vacuum signal

[BLB350]

Hello Mike, et al. I am running 2 front heads on my 2000 FXD TC with two separate intake manifolds. I installed the stock ignition vacuum sensor on the front cylinder intake manifold and wonder if it is sensing enough of a vacuum to advance the ignition properly (at small throttle openings). Obviously, on a stock intake manifold, both cylinders work to create a vacuum, but with 2 intake manifolds and only one sensor, the sensor sees only one cylinder. Don't laugh, but I am getting really lousy mileage, and I think it is more related to the ignition not advancing enough (at small throttle settings - cruising down highway) than it is to having 2 carburetors. Apart from the 35 mpg, the motorcycle runs very well. It has just been my experience that the 50 degrees (or so) of advance at high vacuum throttle settings really ups the mileage.

Please give me your opinion on one cylinder creating enough "pull" for sensor?
And is there an inexpensive vaccum gauge I could buy to hook up to vacuum port on back of Mikuni HSR to get a reading while the engine is running?

[Mike]

You could be right. It's still seeing a pretty strong signal but the timing from one to the next, at idle, means the overall signal may be reduced. We've run a "T" with a hose to each intake runner on the Baisley dual carb stuff to solve this issue. The signal is smoother. I cannot recommend a specific gauge to use but it will have to be heavily dampened (liquid filled) to smooth the signal well enough to get an accurate reading. The needle bounces all over the place, at idle, with standard gauges. This is actually a minor problem for the MAP sensor with heavily cammed combo's.
Mike

[BLB350]

Thx again for your experienced input Mike. It's sure nice to have somebody to talk to who is so knowledgable! To improve my idle quality, I do run a vacuum hose connecting the vacuum ports on the backside of the flat slides. Perhaps I should tap into the manifolds themselves.

[Mike]

The manifolds would be better. Mike

[choochoo]

also since you are in effect only receiving 1/2 the signal, you may want to use a "T" to connect into both cylinders.
Using both will give you a stronger and smoother signal with less fluctuation.
Used to see this all the time in high lobe cams in moderately to highly modified auto engines.
If all else fails you may need to add in a small vacuum reservoir. and tap off that for your signal.
what it does is act like an accumulater, itstores vacuum up for use as needed.

[Mike]

In my above reply regarding the T, it would be used to combine the signal from each manifold into one. The accumulater has been discussed at times (as well as restrictors) but we have not tried it.
Mike

[choochoo]

Mike,
I realize you had mentioned a T earlier in your post, But got to typing quick and forgot it was there.
On the accumulator : Some times when the cams are too radical you just can't get a strong enough
steady vacuum signal direct from the engine.
As you know the higher the lift and longer the duration, the worse the idle gets and the less steady and
amount of vacuum.
That's why I suggested the accumulator. Ideally it stores enough vacuum to let other items that use vacuum,
do so more efficiently.
It was just a thought.

[Mike]

Your thoughts are correct on the accumulater, the old windshield wipers not working at idle deal. I apprreciate your input on this. Funny thing is H-D guys are TRYING to get there bikes to idle like that.
Mike

OK guys, help me out here. I'm envisioning something that looks like an in line gas filter, but with no guts in it. One end plugged into manifold and the other end receiving the vacuum sensor. Should I look in SUMMIT catalog, or some automotive place like that?

[choochoo]

That's where I would look. I haven't seen any in the bike catalogs. But then again I haven't looked.
One note.
As above higher lift and longer duration the less vacuum at idle. But that's also true across the throttle spectrum, til you get to WOT. No noticeable vacuum there in most any engine.
When you're picking throttle off one carb only you are getting HALF the VOLUME of vacuum.
When you go to a more radical cam it is imperative that your carbs are adjusted to optimum settings, so you will need to balance the carbs closely.
with a t in the line you will get a larger volume of vacuum and a steadier signal. Vacuum is created in the manifold when piston is on the intake stroke.
When one cyclinder is intakeing the other is exhausting. so the t helps to smooth this out.
Realize also that under optimum conditions at sea level and engine only produces a steady vacuum of around 21 inches. You lose approximately one inch for every 2000 feet you go up.
How much vacuum does the sensor need to operate properly.
I've never had one on any of my bikes, but I assume it works like the vacuum advance canister on the distributor of older cars? correct?
If so maybe there is one available that requires less vacuum to operate?
Another question I have on your carbs.
Are they vacuum dependent to regulate the flow of fuel through the main circuit?
Holley carbs on cars are built this way.

[BLB350]

The ignition module has a whole family of curves (7 or so) that advance the timing at high vacuum conditions, primarlily to increase gas mileage. Depending on what the vacuum is, anywhere between 0-18" Hg (working off of memory here!) a different advance will be in effect. The carburetor is a Mikuni and works off of a venturi in the throat to draw fuel into carb throat.
The only vacuum canisters I could find on the Summit website are for brake assist with long duration cams. They have a check valve in them (to "store" the vacuum for brakes) and are much too large. Obviously a check valve would not work in this case.
I do not have long duration cams - 240 degrees (@ .050"), 38 degrees overlap. Thjis is not radical for a HD.