Vacuum gauge reading

[Anonymous]

I'm synching the cv carbs on my 81 gs550 using 4 vacuum gauges with dampener valves in each line. When I dampen the needle so it only moves a little I get a reading near 5. This reading to me is very low because I'm used to intake manifold vacuum readings on car engines. Can someone confirm this low reading is ok? I've checked cam timing and spark timing and they're ok. Intake boots and "O" rings are also Ok. I spent most of the day yesterday using the search function on this forum and found some great stuff. The closest one said the reading doesn't really matter it matters that they're all the same and that maybe you should set the middle 2 cyls. to run a little richer (slightly lower vacuum reading) so they're cooler. What should the needles read?
I enjoy reading all the different viewpoints and responses I find here. This forum is the best.

[earlfor]

Re: Vacuum gauge reading

Reducing vacuum level will lean mixture, NOT richen it. Reducing vacuum decreases intake velocity and and the venturi effect. Less fuel will be drawn. Intake air volume for a given rpm will remain constant, but velocity will vary with the carb slide heights. Velocity decreases if volume (rpm) is constant and throughflow area (slides lifting) increases. This is why I disagree with the view that it does not matter what the vacuum level is, and only that all carbs have the same level is important. Optimum vacum levels depend on a lot of things, but most of them are not something that can be easily changed. Generally, if you set vacuum of any of the GS's to
20 inches of mercury at 2300 rpm, you will get a plug burn in the medium to light brown range. (assuming you have stock jetting, intake filters and exhaust.

Earl

Ok.
I spent most of the day yesterday using the search function on this forum and found some great stuff.

**The closest one said the reading doesn't really matter it matters that they're all the same and that maybe you should set the middle 2 cyls. to run a little richer (slightly lower vacuum reading) so they're cooler. What should the needles read? **


I enjoy reading all the different viewpoints and responses I find here. This forum is the best.

[Anonymous]

Earlfor. Ok great, Thanks, I agree. The 20 in. of mercury sounds more like it. I can only get a max reading of about 15 on the vac. gauge when letting the engine coast down from higher rpms like 3000. When it's idling I only have about 2 in. of mercury indicated on the gauge. Can you maybe think of what could be wrong. The cam timing marks align just like I've found on this forum and the timing and advance work great.

[earlfor]

To sych the carbs (set vacuum levels) the engine must be running at a steady/constant speed. I set the rpm with the idle adjustment knob and never touch the throttle grip at all. As I "tweak" back and forth between the carbs, rpm usually increases and I have to keep lowering it with the idle adjustment knob. When I'm finished, the rpm is the same as it was when I started making the adjustments. It can take a few minutes, so I usually put a floor fan in front of the engine to help with cooling. (south Fl) :-) Also, the engine needs to have been run long enough for it to be at or close to normal operating temps before doing any carb synch'ing.

The only thing I can think of that would cause loss of vacuum levels is an
intake air leak between the carbs and cylinder head. It is odd that all 4 carbs would have air leaks if that is the case. My impression is that you are using vacuum guages and not mercury tubes. Vac guages have to be calibrated/ synchronized to each other before each use, so it is possible that your guage reading of 15 inches is in truth what is registering from an actual vacuum of 20 or more inches.

When I am working on a bike for someone and am not familiar with it, the first thing I usually do if I am going to sych carbs is pull the plugs to see which one is burning closest to the correct color. Then I warm the bike up and with it running measure the vacuum on the optimum cylinder. That gives me an idea of the baseline vacuum level. I adjust to a value either higher or lower depending on the color of the "control" plug.

If you measured your vacuum levels relative to your plug burn before making any changes, you can synch them that way. It is the plug color and condition that is important and numerical vacuum value is only a measuring guage to use in achieving that.

Small changes in vacuum can make a big difference in plug burn.

With the engine off and the slides at rest, are they seated all the way closed in the carb or are they raised slightly?

Earl

Earlfor. Ok great, Thanks, I agree. The 20 in. of mercury sounds more like it. I can only get a max reading of about 15 on the vac. gauge when letting the engine coast down from higher rpms like 3000. When it's idling I only have about 2 in. of mercury indicated on the gauge. Can you maybe think of what could be wrong. The cam timing marks align just like I've found on this forum and the timing and advance work great.

[Anonymous]

Earl
You asked if the slides were all the way down with the engine off. Yes they are.
A little background on the bike may help. I got it recently and it wouldn't run. It came from a lein sale. I finally got it running on 2 cyls. and found the igniter bad. Replaced the ignitor with a Dyna III. Ok now it runs on all four but really bad. The carbs had been messed with and I think I now have them in good shape. I'm experimenting with the float level. It seems low because the carbs won't pick up the fuel for high rpm's until I put my finger near the slide increasing the velocity past the needle once one carb picks up fuel the others follow suit with the increased rpm. The only picture I found on the forum for setting the float level is an unclear pencil drawing. So I set it as many ways as I can at about 7/8". It seems that when I set the fuel level very high the carbs pick up the fuel better.
That said, the thing won't idle right and is hard to start. It starts like a car that has the ignition timing set way to retarded. When it does idle, and then I add some carb spray to the air intake the rpm picks up to about 2000 then when I use the thumbwheel to turn the rpm back down it idles for maybe 3 seconds then slowly drops off and dies unless I add more carb spray to the intake. The intake boots look great. I've replaced their "O" rings with ones from the dealer. The clamps holding the boots around the carb spigot seemed like they weren't squeezing tight enough so I put a couple of layers of tape between the boot and the clamp. There's still a very small leak around the boots somewhere though because when it's idling I can spray around the boot area on all four cyls and the rpm picks up slightly sometimes enough to make it idle really fast like I mentioned above (2000 rpm).
About the vacuum gages. I checked the set of 4 against another one I had and the reading is the same. I can get the needle of one gauge at a time to go up to around 17 if I stop the inflow of air by putting my hand over the carb intake. I just can't believe it even runs with a reading of only 2 or 3 at idle.
I thought that maybe something happend since the last time I checked the compression so I checked it again. All cyls are near 100 psi.
This one really has me stumped. It's probably something simple that I am overlooking (I hope) and your help is really great.

[earlfor]

OK, just going to intersperse my comments. Seems easier that way.

[quote="double,d"]Earl
You asked if the slides were all the way down with the engine off. Yes they are.
***************** when my slides are all the way down, engine off, there is a 7/64" gap between the bottom edge of the slide and the carb body. (a 7/64" drill bit just slips through the opening) That what you have?**********



A little background on the bike may help. I got it recently and it wouldn't run. It came from a lein sale. I finally got it running on 2 cyls. and found the igniter bad. Replaced the ignitor with a Dyna III. Ok now it runs on all four but really bad.
*********** How sure are you that you have the timing set correctly?
750's set to 32 deg BTDC, but I am not sure that is correct for a 550.
************



The carbs had been messed with and I think I now have them in good shape.
************** Did you disassemble them and soak them to dissolve varnish build up in the passageways that are internal in the carb body?
If so, what did you soak them in and for how long?
***************


I'm experimenting with the float level.
**********
Float level should be 21mm. That would be about 13.2/16ths or think of it as 13/16ths on the high side of the mark. :-) Float level is measured with the carb upside down, the float bowl gasket removed and the float resting with only its own weight on the float needle. The distance between the metal surface the bowl gasket was sitting on to the top edge of the rounded plastic float should be 13.2/16ths inch.
************




That said, the thing won't idle right and is hard to start. It starts like a car that has the ignition timing set way to retarded. When it does idle, and then I add some carb spray to the air intake the rpm picks up to about 2000 then when I use the thumbwheel to turn the rpm back down it idles for maybe 3 seconds then slowly drops off and dies unless I add more carb spray to the intake.
************ It should run on choke alone and you do not need to touch the throttle. Start it up on choke. Dont touch throttle. Adjust rpm by increasing or decreasing choke. While it is running on only choke, reach over and touch the header pipes at the exhaust ports (carefully) They should all feel the same temperature. if they do not, then you have a choke circuit problem on the cold cylinders. Depending on which cylinders are the problem, it could also be an ignition problem. Let me know what you find
**************




There's still a very small leak around the boots somewhere though because when it's idling I can spray around the boot area on all four cyls and the rpm picks up slightly sometimes enough to make it idle really fast like I mentioned above (2000 rpm).
*********** Sorry, no cigar on this one. :-) We cant have any leaks as
a small one messes things up. Its somewhat like being "slightly" pregnant.
:-) :-)
***************



About the vacuum gages. I checked the set of 4 against another one I had and the reading is the same. I can get the needle of one gauge at a time to go up to around 17 if I stop the inflow of air by putting my hand over the carb intake. I just can't believe it even runs with a reading of only 2 or 3 at idle.
**************The only other possibility I can think of right now is that vacuum reaches the carb diaphram chamber through an opening in the side of the slide/piston. You might remove the slides and examine them closely for anything blocked or unusual, or one different than the others in any way.
******



Still thinking on this one. :-) :-)

Earl

[KEITH KRAUSE]

Two questions about the tool you're using. Does it have a vent cap that needs to be removed? Do the adapter tubes have their rubber washers or o-rings to insure a leak proof fit?
You have to fix ANY intake leaks. You DO have the filter(s) on while trying to sinc'? As Earl said, the float height needs to be set correctly. Sounds like the pilot circuit (at least) is clogged up and needs cleaning. Have you moved the mixture screws? Ignition timing and tappet clearances need to be correct. Are the diaphragms in good shape and seated correctly?

[Anonymous]

Earl and Keith, Thanks for your responses.
I'll respond to your questions later but right now I'm wondering if the exhaust systems ever get clogged on these gs550's?

[earlfor]

I guess anything is possible.

Earl

Earl and Keith, Thanks for your responses.
I'll respond to your questions later but right now I'm wondering if the exhaust systems ever get clogged on these gs550's?

[Anonymous]

I worked on the gs550 today and have new news. First of all I took off the exhaust system and blew compressed air through it and it is just fine with no restriction.
Now to answer your questions.

[quote="double,d"]Earl
You asked if the slides were all the way down with the engine off. Yes they are.
***************** when my slides are all the way down, engine off, there is a 7/64" gap between the bottom edge of the slide and the carb body. (a 7/64" drill bit just slips through the opening) That what you have?**********

I think maybe I didn't tell you my carbs are the cv type. I did have some trouble with the slides returning to rest without the engine running and cleaned that up now they return all the way down every time.

*********** How sure are you that you have the timing set correctly?
750's set to 32 deg BTDC, but I am not sure that is correct for a 550.
************

First I set both pickups using a test light holding the advance all the way advanced to the advance mark. (there wasn't an advance mark on 2&4 so I measured and put a mark there.) Then I double checked the timing with a strobe type timing light with the engine running at idle and at higher full advance rpm. and it was almost right on the marks.

The carbs had been messed with and I think I now have them in good shape.
************** Did you disassemble them and soak them to dissolve varnish build up in the passageways that are internal in the carb body?
If so, what did you soak them in and for how long?

I didn't use the bucket and basket type soak but I did use B12 carb spray through all the passages and made sure all passages were clear. I drilled out the idle mixture plugs and cleaned there too and checked the "O" rings there. The floats were all bent funny so I straightened them and set the level to .88" +?- .04" according to a post by first timer on May 16. 2003. The float drop stop on one carb was bent wrong so I set it like the other 3 carbs. I found the #3 carb (from the left) varnished a little and most of the passages were clogged. Why just one carb is beyond me.

I'm experimenting with the float level.
**********
Float level should be 21mm. That would be about 13.2/16ths or think of it as 13/16ths on the high side of the mark. Float level is measured with the carb upside down, the float bowl gasket removed and the float resting with only its own weight on the float needle. The distance between the metal surface the bowl gasket was sitting on to the top edge of the rounded plastic float should be 13.2/16ths inch.
************

I spent hours reading the past posts on float setting and the question I read lots is where to measure to on the float. Most of the time the answer is measure to the bottom of the float. Well if you hold the carb right side up and the float is hanging down the lowest part of the float is the large radius curve. If you turn the carb upsidedown with the float setting on the spring pin of the needle the highest part is the flat part that the metal connects to. So that's why I'm experimenting and also just to see what difference it makes when I change it.
All those hours of reading did make me double check my tank valve. I have the type that requires vacuum. I found the filter screen in the tank in backwards so that when the handle was in the on position it was on reserve. The long tube inside the oval shaped screen was lined up with the resreve position. So now just to eliminate any question I'm running all the time with the valve on prime just for troubleshooting reasons.

That said, the thing won't idle right and is hard to start. It starts like a car that has the ignition timing set way to retarded. When it does idle, and then I add some carb spray to the air intake the rpm picks up to about 2000 then when I use the thumbwheel to turn the rpm back down it idles for maybe 3 seconds then slowly drops off and dies unless I add more carb spray to the intake.
************ It should run on choke alone and you do not need to touch the throttle. Start it up on choke. Dont touch throttle. Adjust rpm by increasing or decreasing choke. While it is running on only choke, reach over and touch the header pipes at the exhaust ports (carefully) They should all feel the same temperature. if they do not, then you have a choke circuit problem on the cold cylinders. Depending on which cylinders are the problem, it could also be an ignition problem. Let me know what you find
**************

It will start and run with the choke on. Today was the first time it started cold without putting my hand over the carb air intake to richen the mix enough to make it fire. I think the hand is probably necessary because of the low vacuum readings. I need the hand trick when the engine is warmed up also. I can adjust the rpm using the choke. The temperature of the pipes while the engine is warming up seems the same on all exhaust pipes at the head. They are very hot ouch.


There's still a very small leak around the boots somewhere though because when it's idling I can spray around the boot area on all four cyls and the rpm picks up slightly sometimes enough to make it idle really fast like I mentioned above (2000 rpm).
*********** Sorry, no cigar on this one. We cant have any leaks as
a small one messes things up. Its somewhat like being "slightly" pregnant.

***************

Thanks to Keith's post I zeroed in on the "O" rings for the vacuum gauge adapters and that's where the leaks were. There were no "O" rings. Now when I spray around the boot area there's no rpm change.


About the vacuum gages. I checked the set of 4 against another one I had and the reading is the same. I can get the needle of one gauge at a time to go up to around 17 if I stop the inflow of air by putting my hand over the carb intake. I just can't believe it even runs with a reading of only 2 or 3 at idle.
**************The only other possibility I can think of right now is that vacuum reaches the carb diaphram chamber through an opening in the side of the slide/piston. You might remove the slides and examine them closely for anything blocked or unusual, or one different than the others in any way.
******

On the cv carbs there's a passage at the top of the air intake part of the carb. that's oval shaped and connects through a passage to the chamber above the diaphragm. I think I remember finding the passage in the slide that exposes the bottom of the diaphragm to manofold vacuum and they were clear. I didn't take the needles out of the slides but I did feel that they have spring tension holding them upward. I'm wondering if these needles have adjustments in case I need it later. I've read some do and some don't.
It seems to me that if I stop the inflow of air to one of the carbs with the engine running it should make the vacuum reading go up to at least 20 in. of mercury it should also go very high when the engine is decelerating. That's why I took the exhaust off. There's got to be some reason for the low vacuum readings. I checked it again with another gauge (2 total) and all 6 gauges read the same. I opened the dampner valves in the lines to the gauges a little and now at idle or even at about 2000 rpm they read at the top of the pulses about 7" of mercury. I looked at the gauge readings when the engine is just cranking not running and they pulse up to near 5" of mercury.

Keith asked
Two questions about the tool you're using. Does it have a vent cap that needs to be removed? Do the adapter tubes have their rubber washers or o-rings to insure a leak proof fit?
You have to fix ANY intake leaks. You DO have the filter(s) on while trying to sinc'? As Earl said, the float height needs to be set correctly. Sounds like the pilot circuit (at least) is clogged up and needs cleaning. Have you moved the mixture screws? Ignition timing and tappet clearances need to be correct. Are the diaphragms in good shape and seated correctly?


No the tool has no vent cap. I fixed the "O" ring leak.
The machine came with the connector to the air filter gone. the air filter and it's box is there but the parts inbetween are missing. I've worked on a number of 4 cyl. motorcycles in the past and they all would run pretty good with the air cleaners detached from the carbs although the air cleaner in place at idle slowed the rpm slightly. Do you think this particular carb must have the air filters connedted to idle? This thing is so far from idling right that I can't believe the air filters off could be the reason.
When I had the cam cover off to check the cam timing I felt each puck to see if there was clearance. I wanted to make sure the valves were closing and they all had clearance.
Today I found that it seemed #1 carb on the left was slightly lean at an idle of around 1000 and tried adjusting the idle mix needle and couldn't richen it enough to make a difference. On this carb when I put my finger very close to the slide at the bottom then it starts picking up fuel and the rpm goes up to almost 3000 and stays there. Then when I turn the idle down with the adjuster between #2 and #3 carb the thumbwheel, that carb leans out and soon the engine will die if I don't either put my finger in there again or turn up the idle quickly. I'm wondering if maybe I could increase the fuel level in the bowl and stop this problem. I want to make the thing someone described to check fuel level. The thing with the clear hose connected where the bowl drain plug is. He said the level should be about 1/8" below the bowl gasket surface. Sound right?

Thanks for all your time. I'm sure there's lots of gs's now on the road that would be in the bone yard if it wasn't for you. May your names be remembered throughout history. Maybe some time I can be of assitance to you.

[KEITH KRAUSE]

As for measuring your floats, I know about the shape of your floats. With the carb upside down, you measure from the carb body (no gasket) to the highest part of the float. Yes, the "bump" where the float and arm merge is the high spot.

[earlfor]

I knew if I asked enough questions and got you to talk enough, you would tell me what was wrong. :-) :-)

Stop doing anything with trying to adjust the bike until, you have the carbs connected airtight to the airbox, the airbox filter in place and the airbox SEALED. Until that is done, you can pull your hair out until you turn blue and the bike isnt going to run right regardless of what you adjust.

Holler when you have done that.

Earl

[quote="double,d"]
I worked on the gs550 today and have new news. First of all I took off the exhaust system and blew compressed air through it and it is just fine with no restriction.
Now to answer your questions.

[Anonymous]

running rough

it may be off topic a little but an eliminator i recently purchased with onlt 6000 miles had been sitting in a garage and mice had built a nest in the exhaust.when i fired it up even a snake skin came blowwing out!!!!!!i have also found dirtdauber nests in openings

[Anonymous]

Snake skin, wow. Watch out those things bite.
Earl, thanks. I'll search for the connector for the air box and let you know how that works.
I had said something backwards earlier
I said;

On the cv carbs there's a passage at the top of the air intake part of the carb. that's oval shaped and connects through a passage to the chamber above the diaphragm. I think I remember finding the passage in the slide that exposes the bottom of the diaphragm to manofold vacuum and they were clear. I didn't take the needles out of the slides but I did feel that they have spring tension holding them upward. I'm wondering if these needles have adjustments in case I need it later. I've read some do and some don't.

I meant to say that atmospheric pressure air is exposed to the bottom of the diaphragm and vice versa for the intake vacuum.
I've been searching all day for what the normal vacuum reading should be and I found it for a different bike but that should at least be an indicator. That site said set the vacuum level to between 200 and 250 mm of Hg. I think I translated that to mean between 7.8 and 9.8 in. mercury. If that's true I've been barking up the wrong tree.
Thanks for making me think.

[earlfor]

Hang in there, its just a problem of sorting out the pieces. :-)

Earl

That site said set the vacuum level to between 200 and 250 mm of Hg. I think I translated that to mean between 7.8 and 9.8 in. mercury. If that's true I've been barking up the wrong tree.
Thanks for making me think.