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Backfire Issue, need advice.

Old 04-20-2016, 03:13 PM
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Default Backfire Issue, need advice.

Hi Guys, Thanks for the advice last June on my blown up motor. I got the boat going about November. And it ran great. 489 strokers, Howards roller cams (part #120405-10) cast iron 049 heads with 2.19 intake/1.88 exhaust, 9.2:1 compression

About January I got a deal on a set of Weiand 256 blowers to go on my engines. I talked to various people and was told the cam was not ideal, but that it would work with low boost (4-4.5 psi) I'm running holley 0-9015-1 carbs with vacum secondary. 2 Carbs per motor. I have Holley pressure regulators, that are putting out 7psi. I was told I didn't have have to boost reference the carbs with this application. My HEI distributors would not fit the blower intakes so I got some MSD units with a traditional coil. They advance 18 degrees so I was setting the timing to 14 degrees at idle. (what do you guys recommend with my setup) I'm running super unleaded.

It doesn;'t want to idle, and when given throttle backfires thru the carbs and exhaust. I dont want to damage anything obviously.

I messed up last week and put too small an intake gasket on it (weaind blower intake large rectangular port, heads large oval port) and was blowing air/fuel/combustion out. Didn't take long to realize what I had done and now have the larger gasket on.

The plugs are super black from that screw up. I have new ones coming. Gonna try that and see what happens, but I don't think that's the problem.

Any thoughts? Thanks in advance.

Last edited by hblair; 04-20-2016 at 03:14 PM. Reason: head info
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Old 04-20-2016, 04:58 PM
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Without being there to diagnose, my first guess is that You put the distributor in wrong (orientation wise) or you have some ignition wires on the wrong spot.
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Old 04-20-2016, 06:29 PM
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or the carbs are way out of wack.i don,t want to sound rude but you might be wise to get someone with blower setup experiance to help you.it would be easy to ruin the engines with a tune up that is incorrect.
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Old 04-20-2016, 06:39 PM
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To much fuel being delivered IMHO
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Old 04-20-2016, 08:56 PM
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Thing though is popping thru intake and exhaust.

Typically:
Too much fuel - exhaust backfire
Too little fuel - intake backfire

Way too early combustion - intake backfire
Way too late combustion - exhaust backfire

I've put wires/caps/rotors on chevy motors like a million times. I have installed wires in wrong position a couple more times than I'd like to admit. That's why I like to put # indicators on each wire when doing them. Easy way for quick confirmation.

That said, installing a distributor because of the oil pump drive shaft moving on removing distributor, many people install the distributor without turning the oil pump driveshaft a little, and end up a tooth off. Boom, boom, pop, pop.

Edit in: yeh, fuked up plugs can do the pop, pop too.
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Old 04-20-2016, 09:05 PM
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Yeah SB, I was thinking the same thing. Distributors not 180 out. We replaced the plugs with new ones. Wires are routed to the right plugs. All cylinders were firing, all plugs are equally black.

Roger that Mike. I know no one in my area with experience with dual carb blower setup like this.

That's what I'm thinking Mseuro.we went from a 76 main jet all of the way down to a 65, same thing. Hmmm...
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Old 04-20-2016, 09:19 PM
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Timing is WAY off or way lean. Popping through the carb is usually lean. When you give it gas does it pop and die or does it just keep popping?
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Old 04-20-2016, 09:26 PM
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I posted that before I saw your last post SB. Sorry about that.

I pump it twice before starting, When it starts it hits on all cylinders and revs like a nice normal startup, then starts this barely idle while backfiring deal, when given throttle, backfire and dies
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Old 04-20-2016, 09:53 PM
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All plugs look like this...



Sounds like a tractor...
http://youtu.be/E0amIJX5ibk
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Old 04-20-2016, 10:35 PM
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I'm going to throw some old school logic at you on this.

I would base line the timing at TDC with a static timing light...No1 at TDC and the distributor lighting up on the mark. Fire it up and let it run a bit. Then shut down...set up you timing light and start it making sure you don't see the timing mark wobbling around. Loosen the distributor enough to turn it a bit and bring it up to 8* BFTDC.

I think 14* initial is WAY too much. If this works...figure out what you want to have total advance at 3000 and have a distributor guy set those MSD's up.

I had a similar issue with my Vortecs setting them up and at the end of the day I found broken springs in both my distributors. You didn't mention the history of your distributors but if they aren't new...I would guess that's where your problem is.

You might call the guys at DUI in Memphis and talk to them about this. They have a "Small Top" replacement for the HEI's that might fit your set up. I sent my DUI's I bought on Swap Shop here to them and they set them up.

My Dad and Grand Dad always used to say that if air, fuel and spark all get there at the right time it will run good. Knowing that and 18436572,,,there is an answer to your problem LOL !!

Good Luck !!

Last edited by sprink58; 04-20-2016 at 10:51 PM.
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