Weird idea for checking for vacuum leaks?

[DSA]

Hi all,

76 GL1000 here. I've heard of methods of checking for vacuum leaks such as spraying carb cleaner around the potential leak sites, but these haven't worked very well for me (either that or I don't have any air leaks). I was thinking that one possible way to check might be:
1. Drain the carbs
2. Seal off the fuel line from the pump to the carbs.
3. Seal the airbox snorkel- this might be tricky, but maybe a large rubber cork in the snorkel would do it?
4. Remove the airbox breather tube
5. Apply a vacuum pump to the nipple on the airbox where the breather tube attached.

If it holds vacuum, I would conclude that I have no leaks.

Thoughts?

[Lucien Harpress]

The only thing I can think that might goober up the results is that I'm not sure there's a crank position that has all 4 intake valves closed at the same time. If there is, it's worth a try if you want to put the work in to sealing everything up.

The other thing to remember is that air leaks are only super-important downstream of the carburetor. Things like minor leaks around the air filter, the filter housing and the plenum, etc., don't really affect the air-fuel mix down the way (at worst allowing a bit of non-filtered air into the carb throats). But they may not be air-tight enough to hold a vacuum because they were never designed to need to? If that makes sense.

In any case, it might be worth a try.

[DSA]

That does make sense - all intake valves would need to be closed, and that may not be a condition that exists.
As an alternative, I could disconnect the hose from the carb plenum to each carb, and put vacuum through that instead, one at a time. I'd have to rotate the engine to get each intake valve sealed for this to work. I'd lose out on knowing if I have a leak upstream of the carbs, but as you said, that isn't as important.
Thanks for the advice!

[gltriker]

Hi all,

76 GL1000 here. I've heard of methods of checking for vacuum leaks such as spraying carb cleaner around the potential leak sites, but these haven't worked very well for me (either that or I don't have any air leaks). I was thinking that one possible way to check might be:
1. Drain the carbs
2. Seal off the fuel line from the pump to the carbs.
3. Seal the airbox snorkel- this might be tricky, but maybe a large rubber cork in the snorkel would do it?
4. Remove the airbox breather tube
5. Apply a vacuum pump to the nipple on the airbox where the breather tube attached.

If it holds vacuum, I would conclude that I have no leaks.

Thoughts?

I gather your bike's 1976 engine still, "Won't idle when warm"

Nope. Waste of time and energy. If you were to go to the extremes of closing all 4 cylinders' zero leakage intake valves and utilize a pressurized "smoke test" to check for voids in each carburetor intake tract, smoke would immediately appear leaking to atmosphere past both exposed ends of each carburetor's throttle shaft and choke shaft.

Put the Carbtune Pro aside and use a carburetor idle speed synchronizing set-up with 4 vacuum gauges, or a liquid manometer like the Motion Pro set-up Randakk displayed in his website carburetor synchronizing technical article.
https://www.randakksblog.com/carb-synch ... /#more-383
Why, you inquire?
The fact that your Carbtune Pro is displaying 14cm/hg (~5.5in/hg) , regardless of whether your engine valves' lash clearance is set at 0.004" or 0.006' is not making sense to me. Whiskerfish had performed an exacting collection of synchronizing numbers for Randakk utilizing "mercury sticks".
viewtopic.php?t=18015&hilit=Vacuum&start=15

note on page 2,post#16, his carburetors synchronizing "mercury sticks" displays approximately 20cm/hg. (~7.8in/hg ) when testing a 1975 GL1000 at 1000rpm, with valve lash set at 0.004". I process those liquid columns manometer aka "mercury sticks" synchronized vacuum numbers as being far more trustworthy when compared to the Carbtune Pro with metal rods.
Until I get my 1975 GL1000 running, again, to synchronize its carburetors, again, with my 4 vacuum gauges synchronizer panel, then checked again with the Carbtune Pro which I haven't compared with the vacuum gauges yet, I believe you are chasing your tail with the Carbtune Pro.

Although this next test doesn't apply to your 1976 GL1000, Whiskerfish also tested a 1978 GL1000 in post #19. His "mercury sticks" displays 24cm/hg (~9.4in/hg) at 1000 rpms.

There are MY thoughts, Jai

[gltriker]

Yes, I have with a Mityvac vacuum pump tested each of the 4 channels of my new Carbtune Pro and find its cm/hg converted to the Mityvac pump gauge display of in/hg results are seemingly quite accurate in comparison to the other and each channel will hold their vacuum readings until the Mityvac pump is released, too.

I hope to start trike's engine in another week or two. Yes, although its engine ran very well with all 8 valves lash set at 0.006", 3 years ago, I have reset the engine's valve lash back to 0.004" and will start the synchronizing session with my 4 vacuum gauges panel.
After I am satisfied the 4 carburetors' idle speed synchronization results are stable, I will employ the Carbtune Pro to compare its dynamic behavior to the 4 vacuum gauge panel.

Here's where that information will be collected:
viewtopic.php?t=78145

edit Saturday 9/16

Thinking about my claim the individual Carbtune Pro channels held vacuum until released I am not sure whether that statement is true.
I was utilizing my cell phone in one hand, the Mityvac pump in my other hand and trying to keep the cable tie suspended Carbtune Pro display from twisting away from the cell phone camera while operating the vacuum pump.

A bear cub with boxing gloves?

The individual hose connections from Mityvac to Carbtune barb fittings may have been intermittently leaking too.

Since I don't know how the Carbtune "manometer??" operates internally, I have to withdraw my questionable statement.

The fact the Carbtune utilizes an imprecise section of nylon tube restriction within several inches of the intake manifold synchronization fitting causes me to question why it isn't positioned closest to the Carbtune Pro, itself.
Their manual states this

The restricted part of rubber tube must go towards the engine, not the Carbtune.


Why am I obsessed with that alternative?

My 4 vacuum gauges synchronization setup has the maddeningly sensitive to gauge needle damping adjustable fish tank valves very close to their respective vacuum gauge. Each adjustable valve's screw is almost completely closed when its assigned gauge needle oscillation is sufficiently snubbed for better accuracy results.

Back in May, I Reversed the 4 gauge vacuum hoses -end for end- to place the adjustable fish tank valve closest to the intake manifold.
Although trike's engine was running like crap with questionable liquid, I had to unscrew each of the 4 fish tank valves to the extreme point of Almost completely disengaging their threaded connection to have each gauge needle show a sign of the typical vacuum gauge twitching behavior.

Jai, I need you to reverse your Carbtune Pro synch hoses end for end and run the engine, in a, as prescribed by experts, carbs synching session.

Does that end to end hose restrictor position swap over appreciably change the Carbtune Pro's cm/hg vacuum reading display number?

Thanks.

[DSA]

Not quite - the "won't idle when hot" issue has been gone a while. Where I am now is that the bike will idle just fine - but if I give it gas, it revs up to about 3000 RPM and stays therefor a while. It was backfiring through the exhaust quite badly while doing that. I changed the ignition coils (but kept the points), and now the backfiring is much better, but it still sticks at 3000 RPM. I've checked the advance mechanism; it isn't sticking. There's plenty of slack in the throttle cables, and it snaps back nicely.

Next things to check for me are air leaks (hence the questions above), and perhaps to check for sticking slides. As a quick check, I think I'll try tapping the top of the carbs in turn with a mallet when the RPMs are hanging; if that drops the RPM, maybe I've got my cause?
One other thing I want to try is just replacing the gas in the bike - it's getting pretty old at this point because idling doesn't use much. I doubt that will fix it, but will do so anyway.

As to the carb, sure, I can reverse the tubes and try that too. I think I've done so before? If I recall, it didn't change the readings as much as it made them fluctuate a lot more.
I would caution against taking my carb readings at .004" too seriously - I've done a ton on the bike since then, and the state of it was somewhat questionable when I did that. As soon as I get to the point when the bike is running properly, and I trust what I am measuring, I will do a back-to-back test at .004" and .006" and report results.

[gltriker]

Wait a minute... " tapping the top of the carbs in turn with a mallet" struck a chord... have you closely examined the composite button on top of all 4 vacuum piston covers?

[CYBORG]

Still sounds , to me, a lot like the carbs need a sync

[gltriker]

Jai,
I haven't been successful to locate any individual cylinder compression testing results numbers you should have performed on your bike's engine with correct dynamic cylinder compression testing equipment. Numbers not speculation

Numbers. Please.

correction!Yes, I've been poring over older discussions! Again You did post a generalized answer to my request for individual cylinder compression test results.

snip- Cliff: Compression numbers are all 160+. At this point, they need to be individually tested... again

[redglbx]

I’m with Cyborg as this sounds exactly like a carb sync problem, somethings off. If you have a slide hanging it should be flooding the engine. Have you done anything with the anti-backfire valve on the #1 carb by chance ?

[gltriker]

Getting Serious Now

https://www.randakksblog.com/lazy-slow- ... /#more-983

Ciao . gotta get my own bike (trike) up and reassembled and work on getting my old carcass on and off it safely.

[DSA]

Big update and great news!
To date, I'd synched the carbs several times using my Carbtune pro. But multiple people on here saying that my issues sound like unsynched carbs and Cliff's suggestion that maybe my Carbtune isn't working properly got me thinking. So: I re-read the Carbtune manual. It tells me I am supposed to be hanging the carbtune off the handlebars of the bike - I hadn't been doing this. Instead, I'd been laying it on the false tank, almost horizontally. I did as the manual said and hung the instrument off the bars, and lo and behold...it indicated no vacuum. On any of the cylinders. That obviously didn't make any sense, since the bike was running. If I blipped the throttle, I got a briefly got a reading, but only from 3 of the cylinders. If I angled the carbtune so that it was more horizontal (what I had before), I started getting a reading from all 4. But now my confidence in the reading was pretty badly shaken. So back I went to the manual - and it said that for bikes pulling low vacuum, one alternative is to hand the carbtune upside down. Gave that a try, and now I had readings. Using this, I resynched the carbs. Let it sit overnight, then took it for a test ride just now.

The difference is night and day. No more hanging at high RPMs and sitting there backfiring. Much better throttle response and power. All the way from fully cold to warmed up. Took it up to 5000 RPM with no problems. In short - something approaching an actual motorcycle!

There's still things to fix. if I am at something just shy of steady throttle (meaning very slow decel), I get some backfiring. No issues at heavy decel (fully closing throttle from open, even if I pull in the clutch), so I do not think it the issue is the air cutoff valve, which I rebuilt when I did the carbs. I also find it hard to believe that this bike is so slow. I know it's from the 70s and it's not going to be as quick as a modern day liter bike, but still- this cannot be it. Going by feel here because the speedo doesn't do anything after about 25 mph, though I am clearly still accelerating.
I think first order of business is to stay on the carb sync. But I certainly don't trust my carbtune anymore: I am going to check if the guy down the street with a 70s Guzzi has something he can lend me (surely he must), or failing that, this bike can at least now be ridden 6 miles to the local Honda dealer, where it turns out the head mechanic loves GL1000s and has one on the lift right now.

Thanks again all for sticking through this, and for the excellent advice of re-assessing carb sync, even as I told you with utter certainty that I had already done it. Glad I eventually questioned myself on it...but wish I'd done it sooner.

[gltriker]

Jai- Very pleased you had figured out how to use the Carbtune Pro with metal rods


I just found this youtube video that shows the viewer exactly what the universally employed, go-to 4 vacuum gauge panel looks like, the in-line fish tank adjustable flow restrictors used to snub the oscillations of their respective vacuum gauge needle, and the vacuum gauge needle adjustability, too. Although the motorcycle he is testing isn't a carburetted 4 cylinder GL1000, note how close the engine running synchronization vacuum gauge numbers of the 4 cylinders are reading out. It appears he may have made a very slight synchronization adjustment there, too.
At no point in the entirety of this video are we actually seeing a tachometer readout less than 2000rpm.

Remember, the GL1000 engine curb idle speed must not exceed 900ish rpm during the entire carburetors' idle vacuum synchronization session.

Then, the Carbtune Pro is connected to that same bike's vacuum synchronization adapters. I first noted the 2 outboard channels matched , and the 2 inboard channels matched. The fellow started making adjustment to equalize the 4 channels' vacuum displayed in cm/hg readout and the running engine idle speed audibly increased while he was making the hidden linkage adjustments. hmmmmm?

I was hopefully expecting he would reinstall the 4 vacuum gauges synchronizer setup to compare with the Carbtune Pro readjusted idle vacuum synchronized engine. I was crushed when he didn't
The comparison of where the 4 vacuum gauges numbers were first displaying, prior to the changeover to the Carbtune Pro, then tested with the 4 vacuum gauges, again, would have been interesting to see how much the Carbtune Pro readjustments had altered the initial 4 vacuum gauges synchronized readings.

Maybe my advisement to set the Carbtune Pro aside and dynamically idle vacuum synchronization test your GL1000 engine with the go-to 4 vacuum gauges and/or liquid column manometers will add some credibility to my suggestions and make better sense to you now.

https://www.bing.com/videos/riverview/r ... ORM=VRDGAR

[DSA]

Worth doing for sure. Yeah, he kinda missed the punchline, didn't he? Still, a good video.

Further update: After fixing the disconnected wire between the condenser and one of the ignition coils last night, I rode it again this morning. This is it! It it now impossibly smooth, no backfiring everywhere, and revs so easily you'd swear it barely has any flywheel mass. It also pulls HARD - where before I was wondering why it felt so slow, I'm now pleasantly surprised at just how quick it is.

Now that the bike is fully rideable, I'll spend some time on a few go-backs. First, will try and sync using vacuum gauges and compare to the carbtune results. Second, will do that test where I check vacuum vs valve lash - I think it's safe to say that all my previous results should be ignored.
In addition to that, I've got some electrical work to puzzle my way through, as well as figuring out why the speedo is laggy/doesn't read very high (showed 30 when the speed trap on the road read 60...and it sure felt a lot more like 60 than 30 from the saddle).

[redglbx]

Personally I’m not a Carbtune fan, I have a CTune Pro and it constantly gives me trouble with slow or lazy response, so most times I reach for my vacuum gauge set as it just works. Generally takes a little time to properly calibrate them before I use them but I do find them more reliable than the Carbtune.

I may take some time once I get into my 76 and disassemble and thoroughly clean the Carbtune,,,not a fan !