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Old 06-16-2011, 11:05 AM
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SBC Backfire through carb

Hey Guys,

I recently dropped my SBC into my boat. It is a fresh rebuild with new vortec heads, Crane Cam, new lifters, primed oil pump, little gas in the carb and what I thought to be a little advance timing. Hooked everything up and tried cranking it over. When I crank it over, it wont start up, but tries to. At the end of one crank session it backfired pretty bad through the carb. I thought maybe I was 180 out. I tried adjusting the distributor 180 degrees from the point it was at at when i stopped cranking. Same same, backfired again, this time with a slight "wooooopmp" along with the small pillar of flameage. When I initially set the timing, the cam was set to fire at 6. I aligned the cam and crank dots, and set the distributor to fire at 6 on the cap. Does anyone have any insight? I am about to drop the stupid boat off at a mechanic and let him figure it out (I deploy in 3 weeks and I am tired of fiddling with this thing, I want to get out on the water.) Thanks for the help guys!

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Old 06-16-2011, 11:31 AM
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Hey Jay,

I do not know if the issue is the same for you but, when I rebuilt my carburetor, put it back on and started the truck, I was also getting backfires and flames shooting out the carburetor. I had forgotten to install the base gasket, resulting in a vacuum leak. Once I corrected that, the flames and backfiring went away. There was a bit of a skip but it was ignition-related. You can see the discussion and a couple of videos showing before and after here: "Rochester Quadrajet 4MV Carburetor: Removal, Disassembly, Rebuild (Rookie Level)", Page 6 and Page 7.

Hope this helps.
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Old 06-16-2011, 11:38 AM
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Brand new gaskets on everything, Felpro or Holley specific. I am going to manually turn the engine over to #1tdc and see what happens. Question- I have a compression tester, if I installed that into the number one cylinder and manually turn it over, it should still read some compression if it is on the compression stroke right?
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Old 06-16-2011, 06:43 PM
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Jay, just pull the plug and put your thumb over the hole, pull the coil wire and tap it over with the key, you'll feel it coming up on compression, then watch your pointer to stop it on TDC. Pull the dist cap, make sure the rotor is pointed at #1 wire, dist just snug and it should start, maybe a small adjustment while cranking, then time it right away, provided your valves are adjusted correctly. Guessing it hasn't been broken in yet?

I'm down here in Georgetown, hate to see you deploy, thanks for your service.
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Old 06-16-2011, 07:31 PM
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Thanks for the kind words. The problem I having with the thumb over the plug hole is the fact that I can only get fingers in there to put the boot on lol. Marine manifolds, plus alternator and multiple water hoses in that area make it difficult. I would take a valve cover off but that requires pulling the exhaust manifold off too. No winning for me tonight. When I lined everything up with the timing cover and valve covers, everything was kosher. When the engine builder put the cam in, he set it to fire at 6, (dots lined up 6 to 12) I put the distributor in that way. I am so lost. This boat is defeating me (even if it runs, I still have a trim issue to deal with grrrr). Thanks guys for the help! If you get bored in Georgetown this weekend and want to come help out, let me know lol.
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Old 06-16-2011, 08:35 PM
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Jay Gadsby,

I would put the distributor back where you had it. I think you have a wire crossed. Check the firing order and I think you will find your problem.

Scholman
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Old 06-16-2011, 08:45 PM
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I went through all the wires today and all were in the correct position. a couple of them were touching (if that makes a difference). Could my Thunderbolt IV interface be advancing timing too much? There is no mechanical or vacuum advance on this thing, but maybe a bad box causing an issue? And stupid question. The cap is a vented cap (with a vacuum port out of the back) does that need to be hooked to the carb? The carb has a blocked off vacuum line right now. Hind sight is always 20-20. I would have just bought a crate 454 if I had known this was going to be like this.
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Old 06-16-2011, 09:04 PM
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Any chance you're confusing the #6 being at TDC with the marks aligned, with trying to make it fire as #1? You have two chances to have #1 at TDC on the pointer, give it a shot. When ready to fire #1, looking down on the motor from the front, #1 will be at about 5 o'clock on the cap. If it sneezes the carb, 180 it.

I don't think that Thunderbolt vacuum needs to be connected to start, maybe for advance later on, not familiar with the system.

You're tilt thing must be a hydraulic issue? What outdrive? I have an Interceptor with a 5.7, and Alpha I, Gen II, but never had a tilt issue. You have the manual?
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Old 06-16-2011, 09:27 PM
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Quote:
Originally Posted by Jay Gadsby
When I initially set the timing, the cam was set to fire at 6. I aligned the cam and crank dots, and set the distributor to fire at 6 on the cap. Does anyone have any insight? I am about to drop the stupid boat off at a mechanic and let him figure it out (I deploy in 3 weeks and I am tired of fiddling with this thing, I want to get out on the water.) Thanks for the help guys!
The way you had it first was correct, if the cam and crank dots were aligned and you installed the distributor w/the rotor pointing to cap terminal #6. It just needed a shot of fuel down the carb to richen it up, and/or the choke set, to help light it off.
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Old 06-16-2011, 09:30 PM
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Is there a chance that the marks aligned on the cam and the crank timing gears aligned at 6 o'clock on the cam and 12 o'clock on the crank can have #6 at TDC on the exhaust stroke? I am just going by the word of the engine builder to line up the dots and fire it at 6. I know when I put the balancer on with everything lined up and rotated the block over to #1, the line was pretty much where it needed to be on the timing pointer. I will try just pointing it at that like you said and see what happens. I am just worried that the break in is being completed by my starter lol. I am running Royal Purple break in, and I did prime the puimp (got nearly 40psi out of my craftsman drill).

Oh and on the trim, the cylinders appear to be frozen on the outdrive (Alpha 1 Gen 1) The pump works both up and down, just no actuation. I removed the cylinders from the transom and still cannot compress them even with no hydraulic pressure in the system. my luck... Might be time for a new boat lol. I love my Pachanga, but am thinking a Baja Outlaw may be more fun with less headache.
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Old 06-16-2011, 09:31 PM
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Quote:
Originally Posted by cobalt327
The way you had it first was correct, if the cam and crank dots were aligned and you installed the distributor w/the rotor pointing to cap terminal #6. It just needed a shot of fuel down the carb to richen it up, and/or the choke set, to help light it off.
I did that, it fired off a bit( read- blap, blap out of exhaust), stalled out with a resounding backfire and 3 foot flames out of my brand new Holley.
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Old 06-16-2011, 10:02 PM
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Quote:
Originally Posted by Jay Gadsby
I did that, it fired off a bit( read- blap, blap out of exhaust), stalled out with a resounding backfire and 3 foot flames out of my brand new Holley.
If the valves are positively adjusted correctly- and you are absolutely sure they're not so tight that a valve is being held off the seat, then the ignition timing needed to be advanced.

Last edited by cobalt327; 06-16-2011 at 10:09 PM. Reason: Speeling.
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Old 06-16-2011, 10:06 PM
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I have never seen a timing set that put #1 at TDC of its compression stroke when the dots were aligned at 6 and 12, cam and crank gears, respectively.

If the crank gear has multiple keyways, that can be the source of an error.
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Old 06-16-2011, 11:31 PM
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Wonder where this guy is really located

Quote:
Originally Posted by caaraa
I am just going by the word of the engine builder to line up the dots and fire it at 6.
JUST SO YOU KNOW I'm working on getting you banned the hell outta here. You cannot keep cutting/pasting other peoples work w/o it making one bit of sense. You have yet to post anything original or even anything w/any meaning or relating to this forum.

I know what you're doing- hoping that a few posts will make you look legit, before you start spamming. Ain't gonna happen, you are not nearly as clever as you think...
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Old 06-17-2011, 07:47 AM
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IIRC, #1 was at TDC on the exhaust stroke when I had the dots lined up at 6 and 12 cam and crank respectively. #6 should have been the guy on the compression stroke. The valves were adjusted to the manufacturer's spec as far as that goes (zero lash, half a turn). I am about to go out and try this all again.

And who are you trying to get banned???
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