671 Boost Lines
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671 Boost Lines

  1. #1
    Junior Member Huffa350's Avatar
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    Default 671 Boost Lines

    Hi there, I am running 2x 600DP Holley blower carbs on top of 671 blower. Question I have is, is it normal to see some fuel coming down the boost reference lines? When I pull the lines off down at the tee where the boost gauge is hooked into as well on back of manifold, some fuel runs out the lines. I have been having some surging issues and wondered if that could be something to do with it. Engine tuner/dyno man tells me its normal. Thoughts please

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    Senior Member ol guy's Avatar
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    Quote Originally Posted by Huffa350 View Post
    Hi there, I am running 2x 600DP Holley blower carbs on top of 671 blower. Question I have is, is it normal to see some fuel coming down the boost reference lines? When I pull the lines off down at the tee where the boost gauge is hooked into as well on back of manifold, some fuel runs out the lines. I have been having some surging issues and wondered if that could be something to do with it. Engine tuner/dyno man tells me its normal. Thoughts please
    FWIW. If you are running a blower with boost reference you must? Be running power valves? If so I would bet the power valves are blown and allowing fuel into the vacuum lines creating the "drip". If it were me I would remove the power valves and replace with plugs and recheck. M

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    Some guy obnoxious001's Avatar
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    Quote Originally Posted by ol guy View Post
    FWIW. If you are running a blower with boost reference you must? Be running power valves? If so I would bet the power valves are blown and allowing fuel into the vacuum lines creating the "drip". If it were me I would remove the power valves and replace with plugs and recheck. M
    If he plugs power valves,, he needs to know to add 6-8 jet sizes to that metering block to bring mixture back to close to the same as with the power valve.

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    Senior Member ol guy's Avatar
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    Quote Originally Posted by obnoxious001 View Post
    If he plugs power valves,, he needs to know to add 6-8 jet sizes to that metering block to bring mixture back to close to the same as with the power valve.
    I agree O.B but it is better than wondering if with a forced induction if is actually opening the power valves. I prefere to jet fatter and eliminate the ? .Mark

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    Junior Member Huffa350's Avatar
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    Ok, yes boost referenced with power valves. So is it wrong that I am seeing some fuel down those lines? I will check or replace power valves.

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    Senior Member ol guy's Avatar
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    Quote Originally Posted by Huffa350 View Post
    Ok, yes boost referenced with power valves. So is it wrong that I am seeing some fuel down those lines? I will check or replace power valves.
    Yes that vac line in the metering block is a vacuum port and if fuel is coming out the power valve is blown. replace it and recheck. M

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    Junior Member Huffa350's Avatar
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    So theres definitely something up contrary to what dyno tuner is say? He's telling me its normal but got me changing jet sizes. If boost lines were letting fuel go on dyno, will correcting this issue change the whole tune? As it is I think hes got odd jetting @ 64 primaries & 81 secondaries

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    Senior Member ol guy's Avatar
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    Quote Originally Posted by Huffa350 View Post
    So theres definitely something up contrary to what dyno tuner is say? He's telling me its normal but got me changing jet sizes. If boost lines were letting fuel go on dyno, will correcting this issue change the whole tune? As it is I think hes got odd jetting @ 64 primaries & 81 secondaries
    Not knowing your'e engine size and ? the factory setting on jets for a L4776 dp is 66 primary with a 6.5 power valve and 73 secondary plugged power valve. My opinion but I smell fish! M

  11. #9
    Junior Member Huffa350's Avatar
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    Oh yip ta, engine is just 350 Chev, bout 8:1 compression

  12. #10
    Junior Member Huffa350's Avatar
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    So....... I installed new power valves and ran engine up under load on trailer, instantly idling mint and no more fuel coming down boost reference lines, no surging under load, off to the river, started mint, backed off the trailer and idled a while, picked up the bottom so turn key off to drop shingle, go to restart and it does a pop back thru carbs and blows power valves again, got it started but running like crap and flammed out completely. Why is it poping back thru carbs? Doin my head in!

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