My Suzuki RG125F Project

[Steve Bass]

D-Angle said:

Still trying for that first start I'm afraid. The no spark problem turned out to be the CDI box, after much searching I tracked down a replacement in France, fitted that and replaced the coil while I was at it. Presto, a spark!

I stripped and cleaned the carb so there should be no fuelling issues, however the spark plug isn't wet when I take it out after trying to start it. I have finally performed a compression test and got 100 psi. Everything I can find suggests I should be getting a minimum of 110 psi in a healthy engine. It would explain no fuel too, as with no compression the fuel won't be drawn into the cylinder.

So as far as the holy trinity of fuel, spark and compression goes, it appears she had... none of the above. Is there any way to tell if the compression problem is being caused by the piston rings or the crank seals? The rings looked OK when I pulled the cylinder off, crankshaft seals will mean pulling the motor out again and completely disassembling it, which I will do but only if necessary.

100 psi isn't low enough to not draw fuel up. It might not run as well as it could but it'd run.
To check the rings, simply pull the barrel and put the rings in the bore without the piston. Then measure the ring gap. As rings wear they get thinner so they open out in the bore, which makes the ring gap bigger over time.
imply check against specs for this measurement. But if you're refreshing a motor, new rings should be a given...
If you have spark, a simple way to get her fired up is to squirt/pour a bit of premix fuel down the inlet. Kicking it over will draw this up and should fire.
As for the carb not getting the plug wet, are you sure the choke circuit is working. Also check the float level and float valve. It's well worth replacing these as they can stick with age. If there's no fuel in the bowl, or the level is too low, it'll have trouble drawing it up.
So try the fuel down the neck first to see if it fires and work from there.

[Fastdruid]

If you are going to pull the top end I would also (and I say this from bitter experience) check the rings sit in the grooves properly as well, I had a set once which were a smidge too tight in the grooves, they stayed "squished" into the piston rather than floating and so had no compression and the bike ran like a dog (although it did run).

[D-Angle]

Steve Bass said:

But if you're refreshing a motor, new rings should be a given...

Yep. Given that the piston looked brand new (see pics earlier in the thread) I assumed the rings must be good. Chalk that one off to experience I suppose. I am going to test for loss of compression from the cylinder head and inlet manifold too though. I'll get new rings, see what that does for the compression, then one way or another it's back to the carb to see what it's having a huff about.

[Fastdruid]

You could always try a leakdown test as well.

Basically remove the carb and exhaust, plug the holes and pressurise it to ~6psi. It should hold that pressure for at least 6 minutes.

Although this site says 5psi for 5minutes it's pretty much the same deal: https://www.diymotofix.com/blog/how-to-do-a-diy-tw...