Vacuum advance port from distributor

I see a major problem with that, IT'S AUTOMOTIVE AND NOT MARINE GRADE.

That whole unit needs pulled along with all of the other car crap on the engine and needs replaced with marine specific parts.

That would make sense. i will look into replacing the distributor. To be clear this was not my doing. The engine setup came with the boat. It runs fantastic, I just thought I may be missing some top end because of no vacuum advance. But that makes a bit more sense now.

Ok, I see this post is really old but I became a member just now to offer some info on this for anyone searching. A automotive grade vacuum advance dis CAN, CAN, MOST DEFINITELY, ABSOLUTELY CAN, WORK!! PERIOD!! And can even give a performance increase on carburetor engines. Here is the real explanation.

For this example we will use say an 86 range 5.7 mercruiser with a Thunderbolt IV ignition.

1. The stock marine unit has an iron gear, magnetic pick-up, external coil, No advance to speak of, and a seal to keep water out with a vent tube on one side of the housing. Now before anyone tries to give flack, NO they do NOT have mechanical advance! On this particular model (carbed thunderbolt) the ignition amplifier mounted on the riser acts as an electronic advance but is vague. Basically you are set at 10 btdc at idle and the unit will increase the advance to 24 deg on top of that starting at 1000 rpm and becoming full circle (34 deg) by 3800 rpm. Now in stock config in proper working order this is pretty good. However depending on engine modds and prop sizing (load on the engine), that pre set config is not perfect. Vacuum advancing can improve every aspect of this.

2. I hear allot of people suggesting hooking the vacuum line to the intake manifold, DO NOT DO THIS. There is allot of speculation from tuns of different guys or gals on these forums on how the vacuum works. Let me sum it up, Vacuum will generally be about 17inhg at idle in the intake, this goes to ZERO at WOT however again, the carb venturi vac raises with engine rpm to WOT. So in short if you hook the advance to the intake you will get the adverse effect of full advance at idle and retard at WOT causing detonation and a grenade eventually. For holley the correct location is on the front metering block on left side looking at it for Rochester, carter, afb, edelbrock check the manual or Internet as they vary.

3. Now that we know where the vac hooks up lets talk distributor modifications from auto to marine spec and the advantages of having the vac advance. First you will need to change the dis gear from the factory auto steel unit to an iron one for marine use. The housing from there up to under the rotor is the same, now buy the water tight gasket used for truck 4x4 and this will keep the cap water tight for moisture or spray, next there is a water stable coil (coated) for the top mount HEI. Now you are marine spec, No sparks, Water resistant, and the correct drive gear.
Lets talk advantage. You have installed your new distributor set it to spec and are wondering what to do, well with the HEI system you only have to get the connector for the coil pos and neg, then hook the pos and neg wires from your old coil set up (coming from the ignition switch not the ones for the mercruiser amplifier box). You can now chunk the old coil amp box and so fourth, Saves quite a few hundred dollars if your mercruiser box or dis is bad.

4. With this complete you are ready to take full advantage of the poor mans computer corrected spark advance used in newer efi versions. You can now set the dis to 10 btdc, hook up to the water hose start your engine and have a timing light ready that has advance adjustment on the back of it. With the motor running set the light to 32, 34, or 36 (depending on engine mods) raise to 2000 rpm and check to see if the timing tab lines up with the balancer at zero. If it does not do not turn the distributor, instead there is an adjustment in the dis under the cap that allows fine tuning of the advance. Adjust this screw and slide rod in or out to give desired advance on vacuum. Now rev the engine to be sure it does not go over your specified setting for total timing of 32, 34 or 36 (most mildly modded sbc like 36 total). Once this is accomplished you are ready to reap the benefits because at light throttle or a heavy load the vacuum will not be engaged yet thus keeping the engine retarded for longer creating more torque where needed, when your up on plane and cruising the vac will adjust with engine rpm up or down creating a more usable stable power band than the factory all or nothing set up. and finally at WOT high rpm you will get max advance to your setting not the factory so this means more HP. And my personal favourite, when you gig it out of the hole you accelerate faster because even though you are WOT from a stand still the dis stays retarded and rises only with the RPM because there is more venturi vacuum the faster the rpm is hence, NO BOGGING.(due to timing ofcorse). This allows the use of hotter cams and or bigger carbs with better performance and no bogg.

Point is that a vac advance HEI automotive dis not only CAN be used but is a better option in my opinion for the over all performance and longevity of the engine. If anyone has any questions regarding this thread or tech specific quarrels feel free to message me, but for anyone who has the barrel sight not to see this advantage and thinks that factory murcruiser is the only option, not only are you wrong, but your costing people reading these allot of needless money and just to be passed on the water by someone like myself who likes to be realistic and live outside the MERC BOX. Thanks guys for letting me on this forum and glad to help anyone who needs it.

Long ago I was told that distributors with vacuum advance are not USCG approved because it may provide an ignition source when the engine back fires. Maybe wrong but I haven’t seen a vacuum advance distributor that is USCG approved.

The guy is wrong in so many ways to list but no one said it would not work they said it is a a time bomb waiting to blow

The 1st thing I thought last night when I saw it posted was just how many Correct Crafts use Mercruisers with Thunderbolt ignition? And just by changing your distributor your still not going to keep up with Joe anyway

sprayin99 wrote: 8122pbrainard wrote: Nautiqueseventy wrote: The guy is wrong in so many ways to list but no one said it would not work they said it is a a time bomb waiting to blow John, I agree and didn't even bother with a comment till now. It wasn't worth it this morning! You probably don't know him being new to the site but this is a case where I wish Chris was still around. He would have had plenty to say and backing it up with a knowledgeable background - not just a "boat rodder" opinion. Yes, I'm waiting for the big BANG!! Welcome to CCfan. tell us more if you'd like. Boat?? Nice guys, I really would like to know all the ways I was wrong? (hell with the coast guard there are NO sparks period.).

Well, these guys know their stuff, that's for sure. Especially when it comes to safety.

You might disrespect the CG, but they are there to protect some people from themselves. I think they can give a citation for non-CG approved equipment on a boat. Some of them will check too.

We really want to prescribe legal installations on this site. No disrespect to your knowledge about HEI systems.

Little off topic here but I've always run a locked distributor for best performance without issue long as comp ratio is not too high (10 to 1) I assumed the retard was for starting purposes? Any other benefits?

No advance at all distributor locked in place

Anywhere from 34 to 40 btdc, If I remember right the boat I set at 34 initially, haven't got back to foolin with for 110 hrs run time. Should be broke in by now...

I don't necessarily disagree with the original post, but as it pertains to correctcrafts there are some issues..


Let’s get safety out of the way...

the biggest thing you need for a marine approved distributor is explosion proofing. In general manufacturers make these devices explosion proof by eliminating the vent in the cap, controlling all gaps (both in the distributor body and between the body and cap.. newer designs rarely use a gasket but have a tested interface that usually involves steps to eliminate line of sight openings) and then a controlled vent with a fine wire mesh is sometimes added to the body of the dist.. possibly in the location where the vacuum advance mechanism penetrates the body on the automotive version. Again this all varies; my assumption is that each of the designs is then tested by an independent body to a specific standard… I have designed and tested “explosion proof” electrical enclosures for mining industry use and am fairly familiar with the concepts and will say that I have attempted such a conversion once or twice for my own marine use… But I have also experienced first hand a bilge fume explosion and subsequent fire of my own as well as seeing the end results of many of them so I have a healthy respect for those possibilities.   I would recommend extreme caution be used, it ain’t rocket science- but you can turn your boat into a rocket if you don’t get the science right.   
-     

Not safety related but the gear on the dist has nothing to do with the marine application but instead with the cam material – which is usually determined by the cam type. If it was a stock engine and it came stock with an iron gear on the stock dist you are doing the right thing by replacing it with iron. Not all marine engines however will get along with an iron gear.

The reasons for a marine approved carb are not explosion proofing but rather to prevent gas from getting into the bilge to begin with…and they are not trivial features to fully duplicate. Gottaski points out there aren’t ported vacuum sources on marine carbs, I slightly disagree. I have seen some certified marine carbs that did not tool up a different base and simply put plugs over the ported vacuum ports, I have two marine Barry Grant carbs that are examples. The moral of the story is however that you should use a marine carb... and it is unlikely that a marine carb (and certainly not the case for the majority of the users of this forum) will have a ported vacuum source.

Assuming you can work it out so you don’t blow up you make some decent points for a light, planing, i/o boat with a mercruiser with thunderbolt ignition. They just aren’t relevant to 99.9% of Correctcrafts for a couple reasons.

First they generally aren’t mercruisers. The carb versions mostly come with mechanical advance distributors that can be adjusted and recurved to allow full advance at the appropriate rpm and are not limited by a preprogrammed curves.. exceptions are carbed protec boats. The fuel injected versions need to follow the factory map as it controls more than just the ignition timing. In general they are also pretty good, adding an unexplained vacuum advance to them would likely cause the knock sensors to retard your timing .

Second, even when not under the serious loads experienced pulling a skier these boats do not really get up on plane and run under light loads like an relatively small i/o with trim can do. The load is very linear, these boats can maintain speeds that a typical I/o cant that fall between the hull speed and the planing speed. Technically they do climb the bow wave and “plane” but not the way an I/O will. They just don’t have a bow up cruising mode like you would see on a planing I/o or even on a lightweight v drive or inboard that runs a high rake prop and no hook or a rocker the back hull. Therefore the situation where a vacuum advance could help take the level of loading at a certain rpm into account is simply not present. In general in an inboard correctcraft (even more so than the typical boat) rpm is a very good indicator of load on the engine, therefore a mechanical advance properly mapped is the best case scenario.

I hope you aren’t put off by the tone of any of the responses, when one leads by telling us if we don’t get outside the MERC BOX then we are going to be passed on the water… a little good natured kickback is likely to follow.. especially when you are on one of the few boating sites where Merc has no real relevance. Besides plenty of us think outside the box… I run a mechanical secondary carb, a relatively huge cam, and a single plane intake manifold and my boat takeoff compares favorably to that of a 1200cc two stroke jetski.. and there isn’t a merc part on it.

Marty,
I forgot to comment on your vac advance on the Super Saber. Universal was in it's final days before going under when they came out with the V6. They did as little as possible to the purchased engine which involved exhaust manifolds, a RWP and paint. (even keeping it a standard LH rotation) It was a "quick and dirty" attempt to get some cash going again. They found that the V6 did work well with the vac advance and suspect it was do to the unusual torque curve. The added expence of changing the distributor wasn't needed. Through the years of Buick, Jeep and even OMC that V6 had many changes. The biggest was probably changing the firing order to try and smooth it out. That V6 is the only marine engine I know of that came from the factory with the vac advance.

sprayin99 wrote:    Anyhow the boat I originally described was an 86 dixie super skieer, on this test boat which was originally powered with a merc 5.7 carb, the original performance was about 55mph, after building up the engine, tweaking here and there and completing this modification, I am seeing 68-73 on the gps depending on the water itself.

are those real numbers?
if so pretty impressive for a 5.7 sbc...that dixie is probably a fast hull...

I'm still learning a lot about this great boat, this forum is very helpful, thanks guys.
My 1986 2001 with 351 is bogging and not revving up. I just had the carb professionally rebuilt. Now diagnosis points to a timing issue. Saw this thread and noticed I have a vacuum port on my distributor that is hooked up to nothing.   Does this mean I have an automotive distributor? I am having trouble posting photos with my iPad or I would post a photo.

Is this a good stock replacemert distributor in the link below?

http://www.nautiqueparts.com/product/distributor-breakerless-mallory-left-hand-rotation-351-ford/

Any distributor recommendations? Thanks very much for everyone's advice and different points of view.

If you pull your distributor out of the engine and look at the gear it will look like this for a

Reverse rotation engine \\\\\\

Normal rotation engine ////////