Holley carb tuning - Page 6
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  1. #51
    Exceeded sell by date Plumpcars's Avatar
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    No apology needed, I never read your post as inflammatory. As I said I've never run a 292, have run a 280, but like you I would expect a low vacuum reading. I'm not sure that the vacuum advance would help although it may work with the Rhoads lifters and connected to manifold vacuum. Sometimes these things are worth a try, especially when it just means switching a hose around. I would also be looking at the complete timing curve/initial/maximum etc and making a note of exactly what was set to where.
    I think the problem is that we are all trying to help here but it's hard enough when the cars in front of you let alone over the 'net!

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    Rods 'n' Sods Junkie SpannerPete's Avatar
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    Quote Originally Posted by Plumpcars View Post
    I think the problem is that we are all trying to help here but it's hard enough when the cars in front of you let alone over the 'net!
    I realise it's nearly impossible to solve this over the web but the information I'm gaining from you knowledgable guys is incredable. Armed with this I hope I can go forth and solve the problem. I REALLY do appreciate all the help.

  4. #53
    Carburation 'sucks' Roscobbc's Avatar
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    That cam of your is really quite 'stiff' for a small block car IMHO - even though the lift isn't really excessive. Rhoads bleed-off lifters should 'soften' things off a bit - you are lucky to have as much vacumn as you have ! - some might even consider a 244 degree cam marginal even for a street big block powered car. 'Modern' school of thought now tends to be less duration, instead using far more aggressive opening and closing ramps on cam - i.e. get the cam open as quick as possible, hold it open as long as possible and then close it as quick as you can. Idea being to get best duration but without bleeding things off with too much overlap . Its what 'yer roller cam does best - you do need good quality valve train components though.

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  6. #54
    Rods 'n' Sods Junkie SpannerPete's Avatar
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    I've been trying a few things out.

    Tried using the manifold vacuum port instead of the timed port. Doesn't make much difference.

    Fitted two lamda sensors (one to each exhaust) and check the mixture. As far as I can tell the mixture seems to be correct at 2000rpm but lean at 1400rpm. I would like to richen it just to see it it cures the problem, is the only way to adjust this with the main jet? I don't want to mess things up further up the rev range.

    Thanks

  7. #55
    I'm Not Jed Clampett stueeee's Avatar
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    Doubtfull that you'd be on the main system at 1400 RPM in a light car. Did you check how much of the transition slot is showing below the primary side butterflies as per a previous post? If you've used any significant amount of it at tickover, it will definitely go lean on transition.

    Holleys aren't really any different to other fixed jet carbs i.e. the bit that's working as you gently accelerate from tickover is mixture screw - pilot jet - main system

    There has been a suggestion of lengthening the transition slot in this thread. I've never tried this. But I've drilled the pilot jets out in this situation, which has allowed me to wind back the tickover adjusting screw which means you have more slot for transition. And you can get the tickover mixture correct by adjusting the air/fuel bleed screw. And now you have a richer mixture on transition because the motor's now running on your enlarged pilot jets.

    On some Holleys the Jets are visible when you take off the primary metering block. But on other models they're buried inside the metering block wells, so you may need to pull the brass plugs out to get to them.

    BTW, only enlarge the pilot jets a tiny amount at a time. A small increase in diameter makes a huge difference to the mixture. I use this series of drills for this work. 19 pc Microdrill Set Numbers 61 - 80
    Stuart.
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  8. #56
    Exceeded sell by date Plumpcars's Avatar
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    Have you tried richening up the idle mixture screws as a trial? Will only cost you a few minutes to judge the effect.

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    Rods 'n' Sods Junkie SpannerPete's Avatar
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    Very little transition slot showing (looks like a square!) so I guess I need to try richening the idle mixture. I didn't realise this was still working once cruising.

    Many thanks for replies, I'm sure I'll crack it eventually.

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    check the idle slot on the rear as well

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    After a busy weekend decorating I've just found time to check the idle mixture screws. I've tried adjusting them half a turn at a time and then tried it. I went up to 2 1/2 turns out. It did make a very slight improvement at about 1 turn out but no miracle cure.

    I'm getting to the stage where I think I need to take it to an expert. Is there recommendations for someone in the Beds/MK area that really knows Holleys and Chevy's?

    Thanks

  12. #60
    Rods 'n' Sods Junkie SpannerPete's Avatar
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    I know it's dragging on but I'm still tryng to sort this one out...

    Went down the wrong road i think as I'd convinced myself there must be a vacuum leak on the inlet manifold so I've spent time trying to find it and eventually changed the gasket. No difference.

    Also tried a different carb, although only a 500 cfm and I had to wind it right open to get it to idle I just wanted to check if the problem still existed. It did.

    So I started supecting the timing, which a few of you guys have suggested might be a cause. After advancing the idle timing and it not getting any better I asked my brother for some help. He's got mappable engine management on a kit car (Vauxhall red top 4 cyl), looking at the timing for that it starts at 16 degrees at idle but drops down to 8 degrees at 1200 (no load) before increasing again at higher revs.

    So I set my idle timing to about 6 degrees (idle now very rough) and disconnected the vacuum advance (aiming for about 10 or so degrees at 1400rpm, the problem speed) and the problem is very much improved. Obviously this has put the timing at idle and further up the rev range completely out, so have I found the problem or is changing the timing just masking the real problem?

    If this is a cure I think I need 14 deg at idle, 8 at 1400 and then up to 36 at 3000. I see MSD do a digital programable ignition but it looks expensive is there other ways to get round the problem?

    Thanks

    Pete

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