Topic: How much compression does an engine really need to just at least fire?

[drumstyx]

I've got a rare little Honda C65 I picked up (basically a C50 frame with the engine from an S65, which is just a slightly smaller top end version of the C70 motor) and after getting the cylinder unstuck, of course there's not too much compression, around 70psi. Book says 170psi is spec.

Ok, that's fine, I didn't expect much, but I'd like to hear it fire a couple times before putting any money into it.

I finally got a spark, ignition timing *should* be set right, you'd think squirting some fuel down the intake I'd at least get *something*, no? Even just a little puff? Kicking it over sounds just like when there's no fuel, just the pooting sound of plain air coming out of the exhaust.

If it's to be expected that 70psi would yield precisely 0 combustion, I'll move onto a new top end, but I just want to make sure I'm not missing something ignition-wise.

[strynboen]

use to oil mine cd 50 heavy. in the spark plug hole.to get kompresion up...specily at first staret up after a cylender repaeir..it can just not suck the fuel up..vitaut some oil in the piston rings..

[scottly]

Try push starting it in 1st gear. If the motor spins fast enough, it will build pressure faster than it leaks past the rings. Well, sometimes.

[spotty]

i used to have a Lambretta 150 that had no discernible compression at all but if you kicked it over dozens of times in a row really quickly it would eventually build up enough momentum to fire and run, though run was an entirely subjective term as it was flat out at about 35mph, it was really embarrassing being passed by pushbikes uphill and the like, for that matter my sisters yamaha 50 auto scooter was quicker...... 

[MoMo]

It should start with 90-100, as mentioned earlier squirt some oil in the spark plug hole

[drumstyx]

It should start with 90-100, as mentioned earlier squirt some oil in the spark plug hole

That got it to bark! 20w50 in there got me *just* about 90psi on my hardest kicks, and I even got it to start -- as in rev a tiny bit -- with starter fluid. Tried some gear oil to see if heavier oil might help, but no luck, never got it as good as that first little half-second rev.

Anyway, I knew it wouldn't run well given that horrendous compression, just wanted to hear it try a bit.

Now my legs hurt from kicking the damn thing.

[drumstyx]

So let's say I want to do a real 3rd-world job on this -- absolutely as cheap as possible to get it running, labour cost being 'free', and no consideration for doing things "wrong".

Could I get away with just a hone and piston rings? Yes, I know I should replace gaskets, I know my exhaust valve looks like trash, I know I *should* bore or replace the cylinder, etc. But I want to see if I can get it running with nearly nothing, and popping the head/cylinder on these things takes 10 minutes tops.

[MoMo]

give it a shot, if the new ring end gap is good you might get away with it...Larry

[drumstyx]

Welp, the rings were not coming off the old piston, simply impossible.

All good though, because I mic'd the bore and the piston and found the bore is actually in great shape, but the piston wasn't in fantastic shape anyway. $15 more to include the piston.

2-6 weeks shipping wait though, of course...

Ah well, on a positive note, I spent all evening lapping the valves and got the leak out of the exhaust valve!

I was gonna get this thing running and then sell it to someone to restore, but after all this work, I'm growing fond of it...

[70CB750]

When I got my 70 CB750, the compression was low.  Once I got it on the road, the compression improved quite a lot.  I figured the rings were stuck.

Sometimes it helps to get the engine running and ride the bejesus out of it.

[strynboen]

you can boil the piston in diesel fuel(be care it not exploes...it vill free the rings up..do it autside..in a safe distance from anything

[drumstyx]

you can boil the piston in diesel fuel(be care it not exploes...it vill free the rings up..do it autside..in a safe distance from anything

Haha! That about takes the cake for a "3rd world job". I want to get it going cheap, but I'll pay $20 for a piston to not have to boil diesel. Gosh, the neighbours would think me mad!

[strynboen]

ha ha ..ok you go the safe vay..i did it on my cb 77...the engine vas frozen but the hone marks vas still there ..so a low mile engine..had to save it ..even i had to do danger 3 vorld stuff....
there vas nice big flames...as the thik  vhite damp ignited....