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Mac_Muz
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   Posted 6/24/2003 1:00 PM (GMT -7)    Quote This PostAlert An Admin About This Post.
It seems to come up over and over again some problems just nag at bike owners.
So I guess this might be the first of a series of tech tips.
For those of you that don't know me, I was a foreign car tech, and was not ever a bike tech................................................................................

Many things work the same way more or less. To me pretty much theory just is.. By rights this should be the 4th in a series.

Compression is the first test of all tests...

Spark is the 2nd test

Fuel is the 3rd test, and each of the above has a routine in order to follow, so you are not guessing.

Vac Leaks:

A vac leak is unmetered air allowed in error to get by the intake. This effects carbs and injection equally bad....

You do not want a vac leak on any engine ever, but they are very common.

Sometimes you might not know it if the leak is small, and other times a engine won't start at all. So symptoms run a broad range of what a vac leak can do.

Some symptoms are a sudden lean condition, which can result in loss of power instantly and a matching increase in fuel consumtion.

This might be found to be true if  heat cracked a vac line to the petcocks while you were riding, or the line just lifted off

Other types of symtoms cause what is known as "Hunting" Which is idle RMP that will not stay correct. The idle goes up to a given range maybe even to 2,200 rpm and then will drop to 600 RPM and go right back up as if a demon has the grips.. Or the bike might go to 2,200 RPM and stall forcing youi to restart.

Lesser leaks might effect idle, and what was correct yesterday suddenly is high today.

Turning down the over all Throttle linkage screw (THE ONE MAIN ONE MADE FOR FINGERS)
will work to lower idle sometimes but is a mis-adjusting when you should not do that..

If the leak becomes worse the idle will do what ever the leak demands..

The leak leans out the correct mix of 14% to 17% fuel to air and makes the mix undeterminable, ALWAYS lean...

On bikes each carb can have leaks, and manifold mount for any carb can have vac leaks.

Any throttle plate shaft can leak on either end. Any vac lines can leak on either end.

And any test port can have a bad cap, and so also leak.

Most bikes don't have vac operated accesories, with the one exception of vac operated petcocks. A vac operated petcocks WILL say, Pri = prime, On/Run, AND Res = reserve.

There is NO OFF setting..... Also the petcock will have 2 lines each... One line is for fuel and the other is a vacuum line telling the petcock the engine is MAKING vacuum, and to turn on the petcock diaphram to pull open the on off valve with in the petcock.

The way a internal combustion engine works creates vacuum. I have never seen any bike with a vac pump.

In my experience vac lines in general do not deal with heat and weather well. They crack, split, and become brittle, and should be replaced once a year, as well as gravity feed fuel lines.

To locate a vac leak you need a can of WD-40 which is probably the best thing you can use WD-40 for.

Also you can use WD-40 to test whether or not idle mix is right. This chemical beats ether hands down for use as a engine starter as well, and will not cause engine damage in moderate amounts.

WD-40 makes what you can't see, and probably what you can't hear findable.

You need to listen to know....

SO to tell if idle mix is right, Spritz a shot right at the intake with a running engine, and listen...

Does the idle go up? Or does the idle go down?

If things are correct the engine has all the fuel it wants and the idle will drop, as the engine wants no more...

If the idle goes up you are lean.......

If you have 4 carbs and all go down but one, then that one is lean.... Why it is lean remains a question.

Maybe the setting is wrong, and the pilot is in to far (mix screw on a car).

[ Often a book setting will say 2, or 2 1/2 turns out. That is a place where a fresh built engine should run to start, and IS NOT always the best mix for any given clyinder]

Or maybe you have a vac leak...and so adding fuel in the form of WD-40 causes the idle to jump to who knows what, and that depends on the unmetered air.

It is possible for a bike to run on 2 clys out of 4, and have the two dead clys fire up above idle speeds as the engine approaches mid range RPM.

So finding leaks becomes a bit of hit and miss, as you spritz about the carbs after a initial shot into the carbs.

Each time you spritz you must listen, so with a air cooled bike you might want a fan on the engine.

Places to spritz are the manifolds looking for loose clamps, throttle shaft ends, and any vac line ends and components vacuum operated. On injected bikes any Throttle body lines, and injector bases, also any vac operated components as you find them.

Often times vac leaks are mis diagnoised as clogged carbs, and bad plugs, wires, pick ups coils and more...     Mac



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djh48382
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   Posted 8/21/2004 8:12 PM (GMT -7)    Quote This PostAlert An Admin About This Post.
Gee...Thanks....you have just described all of the problems I have been having with my 1980 Yamaha XS850 over the past three years.......
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Suzuki GS 750
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   Posted 9/29/2004 12:17 AM (GMT -7)    Quote This PostAlert An Admin About This Post.
You have just taught me that my bike has a vac. operated petcock.
prime, on, res.  I was wondering why there was no off and with winter coming up It kinda had me on my toes.
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1Hawk
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   Posted 10/5/2004 4:16 PM (GMT -7)    Quote This PostAlert An Admin About This Post.
Having worked on cars for over 30 years and tinkered with some bikes in the same period of time I would totally agree with this.  I do however have two problems with some small issues.  First, if the vacuum hoses are treated with spray silicone they will not decay.  I do not mean Armour-all or tire dressing, I mean silicone.  Using this will keep the hoses soft and they cannot freeze; they will also last a very long time as will radiator hoses.  Second, the use of WD-40 does work however, I would rather not use it as it has a very low flash point, which means it can be extremely volitile.  If this is what you use be cognizant of this.  Especially if you are sprying it over a hot exhaust system.    Other than that, this was very good.


Hawk


In Nature, as in Society, most Creatures are Friendly.  The Secret, is Knowing those that are Not.  Therefore, Knowledge becomes the Key to Understanding.

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CaddmannQ
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   Posted 10/5/2004 5:05 PM (GMT -7)    Quote This PostAlert An Admin About This Post.
One other thing to mention (I have on other threads) is that bikes with CV (constant velocity) carburetors are prone to get cracks in the rubber diaphragm inside the carb, which is what raises the slide as a vacuum is drawn above it.

This internal vacuum leak can make the bike idle erratically, and not want to rev up, to rev slowly, or sometimes to bog down and then rev suddenly.

They are always "keyed" so that they only fit in one way, and must be installed carefully to avoid damage.

THe WD-40 trick doesn't work as well here, as often the bike is already running a bit rich as the slides won't rise to provide air.


Cadd
Clovis CA
VROC #11619
2004 Nomad

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Hiputong Stekward
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   Posted 6/16/2005 7:47 PM (GMT -7)    Quote This PostAlert An Admin About This Post.
 You all say the things so sophisticated that I feel that I am an idiot though I am one motorcycle lover. But I think now
that I can know more about this by repeating reading the contents here.  Of course, I need some here can instruct something!
 
hiputong  Stekward


Motorcycle is the most favourite for young!  email : hiputong@hotmail.com    MSN: hiputong@hotmail.com
 
 

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CaddmannQ
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   Posted 6/20/2005 3:06 PM (GMT -7)    Quote This PostAlert An Admin About This Post.
It takes both study of the theory as well as the techniques, plus hands-on practice.

We try to guide people here, but here is no substitute for serious technical study and practice.


Cadd
2004 Nomad 1500
VROC #11619 Rolling Blunder #128

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Hiputong Stekward
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   Posted 6/20/2005 6:50 PM (GMT -7)    Quote This PostAlert An Admin About This Post.

I try to explain the words:
  Spark :   there is a spark button, which is used to start the motorcycle,  on the head of motorcycle.
  what is the Fuel (#1 say :Fuel is the 3rd test,)
  What is Vac Leaks?
 
hiputong  Stekward


Motorcycle is the most favourite for young!  email : hiputong@hotmail.com    MSN: hiputong@hotmail.com
 
 

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CaddmannQ
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   Posted 6/20/2005 10:45 PM (GMT -7)    Quote This PostAlert An Admin About This Post.
By "spark" he means that the spark plugs must arc or fire properly.

By "fuel" he means the carburetors must be recieving fuel, and feeding it to the cylinder.

By "vac leak" he means a vacuum leak. When the piston descends on the intake stroke it creates a partial vacuum in the cylinder. This sucks in air and fuel from the carburetor. But if air can be sucked in through any place except through the carburetor you have a "vacuum leak" and the carburetor will not siphon enough fuel from the float bowl to run the engine properly.


Cadd
2004 Nomad 1500
VROC #11619 Rolling Blunder #128

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Greywolf
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   Posted 11/7/2005 1:29 PM (GMT -7)    Quote This PostAlert An Admin About This Post.
Without a doubt, vacuum leaks are one of the top engine killers of all time - especially 2-Stroke engines.

The reason is that as the fuel to air ratio leans out, the temperature of the burning mixture increases. Also, in bikes that are 2-stroke those that must have oil mixed with the fuel manually on each fill up will have insufficient top end lubrication. (Less of a problem with bikes that have an auto-lube or oil injection system).
 
If you have ever used a welding TORCH (you know, those old gas-bottle rigs...) you know that to heat up the flame you add more oxygen. Carbueration works the same way: Less fuel or more air equals HOTTER!

The high temperatures caused by a lean mixture can easily melt or burn an exhaust valve in a 4-stroke, or score the living daylights out of a 2-stroke.
 
Normally a 2-stroke bike will seize up when this happens, as the pistons and rings first scratch, then score, then outright destroy the cylinder walls. Oftentimes, a piston will weld itself into the bore

In 4-stroke engines, all chaos breaks loose when an engine "Sucks a Valve" and the price of repairs is - well...
 How does new bike grab ya?

Valve heads are big enough to smash a piston easily, especially at high RPM.

Either way, there's a good chance the bike will have a locked up engine.

Excellent post, and I hope nobody here lets a vacuum leak just go on until they get around to fixing it. Delays on a vacuum leak can eat up your motor for good.

 

PS: "fuel" = "petrol" = "gasoline"


This is no Kawasaki... It's a KAWALSKI!

Post Edited (Greywolf) : 11/7/2005 8:42:52 PM GMT

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Mac_Muz
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   Posted 3/12/2007 2:18 PM (GMT -7)    Quote This PostAlert An Admin About This Post.
I wrote that in 2003, and long before 2003 there was no such thing as silicon tubing,or like cloth covered rubber was as good as it got.... I can't believe a silicon tubing can't split, wear loose and or crack either....

I do not mean to spray WD-40 on the exhausts either, and why would you they can't have a vac leak very well, ans if they do they make the mufflers too lean and pop slowing down and who cares.... You still get to go fast.....

The idea was a very in-general post to cover all bikes and all cars for that matter.... ALL of em!!!!!

Finding broken things you can't see so well with the eye is tuff to do... A few tools to show ears where a vac leak is, works and pretty well so long as ya gotts ears....

A cv carb should come under carbs workings and would show as not finding the vac leak this way but still actting like a vac leak...

If that is the case the slide you can see sitting down low in the venturii and not rising up like it should, and if the bike has 4 carbs that would be a no brainer to see with 3 pistons up and one down....


Why tell the truth when the truth is mostly always inconvienient
Ossipee New Hampster "Eat Seeds or Die"
 
 

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steven444
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   Posted 3/25/2010 5:12 AM (GMT -7)    Quote This PostAlert An Admin About This Post.
informative post ........


steven

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Thoratron
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   Posted 5/27/2010 6:34 PM (GMT -7)    Quote This PostAlert An Admin About This Post.
The Mac guy had really useful info, however this info should be credited to it's original author.
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danny80
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   Posted 8/21/2010 3:15 AM (GMT -7)    Quote This PostAlert An Admin About This Post.
The major symptom of a vacuum leak is very poor idle quality, stalling, Spitting back through the Intake, and a very lean condition at part throttle. This is the most common problem when doing carb conversions where either adapter plates are used or new intake manifolds are fitted.

Post Edited By Moderator (Rich_S) : 8/23/2010 4:15:02 PM GMT

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rob001
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   Posted 11/21/2011 11:45 AM (GMT -7)    Quote This PostAlert An Admin About This Post.
I must say that is really interesting and informative article, i am having carburation issues on my bandit 12 since i bought it 6 months ago!! Will get to the bottom of it eventually!! Thanks for the info.
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