How do you precisely measure engine vacuum?

[ELP_JC]

Never needed to actually measure engine vacuum before... until now, so want to do it right (over the weekend).

Engine pulses (vacuum, no vacuum...) cause a gauge or a 'stick' to bounce, as we all know. If we want to measure 250mmHg, like in our case, there are 2 possibilities:

1. Gauge or stick only goes to HIGHEST vacuum, meaning the highest bouncing level is the accurate vacuum reading.

2. Gauge or stick goes BEYOND true vacuum due to momentarily sucking then abruptly stopping the vacuum, causing the gauge/mercury/etc to shoot past actual level, meaning the middle of the bouncing is the correct reading. Having said that, I'd imagine with a spring-loaded gauge, or heavy mercury, there's less of a chance of this happening, than say, with ATF or something lighter. Comments welcome.

So what is it? Let's start the debate.

PS. I'll use a Craftsman vacuum gauge to measure 250mmHg on #3, then mercury sticks to balance rest against #3.

JC

 

[evil_henchman]

Engine pulses (vacuum, no vacuum...) cause a gauge or a 'stick' to bounce, as we all know.

Not if you have snubbers in the line it doesn't.

 

[ELP_JC]

Not if you have snubbers in the line it doesn't.

Mine do (MotionPro mercury sticks). Never tried measuring engine vacuum without restrictors. Have done TB/carb sync a ton of times, but never needed to measure vacuum levels.

Anyway, that's why I said 'precisely'. I know 3mmHg difference is not a big deal, but am mostly curious which of the 2 scenarios is accurate. Later.

JC

 

[radman]

My Morgan is steady as a rock. If a bouncer, note high, then low, and average the two. Dampeners (restrictors) will stop the fluctuations though.

 

[ELP_JC]

My Morgan is steady as a rock.

That makes perfect sense. A big metal rod is much heavier than mercury, just as mercury is much heavier than ATF fluid.

Also worth mentioning is mercury stabilizes at higher rpms, where vacuum is less.

And the restrictors provided by MotionPro are only the size of a small pin, but still allow some bouncing at idle (highest vacuum pulsations).

So will measure the middle of the range then. Thanks man.

JC

 

[ionbeam]

My Morgan instructions say to adjust the tube length until the rods just slightly oscillate to prevent stiction. My Motion Pro instructions say to adjust tube length until pulsing stops. My Motion Pro is virtually rock stable, Morgan oscillates.

Morgan says: "There will always be a small amount of fluctuation and this is necessary for the gauges to work properly."

If you need absolute vacuum accuracy use a mechanics gauge with an inline restrictor. Use a solid wood dowel or plastic rod on a dill press with a vice. Start with a hole around #79 .0145". The restrictor will serve to average the readings.

 

[evil_henchman]

Like Rad, my Morgan is rock steady but restrictors (snubbers) are only half of what's involved. You need the correct length of full sized tubing on either side of the restrictor to make it work. That gives you the correct volume to act as an accululator. The lengths you need before and after the restrictor are different (less before) and are based on inside tube diameter. So do I sound smart yet? :blink:

 

[Constant Mesh]

If you put the flow control orifices at the engine end of the tubes you would likely see a dramatic reduction in the pulsations. Obviously the air volume between the orifice and the mercury increases when you move the orifice closer to the engine. The magnitude of the pulsation is a function of air volume behind the orifice, orifice size, and frequency of pumping cycles.

 

[ELP_JC]

Thanks a lot for your great answers.

I don't think we need dead-on accuracy for this purpose, so my vacuum gauge with a not-so-small restrictor should work. Thanks to you I know that the AVERAGE, or middle of the 'bouncing', is where I need to aim at. And some fluctuation seems perfectly normal as well, and it's not a problem averaging it.

As far as the placing of the restrictors, that's very interesting, and makes sense as well. I had the 'Deluxe' MotionPro tuner for the longest time, but loaned it and never got it back, so ordered another one. The only mercury one I could find anywhere (the 'economy') was paid over a month ago, and never got it. After disputing the charge, finally got it Thursday.

Instructions were vary basic. Pour mercury thru vent hole, cut hose in 4 pieces, insert restrictors 3/4", and insert hoses into tubes. That's it. And those instructions were identical on my previous (old) deluxe model.

Anyway, here's what I did: inserted the restrictors (kind of buckets with a hole on top) with the orifice facing towards the engine, and inserted all of them exactly 3/4". Then I inserted that end on the solid tubes on top of the tool until I had about 5mm of hose between the top of each tube and the bottom of the plastic 'bucket', but couldn't get them exactly the same (hard to insert the darn hose), but close. Is this the best way to install the tuner?? And thanks again to all who contributed.

JC

 

[Jug]

I helped my dealer's mechanic do my 600 mile throttle sync check (he had never done an FJR). We must have missed the part about setting #3 to 250mm/HG. How critical is that step?

I am wanting to get a sync tool so I can do it myself, but when I went looking most that I found were dial types. I guess the EPA has nixed the mercury ones? Anyone using a dial set? are they worth getting?

 

[Vulcan]

I had a cheap vacuum gauge that bounced....one day it broke so I bought a Snap-on.....rock steady. My lesson learned....buy good quality tools and you will find that it is cheaper in the long run.

 

[ionbeam]

I helped my dealer's mechanic do my 600 mile throttle sync check (he had never done an FJR). We must have missed the part about setting #3 to 250mm/HG. How critical is that step? I am wanting to get a sync tool so I can do it myself, but when I went looking most that I found were dial types. I guess the EPA has nixed the mercury ones? Anyone using a dial set? are they worth getting?

Actually, cyl #3 is supposed to be a reference cylinder that you do not touch. Apparently the admonishment 'not to touch' has caused nearly everybody to futz with the #3 right away before adjusting anything else

Motion Pro offers a detoxified carb stick - Sync Pro. A bit spendy at $99. The FJR does not require any accessory fittings. For a bit more money there is the Morgan Carb Tune.

 

[ELP_JC]

We must have missed the part about setting #3 to 250mm/HG. How critical is that step?

I wouldn't say it's critical, but very important, especially at idle and small throttle openings. I'd definitely get it right buddy. And actually you need to set ALL TBs to 250mmHg, but if you set #3 first, then adjust the rest to #3, it's the same thing. Up to you. Remember the pressure sensor is a vacuum sensor, hence the importance. The larger the throttle opening, the less and less important it becomes.

Vulcan, MotionPro gauges are not cheap buddy, and they bounce a little. Actually bouncing is good, since vacuum signal is on/off/on/off/...., meaning the signal is not 'buffered'. I have zero problem synching like that, but brought up the 'bouncing' issue due to the need to precisely measure 250mmHg. Realized NO fluid synch tool is precise at measuring, so will do that with a vacuum gauge, then synch the rest to #3 with my mercury sticks.

As a comment, I used to have a TwinMax electronic synchronizer, but when I took all the bouncing off the dial, it wasn't accurate, so sold it. I really like mercury sticks, and was lucky to grab one of the last in the market.

Ionbeam, you must be confusing the 250mmHg with the 'unauthorized' synch method buddy. Even the service manual calls to set #3 at 250mmHg BEFORE synching the rest, but it says that AFTER the procedure, which is kind of stupid, and probably why Jug's tech missed it. By the way Jug, that's done with the air screws, so no biggie.

What Ionbeam was probably referring to is not to mess with the actual TB butterfly plates; this is the so called 'unauthorized' TB synch.

I'll check mine when doing my normal TB, and if the synch is not too much out of whack at 3K rpms or so, will probably leave it alone for now. But in winter, when it's cold, will definitely get it right. My SV1000 V-twin was within spec, but when I synched it almost perfectly, it made all the difference in the world. I was amazed my hands didn't tingle even after a 300-mile ride, when before that 1/16th of a turn difference, they started tingling after 30 miles. Later gang

JC

 

[ionbeam]

Ionbeam, you must be confusing the 250mmHg with the 'unauthorized' synch method buddy. Even the service manual calls to set #3 at 250mmHg BEFORE synching the rest, but it says that AFTER the procedure, which is kind of stupid, and probably why Jug's tech missed it. By the way Jug, that's done with the air screws, so no biggie. What Ionbeam was probably referring to is not to mess with the actual TB butterfly plates; this is the so called 'unauthorized' TB synch.

I'm not referring to the 'unauthorized' synch at all. No place in my Gen I FSM could I find any reference to adjusting #3. Both the Gen I and Gen II manuals say: "a. With throttle body #3 as standard, adjust throttle bodies #1, #2, and #4 using the air screw" and later is says: "Vacuum pressure at engine idling Speed 33.3 kPa (250 mm Hg, 9.8 in Hg)". It doesn't say to set anything. I believe the FSM is remiss in not saying: "At sea level - vacuum pressure at engine idle.....". If I lived in Denver, why would I want to set #3 to 250mm Hg, same as a coast hugger? I'm sure the factory set #3 to some standard, not to say that someone can't make changes to it. Unless the tuner knows the whats and whys for changing the factory setting for #3 I would suggest they leave it be.

Please supply the Gen II FSM page that says to adjust #3 to 250mm HG 'cause I couldn't find it. Thanks buddy.

 

[radman]

I've given up on using new-fangled gauges altogether, and converted an old dowsing rod for the job. I'd also like to thank Buddy.

 

[Rickster]

Ionbeam, you must be confusing the 250mmHg...

...

Please supply the Gen II FSM page that says to adjust #3 to 250mm HG...

Gen II FSM page 3-9, Step #8

a. With throttle body #3 as standard, adjust throttle bodies #1, #2, and #4 using the air screw #1

On page 2-8 in the specification, it does show at Idle, the intake vacuum should be 33.3 kPa (9.8 inHg) (250 mmHg), but there is no mention of adjusting #3 to 250 mmHg

 

[ELP_JC]

Gen II FSM page 3-9, Step #8 a. With throttle body #3 as standard, adjust throttle bodies #1, #2, and #4 using the air screw #1

On page 2-8 in the specification, it does show at Idle, the intake vacuum should be 33.3 kPa (9.8 inHg) (250 mmHg), but there is no mention of adjusting #3 to 250 mmHg

Thanks for posting that buddy. And there's no specific mention of adjusting #3 to 250mmHg because it's implied. 'A' says to use #3 as standard, then spec says idle vacuum should be 250mmHg. Which means ALL TBs should be at 250mmHg after they're properly synched. By the way, after adjusting EACH air screw, reset your idle speed to 1K (easier there due to the big marking, but can be anywhere from 1K to 1.1K) before proceeding.

And I wouldn't worry if TBs are not spot on to 250. Since the spec for the difference among TBs is 'within 10 mmHg', I'd interpret that spec to be between 245 and 255 mmHg. Later.

JC

 

[ionbeam]

I've written and deleted a couple of replies to the previous post. Let me just say that correct intake vacuum levels will change with altitude. If you live in Denver, don't kill your motorcycle trying to get 250mm Hg, if you can get 220 to 225 call it a day.

Now I'm off with Rad to find a dowsing rod and a cricket to help me gauge ambient air temperature.

 

[Jug]

I certainly appreciate all the information. The good news is my bike did run smoother after the sync. On the other hand I get the impression that the dial gages I see on E-bay etc are a waste of money. So I'll save up for a better set.