giving you a pig rich idle.


There are anti-reversion plates (or spacers) available
While they are not the cure for all that ails you, they do help.

[team39763]

I put the 750 back on there, but it ran like crap -  just like before.  One thing I did notice was it threw sparks while I was tightening down one of the bolts.  I don't know jack about electrical systems, so I may have a hot wire somewhere that's supposed to have a ground or something like that.  Would that explain why my battery is always dead if I leave it connected overnight?  I'm gonna leave it alone for a while since there's gasoline all over the motor from setting the floats and I don't want to light it up.

[VileZambonie]

You see sparks from where? Yeah disconnect your negative battery cable.

[team39763]

The sparks came from the carb bowl screws.  I think it was something with the wires going to the alternator.  I did a crappy job of wiring this motor, so I'm gonna take some time and go through all the connections and see what's out of place.  After moving the alternator wire away and taping it up, the motor ran pretty good. 

[VileZambonie]

Yikes! You better go through that puppy real good. If your B+ is grounding to the block or intake and you have no circuit protection you can kiss that computer good bye.

[team39763]

I'm thinking that I messed up with the alternator wiring and the starter wiring.  I'm gonna go look at some other trucks and see how they're wired(as in what goes where). 

[team39763]

OK I think I messed up.  I have 2 hot wires going to the alternator and no ground, which would cause the alternator to try to ground through the motor?  I don't know.  Could that be my problem?

[VileZambonie]

No. the alternator is case grounded.

[team39763]

Well what wires need to be hooked up to the alternator?  It's one of those with only one wire coming out of the connection and the thick wire on the stud on the back.