Vacuum or mechanical secondaries?

Go vacuum, I've seen big mechanicals on an SB inboard and it wasn't pretty, didn't deliver the responce one expects from a towboat. But unless you are spinning over 5800 or have stroked the engine, 600 cfm is sufficient. Search for "Carburator CFM Calculator" on the net and play.

The 750 will move the range the primaries are effective up almost a grand. This means if your are getting into the secondaries occasionally at 36 mph ski, power control at that range may improve. The trick is tuning everything else.

This carb is likely not plug-n-play...you will need to jet to your engine with lots of plug readings. You can do that with your carb now, and a secondary metering block can be added to your carb to tune the secondaries.

Firstly, What is the symtom you are trying to improve?

TRBenj,

As I recall, you just replaced your cam etc. I suggest you measure your vacuum at idle and under acceleration both light and heavy (pulling a skier) before you buy the carb and give the Tech Rep's a chance to set it properly before you install a new carb. This needs to be done now due to the new cam change. Without a strong vacuum signal you may never get the 750 to work. Like GottaSki said, the larger carb will affect operation and if your vacuum is already running low due to a higher performance cam with even small amount overlap you could be going in the wrong direction.

Surf a while at CarCraft.com where they had an excellent Tech article on the Holley especially the Power Valve opening and engine vacuum.

Good Luck

Gottaski, thanks for the response. Just to fuel the fire, what do you make of the CFM argument in this article?

64, I just read that article on carb tuning tips, good stuff there.

For now maybe Im best off doing the conversion and rebuilding. Im thinking that if I replace the 1" spacer with a 1/2" and plumb in the PCV port, I can close the motor box and still run a 3" flame arrestor.

Looks like youre recommending this kit to essentially convert my 4160 to a 4150. If I convert, which rebuild kit should I get- 4150 or 4160? I already have my jet kit.

I think Ill save the money and make the hiteks the next major engine purchase further down the road.

Oh, and one more Q for the carb experts. I havent read my plugs yet since Ive changed my intake/cam/heads. I know I want to verify that Im not running lean to avoid damaging the valves, so that is next on the to-do list. Would running lean at WOT prevent me from seeing the full performance of the upgrades? Right now the boat runs right up to 4800 very quickly and gets to 5000 right after, but it just wont pull any higher. I can turn both a 15" qnd 15.5" pitch prop to a strong 5k, but no more. On my dad's boat, there is a 200 RPM difference @WOT between the same 2 props. Sound normal?

I didnt mention that I only have a handfull of hrs on the boat since I added the cam/intake/heads. I finished the boat a week before I left on travel and its only been in the water 3 times... hopefully no damage has been done. It will be first on the agenda when I return!

Im ordering the marine 4160 rebuild kit(there shouldnt be a difference between a 50463 and R50463 List # right?), the conversion kit and extra fuel bowl gaskets. I already have the extra metering block gaskets and the jet kit.

Question on the powervalves- I assume that I only need a single stage PV, correct? Do I want the standard or high flow versions?

Yes, std PV, no two-dtage abortion and high-flow is not neccessary, flow is determined by the diameter of the two pvco's.

Well, over the last few weeks a friend helped me rebuild my carb. We got the metering block installed in the rear and installed new jets, 72 in the front and 76 in the rear. I havent re-read my plugs (no time) but I noticed no difference in performance. Still runs great, and pulls up to 4900 RPM with the 12.5x15.5 4-blade Acme. I still have to read my plugs to see where I am, but I should be getting quite a bit more fuel than before. I guess my hopes of gaining more top end was just wishful thinking...

...so I bought some Ford Motorsports 1.6 roller rockers to install this fall/winter!

Funny you should mention that- a friend of ours is a helicopter pilot and that was brought up last weekend!

I dinged up my Acme 470 at the launch a month ago, so Ill be happy to go back to it. Should pick up 200 RPM.

I don't think the 72's are to big for what he has done to the motor that's what I run in mine, 76's are too much had those in for a while too and went back the the 72's

No exactly ... the PVCO's are sized to deliver 25% more fuel than the (likely) 66's that were in there originally .

4% per jet step equals 6 jet steps from 66, so if you were to plug the PV, you would need 66 + (2 x 6) = 78 to maintain the same primary flow, or similarly simulate the same mix on the secondary side without the PV.

Thats where I suggest 78 to start on the secondaries. It appears I suggested 76 before, so my bad, sorry. Its better to start high and work down. You should be plenty safe because your primaries are richened. but be careful, if you optimize the primaries, the rear cyllinders may become too lean. Reconsider the 78's.

Right now, your two circuits are mismatched. You have the equivelent of (72 + 12)=84 on the primary and 76 on the secondary. The throttle bores are the same, so there shouldn't be this wide mismatch, one or both must be lean/rich.


Overrich primaries use more gas, reduce combustion temps, more CO. Richer still can foul plugs, collect carbon on the combustion chamber and ridge on the top of the cylinder. I don't think you are there yet at all. An '79 says with confidence richer suits your combination.

Power comes from making as much heat, at the right time, as the components can safely take.
More heat means better expanding ga$$es, thus more power and effiency.

If you were to disable the secondaries as you tune the primaries, perfomance could be measured by top speed as you do plug readings and adjust primary mains.

Its not feasible to run just on the secondaries, one must tune the primaries then add the secondaries and see the affects, best by maintaning the same soot riing width that was obtained on the just the primaries.

I just can't see the pix here right now, post the links so I can open..

not certain...I would do a few plugs then when you are confident they are similar, you can reduce your sample accordingly. Typically they are similar.

Whoops- correction - each jet step is 4.5% more flow.. That makes the 76 more reasonable...BUT, if '79 sez you need richer, you are not there yet on the secondaries; you just kept that flow the same. Start with the primaries and dial 'em in nice.

thats blocked, i'll have to lookat home.
you saw recent my thread on the topic, right?

Not sure where the error yet, but yes, the mismatch of primary to secondary suggest there is more to gain somewhere. This is where a discilined methodology is critical, no shorcuts.

Remember, the big power is just to the rich side of just too lean...if you know what I mean.

Engines are air pumps, with the essential changes more air can be facilitated. However using a dual plane manifold more cfm can be used as only one half of the carb supplies one half of the motor. A open plenum is just the opposite, unless the dual plane has been notched.
Vacuum secondaries are the best way to go for your application, it gets what it needs at the right time. If you want a quick plug read use a octane booster it will turn the porcelain red for a good read.