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Post by benficacm on Sept 20, 2018 20:23:02 GMT -5
Ugh. 300km into my bbk and I think I bent a valve. I was driving home and rpms dropped and it died. No restart. Good thing it was only a half mile push home.
Got home and started reading posts and testing. Spark was good on case. Compression was good(weird). 110psi both kickstart and starter at wot. Cam timing at tdc was good. (Will NEVER trust that T mark on the flywheel. Mine is off by about half an inch as it pops into tdc.) Flywheel Woodruff key was good. Stator tested good. 74vac trigger and .325vac from pickup. Kill circuit tested good. Swapped cdi anyway. I had an extra one. Was getting fuel from petcock (giggle) but would not start with a shot of starting fluid in chamber(scratched head and swore a lot). All after removing the carb and cleaning jets and other passages. Got a shot of gas in the eye from the actuator on the bowl. Laughed and swore at that. Valve lash was set to .002” intake and .003” exhaust (Close to metric specs in manual). Was able to push the valves with good spring resistance and they opened but I suspected something funky with them. I mean all electrical tests passed. They looked fine to my eyes.
Didn’t want to order a valve tool and lap new valves so I ordered a new head with the valves already installed.
Installed new head and it started right up. Definitely got lucky considering I wasn’t 100 percent sure of the problem. I hate that. I put the old cdi back in just to confirm it wasn’t that, too. I’ll eventually pull the valves off the old head and confirm. Just want to enjoy my ride for now.
I’m thinking of going even higher on the slider weights to keep my love of rpms in check. With 5.5gram sliders, I still get 8500-9000 rpm at take off but gradually get down to 7200 as it “shifts” to higher variator height. Whatta ya think?
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Post by GrumpyUnk on Sept 22, 2018 9:22:29 GMT -5
If by valve tool you mean a valve spring compressor, then you should know that the valves can be removed and installed without use of said tool. Just the other day I used a hammer to tap on the spring and retainer washer. The keepers popped out. I did my thing, and went to re-assemble. Placed the valve in guide, shim, springs, retainer washer onto the stem. Placed both keepers in place so they would fall into position. Compressed the spring & retainer using my hands, and the keepers slid right into place. Viola. Done. No tool needed. tom
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Post by benficacm on Sept 22, 2018 10:06:30 GMT -5
Thanks Tom. I’ll try it. Still pissed that it happened. I may change the lash to 003” and 004”. Seems to be the popular setting. Maybe the manual specs are too tight for a bbk? It gets so confusing when people talk about lash settings and don’t reference metric or standard measurements. Couldn’t find too much on what people are actually using for lash settings. Plenty of numbers but no reference.
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ratdog
Scoot Enthusiast
Posts: 342
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Post by ratdog on Sept 23, 2018 12:56:14 GMT -5
Well my metric setting is still .004” (G) Btw, 110 psi is not “good” in my book. I would be looking for more like 170 psi. I know it’s not a big deal now that you have everything running, but I would love to know what your compression is now with the new head.
Just a thought, running fine until it got hot, then loosing compression is normally a sign of to tight lash. Ofcourse, the compression would normally come back when it cooled.
Pure guess, but I’m betting you burned you exaust valve. Have you tried turning the head upside down since removing it for the new head? Just wondering as my bet is the exaust will be a leaking!
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Post by GrumpyUnk on Sept 23, 2018 13:16:06 GMT -5
Referring to measured PSI using a compression gauge. Please be aware that with the small displacement of these little hummingbirds, some gauges may not work properly, or they may be as inaccurate as mumble-mumble. I ordered a new gauge to have a 10mm adapter as my old(late 60's from JC Whitney) had no small adapter. Tried out the new gauge. Oh. I only have 30psi compression? Who knew. But wait.(30 is from memory, mighta been 55) I can try my old gauge with the new adapter. It read in the 90-100's. Not exactly great, but I had no expectations as I bought it used, in non-running condition. But. Again, they should have read at least in the same ballpark. I figured(maybe) the new gauge needed more volume displacement to register the pressure. When 39mm cycles up and down full stroke, it may not be pumping enough into the Bourdon tube to deflect and show actual displacement. IOW, the gauge is inaccurate with cylinders below XX.Xcc of displacement. I may be way off base, but do take the compression gauge readings with a grain of salt would be my advice. With the older, seemingly more accurate gauge, 110-120psi compression would start w/o problem. FWIW. That and a nickel will get you a donut if you beg. tom
Added: I just built a 47mm with a 'performance cam' and put on the 20mm Keihin. Wow. Zoom. Fastest I have ever ridden.
But. It misfires in mid range, almost as if the spark is being interrupted. The tach that 'reads' pulses in the ignition wire shows steady rpm even with the miss. My take is the carb has problems with either the mix or with something blocking a jet/passage. Have not yet taken it down/apart and examined. It did not want to idle, and I screwed the hard stop all the way w/o much change. Started to tighten the mid mixture screw and idle started to pick up ... retracted the hard idle stop, to close the butterfly a bit, and it seemed to settle, but has the tendency to idle slower and slower and slower, give it a bip, and it may idle fine.. another, and the idle will decay down to a stop. Fun fun fun. tom
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ratdog
Scoot Enthusiast
Posts: 342
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Post by ratdog on Sept 23, 2018 13:51:03 GMT -5
Grumpy, excellent point re compression testers. I have never had the problem with small engines, but I tend to buy a bit higher end tools then most, and so have not had problems that one get from “ The Chinese Store”
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Post by benficacm on Sept 24, 2018 21:07:05 GMT -5
I’ll do a compression test tomorrow on the new head and report back. It’s running awesome. Hope it’s not a ticking time bomb though. Rode it today and was afraid to go too far out. Ha. It needs to earn my trust again.
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ratdog
Scoot Enthusiast
Posts: 342
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Post by ratdog on Sept 29, 2018 14:59:58 GMT -5
I’m so glad it’s running good for you!
With all the care you have taken, it should be fantastic!
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Post by benficacm on Nov 10, 2018 14:37:34 GMT -5
Sorry I never got around to the compression test and taking the other head apart but, I did finish the paint job and figured I’d share a video. Don’t know why they don’t sell them in this color. Love it in matte black. youtu.be/meUT0-Xh1PU
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Post by diynuke on Nov 19, 2018 5:24:08 GMT -5
I’ll do a compression test tomorrow on the new head and report back. It’s running awesome. Hope it’s not a ticking time bomb though. Rode it today and was afraid to go too far out. Ha. It needs to earn my trust again. Do you have any pictures of the old head. I had the exact same problem. but that was just the exhaust valve which had some carbon deposits on it. so i just removed the head and lapped the valve's again and got the scoot running under 2hours. and also installed a leaner main jet. (was my old bbk kit the cheapest of the cheapest.) it was burning a bit of oil tho so that contributed to the failure of course but well that's why It now has an different engine. But yeah the head should just last and last. could be of course that the valve springs aren't strong enough which will cause valve float and bend a valve but ive not encounterd this YET. My 47mm bbk chooches happy at peaks of 11k rpm (very short periods I know the engine really does not like this at all.. normaly only gets to 9200rpm But well some heavy wind helps allot ;D) Okay back to the point.xD As long as the valves are not bent you can keep lapping them. my kymco has been relapped 4x and now its finally tuned correctly so it runs clean now. the tools are kind of handy to have when you need to fix something now instead of having to wait for a new head. And compression. well my 72cc has 150psi with the stock head 110psi is not enough. a stock 139QMB will do 125psi without any trouble. at least the 3 I tested. But you did solve it so
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