Laurentic--Yes the compression ratio was changed. Was it changed enough to keep the engine from running? I doubt it very much. I didn't do an actual calculation on it.---Brian
Brian,
What was the thickness of the spacer under the cylinders? To me it looks like its about 1/8" (3.175mm)? That is going to have a significant impact on the compression ratio of the engine.
The following are assumptions as we don't have all the details, but I think it gives a good idea as to what may still be wrong with the internal combustion process (induction,
compression, ignition, expansion and exhaust).
Assuming that you designed it initially with the a 6:1 compression ratio (as a copy of Malcom Strides Bobcat), using your modified 25.4mm x 22mm bore and stroke we can reverse engineer a few things:
total displacement = 11.15cc with compression ratio 6:1 results in a design intent of 1.86cc combustion chamber volume
adding the volume the spacer adds (assuming 3.175mm) = 1.61cc plus the original chamber volume of 1.86cc = 3.47cc combustion chamber volume as built.
working that out 11.15 to 3.47 is roughly a
3.2:1 compression ratio - likely too low IMHO.
Brian, you have been asked a few times about how the engine behaves with a bump test - its not a pointless exercise - it will help validate if there is compression, and how much is there. We don't need a psi # to know if there is no compression, just spin the engine by hand - is there resistance to compression once per rev and if so does it feel good (snap over TDC) or is it "soft"? Take a video as you do it, and share, if you really want some help from those on this forum.
Suggestions for anyone else looking for help on why their engine will not run:
1. provide all possible info:
Is it built to an existing design or is it a new or modified design?
If modified - what did you change? details matter when things aren't working.
In this case Brian has changed a fair amount I understand:
Cam tower height,
valve length
valve spring
Cylinders spaced up (i.e. reduced compression ratio)
induction (twin carb)
bore and stroke,
o-ring piston ring
crankcase size,
etc. , etc. to the point that I think it is effectively a new design (along with all the possible errors that come with a new design).
2. Can you provide a video? If so it should show if possible:
a. slowly turning the engine over - is there compression resistance as its turned over? For multi cylinder is there resistance as each cylinder comes to its compression stroke? Does the engine "snap" over TDC, or is it "soft" when turning over.
b. If spark ignition, does it spark when turned over (assuming coil ignition or CDI, not magneto) and spark approximately when it should (just before TDC on compression stroke)?
c. slowly turning the engine over, show direction of rotation, and if possible the valve actuation (for each cylinder if multi cylinder)
Regards,
Mike