Squish help.

[RBD]

You know, it could be a bad crank seal? The fastest and easiest way to find any air leak is both pressure and vacuum testing. Yes, it should be vacuum tested as well as pressure tested.

The only draw back is you will probably have some leakage around the power valve linkage areas.

Ron

[blitz11]

Dude. Sorry. Just trying to help. You've had a year. I've had a few posts in this thread. I may be redundant, but maybe it doesn't hurt to replay everything.

We have are three elements at play:

a. engine dynamics (i.e., power valve position) (affecting compression)
b. air-fuel mixture/volume (carburetor/air leaks/reeds/etc.)
c. ignition timing. (spark)

1. You can check the power valve when it is running. Take off the right side cover, and start your bike. You can mark where the PV Lever is at rest (white paint). Go ride the bike, and see if it returns where it should. This takes the PV out of the equation if it returns properly. See if it ends up in a different spot between blipping the thottle and going for a ride.

You could also disconnect the power valve, and secure it in the down position, then ride it (but not too hard). See if that fixes it. IF you take the PV out of the equation, do you eliminate the problem?


2. Carburetor: How do you know that the slide returns exactly where it rests every time?

I might not have made this clear as far as the idle. If the bike is sitting there idling you can rev it up or do anything you want, it generally goes back to whatever the idle was. If you ride it down the trail for a second and stop it may idle OK, it may idle high, it may idle low. Same deal on starting, it may be OK, high or low.

Why would riding it make a difference? Is something moving/not returning/being jarred up/down? What differentiates moving and not moving? When you're riding, you're holding the throttle open longer, and flowing more air than compared to at rest. Could something be wrong?

Could riding it move the power valve/keep it from returning? Test 1 above will help you answer that question.

If you don't keep asking questions, you won't get the right answer.

3. Have you stuck a timing light on this thing to see if the ignition timing is consistent? Could it be that the timing is jumping around? A light on the rotor may indicate a variance in ignition timing which could affect the idle speed.

You talked about connections, but no one has stuck a timing light on this (as far as i know). That might identify something, OR eliminate a possibility. If you can narrow your problem/solution space, you're making progress.

Stick the light on it, or maybe swap ignitions (rotor, stator, and CDI) with the known good bike. If you don't have access, i could send you mine from my 2002 300.

The fact that this is so variable has me doubting that it is low compression. Low compression usually manifests itself in an inability to idle at all. Variable compression would explain something. Variable compression could be attributed to the power valve.

An air leak is pretty easy to track down. You can build your own rig for $30, and then pressurize the system and see where it leads. Having just tracked down a porosity problem myself, it isn't unheard of. You will have to build a pressurization rig, but that's pretty easy.

If it matters, i've been working on motorcycles for 36 years, (4 as a mechanic in a shop), and i have three degrees in engineering. If i've learned anything through all of this, it is to ask a lot of questions. if you close your mind to a possible solution without data to substantiate the closure, you'll never find the problem. Sometimes you have to cover things a few times before you get the answer.

Good luck. Sorry if I annoyed you.

blitz.

[Eric K]

This may not be root of your issue, just an observation... If the slide is too lean the idle speed can be erratic. Sometimes it will be high, sometimes low. It is exacerabated by changes in engine temperature.

Engines respond differently. My '01 GasGas is happiest with a #8 slide. My '06 GasGas is happiest with a #7 slide. Put a #8 slide in my '06 GasGas and the idle is erratic - sometimes it will hang high and sometimes it will idle low. Blipping the throttle would not always bring down a high idle.

[Ud_luz]

Originally Posted by blitz11

2. Carburetor: How do you know that the slide returns exactly where it rests every time?

Other than using two different carbs and insuring the throttle isn't binding I can't answer your question other than both carbs work perfectly on the other 07.

Stick the light on it, or maybe swap ignitions (rotor, stator, and CDI) with the known good bike. If you don't have access, i could send you mine from my 2002 300.

Already swapped ignitions.

The fact that this is so variable has me doubting that it is low compression. Low compression usually manifests itself in an inability to idle at all. Variable compression would explain something. Variable compression could be attributed to the power valve.

Something I cannot easily check when idling.

An air leak is pretty easy to track down. You can build your own rig for $30, and then pressurize the system and see where it leads. Having just tracked down a porosity problem myself, it isn't unheard of. You will have to build a pressurization rig, but that's pretty easy.

Actually I was thinking of this after swapping all these parts the last couple of days. I was going to do a vacuum test. Would a pressure test be better?

If it matters, i've been working on motorcycles for 36 years, (4 as a .

Good luck. Sorry if I annoyed you.

blitz.

You haven't annoyed me at all. I find it an interesting problem. Here's what I've done as a recap.

Swapped carb.
Swapped reed valve assembly.
Swapped CDI.
Swapped ignition. (tried the 2K2 from the other bike in place of the 2K3)
Swapped throttle assembly (even though it's not binding).
Swapped ignition coil.
Swapped wiring harness a couple of months ago to Euro DS.
Swapped plug.


Tested:
Compression=low.
Leakdown=OK.
Cylinder base gasket for leaks.
All circuit grounds.

Inspected:
Piston.
Port alignment.
Visual on powervalve assembly and checked for static return.
Pickup coil for metallic debris.
Stop switch for corrosion (I know grabbing at straws).

Not checked:
Either crank seal.
Crankcase pressure test.
Timing variation. Unlikely unless the CDI is defective which isn't the case.

I've pretty much lived with it since the bike was new but I couldn't find the problem. When I got a donor bike to test with is when I got real interested in resolving the issue. Like I mentioned I'm thinking I have an air leak probably either in the case itself or a crank seal. A couple of other mechanics have been stumped by it too but didn't have the parts to swap or test with.

I suppose I could disable the powervalve and see what happens. If there's no change then I'll try a crankcase vacuum or pressure test. (Advice on this one)

[GMP]

Casting porosity issues occasionally come up (with all brands), and a pressure test will find them. A quick and dirty test is to spray some starting fluid near the suspected area and listen for an RPM change. The 140 PSI compression still bugs me though.

[jostby]

I agree 140 seems low. It would also be easy to check the crank seal on the left side like GMP says. Spray it with starting fluid while it is running. If nothing else replace it to eliminate it as a culprit, it's easy to access and inexpensive. Right side seal would be using tranny oil if it was leaking.

[widebear]

This is a great tool to help diagnose potencial leakage problems.


http://cgi.ebay.com/ebaymotors/Cylin...motiveQ5fTools

[Ud_luz]

Update: I removed the base gaskets in their entirety and used a cut out sheet of paper (.07mm) and copper coat as a sealant. Viola!!!, bike runs great. Compression is now at 175 and the bike idles perfectly.

I still should remove the head and measure squish but so far the results have been quite good. The only thing slightly amiss is the piston is a very tiny fraction higher than the exhaust port but I'm guessing that won't hurt anything.

My assumption is the head deck height was machined slightly high causing the low compression. I'm guessing it needs to be cut down, a base gasket installed to perfectly match the port/piston and off I go.

Now, can anybody explain how somewhat low compression can affect idle? It doesn't on a four stroke, what's the mechanism for problems with a two stroke? I can't really tell much difference in power but the transition off idle is much better and the low/mid range torque seems to be somewhat better.

[iancp5]

Originally Posted by Ud_luz

Now, can anybody explain how somewhat low compression can affect idle?

At a guess I'd say it's having a more optimal squish improving combustion consistency. Less variation in burn between strokes. I'm guessing though.

[penev]

Hi from Bulgaria,
GAS GAS ec 300 2006
Look at this and tell me what you think. This is lower dead point
New piston PROX , new cylinder, everything is great but the piston is not equal to the port.
I put 0.15 0.3 0.5 gasket
squish band 2mm .
Best regards Dimitar Penev