breather tube smoking after new head install  By  endru (Read 7,790 times)

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.

A guy that i was supposed to do a big bore kit on bought that ebay big valve head it came in a kit with cam and jug, he changed his mind so my stepson used the kit. Yesterday his scooter quit running, the head dropped a valve after 700 miles, that head is not recommended left my stepson stranded.

To be honest, I'm think it's less likely that the head is the cause of the low compression. I've got a feeling it's the rings or my timing. I'll still check the head and everything when I tear it apart, but I never really had it running well enough even before I put the new head on. Even with the new gears, fine-tuned sliders and of course the bbk, I couldn't get it to go over 45 on a downhill. I would always feel the engine skip a beat at the higher rpms, and the idle was rough then too. The new head made it worse, probably because it lowered compression even more.

Maybe I gapped the rings improperly, maybe the cylinder got scored, I'll just have to feel around in there when I get it taken apart this weekend. I'll order some 50mm rings in the meantime, I think they're like $15 so that I can try them out just in case.

It could instead be bad timing. Like I mentioned, the ignition seems to skip at higher rpm's, and more importantly at idle I often get backfiring into the carb which I can hear through the air filter. Is there a way to test the cdi with a multimeter or something, or should I just buy a cheap, non rev-limited cdi and see if it helps? I would love to get a cdi with an advanced timing curve, but based on previous threads on the forum, I have the impression that nobody's cdi can be guaranteed to actually advance the timing like they claim.

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??

Endru,
Did you hold the throttle wide open when you where checking the compression? The throttle HAS to be wide open so it sucks in as much air as possible to get a good reading. If not you will get a low reading.

Now to see if the rings are your low compression problem, pour a little bit of oil down the plug hole and re-check your compression, IF the compression reads a lot higher then its your rings that are the problem.
Alleyoop

[replyingto=tvnacman]tvnacman[/replyingto]John

The professor scooter url is not to a honing tool, It is to that BMX TMS 260cc EFI air /fuel screw setting

Steve