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07-12-2008, 02:17 PM #1
Why : No vacuume advance on inboard boats
Why don't boats have vacuume advance?? Most of my usage is wakeboarding at 2,100 rpms.On the 350 ci chevy. engine, the spark is at about 22* at that rpm. While the boat is underway, i can advance that distributor up to about 30* and the boat speeds up wonderfully. Exhaust temp comes down. (touching ex.manifold). But I don't go any faster than that so not to ping., . I undo it back to stock. So,,,, my question,,so far, no one seems to know the answers. Any one know something on this? I'm starting the proceedure to change dist. shafts with vacuume. I have found another that has done this, with great success. Thanks everyone who helps.
Last edited by Holy Smoke; 07-12-2008 at 02:36 PM.Jay :happy: Waxhaw, N.C.
70 Chevy C-10 Stepside from 86 lowered 2" front 3" back, 383ci/350hp. elec. seats,functioning cowl, wood bed, hidden hitch, hidden tach, long headers, duel 3" exhaust. perform int., semi-aggressive cam (no specs.)
69 Chevy C-10 Fleetside 350 ci 3spd. mild cam, 270hp,stock.duel 2.5" exhaust. Lowered 2" made own blocks
89 Ski Marlin Boat 350 ci:party: pulling machine. 7 skiiers.
07-12-2008, 02:53 PM #2
That may just be the particular engine model/manufacturer, ie Mercury, Force, etc.
My Glastron with a Volvo Penta 350 has vacuum advance, to get vacuum advance I had to order the carburated engine.
The Mercury engine package for this same boat only comes with electronic advance whether carb'd or injected.
When I discussed this with a Glastron rep I was told vacuum advance engines had more throttle response issues, when accelerating the engine vacuum couldnt keep up with the timing advance. I had a small vacuum canister in the engine compartment that was supposed to assist with vacuum under high loads. After I Supercharged the engine I had to install a larger vacuum canister to assist with vacuum advance.
07-12-2008, 03:18 PM #3
Thanks so much for the info. TB . Yes, my model is mercruiser, 1989. Mine is carbed, with no electronics. just plain and simple. The canister to assist with vacuume under load has me confused. was the Vacuume advance the only way it advanced?
07-12-2008, 03:55 PM #4
Newer model marine engines only use vacuum or electronic advance (late 90's to the present) no mechanical assist. Vacuum for most carburated, electronic for injected and some carb'd. I went vacuum because I've had experience with electronics getting wet. I wanted mechanical advance or a vacuum/mechanical assist set-up but for some reason it isnt available anymore on popular engine brands, some of the lessor known manufacturers still use it (Force, Sea Time, etc.)
Vacuum is the only advance, at high RPM's you loose vacuum and there isnt enough to adjust and maintain the timing advance so a vacuum canister or resevoir is needed since an I/O is usually either at high RPM's pulling skiers and tubes or idleing picking them out of the water.
Superchargers compound the problem because they run such low vacuum #'s under boost that without a canister you end up with no advance under full throttle.
A theory I've been told regarding the change to vacuum and electronic advance has to do with boat use/sales in the state of California, the theory is that mechanical advance can be adjusted more than CARB would allow for and engine manufacturers were restricted to these two types of timing advance for boats sold or used in California. This theory may be true since many of the lessor brand engines are restricted for sale and use on many California waters and cant be registered in the state.
07-12-2008, 05:01 PM #5
In reality, that vacuum advance isn't an advance, but a retard mechanism. It's to prevent faltering (and resultant emissions) under sudden throttle while at idle. When you jab the throttle, the vacuum drops abruptly and the distributer is retarded momentarily until the vacuum returns. Otherwise, an idling engine may faulter. A similar situation occurs with 4-barrell carbs. Without vacuum regulated secondaries, a sudden throttle at idle can result in too large a vacuum drop, over carbring, and naughty emissions. Before computer controls, emissions legal cars had vacuum controls. Now, with EFI, faltering emissions are a thing of the past.
Many pre-EFI street/strip cars ran successfully without vacuum controls. You learned to control jab and faltering. It wasn't unusual for a dyno turner to place little steel balls in the vacuum control hoses to maintain the OE look.
Assuming a non-EFI engine, operating your boat at 2,100 RPM is above the speed where a throttle punch is going to cause a falter. You don't need a vacuum advance; but, it sounds like you do need your distributer recurved to match your application. With few modification, an experienced turner will eliminate the vacuum hose, jet and adjust your carb, and recurve your distributer (selecting the correct weights and springs). This will make a significant difference in existing performance.
Remember, only so much advance can come from your distributer... at higher RPM, you get to the point when you need to changing the cam timing. These days, an adjustable timing set-up is prohibited on street cars, but they are still available for off-road. At your 2,100 RPM, the stock cam timing setting should be good. But, your tuner might recommend an adjustable timing set and make adjustments for more low or high end power.Are we having fun, yet?
07-12-2008, 05:27 PM #6
In the mean time, I am using much lighter spring on the mech. advance which helps idle. I have enlarged my timing advance capabilities. Instead of a mech advance from 10 - 28 (we'll say,) I now can start at 4* and go to 30. I do not set my timing at idle, I set it at full advance. (2500 rpms). At 2,100 rpms, I have it at 30, but at rpms of 4000, I have to have it back at 26*. I need it to be about at 28 at idle, 30 at 2000, 28 at 3600, and 25 at 4000. And at any time be able to put it under load for mutiple skiers, or more people in the boat , and then be able to cruise with just 2 people and keep the burn in the combustion chamber and not finishing the burn after the exhaust valve.
As for the Canister, If your at high power for a long time , I would guess that that canister is going to be loosing it's vacuume anyway.
From one squid to another. Congratulations on the Retirement.
Holy Smoke HM2
07-12-2008, 07:31 PM #7
The canister holds good vacuum even on long high RPM runs (I hooked a vacuum gauge up due to issues I was having with the Supercharger). Without the vacuum canister as Zippy pointed out the engine falters at take off from idle and when dropping the throttle back to idle after a high RPM run.
I'm running a modified Pro-Jection injector set-up below my Supercharger that complicates adjustments to timeing, its an early version that has no tuning capability, no sensors in the exhaust, and doesnt tie into the timing, unfortunately on a boat this is about the only type injection system that can be used because of the lack of exhaust sensors.
Thanks on the retirement congrats, I retired 4 years ago, went back on recall 3 years ago for 1 year and have been fully retired 2 years now. I still miss active duty.
07-12-2008, 07:59 PM #8
If I'm ever in Texas, I've got to come and see that boat. Sounds fast.
I think I will gently install the dist. and plumb off the secondary dash pod for the top choke on the secondaries. I forgot to mention I have a quadrajet. That I'm not tuning.(to difficult ). And run plenty of test before expecting to much to soon. Unless I hear good points otherwise.
07-12-2008, 08:58 PM #9
Gotta make that visit soon, if all goes well I'll be trading my Glastron for a new Chris Craft
Not really a ski boat, but we hardly ski anymore mostly pull a tube or cruise and I fell in love with the lines and handling after I took one out a few weeks back. I've always wanted a wooden boat for the clasic lines, I just never wanted the upkeep that came with them.
I'm surprised you still have the Quadrajet on, most times thats the first thing that gets swapped out, usually for a Holley or an Edelbrock.
Last edited by tbplus10; 07-12-2008 at 09:00 PM.
07-13-2008, 05:02 AM #10
Your right, on the carb change. I've changed my trucks to Edelbrock then put in an O2 sensor and gauge and tuned to get perfect ratio for our elevation. 800 ft. Big help on gas milage. But paying for a marine carb. . I guess I will be running a different Dist. oops! I use a different engine cover to ventalate and breath colder air. No fumes. Thanks for the great knowlege on the boat.
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