Hello Ken
See page 70 of the free online 180-81 shop manual in the files section of this group. That picture shows how the alignment should have been at teardown in order to get TDC of the compression stroke. Notice that in 4 stroke engines the cam runs at 1/2 the revolution speed of the cranlshaft. So one gets two TDCs on the crankshaft mark (and piston at top of the cylinder) for every one revolution of the camshaft(and its sprocket).
If the crankshaft, keyed to the flywheel with the woodruff key shows "T" as in page 69, and the index mark on the head at ~9 O'Clock and the 'o' on the cam sprocket all line up, you are at TDC compression, and the valve tappet adjustment can be made as both valves are closed. If you are on the other TDC (just rotate the crankshaft one more revolution [c'cwise]) you will see that the camshaft has rotated 180 degrees, the exhaust valve should be open and the 'o' on the sprocket is on the other side facing the crankcase.
On the compression stroke (& also the exhaust strike!) towards TDC, that is the 'F'ire mark on the flywheel for the points to be opening. If you took apart the mechanical advancer and put it back together wrong, the lobe will be wrong and won't open the points at the right place/time. If the advancer is stuck partially or fully advanced, it will be nearly impossible to set the timing properly. Ditto if the points rubbing block is worn down too much.
Maybe it is the head gasket? Maybe you forgot a base gasket &/or head gasket?? Maybe a valve is 'stuck' open? 125 lbs is not too bad a compression reading. 150 is better though. Let us know what you find...
Ron in NJ
--- In Honda-C70@yahoogroups.com, "joe70amx" <joe70amx@...> wrote:
>
> Had the same problem on my last rebuild. It was not a valve hitting it was the piston hitting the head gasket. I had a lot of s65 gasket kits (same except for the head gasket) from past rebuilds thought I would save buying a c70 kit. The head gasket looks like it will work BUT it is just small enough to stop the piston at top dead center. Did you replace the piston when you tore it down? was it the correct dome top? with the cover off on the top of the head you can see if the cam is on the bottom of the stroke (lowest part of the cam lobes)the valves should be closed if not you have a valve hanging up. If you got it to cycle and fire before the teardown the problem should not be a valve hitting. A valve lap may help raise the compression.
>
> --- In Honda-C70@yahoogroups.com, Ken Teng <k.teng@> wrote:
> >
> > Thanks Baby Huey for responding, my Passport is a 1981, I did the compression test before I decided to remove the head. The bike would fire up a bit chug chug backfire when I put some gas in the spark plug hole. The compression is supposed to be about 175 psi. I only got max 125 psi so I decided that it would be good to remove the head & take a look. Following the manuel for engine tear down I lined up the 0 on cam sprocket to 9 oclock position (V-grove on head) & Tdc mark on ignition advancer (on crankshaft) to arrow at 11 oclock on stator. It appeared that the timing was not correct so I mucked with it but was alittle unsure that this was the problem. Turned over by hand & valves were hitting pistom now. Have removed the head now cylinder bore looks good but I think that I will probably lap the valves.I did not no that there is a mark on the timing chain as well the manuel didnt say anything about that or maybe I misread. Thanks for youre
> > help, Ken
> >
>
1970-73 C70 Honda Service Manual: http://tinyurl.com/6ebwtw
1970-73 C70 Troubleshooting Guide:
http://tinyurl.com/6ebwtw (scroll to sec.7.1 page 101.jpg)
1980-83 C70 Honda Service Manual: http://tinyurl.com/hu42c
1980-81 C70 Troubleshooting Guide:
http://tinyurl.com/hu42c (scroll to sec.18-1 page 170.jpg)
1982-83 C70 Troubleshooting Guide:
http://tinyurl.com/hu42c (scroll to sec. 20-28 page 205.jpg)
1980-81 C70 Maintenance Schedule: http://tinyurl.com/z4zn6
1982-83 C70 Maintenance Schedule: http://tinyurl.com/hw35c
More resources for C70 Passports and Cubs:
http://www.shlaes.com/Vehicles/Scooter.htm
http://www.velodrome.com/HondaC70/HondaC70.html
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