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Thread: Carb tuning by vacum?

  1. #1
    flat broke
    I'm going to be setting up a new carb on the boat come Sunday and I figgured I'd try going about it the "right way" rather than just making changes, reading plugs and fumbling around through trial and error.
    To set up the powervalve, I know I need to make sure I use a powervalve that has a lower vacum rating than my vacum at cruise. But how much lower should it be? As a hypothetical, if I was pulling 6 lbs at a 3500RPM cruise, would I go with a 5.5?, a 5? How far in the powerband do I want to take my reading for where the PV should open? If I spin to 5k WOT where would I consider my cruising RPM? highest possible without the secondaries? whatever feels comfortable for the motor, or base it off of my actual typical cruising MPH?
    Prior to setting up the PV, I'll need to tailor the idle circuits. I've always used the tach as my gauge for this routine. With the primary and secondary throttle shafts set so that I'm not bypassing the idle circuit, aim for the highest RPM without an off idle stumble. Is this an accurate way of handling things? Could vacum be used as an indicator for this portion as well? If so, set it up to pull the least amount of vacum, or the most?
    This is stuff that a lot of folks would consider remedial, but I'd appreciate any input that the pros such as Info, Gofast, and the like can offer up.
    I've always had good luck with more "shade tree" techniques on cars where you could just go haul ass around the block to check your work. But after dicking with setup and tuning during trips, I'd rather take a more direct and proven approach to nail things down with a higher degree of confidence.
    Thanks in advance for the input,

  2. #2
    With the butterflies closed, the idle circuit should be set to the highest idle speed/ smoothest idle. You may have to lower the idle speed adjustment in order to realize any changes in the idle speed using just the mixture screws..Once that is established as close as possible, (as exotic cams and intake/exhaust upgrades from stock sometimes make these adjustments tough to dial in). Be sure the accelerator pump actually squirts as soon as the throttle shaft moves the slightest bit, and the fuel should continue to flow through the initial opening of the primaries. Most of the holleys I've seen had 6.5 Power valves when new.. This is a ballpark number that works with most automotive applications, however boats aint cars.. As far as a starting point, 6.5 isn't far off. You will have to find out how your throttle response is, throughout the power range of your engine, and the demand of your hull. If the engine seems to stumble when nailed from a cruise speed, the pV may be too low, or the pump timing may be early or late. These fine tuning areas must be evaluated in the real world, under real use. It is a common misconception that there should be an abrupt increase when the secondaries open, this is wrong! The sudden jerk you feel is actually a bog, and a recovery. The secondary action should be a gradual increase in speed/power, a smooth transistion into WOT. And yes, the PV plays a role in this transistion, and you may want to monitor the vacuum as you experiment with your particular set up for a better idea what PV will work the best.. Some tuners just block them off and tune around them, but again, the real world, under real use is the only way I know to fine tune carbs/timing.....Just my .02 Moneypitt

  3. #3
    On my jet I backed the boat into the water and fired it up. Strapped down of course. I then started it and let it warm a bit then took a loaded vacuum reading at idle 9", then I applied throttle slowly and vacuum went up to 10.5" momentarily before dropping below my idle vacuum. I then installed an 8.5 PV. I have my idle at 900 in the water, and made sure the PV wasn't leaking at an idle. When I had my 6.5 PV and tried to pull a skier I had a small miss while at part throttle and this cleaned it up. There was a couple hundred rpm range just above idle where the pump didn't put a hard load against the engine to drop my vacuum to open the PV and I was getting a lean misfire. at full throttle I was ok, just in family waterski mode it sounded bad. Just my experience

  4. #4
    Dennis Moore
    Most marine engines need the idle speed set with the intake manifold vacuum at its highest point. This requires adjusting the ignition timing, idle mixture screws and both primary and secondary idle speed screws (Holley).
    The adjustments should be made in the water with the drive in gear (sterndrive or V drive). Rotate the distributor until the engine idles the best (don't worry about the ignition advance numbers for now). Adjust the idle mixture screws until you have the highest intake vacuum. You will have to be constantly readjusting the idle speed screws to attain the correct idle speed. After the idle is set, go back to the distributor and see if the vacuum increases by rotating the distributor again. Once you have the highest intake manifold vacuum, by adjusting the ignition and idle mixture screws, check the ignition advance with a timing light. It should be around 14 - 22 degrees advance at an idle (with a mild camshaft, 20 - 24 with a big camshaft).
    Readjust the advance curve (recurve the distributor) so that when the timing is set at an idle, 14 degrees advance for example, you will have 34 degrees total ignition advance at 3500 rpm. This would mean that you have recurved the distributor to give you 20 degrees advance (14 degrees at idle + 20 degrees distributor = 34 degrees total @ 3,500 rpm).
    Usually you will only be able to limit your distributor to 20 degrees advance (most ignition systems) which would mean that you will have to set your timing at idle to 14 degrees (14+20=34).
    With a Holley carb and a big camshaft, to maintain an idle, you may have to open the primary idle speed screw to a point were you won't have any adjustment with the idle mixture screws . In this case, close down the primary butterflys with the idle speed screw and open the secondary butterflys with the secondary idle speed screw.
    As far as the power valve is concerned, throw in a 5.5 and fergetaboutit!
    Dennis Moore

  5. #5
    After the idle is set, go back to the distributor and see if the vacuum increases by rotating the distributor again. Once you have the highest intake manifold vacuum Dennis, Good information but I have a question, are you saying there will be a point on the initial timing at idle where the vacuum will top out and start to fall. Also if I have already drilled 1/32" holes in my primary plates will this method still work for me if I try it? My Cam is 260/270 at .050" and I needed the holes just to get the throttle plates back to somthing normal. Its a jet drive also. Thanks Dennis Sorry to Rob the post Chris.

  6. #6
    Dennis Moore
    Yes, you would think that the engine would keep increasing vacuum if you keep increasing ignition timing but it won't. It will top out at a certain point and then start to drop.
    Before smog controls on car engines the vacuum advance would bump up the timing to around 24 degrees advance at an idle. It was connected directly to intake manifold vacuum. The initial was set at 4 degrees advance at an idle and the vacuum advance would add another 20 degrees. Those engines idled very smooth but were dirty.
    Many engine builders will use the V6 ignition module on the Mercruiser TBIV ignition system to increase the idle quality of a big cammed engine. This ignition module will only provide aproximately 10 degrees advance. To get 34 degrees total, the idle timing is set at 24 degrees initial and the engine will idle great! Much better than locking in the timing at 34 degrees with no advance mechanism (like some guys do).
    Drilling the holes in the primary butterflys is the same as opening the secondary butterflys. Sometimes both are necessary (particularly with long duration camshafts like yours).
    Dennis Moore

  7. #7
    flat broke
    Thanks for the input.
    You're not hijacking my thread as you were asking a similar question. I'm running 270degrees at .050 so we're in a real similar situation in that regard.
    Hopefully tomorrow afternoon brings about a good state of tune that will get the boat running well for Parker in October. I've got some RPM that I've been waiting to see as I'd be really happy to get this pig up past 70 again.

  8. #8
    flat broke
    Well, yesterday evening went well with newly established baselines for timing and mixture. The new carb is absolutely amazing and is only going to get better with time and tuning. Thanks for everyone's input previously as it helped confirm a lot of what I read, and also gave me a couple of pointers that helped tie some things together.
    The only area that came in contrary to the advice received here was on the PV. I have a 6.5 in there right now and I'm only pulling 4.5-5lbs of vacum at idle, so I'll probably drop the PV down to a 4 or 3.5 to keep it from dumping fuel at idle. This should help make the idle circuit more effective, though it seemed to be working well last night.
    I ran out of time to do any plug checks for main jetting info, but I'd bet on it being a tad fat on the big end. Back to the lake on Sunday again to do some plug checks/jetting changes and run the different PV to see what happens.
    Regardless, the boat is running MUCH better and will hopefully be up a couple more hundred RPM by the end of next weekend.

  9. #9
    Blown 472
    The power valve only addes fuel to the main circut when it is being used.

  10. #10
    flat broke
    If that is the case, and I don't see any fuel dripping through the boosters at idle, would I assume my vacum reading is off? If no fuel is in the boosters at idle and I'm truly metering off of the idle circuit with the 6.5pv with 5lbs of manifold vacum, should I even worry about it?

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