Vacuum Draw (amount) for XJ650J?

Hello,

This may have been answered already (somewhere), if so apologies in advance, but I can't find it here or via google.

I am wondering "how much" draw occurs per RPM (at location? I don't know? Any reference point would be a start I suppose, but I imagine a vacuum scale against an RPM scale would've been created at the intake manifold vacuum port.)

-- In other words -- what does the "vacuum curve" look like against an RPM scale for the non-yics XJ650J engine (or heck any reference 650 engine)? (ideally, measured at the vacuum hose port already on the intake manifold).

?

Thank you

 

Uncap one of your vacuum ports and attach a gauge.

 

the vacuum will change depending on the load, say at 3K, it might be 6 with no load on the stand but going up a hill to keep 3K you add throttle and the vacuum drops or let off the throttle at 3K the vacuum goes up.
when i used a gauge to sync, i think it was 6 or 7 at idle, synced, but i wasn't paying attention to the numbers

 

Agreed - and the 6 or 7 is inches of mercury, out of a possible max of 30
This is only a few pounds of vacuum. It's also an "average pull" after a small restrictor. Add a check valve and see !!

A V8 car pulls 17-20 inches

 

I have 4-cyl vac guage set -- I haven't yet put the carbs back on (if you wish, see my xj650 restoration thread http://xjbikes.com/Forums/viewtopic/t=35179.html
--

So, I'd like to simulate various rpm increases through various/increasing vacuums to see that the slides are behaving consistently -- particularly initial lifting.

I am going to guess that the slides will lift in the -26 to -30 ccmHg range, but I've never run the guages on the bike with the boots off to "see" when the slides first come up (rpm and vacuum level)...

I've rebuilt these carbs and am thinking about a quick sumulation vacuum test with a shop-vac with variable speed/suction control -- I'll put all four guages on the ports -- I wanted to see an as-designed reference graph (I dunno, did Yamaha publish one -- can we calculate one to see where by design the slides are supposed to start to lift?)

Thanks

p.s. I'll create my own graph soon I suppose

 

I think I remember idle is at about -14 or -15 ccmHg on my 650 at 1050 or so (if I remember corectly) which if converted to english is about -3psi if I am converting OK

 

here's some youtube to watch
load noload
shopvac
there's one there with a shopvac

 

Thanks


RPM's don't matter - vacuum matters (is the lesson) - the load varies the vacuum... (well I guess the position under load of the butterfly determines vacuum)

http://www.youtube.com/watch?v=sMKGMCim ... er&list=UL
http://www.youtube.com/watch?v=tR63vrfhwr8


I believe I'd need to modify my test if my goal is to ensure rise-consistency across all four slides -- by drawing the same vacuum thru all four intake manifolds (carbs on the rack) and opening the common-throttle link to see if all 4 vacuums stay the same (i.e. they are all synched) and that all four slides rise at the same throttle position and therefore the same vacuum (which the four connected guages will show).

http://www.youtube.com/watch?v=g0RQ47PNfYA

He's working with one slide in the above video, I'll try the same with all four carbs, one vacuum source, and 4 vac guages off the ports.

I'll try to get thsi set up over the next week and post results/measures of vacuum here.

Thanks much

 

Isn't this nothing more than trying to "quantify" a vacuum sync?

I'll write down the readings from my CarbTune Pro the next time I sync a bike. I've never paid attention to the actual numbers, just to comparing the four tubes.

 

A well running engine should pull about 10" Hg at idle. Under load it will be less. If you're only pulling 6 or 7" you have a cylinder that isn't running right.

Your slides are not activated by the manifold pressure but by the velocity of the air moving under them. That's why they're called Constant Velocity carburetors.

If you want to confirm they move the same compare the force required to compress the springs a certain distance - the springs are the only variable under your control.

 

Except maybe the position of the throttle butterflies in relation to each other? In other words, the vacuum sync?

 

Yeah, but that's at idle. The farther open the throttle the less impact of minor variations.

 

-- I just rebuilt the carbs. So, I want to test to ensure they all four operate (atleast) consistently before I bolt them back on. It will be a nihtmare tuning particularly mid-to-high end if the slides are behaving differently.

It seems someone should build a test-bed or simulator for carb racks? In order to accumulate real data to support both an understanding of how these work and data to better understand tuning?

I simply want to see if the slides all behave consistently. In other words, they all lift (initially) at the same velocity or vacuum -- it's a vacuum that lifts them -- the velocity induces the vacuum...

Yes Fitz, it is a "quantified sych" I suppose -- and the carbs have to be "synched" for this to work, so I will also do a simulated synch (i.e. hook up the guages - all 4 - start the vacuum through all four intake manifolds concurrently and synch to a consistent guage vacuum).

THEN, assuming the vacuum I connect to all four intakes is strong enough and pulls enough flow through the throats, I'll see if all four slides rise at the same vacuum and that that they do so consistently.

I guarantee that the Yamaha engineers had all of the flow characteristics and attributes and slide lift positions and various RPM's under various loads, fuel ratios, etc. mapped and probably graphed... I'd loved to see that data.

Thanks

p.s. not home this week but should be able to get this all hooked up next week... just sitting around thinking about this

 

*deleted* I forgot to quote Carl see below***

 

VERY true.

But you forgot one other important variable; something that will greatly affect the volume and velocity of said air, especially at WOT when the throttle butterflies have been pretty much taken out of the equation.

AND, it's (within a "window") under our control! Well, sorta, that's the "window" part...

VALVE CLEARANCES.

 

Gentlemen,

I gotta tell ya' you're confusing me!

Springs are not the only variable one can control -- I should be able to get a consistent draw (velocity) through the racked carbs -- I should also be completely able to vary that draw - I should be able to leave the butterflies closed (i.e. closed throttle) to "simulate" idle at a low vacuum (with a variable speed shop vac or other draw source) -- at say -14 to -16 cm/hg -- which IS what the engine draws at idle with valves in-spec. I should be able to synch the rack under this simulated condition -- I should then be able to lower the pressure in the manifolds and watch that lowered pressure on all four gauges and open the butterflies to various positions and observe the slides -- repeating this procedure with ever decreasing vacuums (to -30 or -40 or -50 cm/hg) simulating load vacuums and therefore slide behavior.

That's what I intend to try -- My proposition is that this simulation will provide the following:
1. A selfish comfort that my slides behave consistently and in a "healthy" as-expected manner given various vacuums - (so I may observe this before bolting them back on)
2. Will provide some data for slide movements (initial, half, full lift, etc.) against various vacuum levels for publication here to aid in understanding with data

That's the proposition

 

We were discussing the "in-service" variables as it were. You can vary your lab parameters all you want; the carbs are going to react to the airflow through your motor and the only thing you can do to "control" that is sync the butterflies (vac sync) and ensure the valves are in spec.

Bench-test away; it's very interesting but probably not related in any way to the conditions presented to the carbs by your running motor. You won't be ensuring any sort of synchronized reaction; that's a function of the tuning steps above.

Interesting, though.

 

CONCEDED -- If they lift differently under this simmulation -- ONE variable MAY be the springs -- Others may be:
1. Pin holes in diaphrams
2. Pistons "cleaned" inconsistently
3. Bore holes not polished consistently
4. Diaphrams of inconsistent "rubber" flexibiliy
5. Diaphrams incorrectly seated under the plastic snap-ring
6. Blocked passages

There may be others

 

All of those other variables were assumed to have been eliminated by proper carb service.

You can't produce "publishable results" if everything isn't as 100% as possible to begin with.

I was assuming "scientifically sound" carbs as a basis for this experiment, sorry.

 

I agree and I only want to simulate for expected consistent carb slide behavior ... I agree actual operating engine issues effect results ... I've specd valves ... Carbs are refurbished so I expect 100% sound working ... But I am proposing testing the Carbs via bench simulation to ensure that assumption... If they appear to operate well the data may be good... If they don't I'd rather see it there than have that variable introduced to the bike where you may not be able to actually see an issue... Make sense?

 

There's no guarantee that one's carb service will catch all issue where would you rather find out? On the bike you might not even know you've missed an issue and those issues combined with others may overwhelm one's patience ...

 

Except I doubt that the results of this experiment will be anywhere near enough to real-world operating conditions to "uncover" an omission in the carb service process.

Most carb issues stem from the tiny fuel passages not being cleaned properly; I really doubt that slides not lifting together has much to do with it, or at least not all that often (unless you're trying to sync a motor with horribly out of spec valves.)

Like I said, your theory is sound and it's an interesting experiment. It's value as a potential "diagnostic aid" is limited however. The carbs react to the motor they're on.

 

this might be great, might be a waste of time so here's a few things to think on.
how to assure each carb is presented with the same vacuum, one source has to be divided equally 4 ways, that can't change as the slides move. how can this happen with 1 vacuum source, when 1 lifts it changes the vacuum presented to the others.
and how would you fix something that you thought was wrong?
is gas a lubricant for the slide, does it aid the seal of the slide in it's bore?
your about to find out.
remember sometimes ignorance is bliss

 

Understood... Fitz and makes sense and Polock's points make the proposition using a single vacuum even more suspect. I've been thinking more and have "slept" on the idea -

Through this simulation on a test bed with a known, variable measured vacuum, I was hoping to both "watch" the carbs behave relative to each other, learn something, and to at least see if my own carb restore had mechanical issues.

Look, for example while I am confident I've thoroughly cleaned and replaced *some* required parts - I replaced:

1. all 8 fuel jets
2. throttle shaft seals fuel supply tube o-rings
3. bowl gaskets.
4. pilot mixture washers & o-rings & springs
5. four emulsion tubes
6. totally cleaned the bodies and polished bores and cleaned pistons and clunk tested etc., etc.
7. set float levels
8. bench synched

I even bought Chac's "rNr" fluid and soaked the rubber diaphragms... I did not however replace the diaphragms - I can see no holes through as careful an inspection as I can do.

So this is a cost tradeoff - the rebuild on the carbs to date has been relatively speaking expensive… Replacing diaphragms and reseating them with new plastic circle-retainers is a special skill and appears VERY VERY expensive.

But, do I trust 30-year old "rubber"? They seem OK…

So other than clunk, how to know that the basic mechanical properties of the piston-diaphragm assembly is OK? and that they are consistent among each other?

This proposed test was my solution (in addition to hopefully learning something and gathering some data).

Thanks,

p.s. I'd still like to know the as-designed data on the Mukini hsc32... like to have the data Yamaha certainly must have regarding these carbs (I posted earlier that the Yamaha engineers must have had had all of the flow characteristics and attributes and slide lift positions and various RPM's under various loads, and resulting fuel-air ratios, etc. all documented/mapped out and probably graphed)

 

First off, HSC32s are Hitachis (your carbs.)

I can tell you this about Mikuni carbs' diaphragms: I have (2) 550 Secas; one has 27K miles on it and one has 15K miles, one 9K and the other 8500 since being throughly gone through and recommissioned.

Both bikes are still running the original diaphragms, and they're fine. Both bikes are stone reliable, and return 52~56MPG while producing performance results that match what was published in the motorcycle magazines back when they were new. Plugs all textbook.

If your diaphragms are pliable, the retaining rings not broken, and you can't see any holes, THEY'RE FINE. You're way over-worrying this.

Get your float levels wet-set as precisely as possible; do a good precision bench sync (make sure the rack is straight AND true) and use a 4-stick manometer to get an absolutely perfect vac sync; tweak your mixtures and ride the bike.

The slides'll do their jobs.

 

If you (or anyone else) ever comes up with this info, please do share!

I think these are secrets that are more closely guarded and inaccesible that how to make nuclear bomb triggers.......

 

Thanks yes Hitachi

 

Thanks yes Hitachi

 

Quite honestly, I'll bet ONLY Hitachi, not even Yamaha, have said data. Yamaha provided specs; and I'm sure Hitachi provided not only a number of potential solutions, but an engineer or 20 to help get them set up. And I'll bet they didn't share some of that data, even with Yamaha.