Honda G30 OHV 30cc Engine Timing

[unxgtrwd]

Hello people. I have a 30cc OHV engine from a Stihl FS65-4 strimmer. On the inside of the plastic casing it says Honda G30, so I guess this is the model of Honda engine.

I have taken it apart for a learning experience. Everything seems in good order, so it is just a case of putting it back together... However, I have not been able to find any technical details for this engine, and am not sure how to set the valve timing correctly.

On the nylon cam gear, there are some marks, two of which look to be marker pen - I guess these marks corraspond to the position of the cam lobe at TDC?

How do I set the timing on this? And does anyone know for sure which model of engine this is/have a service manual for it? Also, what is the mechanism on the back of the cam lobe for?

If any more pictures are required, no problem.

Thanks

 

[btrueblood]

"I have taken it apart for a learning experience."

Then learn the lesson: note the timing marks before disassembly. Photos are cheap in this digital age, and worth their weight in tritium at the stage you have now reached.

Good luck.

You might want to talk to somebody in a small engine repair shop, see if they can let you look at a repair manual.

 

[unxgtrwd]

Indeed this is a lesson learnt.. although I guess now I will have to think about the theory of each of the 4 strokes, and figure out the position of the valves at TDC/BDC.. and if I come close to correct the marks on that cam gear should mean something more obvious I hope

From what I think I know about 4-stroke theory I can figure:

TDC to BDC = intake = @TDC intake open 100% exhaust closed 100%
BDC to TDC = compression = @BDC both valves closed 100%
TDC to BDC = combustion = @TDC both valves closed 100%
BDC to TDC = exhaust = @BDC exhaust valve open 100% intake closed 100%

If it was as exact as that it might not be too hard to figure it out rotationally where to mesh the cam gear to the crank, but would I be right in thinking that the valves will probably open slightly out of sync with TDC and BDC for reasons of performance, tuning and other stuff I didn't think of yet?

If I try and run the engine set like my list with 100% valve open at TDC/BDC, will I break something? Or can you give me a sensible setup to work from/tweak?

Any help appriciated

 

[j79guy]

unxgtrwd: Likely the closest you will get, short of reading the correct manual, or bumping into an experienced Honda tech, is to set the camshaft so that the middle of the valve overlap period is at, or as near as you can get to TDC. About as simple as I can put it.

Robin Sipe.

 

[patprimmer]

J9 is correct.

If it is a single cam, use the tooth that sets it as close to split overlap as possible.

Split overlap is when the inlet and exhaust valves are equal distance off their seats.

Most engines require split overlap to be within about 6 deg of TDC, but not actually at TDC. One tooth will be a lot more than 6 deg, so within about 6 deg will be the correct mesh.

If it's a win cam you are in a lot more trouble and will need to put the engine with #1 at TDC then tun each cam so the valves are a little off the seat, then look for marks, or holes that align so you can drop in a dowel to lock it there while you set the other cam then engage the gears or belts or chains or whatever. It is entirely possible to set the cams with to much or to little lobe centre separation which will result in to much or to little valve lift at TDC.



Regards
Pat
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[unxgtrwd]

Thanks guys for pointing me to valve overlap. I read up on this, and from that I was able to figure out the cam. There is a little notch that keeps the exhaust valve briefly open at the start of the intake stroke, which creates the point of overlap. The marks on the cam also make sense when I put the piston at TDC, with the cam meshed to start at the intake valve. I have turned the crank by hand and the valves open in sync as per my previous post, and overlap at the right time.

I have yet to put the engine back together, I'll leave that for the weekend. I hope it will start and not destroy itself

This is really just a primer because I have a rust bucket Triumph Herald in the garage to restore...

 

[dicer]

Just aline the marks.