fingertip compression test?

[Mandres]

Can anybody explain the fingertip compression test to me?  I re-installed the newly-rebuilt carbs (airbox not installed), pulled the plugs, put my thumb over one of the plug holes and turned the engine over with a socket wrench.  From what I've read I'm supposed to feel a puff of air right coming from the plug holes right?  I didn't feel anything...  Did I miss a step?  Am I supposed to plug the exhaust first? 

I have no reason to believe this bike has engine damage; oil level is fine with no leaks and it turns over smoothly when I crank it.  The replacement battery hasn't come in yet so I have not tried starting it.  Please advise...

Thanks,

-M

[GeeP]

Take out the spark plugs.  Put your thumb over one of the holes and crank 'er over.  If it pushes your thumb off the hole you have compression.  If it doesn't push your thumb off the hole, one of the valves is leaking badly.  It won't tell you anything more than that.  Well, unless you have a hole in the piston.

As to why you don't feel anything, are you sure you're turning in the correct direction.  (Clockwise as facing the signal generator) 

[Mandres]

well shaZam!, I guess I did it right then.     I don't feel anything, from either plug hole.  I can hear air blowing but I can't tell where it's escaping from. 

so what's my next step?  Do I need to pull the head?

thx
-M

[Chris_B]

Did the bike have compression before rebuilding the carbs? That wouldnt change anything compression wise...try cranking it over with the start button instead of a wrench.

[Mandres]

I don't know.  After I bought it the first thing I did was pull the clogged carbs for rebuild.  The last owner told me the bike had been sitting for a long time.  He lowsided at 5 mph. and hurt his leg and never rode again.  There is no sign of damage to the engine from the crash. 

This is really discouraging;  after battling with the carbs for the last few weeks I finally got them sorted out.  I rebuilt the petcocks and POR15'd the gas tank too.  I was hoping to have my battery in by next weekend and fire her up for the first time.  Doesn't look like that's gonna happen 

[Mandres]

tomorrow I'm going to try the flashlight trick suggested by Srinath to check for bent valves.  I found this description in an old post:

"Take off tank, seat, side covers (optional), airbox, carbs, valve cover, round engine side cover, spark plugs and exhaust. bring right cyl to top of compression stroke (manual). Shine light in intake port and look in spark plug hole, repeat for exhaust port. Then bring left cyl to TDC top of compression stroke. Shine light in both ports and look in plug hole. In both cases the plug holes should be dark.
Then bring right cyl to middle of intake stroke and repeat flash light but this time you should see light, then repeat exhaust and other cyl. Trust me this is easier than any other method for checking for bent valves etc. Also when the right cyl is at TDC top of compression stroke look for cams being in the right position. Get a small powerful flash light. Really helps.
Cool.
Srinath."

Can anyone expand on this procedure?  Where exactly are the intake/exhaust ports?  I'm not sure where I need to position the flashlight.

Thx,

-M

-edit  Something else just occurred to me; the carbs were dry when I tried the compression check.  Does there need to be fuel in the bowls for this method to work? 

[runsilent]

I just reread this thread and it sounds like you are trying to check the compression with the finger by turning the bike engine over with a wrench.  Won't work that way.  You have to spin the engine over with the starter motor to hear the compression pop.  In fact if it has sat for a long time without running you should spin it over a bit to get some oil thruout the engine.

At normal cranking speed it will blow your finger out of the spark plug hole with a loud pop.  If you don't have a functioning battery in the bike you can jump it from a car or other bike battery.  Turn off car engine when jumping a bike to prevent possible damage.

The term cranking an engine refers to turning it over with the starter motor and not cranking it over with a wrench as you described.

Also if you have compression then throw away all the old gas from the tank and start with fresh gas.  Put the fuel selector valve on prime to fill the carbs with new gas. 

[Cal Amari]

The intake ports are the holes where the fuel and air from the carbs enter the engine. They are connected to the carbs by what Suzuki refers to as the "intake pipe assembly", though most of us simply refer to them as the intake manifolds.

Check number 21 in this LINK to see the intake manifolds; the air/fuel mix passes through those manifolds from the carbs into the cylinder head, through the intake port, past the open intake valve, which then closes as the piston comes up to compress the mixture before the spark plug ignites it... I imagine you understand the four-stroke cycle...

Anyway, the terms "intake port" and "exhaust port" simply refer to the holes in the cylinder head where the fresh air/fuel mixture enters the engine from the carbs ("intake port"), and where it exits the engine through the exhaust pipes ("exhaust port"); IOW, the ports are the holes in the cylinder head. In normal operation, all four ports (two intake, and two exhaust, one of each for both cylinders) are sealed off by the valves at "TDC" ("Top Dead Center") of the piston on the compression stroke; TDC is when the piston is as high as the piston can go in the cylinder before it starts to travel down again. Logically, "BDC" ("Bottom Dead Center") is at the lowest point of piston travel, the point where the piston is about to start moving up in the cylinder again, as the crank turns.

Keep in mind that you have to follow the instructions for setting each cylinder to TDC on the COMPRESSION stroke so that both valves (intake and exhaust) are in the proper position (closed). At TDC on the EXHAUST stroke, the exhaust valve opens, allowing the burned gases to escape through the exhaust port, into the exhaust pipe.

Check the information in these links for more details; there are some excellent animations to help you:

http://oldmanhonda.com/MC/MC.html

The repair hints page is very helpful:

http://oldmanhonda.com/MC/RHints.html

The see-through engine (based on the old Honda CB/CL/SL 350 twin) is similar in many ways to the GS engine, with several obvious exceptions. The Honda uses a single overhead cam, while the GS has two cams, one to control the intake valves, and one to control the exhaust valves. Also, the Honda ignition plate (with BREAKER POINTS!) is at the top of the engine (on the left side), while the GS ignition plate is behind the ignition cover at the bottom of the GS engine on the right side; of course, the GS uses an electronic ignition, rather than points.

http://oldmanhonda.com/MC/engine.html

One of the best features of the see-through engine is that you can easily modify the illustration to remove the most of the components, to focus only on the parts that interest you. Give it a try...

There is a lot of good information there; next take a look at this:

http://auto.howstuffworks.com/motorcycle.htm/printable

The working engine in that illustration is even closer to the GS design, again, with the exception of the cam; the GS uses two cams (a double overhead cam / DOHC design), while the illustrated engine on the HowStuffWorks.com page shows a single overhead cam (SOHC).

Hope this helps you understand the concepts a little more clearly; now, you not only know what the "ports" are, you have some working engine designs to examine to make things easier to understand.

If you still have questions, we're here to help; we want you to rescue that GS and bring it back to life!

[Mandres]

I just reread this thread and it sounds like you are trying to check the compression with the finger by turning the bike engine over with a wrench.  Won't work that way.  You have to spin the engine over with the starter motor to hear the compression pop.  In fact if it has sat for a long time without running you should spin it over a bit to get some oil thruout the engine.

At normal cranking speed it will blow your finger out of the spark plug hole with a loud pop.  If you don't have a functioning battery in the bike you can jump it from a car or other bike battery.  Turn off car engine when jumping a bike to prevent possible damage.

The term cranking an engine refers to turning it over with the starter motor and not cranking it over with a wrench as you described.

Also if you have compression then throw away all the old gas from the tank and start with fresh gas.  Put the fuel selector valve on prime to fill the carbs with new gas. 

Really!!!  Oh man, what a relief.  I'm going to wait till my battery gets here and then I'll try it again the right way.  Thanks for the links too Cal; there's a lot of awesome info there!

-M