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I finally got the top end reassembled with the new rings. The compression tester still measured exactly 40psi. So I figured it was definitely the compression tester that had the problem. Tried it out on my car and it got 170 or so... with a different adapter though. So maybe only the adapter that fit my scooter was defective ... just to be sure I put a little oil in the cylinder. If it were a defective gauge then it should have stayed at 40 psi, right? Well with a little oil it went up to 60 psi.

3 thoughts:
- The cylinder wall is defective, hopefully can be fixed with a honing.
- The new rings are bad or installed improperly. Unlikely though, because I installed them with the logo-stamped side up and none of the gaps line up. Silver ring was at the top.
- The tester adapter is in fact defective, and the little bit of oil that got poured in just raised the compression above what was normal and pushed just a little more air through the leaky adapter.

Any ideas what's going on here?

That's really good to know. I mean, it was pretty tight when the hose began turning in the adapter, so I thought it was nonetheless tight enough. I'll do the superglue and see what effect it has.

Those are very good points John, thanks for your post. No, I didn't hone the cylinder, that takes way more skill than I have to keep from messing it up. I finally had the time yesterday to pull everything back apart and do some inspections. The head is just fine, no leakage was found. The gaskets are all intact, all the crankcase bolts were still tight, and most importantly the cylinder walls had no scoring or abnormal marks. SO it must be the rings, right?? The rings looked fine, but from what I've heard, it's hard to diagnose bad rings just by sight. Now what I did find is that the end gap was slightly too large. For a 50mm bore I think it should be about 0.2mm for the top compression and 0.25mm for the bottom compression rings. They were both .05 wider than that... so 0.25mm and 0.30mm respectively. Would this cause the tremendously low compression that I had? Maybe I goofed on the gap spec. when I installed them... I used the same formula this time as I did when I first installed. In any case, I'm going to use a tighter gap this time, possibly even lower than what it should be. LUCKY me the new set of rings I ordered came in today (a day earlier than expected), so I can get them installed right away.

I was a non-believer about cdi's BEFORE I read your recent post with millsc. Maybe 2 months ago people were posting calling everyone else liars about their cdi claims, so I just figured it was a lost cause... but the recent one makes me think otherwise, so I'm willing to give yours a try. Not only have I wanted to get a good cdi anyway, but I really do suspect there's some ignition problem. It wouldn't cause compression issues, but I get backfiring into my carb and skipping at full throttle. I'm 100% sure the that the cam holes were lined up when the flywheel mark was at TDC, so my static timing should have been good. When I rebuild today I'll double check it all again. I'm sure your CDI will do what it claims, I'm not going to bother testing it. Thanks for the PM, I'll go ahead and order that today

BTW - what's a CCV you were talking about??

So I went to harbor freight and got this compression tester. www.harborfreight.com/quick-connect-compression-tester-95187.html I hooked it up, opened the throttle and ... 40 psi. I tightened it down into the spark plug hole as tight as I could by hand (the coupling between the threaded insert and the hose started rotating, so I figured it wasn't meant to get any tighter) and tested it again. I tried warming up the engine for 2 minutes and then tried it again... same reading.

What the heck, how is the engine even running at 40 psi?? Maybe the gauge is junk, I don't know.

Well if I trust the gauge then I'll need to take apart the top end again and double check the gaskets and rings. What do bad rings look like? I mean, will there be noticeable flaws or are they just hairline gaps? Speaking of gaps, I'll check the ring gap as well. Who knows, maybe I installed the rings improperly back when I first put the bbk on. I'll check the head for leaky valves... I've heard you can flip it upside down and pour some gas into it to see if the valves are tight.

skuttadawg - a 45 idle jet? Wow that's big, but I'm guessing the 2T's run generally larger idle jets than 4T's. I ordered a 38 idle a while back, but discovered it was the wrong fit. Apparently Keihin carb jets are not all made alike. So I have a 38 ready to go for the new carb. EFI - you probably remember a discussion about that from the old scootdawg site. It intrigued me then, but I'd probably upgrade to a motorcycle before I made that kind of investment on my scooter.

@buford - Yeah "racing head" is hardly accurate. The only people I know of online that port heads is scrappys, and from what I've heard they do a great job. At some point I may see if I can send in this one to be ported. Might be fun to try out myself though... I have a dremel and I could take a practice run on the stock head first.

Anyway, as far as intake port measurement, you bring up an interesting point. I will measure it then to get an exact number, but I can tell you now the spacer from the stock head matches the new port diameter, so it probably is only 19mm. And if I got a 24mm carb, it seems like it would bottleneck right there at the 19mm intake opening. I'll try it without the spacer once just to see what that does. Do you know specifically why that spacer might cause problems? I've got a gasket underneath it and there's the o-ring on the manifold, so if it's tightened down securely I can't imagine any outside air leaking in.

sickopsycho - Thanks for the plug on the compression tester and the psi numbers, that's exactly what I needed.

That's a lot of good info, thanks everyone. Does anyone now about the main jets question - are the 19mm carb main jets compatible with the 24mm carb?

Right now I just want to throw my scooter over a cliff. I just simply cannot get it to run right. Nothing I tried helped. Among many other things, the carb is cleaned and the stock exhaust is on, but it still has an ugly idle that I can't fix with any carb adjustments, and the scoot sputters and skips when I give it throttle.

I'm beginning to wonder if the idling problem and the poor performance when giving it throttle are caused by the same thing. Could it be that my carb is crap? Or is the intake on it too small for my current setup? Could my cdi be bad and misfiring at all rpm ranges? I guess the only way to know is to buy a new carb and/or a new cdi, but if neither solves the problem then I just spent all that money for nothing.

So I guess my questions for the forum are 1) Can a bad cdi cause rough idle and what sounds like sputtering/misfiring, or does a bad cdi just stop firing altogether? 2) Is a 19mm carb ill-suited to a 50mm bbk, large valve head, and free flowing intake/exhaust setup, to the point that it would cause the described problems?