Thread:

Jetboat timing/advance setups and Vacuum info

When you read "lock out timing" or "lock out advance" it is not the vacuum being discussed, it the ENTIRE advance mechanism that is locked out. No vacuum advance, no mechanical advance.

But lets take the vacuum advance since you brought that one up and you are running it.
If the vacuum canister is hooked to the manifold, and the throttle is pushed wide open, what do you think the vacuum advance is doing to your timing? If you were in your car driving down the hi way at 2500 Rs and floored it, what happens to the timing?


The vacuum advance as very very little to do with the openig of the secondaries. They are controlled by a spring inside the secondary vacuum diaphram on the side of the carb.

Originally Posted by Chop Shop

I know the vacuum advance has nothing to due with the secondaries.

I guess what I was trying to say is that I had seen folks mention locking out dist advance. It made wonder how a boats vacuum needs/offerings differ from how a car operates.

If the boat vacuum demands are strange compared to a car and I dont understand them, then it makes it frustrating to find powervalve and secondary adjustments for me.

Also I am new to tunnel rams and have been toying with them lately as my friends by boats after I infected them with "hotboat disease". How does a single carb vs a dual carb affect engine vacuum?




Help me understand how a boat uses/develops vacuum?

Load and throttle opening is what effects manifold vacuum. The further the throttle is opened, the less manifold vacuum you will see at any particular RPM. Boats, jets, outdrives, and v drives all load the engine harder than a car does(unless its going up hill towing a trailer).
So you have the throttle open further to acheive the same RPM. Thus, the MANIFOLD vacuum is usually considerably lower all the time.

The vacuum advance in the dizzy RETARDS the timing when the manifold vacuum falls, and ADVANCES it when the vacuum is high. Max advance on the freeway cruising with light throttle. You push the throttle open and the timing RETARDS.
So if you have a vacuum advance on a boat, the more you open the throttle, the vacuum advance is trying to RETARD your timing.

Multiple carbs (t rams with 2 4 bbls) lowers the manifold vacuum even further, retarding the timing even more, or at least maxing the retard at a lower throttle opening due to more throttle blades.

Mutliple carbs also have less airflow thru each barrel than than a single 4 bbl at the same throttle angle. The secondaries are opened by AIRFLOW THRU THE CARB, not manifold vacuum. Less air thru each barrel means less signal to open the secondaries. If the secondaries have a control spring for a single 4 bbl setup, then its going to take just as much air thru BOTH carbs to get the secondaries to open fully. All the carb cares about is how much air is passing thru it. If you cut that air in half, it not going to open the secondaries the same as if it was only one carb.

Make sense so far?

I am going to take a stab at this using different terminology..

Don't taze me GN7...

There simply is no need to run different advance amounts on an engine that does not have to "pull a gear"...

A jet pump absorbs little/no horsepower at lower RPM's, but on the top end will absorb everything the engine can develop. This is virtually opposite what a car engine does. A car has to pull a large load from the bottom end, meaning that with low/no vacuum at the take off putting the engine under an extreme load when running through the gears, this is where a lower rpm advance comes in, keep the engine from detonating itself to death. Once the rpm level increases it brings in more advance as the load decreases.

On a jet boat however, the load increases greatly after you have already passed the point where lower ignition timing will help you. Virtually the only thing a lower initial ignition timing will do for you is aid in the starting of the engine, especially when warm.

A vacuum advance mechanism increases the ignition timing as manifold vacuum increases, which for lack of a better phrase will "never" happen on a jet pump.

Get rid of the vacuum advance on the distributor (you could just simply disconnect the signal hose and cap the carburetor port) and set the ignition timing for above 2500 RPM to be at approx 34*BTDC and a typical mechanical advance is good for approximately 14* advance, putting you in the 20* range at idle, which theoretically should be enough to get it to start reliably.

You should see a performance gain and you can use manifold vacuum readings to dial in the max BTDC ignition timing.

Right now, they way your set up is, you should be losing some timing right when the engine could really use it.

Hope this clarifies at least a little something something...

Originally Posted by Chop Shop

Yes that makes great sense. You have an easy way of explaining things.


What about a single carb setup on a single plane vs a single carb ona TR?

Ive also been told to put a powervalve block off in the carb/s??

So on ANY jetboat with a performance engine I should not have ANY vacuum advance or ANY mech advance, correct?


If so with that out of the way then I guess my problems are more based on how to make an engine with very low vacuum perform well. Guess I need to learn how to tune carb/carbs with very little vacuum.

If you go with a total lock out, you will more than likely need to install a separate ignition toggle to get the engine cranking before you hit the spark...

Carburetor tuning will not vary a lot. As the velocity drops, the AFR will remain fairly constant, the carburetor has no idea how fas the engine is turning, only the vacuum signal that is being generated. I am on the fence as far as the PV block off. I would have to have a pretty compelling reason to do so because it will effect the overall jetting.

Originally Posted by Chop Shop

Yes that makes great sense. You have an easy way of explaining things.


What about a single carb setup on a single plane vs a single carb ona TR?

Ive also been told to put a powervalve block off in the carb/s??

So on ANY jetboat with a performance engine I should not have ANY vacuum advance or ANY mech advance, correct?


If so with that out of the way then I guess my problems are more based on how to make an engine with very low vacuum perform well. Guess I need to learn how to tune carb/carbs with very little vacuum.

The vacuum advance is there to hold off low rpm detonation under full throttle, full load. You simply cannot duplicate the low rpm, high load in a small jet boat. MAYBE in a 30 ft single engine V bottom outdrive, but never in a small jet. Try holding the throttle wide open and hold the RPM to 2500 in your jet.
If you have no low RPM load, then you have no or very little need for low RPM timing curve.



There is a slight difference in tuning a carb for single and dual plane manifolds, as there is for 2 carbs of the same type originally tuned for a single carb use.
The single plane manifold will usually have a weaker signal(think it as manifold vacuum to make things simpler) than a dual plane. Same holds true for a dual 4 bbl compared to a single. Even a single 4 bbl single plane.

Compared to a carb tuned for a single carb dual plane, and carb will need to be jetted richer. If the same carb tuned for a single dual plane is mounted on a T ram with another carb, you may have to jet it even richer still. Two carbs DOES NOT mean more fuel for the same air flow. Usually less.

The secondaries if a T ram setup will require very weak secondary springs to get the secondaries fully open. Again, the secondaries are not controlled by manifold vacuum drop, they are controlled by VENTURI pressure drop. If you don't change the springs, you have to move the same amount of air thru EACH carb to open the secondaries all the way, or in other words, twice the total amount as if it were only the one carb.
MANIFOLD VACUUM DOES NOT OPEN THE SECONDARIES. AIRFLOW/PRESSURE DROP THRU THE VENTURIS DOES.

Originally Posted by Chop Shop

So should I leave the mechanical advance alone? It sounds like the ONLY benefit of the mech adavnce is to aid in startups? If thats the case would I want to put light springs and heavy weights in so it advances as soon as the engine starts and then stays at its "advanced setting thru all running rpms?

You just need to keep in mind that there is no engine load at the lower RPM on a jet. So it is less important to vary the timing. You would be hard pressed to notice the difference from locked out vs. a mechanical only, the idle quality should really improve with around a 20 BTDC at idle, so even that would a compelling reason to leave the mechanical in action.

As long as the timing is "all in" at around 2000+/-RPM I would not think there would be any gains to be had locking out other than fewer moving parts....Again, just my opinion.

Originally Posted by GT Jets

As long as the timing is "all in" at around 2000+/-RPM I would not think there would be any gains to be had locking out other than fewer moving parts....Again, just my opinion.

The single biggest reason for eliminating the advance mechanism is that its one less thing you don't need. Like tossing the choke. Who boats in 40^o weather?
The ONLY reason for retaining any mechanical advance, even a very very very fast one that is all in by 1500 is for starting. If you can't start a pump gas engine at full advance, you need to rethink your electrical/starter system. With todays gear reduction starters, its easy to start a 12 to 1 engine. A Pump gas engine with a GOOD stock starter is a piece of cake.

There simply no reason to have an advance mechanism on a pump gas jet.