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  1. #1
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    BG Speed Demon 850 problems - carb experts please HELP

    I finally got the GM 572/620 installed and took it on it's maiden voyage this past Monday. The engine comes with a BG Speed Demon 850 (#1563010 - mech secondaries). The carb idles fine (900-1000rpm) and when you're "on the gas" it runs strong and smooth all the way to 6000rpm. The problem is when I'm trying to "cruise" or hold the throttle still for a constant mph. When holding the throttle still, the carb surges and stumbles, NO popping/backfiring through the carb, more of a surge. It feels like it's going from lean to rich, lean to rich which makes the car surge/stumble. It doesn't matter if I'm doing 10mph in 1st or 75mph in 5th, when holding the throttle still it's the same result. As I mentioned it idles fine, but when taking off I feel the same stumble between ~1000-2000rpm.

    I followed the manual that came with the carb and set the butterflies (.020 slot showing) and idle mixture screws (3/4-1 turn out) as recommended. Out of the box the carb was jetted 85/93 with a 6.5 power valve. I have the inital timing set to 20 degrees and it advances to 36 around 3500rpm. Here are the things I've done so far at the recommendation of BG tech support:

    - I went back through the manual and double-checked the baseline settings
    - I changed the primary jets from 85s to 87s - thinking we might have a lean condition - no improvement/change
    - I changed the jets from 87s to 83s - since going up didn't help I figured I'd go the other way - no improvement/change



    Any thoughts on what might be the issue? I'm thinking of plugging the power valve next to take that out of the equation.

    Here are the specs of the carb:
    MODEL #850
    PART #1563010
    VENTURI SIZE:1.562
    IDLE AIR BLEEDS PRI:.070
    IDLE AIR BLEEDS SEC:.063
    H.S. AIR BLEEDS:.028
    BOOSTER TYPE:DOWN LEG
    BOOSTER LEG:.184
    SQUIRTER SIZE:31 STD.
    BUTTERFLY SIZE:1-3/4
    AIR HOLE SIZE:NONE
    PUMP CAM ## 330
    PUMP CAM COLOR:PINK
    METERING BLOCK # P.A-12400
    METERING BLOCK # S.A-12401
    SIPHON BREAK:.028
    E-HOLE POS. #1.031
    E-HOLE POS. #2 N/A
    E-HOLE POS. #3.031
    E-HOLE POS. #4 N/A
    E-HOLE POS. #5 .033
    P.V.C.R. SIZE:.059
    TRANSFER SLOT RES: OPEN
    IDLE FEED RESTRICTOR .035
    MAIN JETS PRI: #85
    MAIN JETS SEC:# 93
    POWER VALVE PRI: 6.5 STD.
    POWER VALVE SEC: N/A
    LINKAGE TYPE: PROG.
    N&S SIZE/TYPE: 110-VITON
    ACC. PUMP DIA. SIZE 30CC

    Thanks!


  2. #2
    Join Date
    Sep 2004
    Location
    Big Stone Gap, Virginia
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    Running lean!!

    Sounds like it's either too lean or need to change the power valve to a lower vacuum!!

    Mark
    Ability is not being able to do something once or twice.Ability is being able to perform upon demand!!

  3. #3
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    What Mark said. I bet the power valve needs a-changin'. What's the manifold vacuum at idle?

    Can you list some of the engine specs?
    -Matt

    Welders: The only people that think a co-worker catching on fire is funny.

  4. #4
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    Unfortunately I don't have a vacuum gauge to measure manifold vacuum. One of the BG techs also mentioned changing the power valve from the stock 6.5 to a 4.5. Is this change in the right direction (i.e. going numerically lower)?

    The engine specs are listed in the URL below (the 620hp model, NOT the 720).

    http://www.sallee-chevrolet.com/fram...ckV8s/572.html

    Thanks!

  5. #5
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    Quote Originally Posted by dhardison
    Is this change in the right direction (i.e. going numerically lower)?
    Probably, but you won't know for sure until you buy a vacuum gauge.
    -Matt

    Welders: The only people that think a co-worker catching on fire is funny.

  6. #6
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    Oct 2004
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    I took the possible power valve problem to an extreme and plugged it off completely. Still no change in behavior. Here's what else I've tried with still no positive result.

    https://www.pro-touring.com/forum/sh...ad.php?t=20927

    If you have any suggesstions I'm all ears!

  7. #7
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    What do the plugs look like?
    -Matt

    Welders: The only people that think a co-worker catching on fire is funny.

  8. #8
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    The plugs are all light to medium brown in color. Not too light, not too dark, seem to be just right. No deposits, oil, etc on the plugs either.

  9. #9
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    Plugging the power valve will lean out a carburetor not jetted for it something awfull. If I remember correctly, the correction factor is something like 8-10 jet sizes.

    You really need a vacuum gauge dude.
    -Matt

    Welders: The only people that think a co-worker catching on fire is funny.

  10. #10
    Join Date
    Jun 2005
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    Greenwood, SC
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    a vacuum gauge is god when it comes to carbs. It'll tell you everything you need to know if you know how to read it. Best 25 bucks I've ever spent.

  11. #11
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    Oct 2004
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    Quote Originally Posted by [email protected] Dynamics
    Plugging the power valve will lean out a carburetor not jetted for it something awfull. If I remember correctly, the correction factor is something like 8-10 jet sizes.

    You really need a vacuum gauge dude.
    Yes, the power valve is good for 8 jet sizes with the Demon carb. But the power valve shouldn't come in to play when holding a constant speed. It only starts opening when you're accelerating and vacuum is moving toward zero.

    So in theory just plugging the power valve without jetting up 8 sizes will cause a lean condition when accelerating, but when holding a constant speed it shouldn't cause a lean condition because the PV should stay closed. Right?

  12. #12
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    Quote Originally Posted by formula
    a vacuum gauge is god when it comes to carbs. It'll tell you everything you need to know if you know how to read it. Best 25 bucks I've ever spent.
    Agreed I need a vacuum guage. When I get one, what type readings will I need to obtain? Idle vacuum? Vacuum when trying to hold a constant speed?

    How do I use this new vacuum info to help diagnose my problem?

    Thanks for the help guys!

  13. #13
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    When you get the vacuum gauge, hook it up to a long enough line for it to reach inside the cab. Check idle vacuum, off idle vacuum, and cruise vacuum.
    -Matt

    Welders: The only people that think a co-worker catching on fire is funny.

  14. #14
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    I hooked up a vacuum gauge and here are the readings:

    idle (950rpm) 6.5
    off-idle (1600rpm) 11
    cruise (2000rpm) 11

    I noticed that when I accelerate and the vacuum drops at or below 6-6.5 the stumble is gone. Same thing when decelerating when the vacuum goes above 6-6.5 the stumble returns. I have a 6.5 power valve it her now so the way I figure is when the PV is opening at 6.5 the extra fuel is aleviating the stumble. My next plan of attack is to leave the PV at 6.5 and go from 85 to 90 jets in the primary. This will provide additional fuel at cruise without the PV needing to kick in. If this works I'll adjust the secondary jets accordingly. Does my logic make sense?

  15. #15
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    I have to ask, have you ever owned an engine like this? If not, is it possible that you're feeling the cam? 2000 RPM is lugging an engine like yours.

    (Disclaimer, I've never owned, or worked on, a BBC)

    The power valve is incorrect anyway, whether it gets rid of the problem completely or not is anyone's guess. Throw a 4.5 in it and see what happens.
    -Matt

    Welders: The only people that think a co-worker catching on fire is funny.

  16. #16
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    I just tried the 90 jets and if anything it was worse.

    I've owned many BBCs, none with this many cubes or a hydraulic roller cam (the cam isn't very radical). It doesn't just do it at 2000 rpm, if I'm 'cruising' ANY rpm in ANY gear it stumbles. The stumble is not tied to a certain rpm, mph or gear selection. I'll try a 4.5 and see, but right now I'm taking it out to test-n-tune night at the drag strip, since there I won't be 'cruising'.

  17. #17
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    I wouldn't do that if I were you. Motors running way lean at full throttle still feel strong if you don't have a base line ass reading with a good tune. They run strong right up until parts of the piston leave the car via the exhaust.

    Try the power valve and we'll go from there.
    -Matt

    Welders: The only people that think a co-worker catching on fire is funny.

  18. #18
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    Quote Originally Posted by dhardison
    but right now I'm taking it out to test-n-tune night at the drag strip, since there I won't be 'cruising'.
    Hey Dan...Are you going to NationalTrails?

    If I wasnt working on that dang barn I would come watch. You going next weekend too??

  19. #19
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    National Trails is having some big 3-day race. I'm going to Mt. Vernon for test-n-tune instead. Hard to say about next weekend......

  20. #20
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    Put the Holly on and then see what happens!!, BG's I have heard are tempermential, (disclaimer) never worked on one but watched the kid down the street put one on a Mustang and it was nothing but problems!

    I have messed with a bunch of Holly and I can usually get them to work,
    Steve68- 1968 Camaro SS LS1 T56, 12bolt 3:90's, C5/LS1 brakes IROC box, 18" Fikse Profil 13s, Deep Fathom Green,Spearcos,


    http://www.ls1tech.com/forums/showthread.php?t=475228 some pics

    70 Nova SS street/drag 454, T400, 3:55, ugly!

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