Compression Value, What Should It Be.

[geoffersgra]

Still struggling with my 315. It just wont go. I have fresh fuel, a spark, the plug gets wet so sure fuel is present. From reading here I suspect the compression may be low. I have measuured it at 145psi. Does anyone have a figure of what it should be. It is possible to turn the engine over by grabbing the flywheel by hand & rotating the engine. Is this normal. Tripple X says you cant do this with his Bultaco. Any comments please.

Geoff

[dan williams]

Both my Beta Rev3s come in around 175PSI if that helps.

[tt5th]

Not sure about the PSI but I certainly can't turn mine over by hand.

[gii]

I'd doubt compression is your problem, I've an old TY mono at the back of the garage that has no discernable compression whatsoever but ten or twenty kicks and it fires up. powerful too even though the piston rattles like it's trying to get out.

I'd guess that either you're not getting enough fuel through or the spark is at the wrong time. Did you ever get the flywheel off and was the woodruff key still present?

If it was me, I'd take the plug out and the airbox off. Heat the plug up with a blowlamp till it's just glowing, then whip it back into the head, just nip it up, then immediately pour a level tablespoon of petrol into the back of the carb and start kicking with around 1/2 throttle. If you still get no sign of life then I'd say the spark is not occurring anywhere near TDC.

The above may sound crude - it is - but engines will run with settings a mile away from ideal and it works for me on bikes, outboards, lawnmowers, anything really The hot dry plug guarantees the spark in the correct place (but not time) and the big slug of petrol in the carb airway provides a combustible vapour.

Edited May 12, 2010 by GII