Help with idle/compression on suzuki gt125.

[Gsa60]

Hi everyone,
I rebuilt the engine of a 1976 suzuki gt125.
I am having some issues to having it to properly idle and the engine seems to have low torque.
I was doing a compression test and it is showing aroud 4.5 bar (around 65 psi) on both two cylinders.
I have read that the compression ratio of this engine is around 6.8. So should I have Around 6.8 bar? Btw its a twin cylinder two stroke.
I tested it with wide open trhottle and used the kick start untill the pressure stabilised.
Cheers

[murdo]

Have you balanced the carburettors yet?

[Gsa60]

Yes

[murdo]

The 6.8 is the compression ratio (as in 6.8 to 1) and has nothing to do with your psi reading (which is fine for a two stroke). A two stroke will never have the higher readings that you will get with a four stroke due to the gas reversal of the ports verses the closed valve of the four stroke.
Suggest you look for an air leak of the intake rubbers (try spraying some carb cleaner or similar around the rubbers and listen for an increase in engine rpm) and maybe check that the timing is correct for both cylinders.

[Gsa60]

Hi thanks Murdo,
So you mean my compression numbers of around 65psi is OK? I was reading people saying that it should be 120. So I was second guessing.
also you mean air leaks just after the carburetor? Ok, I'll try that also.
The mixture seems way too rich and no matter what I do it does not lean out.

[chaterlea25]

Hi,

The mixture seems way too rich and no matter what I do it does not lean out.

Have you checked the float heights?
Until you get the mixture correct the bike not much point in looking elsewhere

John

[murdo]

Are your pipes/baffles clean as a clogged exhaust will give symptoms of running rich. Is air filter clean?

[Rex]

Have you got a manual? These are all basic maintenance checks.

[Gsa60]

Hi everyone,
Exahusts still need to be decarbonized.
Perhaps floats are too high. The thing is I do not have a manual for the gt125. The only manual I find is for the gt185. There it states the height should be 19.9mm. I was checking that just now and even with the metal tang all the way closed i cannot go bellow the 20.5 mm of heigth.
Either something is up with the float or the values are different from bikes.
Cheers for your help on that.
I will keep you posted

[Gsa60]

Hi again,
So i finally got some newer parts.
I turns out that the needle that ataches to the float was too high.
Now, I am able to set the float height as 19.9mm
I managed to source also rest of parts with the same values as the spec.
I am trying to tune the carbs but seems to still run too rich at idle.
I even reduced the pilot air screws from 20 to 15. It is still not enough. The right spark plug is completely black.
What else am I missing?

[Gsa60]

Here is the pic of the plugs.
It is rotated, but the one on the top is for the right cylinder.
Cheers

[murdo]

Right, have you checked to see that the coils are getting 12 volt, that they are within range for resistance (both primary and secondary), that the plug wires and caps have the correct resistance, that the condensers are within spec and the points are set correctly. (Do this to eliminate any variables in the sparks)
Have you cleaned the pipes/baffles yet?
With good sparks and compressions, the next thing to look at is carb float height with engine running. Place a small clear plastic hose on the bowl drain and hold up beside the carb body, then open the drain screw. The fuel will run up the hose to the same level as is in the bowl. Are these the same between the two carbs?