408 SBC Blower motor backfires? - Hot Rod Forum : Hotrodders Bulletin Board
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Old 01-01-2014, 07:13 PM
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408 SBC Blower motor backfires?

fresh rebuild on 400 sbc. Has the 871 blower, dual Holley 700 or 750 cfm blower carbs cant remember exactly which they are, petronix dist. with second strike. First of all the motor will not start without giving it a little throttle. But when it does get going it runs and sounds really good and wants to go. The other thing is after it starts and I rev it a few times it will idle for about 5 seconds on its own then dies puffing out white smoke and fuel from the carbs. My thoughts are carb adjustments need to be made but I have no idea where to start with them. I think they may be flooding out at idle. I have not messed with any settings on them yet. Would rather consult with those who have first. Or if anyone thinks it could be something else please let me know. Any input would be greatly appreciated. Thanks.

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Old 01-01-2014, 07:18 PM
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set float levels first. check timing for total at 3k,around 28 for starters.

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Old 01-01-2014, 07:22 PM
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408 SBC Blower motor backfires?

Ok now the float levels have to be checked while running correct? I will also verify the timing as soon as I can.
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Old 01-02-2014, 01:58 AM
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No, not while running. Pull the carbs and do it on the bench, having the proper gauge helps a lot. Rebuild kits come with one and you can buy them at parts stores, they're cardboard or thin plastic, most people throw them away.
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Old 01-02-2014, 05:59 AM
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408 SBC Blower motor backfires?

Well the carbs are brand new do I still need to get a rebuild kit?
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Old 01-02-2014, 10:14 AM
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Is it OK after it has run for a while (warmed up). Do the carbs have a choke, if not it is going to be finicky until it warms up. If you do have a choke it may be opening to soon.
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Old 01-02-2014, 02:18 PM
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It does have chokes but I hvae had them open. Have not used them at all. Didnt think I would need them because it fires up so easy.
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Old 01-02-2014, 02:24 PM
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do you have electric fuel pump(s)?
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Old 01-02-2014, 06:10 PM
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Yes it is a holley blue electric 110 gph with regulator.
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Old 01-02-2014, 06:18 PM
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no need to start engine to set float levels.
turn on 1 pump,remove site screws(all of them) the level should be just below the threads where you took out the plugs,,,shaking car should allow slight spillage,but only when shaking car,not constant.

what are the regulators set at? 5 ish?
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Old 01-02-2014, 06:30 PM
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Yeah I looked it up on here earlier and will do it as soon as I can. Yes they are at 5 to 7 psi. Gauge is reading right at 6 psi.
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Old 01-02-2014, 06:35 PM
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cut it back to 5 1/2 for now.set total timing at 28 at 3000 RPM no vacuum advance(not likely you will use any anyways)
Make sure you have your carb build sheets with you when you get ready for tune
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Old 01-02-2014, 06:48 PM
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will do just waiting for the time to get around to it lol. My guess right now is the float levels are off from all of the research I have done. Also do you think a 2.5 exhaust will do or should I go with the 3 inch? Because right now its so loud I cant start it in my neighbor hood with out risk of cops being called?
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Old 01-02-2014, 06:52 PM
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https://www.youtube.com/watch?v=0XbA5cXUvyw
this is 3 inch mandrel bent
wish I had room for 4 inch
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Old 01-02-2014, 07:02 PM
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Do you know what mufflers those are?
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