Need some carb help
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Need some carb help

  1. #1
    Senior Member gregb's Avatar
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    Default Need some carb help

    I finally got a chance to run my boat for a couple hours and play around a bit. It's a jet, 383 small block, has a 750DP, the standard 4779 model with 4 corner idle adjustments. Jetting is stock, 71 front and 80 rear, 5.5 power valve. I did an old school thing and put some .012 wire in the idle feed restrictions (all 4) to clean the idle up. I'm not new to this, but I am new to tuning with a wideband O2 meter. Had I not had the meter I would have called it good. However, here's what I get. Idles at 15:1, a touch lean, but it runs ok at that point. As I give it gas from idle to about 3000 rpm it goes down to 14.2, after that it goes lean again, at 3100 it's over 15:1, at 3500 it's about 15.5 and at 4000 on the primaries it's close to 16:1. Not so good. I have to get into the secondaries to rev it past that. If I hammer it and hold it WFO it drops to 11.8 and then stabilizes at 12.5. At the point it's going lean, is it running off the main jets or a combination of the idle channels and mains? If I go up a jet size or two will I have to go down a similar amount on the secondaries to keep the full throttle A/F where it is now? Is the wire method I used to clean up the idle just an outdated method? (it's worked for me on some cars, without the wire in there I couldn't get the idle any cleaner than 13.2, that was on the trailer all the other numbers above are on the water). Or is this something that is going to require some air bleed adjustments. It has 10 inches of vacuum idling at 900 rpm and I have the timing locked at 36 degrees. If you can point me in the proper direction I would be most appreciative, I don't want to just start throwing different jets in there if I need to look at something else, thanks
    Greg

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  3. #2
    Sit N' Spin Jetaholic's Avatar
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    Quote Originally Posted by gregb View Post
    I finally got a chance to run my boat for a couple hours and play around a bit. It's a jet, 383 small block, has a 750DP, the standard 4779 model with 4 corner idle adjustments. Jetting is stock, 71 front and 80 rear, 5.5 power valve. I did an old school thing and put some .012 wire in the idle feed restrictions (all 4) to clean the idle up. I'm not new to this, but I am new to tuning with a wideband O2 meter. Had I not had the meter I would have called it good. However, here's what I get. Idles at 15:1, a touch lean, but it runs ok at that point. As I give it gas from idle to about 3000 rpm it goes down to 14.2, after that it goes lean again, at 3100 it's over 15:1, at 3500 it's about 15.5 and at 4000 on the primaries it's close to 16:1. Not so good. I have to get into the secondaries to rev it past that. If I hammer it and hold it WFO it drops to 11.8 and then stabilizes at 12.5. At the point it's going lean, is it running off the main jets or a combination of the idle channels and mains? If I go up a jet size or two will I have to go down a similar amount on the secondaries to keep the full throttle A/F where it is now? Is the wire method I used to clean up the idle just an outdated method? (it's worked for me on some cars, without the wire in there I couldn't get the idle any cleaner than 13.2, that was on the trailer all the other numbers above are on the water). Or is this something that is going to require some air bleed adjustments. It has 10 inches of vacuum idling at 900 rpm and I have the timing locked at 36 degrees. If you can point me in the proper direction I would be most appreciative, I don't want to just start throwing different jets in there if I need to look at something else, thanks
    Greg
    Since it's a double pumper and you have complete mechanical control over when the secondaries open, I would focus on the primary jetting first, then the secondaries. Try going up on the primary jets and checking the A/F meter from off idle to just before the secondaries crack open until you get the correct A/F ratio. Then worry about the secondaries. Once you get the primaries right, if you had to jet up on the primaries and the secondaries are good where they're currently at, you'll more than likely have to jet down a bit on the secondary side but it all will depend on what the A/F meter tells ya.

    If you find yourself 4 sizes up and it's still lean, I would try a power valve with a higher vacuum rating so it opens earlier and supplies the extra fuel.
    Last edited by Jetaholic; 08-06-2009 at 11:12 PM.



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    Senior Member Bubbletop409's Avatar
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    Sounds like a fairly mild engine and maybe your power valve rating is the problem. The PV is designed to open when you pin the throttle, and if your still pulling over 5.5 vacumn at WOT that will cause a lean condition, then when you get into the sec's their additional flow masks the problem. I would measure vac at WOT and adjust the PV rating accordingly. There are several 4779 models, depending on the dash number. You can go to mortec.com or the Holley site and find the setup your particular carb came with from Holley. But I think most come with 6.5 or higher PV's. Think I'd ditch those wire restricters also.
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    Sit N' Spin Jetaholic's Avatar
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    Quote Originally Posted by Bubbletop409 View Post
    Sounds like a fairly mild engine and maybe your power valve rating is the problem. The PV is designed to open when you pin the throttle, and if your still pulling over 5.5 vacumn at WOT that will cause a lean condition, then when you get into the sec's their additional flow masks the problem. I would measure vac at WOT and adjust the PV rating accordingly. There are several 4779 models, depending on the dash number. You can go to mortec.com or the Holley site and find the setup your particular carb came with from Holley. But I think most come with 6.5 or higher PV's. Think I'd ditch those wire restricters also.
    Based on him mentioning that the carb came with 71s in the primary stock, this crosses to the -6 model. I agree also on the PV rating...seems a bit too low.



    Quote Originally Posted by HammerDown
    Kendall L-427 Super Blu...extreme-pressure (ep) lithium complex keeps my thrusting balls happy
    Quote Originally Posted by back2k5 View Post
    Well I am putting in a nice stereo system in the Carrera...maybe Ill put on some rap and turn the scoop sideways

  7. #5
    steelcomp was here
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    I'm suspicious as well about the PV. Check your vacuum on the primarys all the way to 4000 (or before you open the sec's). You may find you're still over 5.5" vac. Once you open the sec's, the vac drops, PV opens and you get the fuel you need, but until then, no PV. You also may just want to ditch the PV and go 5-6 jets up on the pri's. If you want the PV, then you're going to have to mess with it untill you get what you want. 14:1 with no load isn't the end of the world, but the minute you start putting a load on it, you're going to want to see more fuel.
    At 4000rpm you're way past the transfer slots and running off the main wells.
    Was there a specific reason you did the idle restrictors? I'm not sure I see the need here with timing locked at idle and no real load. Should idle pretty clean and have a crisp throttle w/o the restrictors.
    IF- (and I don't think it's likely) you do find that your PV is opening, then you're just going to have to add some jet on the pri side untill you get what you're looking for, and yes, then you'll probably have to pull some jet out of the sec's to maintain WOT a/f.
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    Senior Member gregb's Avatar
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    Thanks guys, I'll get the vacuum gauge out (didn't even think about taking it with me), ditch the restrictors and up the jets, about the power valve though, at steady state cruise, say 3500 rpms, that should be closed and not part of the fuel curve correct, that should only come into play when I mash the pedal or am at full throttle? The readings I took were steady state except when I was watching it go from cruise to max, I coudn't get a clean RPM read on my LM2 wideband with the HEI so that was the only way I could manually "data log" the AF vs RPMs, hold it steady, write it down. The engine is mild, but it is turning my detailed B impeller 5200, just want to make sure it keeps it up without melting anything. Thanks again.

  9. #7
    Sit N' Spin Jetaholic's Avatar
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    Quote Originally Posted by gregb View Post
    Thanks guys, I'll get the vacuum gauge out (didn't even think about taking it with me), ditch the restrictors and up the jets, about the power valve though, at steady state cruise, say 3500 rpms, that should be closed and not part of the fuel curve correct, that should only come into play when I mash the pedal or am at full throttle? The readings I took were steady state except when I was watching it go from cruise to max, I coudn't get a clean RPM read on my LM2 wideband with the HEI so that was the only way I could manually "data log" the AF vs RPMs, hold it steady, write it down. The engine is mild, but it is turning my detailed B impeller 5200, just want to make sure it keeps it up without melting anything. Thanks again.
    On a jet you'll notice the load curve is not linear. If you were to hook up a vacuum gauge and drive with it where you can see it, advance the throttle slowly and you'll find that from idle to about 4000-4500 RPM the vacuum will dropat a slower rate than the RPMs are increasing. Once the pump starts loading the engine, this relationship between the two readings will reverse...vacuum will start dropping at a faster rate than RPMs are increasing, indicating that the pump is starting to provide a moderate to heavy load to the engine. Note how much vacuum you're pulling when this starts to happen and that's where you want your power valve coming open at. This should get you very close.



    Quote Originally Posted by HammerDown
    Kendall L-427 Super Blu...extreme-pressure (ep) lithium complex keeps my thrusting balls happy
    Quote Originally Posted by back2k5 View Post
    Well I am putting in a nice stereo system in the Carrera...maybe Ill put on some rap and turn the scoop sideways

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